AU2007229367B2 - In Vitro Micro-Organs and Uses Related Thereto - Google Patents

Description

I AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Yissum Research Development Company of The Hebrew University of Jerusalem Address for Service: CULLEN & CO. Level 26 239 George Street Brisbane Qld 4000 Invention Title: In Vitro Micro-Organs and Uses Related Thereto The following statement is a full description of this invention, including the best method of performing it, known to us: 2 Technical Field The invention described herein relates to micro-organ explants, methods for the production of the explants, methods of utilizing the explants, extracts derived therefrom and 5 devices including said extracts for treating disorders in host tissues. The present invention further relates to micro-organs transformed with exogenous nucleotide sequences and methods of utilizing same for delivering the expression products of said nucleotide sequences to host tissues. 0 Background of the Invention Eukaryotic cell culture was first achieved in the early 1950s. Since that time, a wide range of transformed and primary cells have been cultivated using a wide variety of media and defined supplements, such as growth factors and hormones, as well as undefined supplements, 5 such as sera and other bodily extracts. For example, fibroblasts obtained from the skin of an animal can be routinely cultivated through many cell generations as karyotypically diploid cells or indefinitely as established cell lines. Epithelial cells, however, have morphological and proliferative properties that differ from fibroblasts and are more difficult to cultivate. Moreover, when epithelial cells and fibroblasts are grown in the same culture, the epithelial cells are !0 commonly overgrown by the fibroblasts. While the growth of cells in two dimensions is a convenient method for preparing, observing and studying cells in culture, allowing a high rate of cell proliferation, it lacks the cell-cell and cell-matrix interactions characteristic of whole tissue in vivo. In order to study such functional and morphological interactions, a few investigators 25 have explored the use of three-dimensional substrates such as collagen gel (Douglas et al., (1980) In Vitro 16:306-312; Yang et al., (1979) Proc. Nati. Acad. Sci. 76:3401; Yang et al. (1980) Proc. Nati. Acad. Sci. 77:2088-2092; Yang et al., (1981) Cancer Res. 41:1021-1027); cellulose sponge, alone (Leighton et al., (1951) J. Natl Cancer Inst. 12:545-561) or collagen coated (Leighton et al., (1968) Cancer Res. 28:286-296); a gelatin sponge, Gelfoam (Sorour et 30 al., (1975) J. Neurosurg. 43:742-749). For growing epithelial cells in a clonally competent manner, a variety of culture conditions have been employed. For example, epithelial cells, and in particular, skin epithelial cells (keratinocytes), have been cultivated on feeder layers of lethally irradiated fibroblasts (Rheinhardt et al. (1975) Cell 6:331-343) and on semi-synthetic collagen matrices (U.S. Patent 35 No. 5,282,859; European Patent Application No. 0361957). In some cases, the media used to grow such cells is manipulated by adding biological extracts, including pituitary extracts and sera, and growth supplements, such as epidermal growth factor and insulin (Boisseau et al. (1992) J. Dermatol. Sci 3(2):111-120; U.S. Patent No. 5,292,655).

3 Numerous attempts at growing skin in vitro have been undertaken. These attempts typically include the step of separating the keratinocytes in the epidermis from fibroblasts and fat cells in the dermis. After separation, the keratinocytes are generally grown in a manner that permits the formation of a stratified epidermis. The epidermis prepared in this manner, 5 however, lacks hair follicles and sweat glands. Moreover, in such cultures, the natural relationship between the epidermis and the dermis is not preserved. Cultivation methods including growing keratinocytes on non-viable fibroblasts (Rheinwald et al. (1975) Cell 6:331 343 or placing keratinocytes on a dermal substrate of collagen and fibroblasts that is synthetic or has been derived from an alternative source from that of the epidermis (Sugihara et al. (1991) 0 Cell. Dev. Biol. 27:142-146; Parenteau et al. (1991) J. Cell Biochem. 45(3):245-251) have also been undertaken. In some cases, however, separation of keratinocytes is not performed and the whole organ is placed in culture. Attempts to cultivate organs in vitro have been limited to incubating organs in a serum-containing medium (Li et al. (1991) Proc. Nati. Acad. Sci. 88(5):108-112). 5 Most existing in vitro models of the epidermis lack hair follicles, sweat glands and sebaceous glands (for a view of epidermal cell culture, see Coulomb et al. (1992) Pathol. Biol. Paris 40(2):139-146). Exceptions include the gel-supported skin model of Li et al. ((1992) Proc. Natl. Acad. Sci 89:8764-8768) in which skin explants with dimensions of 2 x 5mm2 and 2.0 mm thick remained viable for several days in the presence of serum-containing media. !0 In addition to the drawbacks of cell damage, bio-reactors and other methods of culturing mammalian cells are also very limited in their ability to provide conditions which allow cells to assemble into tissues which simulate the spatial three-dimensional form of actual tissues in the intact organism. Conventional tissue culture processes limit, for similar reasons, the capacity for cultured tissues to express a highly functionally specialized or differentiated state considered 25 crucial for mammalian cell differentiation and secretion of specialized biologically active molecules of research and pharmaceutical interest. Unlike microorganisms, the cells of higher organisms such as mammals form themselves into high order multicellular tissues. Although the exact mechanisms of this self-assembly are not known, in the cases that have been studied thus far, development of cells into tissues has been found to be dependent on orientation of the cells 30 with respect to each other (the same or different type of cell) or other anchorage substrate and/or the presence or absence of certain substances (factors) such as hormones, autocrines, or paracrines. In summary no conventional culture process is capable of simultaneously achieving sufficiently low shear stress, sufficient 3-dimensional spatial freedom, and sufficiently long periods for critical cell interactions (with each other or substrates) to allow excellent modeling 35 of in vivo tissue structure. There is a need, therefore, for in vitro methods of generating and maintaining portions of organs in cultures in which the cells of the culture preserve their natural intercellular relationships for extended periods of time. The availability of tissue and organ models in which 4 cell differentiation, cell proliferation, and cell function mimics that found in the whole organ in vivo would have utility in understanding the mechanisms by which organs are maintained in a healthy state and consequently how abnormal events may be reversed. 5 Summary of the Invention The present invention provides an in-vitro micro-organ culture which addresses the above-cited needs. Salient features of the subject micro-organ cultures include the ability to be maintained in culture for relatively long periods of time, e.g., at least about twenty four hours, 0 preferably for at least seven days or longer, as well as the preservation of an organ microarchitecture which facilitates, for example, cell-cell and cell-matrix interactions analogous to those occurring in the source organ. Typically, at least one cell of the population of cells of the micro-organ culture has the ability to proliferate. The population of cells in the micro-organ culture can, overall, be in a 5 state of equilibrium, i.e., the ratio of cell proliferation to cell loss in the population of cells is approximately one, or the cells in the micro-organ culture can be proliferating at a greater rate than they are lost, resulting in a ratio of cell proliferation to cell loss in the population of cells which is greater than one, e.g., as in a population of cells obtained from neoplastic tissue, or, e.g., a progenitor cell population induced to proliferate in an explant. !0 Preferred organs from which the cells of the micro-organ culture can be isolated include lymphoid organs, e.g., thymus and spleen; digestive tract organs, e.g., gut, liver, pancreas, gallbladder and bile duct; lung; reproductive organs, e.g., prostate and uterus; breast, e.g., mammary gland; skin; urinary tract organs, e.g., bladder and kidney; cornea; and blood associated organs such as bone marrow. The isolated population of cells of the micro-organ 25 culture can, in certain embodiments, be encapsulated within polymeric devices, e.g., for delivery of the cells or cell products, e.g., gene products, to a subject. The present invention also pertains to conditioned medium isolated from the micro-organ cultures of the present invention. In one embodiment of the present invention, the micro-organ culture includes a population of cells which is a section of an organ. Preferably, the micro-organ explant includes 30 epithelial and connective tissue cells. In one embodiment of the invention, the organ explant is obtained from a pancreas, e.g., the microarchitecture of the population of cells is substantially the same as the microarchitecture of the original pancreas from which the explant was derived, and includes pancreatic epithelial cells, e.g., islet cells, and pancreatic connective tissue cells. In another embodiment of the invention, the micro-organ explant is obtained from skin, 35 e.g., microarchitecure of the population cells is substantially the same as the microarchitecture of skin in vivo, and includes skin epithelial, e.g., epidermal cells, and skin connective tissue cells, e.g., dermal cells. The micro-organ culture which is obtained from a skin explant can also 5 include a basal lamina supporting the epidermal cells, an extracellular matrix which includes the dermal cells, and at least one invagination, e.g., at least one hair follicle or gland. In another embodiment of the present invention, the micro-organ culture includes an isolated population of cells infected with a virus, such as a hepatitis virus, e.g., hepatitis B or 5 hepatitis C, or a human papilloma virus (HPV), e.g., HPV-6, HPV-8, or HPV-33. When infected with a virus, the micro-organ culture can be used in a method for identifying an inhibitor of viral infectivity. This method includes isolating a micro-organ explant according to the method of the present invention, which explant is derived from a virally-infected organ, or is subsequently infected in vitro with a virus to produce a population of virus-infected cells in the 0 explant. The explant can then be contacted with a candidate agent, e.g., agent which is being tested for anti-viral activity, and the level of infectivity (e.g., viral loading, new infectivity, etc) in the presence of the candidate agent is measured and compared to the level of infectivity by the virus in the absence of the candidate agent. A decrease in the level of infectivity of the virus in the presence of the candidate agent is indicative of an inhibitor of viral infectivity. 5 The present invention also pertains to a method for producing a micro-organ culture. This method includes isolating, from a mammalian donor subject, a micro-organ explant having dimensions which provide the isolated population of cells as maintainable in a minimal medium for at least about twenty-four hours. The micro-organ explant is then placed in culture. Typically, the explant includes an isolated population of cells having a microarchitecture of the .0 organ from which the explant is isolated. In one embodiment of the present invention, at least one cell of the explant has the ability to proliferate. The cells of the subject micro-organ culture can be in a state of equilibrium, i.e., the ratio of cell proliferation to cell loss in the population of cells is one, or the cells in the micro-organ culture can be proliferating at a greater rate than they are lost resulting in a ratio of cell proliferation to cell loss in the population of cell loss in 25 the population of cells which is greater than one, e.g., the micro-organ explant includes a population of cells obtained from neoplastic tissue. Preferred organs from which the cells of the micro-organ culture can be isolated include lymphoid organs, e.g., thymus and spleen; digestive tract organs, e.g., gut, liver, pancreas, gallbladder and bile duct; lung; reproductive organs, e.g., prostate and uterus; breast; skin; 30 urinary tract organs, e.g., bladder; kidney; cornea; and blood-associated organs such as bone marrow. Organs from which the cells of the micro-organ culture can be isolated also include muscles. In each of these examples, the microarchitecture of the organ is maintained by the cultured explant. The micro-organ culture can be a tissue section, e.g., a pancreatic tissue section which includes p-islet cells, e.g., a skin tissue section which includes epidermal and 35 dermal cells and other skin-specific architectural features, e.g., hair follicles. Cells in the micro-organ explants can also be modified to express a recombinant protein, which protein may or may not be normally expressed by the organ from which the explant is derived. For example, gene products normally produced by the pancreas, and which can be 6 augmented by the subject transgenic method, e.g., to correct a deficiency, include insulin, amylase, protease, lipase, trypsinogen, chymotrypsinogen, carboxypeptidase, ribonuclease, deoxyribonuclease, triacylglycerol lipase, phospholipase A 2 , elastase, and amylase; likewise, gene products normally produced by the liver, and which can be complemented by replacement 5 gene therapy, include blood clotting factors, such as blood clotting Factor VIII and Factor IX, UDP glucuronyl transferase, ornithine transcarbamoylase, and cytochrome p450 enzymes; gene products normally produced by thymus include serum thymic factor, thymic humoral factor, thymopoietin and thymosin a. The micro-organ culture of the present invention can be used in a method for delivering 0 a gene product to a recipient subject. This method includes providing an isolated population of cells from a donor subject, the population of cells having a microarchitecture of an organ or tissue from which the cells are isolated and a surface area to volume which provides the isolated population of cells as maintainable in a minimal medium for at least about twenty-four hours. A recombinant nucleic acid which encodes and directs expression of a desired gene product can 5 then be introduced into the population of cells to produce a population of transgenic cells in the micro-organ explant, e.g., a transgenic explant. The transgenic explant can be administered to a recipient subject. The donor subject and the recipient subject can be of the same species or of different species. The micro-organ culture of the present invention can also be used in a method for !0 identifying agents which induce proliferation of cells of a given organ, including progenitor cells. This method includes generating a micro-organ explant culture according to the present invention, which explant includes at least one cell which has the ability to proliferate. After being placed in culture, the explant is contacted with a candidate compound, e.g., a compound to be tested for cell proliferative capacity, and the level of cell proliferation in the presence of 25 the candidate compound is measured. The measured level of cell proliferation in the presence of the candidate compound is then compared to the level of cell proliferation in the absence of the candidate compound. An increase in the level of cell proliferation in the presence of the candidate compound is indicative of a cell proliferative agent. Inhibitors of cell proliferation can be identified using a similar method. Specifically, when the measured level of cell proliferation 30 in the presence of the candidate compound is determined using the above-described method, it can be compared to the level of cell proliferation in the absence of the candidate compound. A decrease in the level of cell proliferation in the presence of the candidate compound is indicative of an inhibitor of cell proliferation. Another method in which the micro-organ culture of the present invention can be used is 35 in a method for identifying an agent which induces, or inhibits, differentiation of one or more cell types in a given organ, or an agent which maintains a particular differentiated state (prevent dedifferentiation). This method includes generating a micro-organ explant from the organ of interest, the population of cells making up the explant having a microarchitecture of that organ, 7 as described hereonbelow, aleph of at least about 1.5 mm -, and including at least one cell which has the ability to differentiate or is differentiated and has the ability to dedifferentiate. Once in culture, the population of cells is contacted with a candidate compound and the level of cell differentiation in the presence of this compound is measured. The measured level of cell 5 differentiation in the presence of the candidate compound is compared with the level of cell differentiation in the absence of the candidate compound. An increase in the level of cell differentiation in the presence of candidate compound is indicative of cell differentiating agent. Inhibitors of cell differentiation can be identified using a similar method. In particular, when the measured level of cell differentiation in the presence of the candidate compound is determined 0 using the above-described method, it can be compared to the level of cell differentiation in the absence of the candidate compound. A decrease in the level of cell differentiation in the presence of the candidate compound is indicative of an inhibitor of cell differentiation. Yet another aspect of the present invention provides a method for identifying, and isolating, stem cell or progenitor cell populations from an organ. This method generally 5 provides isolating, in a culture, an explant of a population of cells from an organ. As described herein, the explant is characterized by (i) maintenance, in the culture, of a microarchitecture of the organ from which the explant is derived, (ii) a surface area to volume index (aleph) of at least about 1.55 mm- , and (iii) at least one progenitor or stem cell which has the ability to proliferate. The explant is contacted with an agent which induces proliferation of the progenitor !0 or stem cell, e.g., a growth factor or other mitogen, in order to amplify discrete populations of cells in the explant. Subsequently, the amplified progenitor cells can be isolated from the explant. Such sub-populations of the explant can be identified by virtue of their proliferative response. In other embodiments, the progenitor/stem cells will proliferate spontaneously in the culture even without addition of an exogenous agent. In other embodiment, progenitor or stem 25 cells from the explant that proliferate in response to the agent can be isolated, such as by direct mechanical separation of newly emerging buds from the rest of the explant or by dissolution of all or a portion of the explant and subsequent isolation of the amplified cell population. Still another method in which the micro-organ culture of the present invention can be used is in a method for promoting wound healing in a recipient subject. This method includes 30 isolating, from a donor subject, a population of cells having an aleph of at least approximately 1.5 mm and applying the population of cells to a wound of the recipient subject. The donor subject and the recipient subject can be of the same species or of different species. In one embodiment, the tissue from which the cells are isolated is skin and the wound of the recipient subject is an ulcer, e.g., an ulcer associated with diabetes.

8 The practice of the present invention will employ, unless otherwise indicated, conventional techniques of cell biology, cell culture, molecular biology, transgenic biology, microbiology, recombinant DNA, and immunology, which are within the skill of the art. Such techniques are explained fully in the literature. See, for example, Molecular Cloning: A 5 Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch and Maniatis (Cold Spring Harbor Laboratory Press: 1989); DNA Cloning, Volumes I and II (D. N. Glover ed., 1985); Oligonucleotide Synthesis (M. J. Gait ed., 1984); Mullis et al. U.S. Patent No: 4,683,195; Nucleic Acid Hybridization (B. D. Hames & S. J. Higgins eds. 1984); Transcription And Translation (B. D. Hames & S. J. Higgins eds. 1984); Culture of Animal Cells (R. I. Freshney, 10 Alan R. Liss, Inc., N.Y.); Gene Transfer Vectors for Mammalian Cells (J.H. Miller and M.P. Calos eds., 1987, Cold Spring Harbor Laboratory); Methods in Enzymology, Vols. 154 and 155 (Wu et al. eds.), Immunochemical Methods In Cell And Molecular Biology (Mayer and Walker, eds., Academic Press, London, 1987); and Handbook of Experimental Immunology, Volumes I-IV (D.M. Weir and C.C. Blackwell, eds., 1986). 15 Definitions of the specific embodiments of the invention as claimed herein follow. According to a first embodiment of the invention, there is provided a genetically modified micro-organ explant expressing at least one recombinant gene product, wherein said micro-organ explant is a section of tissue comprising a population of cells maintaining the microarchitecture and the three-dimensional structure of an organ from which it is obtained and 20 at the same time having dimensions selected so as to allow diffusion of adequate nutrients and gases to cells in the micro-organ explant and diffusion of cellular waste out of the micro-organ explant so as to minimize cellular toxicity and concomitant death, wherein said dimensions provide for a micro-organ explant with a surface area to volume index characterized by the formula 1/x + 1/a > 1.5 mm 1 ; wherein 'x' is a tissue thickness and 'a' is a width of said tissue in 25 millimeters, and wherein at least some of the cells of said population of cells of said micro organ explant comprise a recombinant gene and express at least one recombinant gene product, wherein said recombinant gene product is Factor VIII. According to a second embodiment of the invention, there is provided a method of delivering a gene product to a subject, the method comprising the steps of: 30 (a) providing a micro-organ explant obtained from said subject, wherein said micro organ explant is a section of tissue comprising a population of cells, which maintains a microarchitecture and a three dimensional structure of an organ from which it is obtained and at the same time having dimensions selected so as to allow diffusion of 8a adequate nutrients and gases to cells in the micro-organ explant and diffusion of cellular waste out of the micro-organ explant wherein said dimensions provide for a micro-organ explant with a surface area to volume index characterized by the formula 1/x + 1/a > 1.5 mm-'; wherein 'x' is a tissue thickness and 'a' is a width of said tissue in millimeters; 5 (b) genetically modifying ex-vivo at least some cells of the micro-organ explant with a recombinant gene to express at least one recombinant gene product; and (c) implanting the genetically modified micro-organ explant in said subject, wherein said subject is a human being. Other features and advantages of the invention will be apparent from the following 10 detailed description, and from the claims. Brief Description of the Drawings Figure 1 is a diagrammatic representation of a micro-organ depicting the dimensions that determine Aleph where x = thickness and a = width of tissue. Figure 2 is a histogram showing cell proliferation in a guinea pig micro-organ culture as 15 determined by BrdU labeling after incubation for different time periods. Figure 3 is a histogram showing cell proliferation in a human back skin micro-organ culture as determined by BrdU labeling after incubation of cultures for 1-8 days. Figures 4A-4D are micrographs showing immunofluorescence corresponding to replicating cells of mouse skin (mag. 50x) (Figure 4A), guinea pig skin (mag. 75x) (Figure 4B) 20 human foreskin (mag. 50x) (Figure 4C) and human foreskin (mag. 75x) (Figure 4D). Figures 5A-5C are transverse sections of human epidermal micro-organ explants. (mag x75) showing tissue architecture at zero (Figure 5A), three (Figure 5B) and six (Figure 6D) days in culture. Figure 6 is a histogram demonstrating the effect on epidermal proliferation of varying 25 thickness (x) of guinea pig skin micro-organ cultures using BrdU incorporation where (a) has been kept constant at 4mm. Figures 7A-7B are micrographs showing immunofluorescence corresponding to proliferating cells in pancreas-derived micro-organ cultures (mag 75x). Figure 8 is a histogram showing amounts of insulin released by adult pig pancreas 30 micro-organ cultures.

9 Figure 9 is a histogram showing 3 H-Thymidine incorporation in proliferating cells in micro-organ cultures of the colon, liver, kidney, duodenum and esophagus, at three days, four days and six days of culture. Figures 10A-10C are micrographs showing active proliferation of hair follicles in 5 micro-organ cultures as determined by immunofluorescence. Magnification 40x (Figure 1OA), 40x (Figure lOB), and 75x (Figure 1 OC). Figure 11 is a histogram showing the size distribution of hair shafts at the beginning and end of the microculture. Figure 12 is a histogram showing the inhibition of mitogenesis in micro-organ cultures 0 in the presence of 2.5 ng/ml TGF-p in guinea-pig skin cultures. Figure 13 is a diagrammatic representation of a micro-organ explant for treatment of chronic skin ulcers showing incomplete sectioning of tissue slices so as to maintain a structure that can be readily manipulated in vivo. Figure 14 is a photograph of the surface of a mouse after replacement of a piece of 5 normal skin with a micro-organ culture; healing, generation of new hair shafts in the implant, and incorporation of the implant into the normal mouse skin can be observed (mag 1Ox). Figure 15 is a graphic representation of the expression of a luciferase reporter gene in a guinea pig skin micro-organ culture after transfection ( of the culture with a plasmid encoding the luciferase reporter gene. to Figure 16 is a graphic representation of the expression of a luciferase gene in rat lung and thymus micro-organ cultures after cationic lipid mediated transfection of the culture with plasmid encoding the luciferase reporter gene. Figure 17 is a graphic representation of the activation of telogen follicles upon treatment with FGF in micro-organ cultures of the present invention. 25 Figure 18 is a graphic representation of the expression of a transgenic luciferase gene in micro-organ explants of the present invention. Figure 19 is a photograph showing neo-vascularization around an implanted micro organ (marked with arrow). Figure 20 is a graph illustrating the relative levels of various angiogenic factors 30 expressed in transplanted micro-organs. Angl - angiopoietin 1, Ang2 - angiopoietin 2, MEF2C - myocyte enhancer factor 2C, VEGF - vascular endothelial growth factor. Figure 21 is an angiogenic factor-specific RT-PCR of RNA extracted from micro organs cultured outside the body for various time points following prepration. Actin - beta actin (control). 35 Figure 22 is a graph representing semi-quantitative data obtained by densitometry of the RT-PCR products shown in Figure2l, normalized to the intensity of the beta-actin RT-PCR product (control).

10 Figure 23 is a histogram representing the gating pattern of common iliac-ligated rats implanted with micro-organs or sham implanted (control). (n) = 13. P values for the three time groups (from left to right) are 0.16, 1 and 0.841. Scores: 0-full functionality 9-total inability to move the limb, 10 loss of the limb.; 5 Figure 24 is a histogram representing the same experimental group as in Figure 23 with the exception that the animals were now exerted prior to scoring gating behavior. P values for the three time groups are (from left to right) 0.0001, 0.0069 and 0.06. Figure 25 is a histogram representing the gating pattern of common iliac-ligated mice implanted with micro-organs or sham implanted. Scores: 0-full functionality 9-total inability to 0 move the limb, 10 loss of the limb. P values for the three time groups are (from left to right) 0.00025, 0.00571 and 0.07362. Figure 26 is an image illustrating a mouse spleen derived micro-organ (marked with MC arrow) six months following implantation into a subcutaneous region of a syngeneic mouse. One of the newly formed blood vessels surrounding the micro-organ is marked with an 5 arrow. Figure 27 is an image illustrating a rat cornea implanted with lung micro-organs from a syngeneic rat. The implanted micro-organ (marked with arrow) is surrounded by newly formed blood vessels. !0 Detailed Description of the Invention The present invention is directed to a three-dimensional organ explant culture system. This culture system can be used for the long term proliferation of micro-organ explants in vitro in an environment that closely approximates that found in the whole organ in vivo. The culture 25 system described herein provides for proliferation and appropriate cell maturation to maintain structures analogous to organ counterparts in vivo. The micro-organ cultures of the present invention provide in vitro culture systems in which tissue or organ sections can be maintained and their function preserved for extended periods of time. These culture systems provide in vitro models in which cell differentiation, cell 30 proliferation, cell function, and methods of altering such cell characteristics and functions can be conveniently and accurately tested. The resulting cultures have a variety of applications ranging from transplantation or implantation in vivo, to screening cytotoxic compounds and pharmaceutical compounds in vitro, to the production of biologically active molecules in "bioreactors", and to isolating progenitor cells from a tissue. 35 For example, and not by way of limitation, specific embodiments of the invention include (i) micro-organ bone marrow culture implants used to replace bone marrow destroyed during chemotherapeutic treatment; (ii) micro-organ liver implants used to augment liver function in cirrhosis patients; (iii) genetically altered cells grown in the subject micro-organ 11 culture (such as pancreatic micro-organs which express a recombinant gene encoding insulin); and (iv) dental prostheses joined to a micro-organ culture of oral mucosa. In yet other illustrative non-limiting embodiments, the subject micro-organ cultures may be used in vitro to screen a wide variety of compounds, such as cytotoxic compounds, 5 growth/regulatory factors, pharmaceutical agents, etc. To this end, the micro-organ cultures are maintained in vitro and exposed to the compound to be tested. The activity of cytotoxic compound can be measured, for example, by its ability to damage or kill cells in the explant. This may readily be assessed by vital staining techniques. The effect of growth/regulatory factors may be assessed by analyzing the cellular content of the explant, e.g., 0 by total cell counts, and differential cell counts. This may be accomplished using standard cytological and/or histological techniques including the use of immunocytochemical techniques employing antibodies that define type-specific cellular antigens. The effect of various drugs on normal cells cultured in the three-dimensional system may be assessed. For example, drugs that increase red blood cell formation can be tested on the bone marrow micro-organ cultures. Drugs 5 that affect cholesterol metabolism, e.g., by lowering cholesterol production, could be tested on the liver micro-organs. Micro-organ cultures of abnormal tissue can also be employed, such as to facilitate study of hyperproliferative or neoproliferative disorders. For instance, micro-organ explants of organs invaded by tumor cell growth may be used as model systems to test, for example, the efficacy of anti-tumor agents. !0 For convenience, certain terms employed in the specification, examples, and appended claims are collected here. The term "explant" refers to a collection of cells from an organ, taken from the body and grown in an artificial medium. When referring to explants from an organ having both stromal and epithelial components, the term generally refers to explants which contain both components 25 in a single explant from that organ. The term "tissue" refers to a group or layer of similarly specialized cells which together perform certain special functions. The term "organ" refers to two or more adjacent layers of tissue, which layers of tissue maintain some form of cell-cell and/or cell-matrix interaction to generate a microarchitecture. In 30 the present invention, micro-organ cultures were prepared from such organs as, for example, mammalian skin, mammalian pancreas, liver, kidney, duodenum, esophagus, bladder, cornea, prostrate, bone marrow, thymus and spleen. The term "stroma" refers to the supporting tissue or matrix of an organ. The term "micro-organ culture" as used herein refers to an isolated population of cells, 35 e.g., an explant, having a microarchitecture of an organ or tissue from which the cells are isolated. That is, the isolated cells together form a three dimensional structure which simulates/retains the spatial interactions, e.g. cell-cell, cell-matrix and cell-stromal interactions, and the orientation of actual tissues and the intact organism from which the explant was 12 derived. Accordingly, such interactions as between stromal and epithelial layers is preserved in the explanted tissue such that critical cell interactions provide, for example, autocrine and paracrine factors and other extracellular stimuli which maintain the biological function of the explant, and provide long term viability under conditions wherein adequate nutrient and waste 5 transport occurs throughout the sample. The subject micro-organ cultures have a microarchitecture of an organ or tissue from which the cells or tissue explant are isolated. As used herein, the term "microarchitecture" refers to an isolated population of cells or a tissue explant in which at least about 50%, preferably at least about 60%, more preferably at least about 70%, still more preferably at least about 80 %, 0 and most preferably at least about 90% or more of the cells of the population maintain, in vitro, their physical and/or functional contact with at least one cell or non cellular substance with which they are in physical and/or functional contact in vivo and form a cell culture of at least about one, more preferably at least about five, and most preferably at least about ten layers or more. Preferably, the cells of the explant maintain at least one biological activity of the organ or 5 tissue from which they are isolated. The term "isolated" as used herein refers to an explant which has been separated from its natural environment in an organism. This term includes gross physical separation from its natural environment, e.g., removal from the donor animals, e.g., a mammal such as a human or a miniature swine. For example, the term "isolated" refers to a population of cells which is an .0 explant, is cultured as part of an explant, or is transplanted in the form of an explant. When used to refer to a population of cells, the term " isolated" includes population of cells which result from proliferation of cells in the micro-organ culture of the invention. The term "ectoderm" refers to the outermost of the three primitive germ layers of the embryo; from it are derived the epidermis and epidermal tissues such as the nails, hair and 25 glands of the skin, the nervous system, external sense organs and mucous membrane of the mouth and anus. The terms "epithelia" and "epithelium" refer to the cellular covering of internal and external body surfaces (cutaneous, mucous and serous), including the glands and other structures derived therefrom, e.g., corneal, esophageal, epidermal and hair follicle epithelial 30 cells. Other exemplary epithelial tissues include: olfactory epithelium, which is the pseudostratified epithelium lining the olfactory region of the nasal cavity, and containing the receptors for the sense of smell; glandular epithelium, which refers to epithelium composed of secreting cells; squamous epithelium, which refers to epithelium composed of flattened plate like cells. The term epithelium can also refer to transitional epithelium, which is that 35 characteristically found lining hollow organs that are subject to great mechanical change due to contraction and distention, e.g. tissue which represents a transition between stratified squamous and columnar epithelium. The term "epithelialization" refers to healing by the growth of epithelial tissue over a denuded surface.

13 The term "skin" refers to the outer protective covering of the body, consisting of the corium and the epidermis, and is understood to include sweat and sebaceous glands, as well as hair follicle structures. Throughout the present application, the adjective "cutaneous" may be used, and should be understood to refer generally to attributes of the skin, as appropriate to the 5 context in which they are used. The term "epidermis" refers to the outermost and nonvascular layer of the skin, derived from the embryonic ectoderm, varying in thickness from 0.07-1.4mm. On the palmar and plantar surfaces it comprises, from within outward, five layers: basal layer composed of columnar cells arranged perpendicularly; prickle-cell or spinous layer composed of flattened .0 polyhedral cells with short processes or spines; granular layer composed of flattened granular cells; clear layer composed of several layers of clear, transparent cells in which the nuclei are indistinct or absent; and horny layer composed of flattened, cornified non-nucleated cells. In the epidermis of the general body surface, the clear layer is usually absent. An "epidermoid" is a cell or tissue resembling the epidermis, but may also be used to refer to any tumor occurring in a .5 noncutaneous site and formed by inclusion of epidermal elements. The "corium" or "dermis" refers to the layer of the skin beneath deep to the epidermis, consisting of a dense bed of vascular connective tissue, and containing the nerves and terminal organs of sensation. The hair roots, and sebaceous and sweat glands are structures of the epidermis which are deeply embedded in the dermis. !0 The term "gland" refers to an aggregation of cells specialized to secrete or excrete materials not related to their ordinary metabolic needs. For example, "sebaceous glands" are holocrine glands in the corium that secrete an oily substance and sebum. The term "sweat glands" refers to glands that secrete sweat, situated in the corium or subcutaneous tissue, opening by a duct on the body surface. The ordinary or eccrinesweat glands are distributed over 25 most of the body surface, and promote cooling by evaporation of the secretion; the apocrine sweat glands empty into the upper portion of a hair follicle instead of directly onto the skin, and are found only in certain body areas, as around the anus and in the axilla. The term "hair" (or "pilus") refers to a threadlike structure, especially the specialized epidermal structure composed of keratin and developing from a papilla sunk in the corium, 30 produced only by mammals and characteristic of that group of animals. The term also refers to the aggregate of such hairs. A "hair follicle" refers to one of the tubular-invaginations of the epidermis enclosing the hairs, and from which the hairs grow; and "hair follicle epithelial cells" refers to epithelial cells which are surrounded by the dermis in the hair follicle, e.g., stem cells, outer root sheath cells, matrix cells, and inner root sheath cells. Such cells may be normal non 35 malignant cells, or transformed/immortalized cells. The term "alopecia" refers generally to baldness, e.g., the absence of hair from skin areas where it is normally present. Various forms of alopecia are noted in the art. For instance, alopecia areata refers to hair loss, usually reversible, in sharply defined areas, usually involving 14 the beard or scalp; alopecia mediacamentosa refers to hair loss due to ingestion of a drug; and male pattern alopecia, or male pattern baldness, refers to loss of scalp hair genetically determined and androgen-dependent, generally beginning with frontal recession and progressing symmetrically to leave ultimately only a sparse peripheral rim of hair. 5 Throughout this application, the term "proliferative skin disorder" refers to any disease/disorder of the skin marked by unwanted or aberrant proliferation of cutaneous tissue. These conditions are typically characterized by epidermal cell proliferation or incomplete cell differentiation, and include, for example, X-linked ichthyosis, psoriasis, atopic dermatitis, allergic contact dermatitis, epidermolytic hyperkeratosis and seborrheic dermatitis. For 0 example, epidermodysplasia is a form of faulty development of the epidermis, such as "epidermodysplasia verruciformis", which is a condition due to a virus identical with or closely related to the virus of common warts. Another example is "epidermolysis", which refers to a loosened state of the epidermis with formation of blebs and bullae either spontaneously or at the site of trauma. 5 As used herein, the term "psoriasis" refers to a hyperproliferative skin disorder which alters the skin's regulatory mechanisms. In particular, lesions are formed which involve primary and secondary alterations in epidermal proliferation, inflammatory responses of the skin, and an expression of regulatory molecules such as lymphokines and inflammatory factors. Psoriatic skin is morphologically characterized by an increased turnover of epidermal cells, thickened o epidermis, abnormal keratinization, inflammatory cells infiltrates into the dermis layer and polymorphonuclear leukocyte infiltration into the epidermis layer resulting in an increase in the basal cell cycle. Additionally, hyperkeratotic and parakeratotic cells are present. As used herein, "proliferating" and "proliferation" refer to cells undergoing mitosis. The term "progenitor cell" refers to an undifferentiated cell which is capable of 25 proliferation and giving rise to more progenitor cells having the ability to generate a large number of mother cells that can in turn give rise to differentiated, or differentiable daughter cells. As used herein, the term "progenitor cell" is also intended to encompass a cell which is sometimes referred to in the art as a "stem cell". In a preferred embodiment, the term ''progenitor cell" refers to a generalized mother cell whose descendants (progeny) specialize, 30 often in different directions, by differentiation, e.g., by acquiring completely individual characters, as occurs in progressive diversification of embryonic cells and tissues. For instance, a "hematopoietic progenitor cell" (or stem cell) refers to progenitor cells arising in bone marrow and other blood-associated organs and giving rise to such differentiated progeny as, for example, erythrocytes, lymphocytes and other blood cells. 35 As used herein, "transformed cells" refers to cells which have spontaneously converted to a state of unrestrained growth, i.e., they have acquired the ability to grow through an indefinite number of divisions in culture. Transformed cells may be characterized by such terms as neoplastic, anaplastic and/or hyperplastic, with respect to their loss of growth control.

15 As used herein, "immortalized cells" refers to cells which have been altered via chemical and/or recombinant means such that the cells have the ability to grow through an indefinite number of divisions in culture. The term "carcinoma" refers to a malignant new growth made up of epithelial cells 5 tending to infiltrate surrounding tissues and to give rise to metastases. Exemplary carcinomas include: "basal cell carcinoma", which is an epithelial tumor of the skin that, while seldom metastasizing, has potentialities for local invasion and destruction; "squamous cell carcinoma", which refers to carcinomas arising from squamous epithelium and having cuboid cells; "carcinosarcoma", which include malignant tumors composed of carcinomatous and 0 sarcomatous tissues; "adenocystic carcinoma", carcinoma marked by cylinders or bands of hyaline or mucinous stroma separated or surrounded by nests or cords of small epithelial cells, occurring in the mammary and salivary glands, and mucous gland of the respiratory tract; "epidermoid carcinoma", which refers to cancerous cells which tend to differentiate in the same way as those of the epidermis; i.e., they tend to form prickle cells and undergo cornification; t5 "nasopharyngeal carcinoma", which refers to a malignant tumor arising in the epithelial lining of the space behind the nose; and "renal cell carcinoma", which pertains to carcinoma of the renal parenchyma composed of tubular cells in varying arrangements. Another carcinomatous epithelial growth is "papillomas", which refers to benign tumors derived from epithelium and having a papillomavirus as a causative agent; and "epidermoidomas", which refers to a cerebral !0 or meningeal tumor formed by inclusion of ectodermal elements at the time of closure of the neutral groove. As used herein, a "transgenic animal" is any animal, preferably a non-human mammal, bird or an amphibian, in which one or more of the cells of the animal contain heterologous nucleic acid introduced by way of human intervention, such as by transgenic techniques well 25 known in the art. The nucleic acid is introduced into the cell, directly or indirectly by introduction into a precursor of the cell, by way of deliberate genetic manipulation, such as by micro injection or by infection with a recombinant virus. The term genetic manipulation does not include classical cross-breeding, or in vitro fertilization, but rather is directed to the introduction of a recombinant DNA molecule. This molecule may be integrated within a 30 chromosome, or it may be extrachromosomally replicating DNA. This term also includes transgenic animals in which the recombinant gene is silent, as for example, the FLP or CRE recombinase dependent constructs described in the art. Transgenic animals also include both constitutive and conditional "knock out" animals. The "non-human animals" of the invention include vertebrates such as rodents, non-human primates, swine, sheep, dog, cow, chickens, 35 amphibians, reptiles, etc. Preferred non-human animals are miniature swine, or are selected from the rodent family including rat and mouse, most preferably mouse. The term "chimeric animal" is used herein to refer to animals in which the recombinant gene is found, or in which the recombinant is expressed in some but not all cells of the animal.

16 L Establishment of the micro-organ culture A salient feature of the present micro-organ cultures and methods, according to the 5 invention, is the ability to preserve the cellular microenvironment found in vivo for a particular tissue. The invention is based, in part, upon the discovery that under defined circumstances growth of cells in different tissue layers of an organ explant, e.g., mesenchymal and epithelial layers, can be activated to proliferate and mature in culture. Moreover, the cell-cell and cell matrix interactions provided in the explant itself are sufficient to support cellular homeostasis, 0 e.g., maturation, differentiation and segregation of cells in explant culture, thereby sustaining the microarchitecture and function of the tissue for prolonged period of time. An example of physical contact between a cell and a noncellular substrate (matrix) is the physical contact between an epithelial cell and its basal lamina. An example of physical contact between a cell and another cell includes actual physical contact maintained by, for example, 5 intercellular cell junctions such as gap junctions and tight junctions. Examples of functional contact between one cell and another cell includes electrical or chemical communication between cells. For example, cardiomyocytes communicate with other cardiomyocytes via electrical impulses. In addition, many cells communicate with other cells via chemical messages, e.g., hormones which either diffuse locally (paracrine signaling and autocrine .0 signaling) or are transported by the vascular system to more remote locations (endocrine signaling). Examples of paracrine signaling between cells are the messages produced by various cells (known as enteroendocrince cells) of the digestive tract, e.g., pyloric D cells which secrete somatostatin which in turn inhibits the release of gastrin by nearby pyloric gastric (G) cells. 25 Not wishing to be bound by any particular theory, this microarchitecture can be extremely important for the maintenance of the explant in minimal media, e.g., without exogenous sources of serum or growth factors, because the tissue can be sustained in such minimal media by paracrine and autocrine factors resulting from specific cellular interactions within the explant. 30 Moreover, the phrase "maintain, in vitro, their physical and/or functional contact" is not intended to exclude an isolated population of cells in which at least one cell develops physical and/or functional contact with at least one cell or noncellular substance with which it is not in physical and/functional contact in vivo. An example of such a development is proliferation of at least one cell of the isolated population of cells. 35 In preferred embodiments, the populations of cells which make up the explant are isolated from an organ in a manner that preserves the natural affinity of one cell to another, e.g., to preserve layers of different cells if present in the explant. For example, in skin micro-organ cultures, keratinocytes of the epidermis remain associated with the stroma and the normal tissue 17 architecture is preserved including the hair follicles and glands. This basic structure is common to all organs, for instance, which contain an epithelial component. Moreover, such an association facilitates intercellular communication. Many types of communication take place among animal cells. This is particularly important in differentiating cells where induction is 5 defined as the interaction between one (inducing) and another (responding) tissue or cell, as a result of which the responding cells undergo a change in the direction of differentiation. Moreover, inductive interactions occur in embryonic and adult cells and can act to establish and maintain morphogenetic patterns as well as induce differentiation (Gurdon (1992) Cell 68: 185 199). 0 Furthermore, the micro-organ cultures prepared according to the invention preserve normal tissue architecture even when cultured for prolonged periods of time. This includes the maintenance of hair follicles, sweat glands and sebaceous glands in skin micro-organs in vitro according to their normal occurrence in vivo (see Examples VIII and Figures IOA -10C), or islets of Langerhans in the pancreas according to the normal occurrence in vivo (see Examples .5 IV, V and VI). Because these cultures can be maintained in controlled and uniform conditions and yet closely resemble tissue in vivo, they provide a unique opportunity to observe, measure and control natural phenomena and the perturbation of natural phenomena arising from disease, aging or trauma. Furthermore, the ready availability of techniques to study individual cells at identified sites on the culture provide insights into the functioning of individual components of 0 the tissue as they interact with each other as well as the whole tissue. Examples of micro-organ cultures prepared according to the invention are described in the appended Examples, and can include a population of cells grouped in a manner that may include a plurality of layers so as to preserve the natural affinity of one cell to another. The proliferation of individual cells or groups of cells can be observed and followed by 25 autoradiography or immunofluorescence. As merely further exemplification, the appended examples demonstrate that the subject culture system provides for the replication of epithelial and stromal elements in vitro, in a system comparable to physiologic conditions. Importantly, the cells which replicate in this system segregate properly to form morphologically and histologically normal epidermal and 30 dermal components. In addition to isolating an explant which retains the cell-cell, cell-matrix and cell-stroma architecture of the originating tissue, the dimensions of the explant are important to the viability of the cells therein, e.g., where the micro-organ culture is intended to be sustained for prolonged periods of time, e.g., 7-21 days or longer. Accordingly, the dimensions of the tissue explant are 35 selected to provide diffusion of adequate nutrients and gases, e.g., 02 , to every cell in the three dimensional micro-organ, as well as diffusion of cellular waste out of the explant so as to minimize cellular toxicity and concomitant death due to localization of the waste in the micro organ. Accordingly, the size of the explant is determined by the requirement for a minimum 18 level of accessibility to each cell in the absence specialized delivery structures or synthetic substrates. It has been discovered, as described herein, that this accessibility can be maintained if Aleph, an index calculated from the thickness and the width of the explant, is at least greater than approximately 1.5 mm 1 5 As used herein, "Aleph" refers to a surface area to volume ratio given by a formula l/x + 1/a > 1.5 mm ; wherein x= tissue thickness and a= width of tissue in millimeters. In preferred embodiments, the aleph of an explant is in the range of 1.5 to 25mm~', more preferably in the range of 1.5 to 15 mm~1, and even more preferably in the range of 1.5 to 10 mm~', though alephs in the range of 1.5 to 6.67 mm t , 1.5 to 3.33 mm~' are contemplated. 0 Accordingly, the present invention provides that the surface area to volume index of the tissue explant is maintained within a selected range. This selected range of surface area to volume index provides the cells access to nutrients and to avenues of waste disposal by diffusion in a manner similar to cells in a monolayer. This level of accessibility can be attained and maintained if the surface area to volume index, defined herein as "Aleph or Aleph index" is 5 at least about 1.5 mm- . The third dimension has been ignored in determining the surface area to volume index because variation in the third dimension causes radiometric variation in both volume and surface area. However, when determining Aleph, a and x should be defined as the two smallest dimensions of the tissue slice. Examples of Aleph are provided in Table I wherein, for example, a tissue having a .0 thickness (x) of 0.1 mm and a width (a) of I mm would have an Aleph index of 11. In Example I, the tissue had x=0.3 mm and a=4 mm such that Aleph = 3.48. In Example III, x is varied and a is constant at 4 mm. As illustrated in Figure 6, proliferative activity is substantially reduced as the thickness of the explant increases. Accordingly, at 900 pm thickness, the number of proliferating cells in a micro-organ culture is about 10 fold less then in tissue from a similar 25 source having a thickness of 300 pm. The Aleph index for a tissue having a thickness of 900 pim is 1.36 mm ', below the minimum described herein whereas the Aleph index for tissue having a thickness of 300 ptm is 3.58 mm~ , which is well within the range of defined herein. TABLE 1: Different values for the surface area to volume ratio index "Aleph", as a function of 30 a (width) and x (thickness) in mm~ 19 WIDTH x(mm) a = 1mm a = 2mm a = 3mm a=4mm a=5mm 0.1 11 10.5 10.33 10.25 10.2 0.2 6 5.5 5.33 5.25 5.2 0.3 4.3 3.83 3.67 3.58 3.53 0.4 3.5 3 2.83 2.75 2.7 0.5 3 2.5 2.33 2.25 2.2 0.6 2.66 2.16 2 1.91 1.87 0.7 2.4 1.92 1.76 1.68 1.63 0.8 2.25 1.75 1.58 1.5 1.45 0.9 2.11 1.61 1.44 1.36 1.31 1 2 1.5 1.33 1.25 1.2 1.2 1.83 1.3 1.16 1.08 1.03 1.3 1.77 1.26 1.1 1.02 0.96 1.6 1.625 1.13 0.96 0.88 0.83 2 1.5 1 0.83 0.75 0.7 Again, not wishing to be bound by any particular theory, a number of factors provided by the three-dimensional culture system may contribute to its success: 5 (a) The appropriate choice of the explant size, e.g., by use of the above Aleph calculations, three-dimensional matrix provides appropriate surface area to volume ratio for adequate diffusion of nutrients to all cells of the explant, and adequate diffusion of cellular waste away from all cells in the explant. (b) Because of the three-dimensionality of the matrix, various cells continue to 10 actively grow, in contrast to cells in monolayer cultures, which grow to confluence, exhibit contact inhibition, and cease to grow and divide. The elaboration of growth and regulatory factors by replicating cells of the explant may be partially responsible for stimulating proliferation and regulating differentiation of cells in culture, e.g., even for the micro-organ culture which is static in terms of overall volume. 15 (c) The three-dimensional matrix retains a spatial distribution of cellular elements which closely approximate that found in the counterpart tissue in vivo. (d) The cell-cell and cell-matrix interactions may allow the establishment of localized microenvironments conducive to cellular maturation. It has been recognized that maintenance of a differentiated cellular phenotypes requires not only growth/differentiation 20 factors but also the appropriate cellular interactions. The present invention effectively mimics the tissue microenvironment. As described in the illustrative examples below, micro-organ cultures from animals (including humans), such as derived from skin, pancreas, liver, kidney, duodenum, esophagus, 20 bladder, bone marrow, thymus or spleen, have been isolated and grown for up to 21 days in culture. However, it is within the scope of the invention to maintain cultures for extended periods of time beyond 21 days. 5 II. Source of explants for the micro-organ culture The subject micro-organ culture can be derived using explants isolated from, for example: skin and mucosa (including oral mucosa, gastrointestinal mucosa, nasal tract, respiratory tract, cervix and cornea); pancreas; liver; gallbladder; bile duct; lung; prostate; 0 uterus; mammary gland; bladder tissue; and blood-associated organs such as thymus, spleen and bone marrow. Accordingly, in vitro culture equivalents of such organs can be generated. The tissue forming the explants can be diseased or normal (e.g., healthy tissue). For example, the organs from which the micro-organ explants of the invention are isolated can be affected by hyperproliferative disorders, e.g., psoriasis or keratosis; proliferation of virally-infected cells, 5 e.g., hepatitis infected or papillomavirus infected; neoproliferative disorders, e.g., basal cell carcinoma, squamous cell carcinoma, sarcomas, or Wilm's tumors; or fibrotic tissue, e.g., from a cirrhotic liver or a pancreas undergoing pancreatitis. Examples of animals from which the cells of the invention can be isolated include humans and other primates, swine, such as wholly or partially inbred swine (e.g., miniature !0 swine and transgenic swine), rodents, etc. III. The growth media There are a large number of tissue culture media that exist for culturing cells from 25 animals. Some of these are complex and some are simple. While it is expected that micro-organ cultures may grow in complex media, it has been shown here that the cultures can be maintained in a simple medium such as Dulbecco's Minimal Essential Media. Furthermore, although the cultures may be grown in a media containing sera or other biological extracts such as pituitary extract, it has been shown here that neither serum nor any other biological extract is 30 required. Moreover, the organ cultures can be maintained in the absence of serum for extended periods of time. In preferred embodiments of the invention, growth factors are not included in the medium during maintenance of the cultures in vitro. The point regarding growth in minimal media is important. At present, most media or systems for prolonged growth of mammalian cells incorporate undefined proteins or use feeder 35 cells to provide proteins necessary to sustain such growth. Because the presence of such undefined proteins can interfere with the intended end use of the subject micro-organ cultures, it will generally be desirable to culture the explants under conditions to minimize the presence of undefined proteins.

21 As used herein the language "minimal medium" refers to a chemically defined medium which includes only the nutrients that are required by the cells to survive and proliferate in culture. Typically, minimal medium is free of biological extracts, e.g., growth factors, serum, pituitary extract, or other substances which are not necessary to support the survival and 5 proliferation of a cell population in culture. For example, minimal medium generally includes at least one amino acid, at least one vitamin, at least one salt, at least one antibiotic, at least one indicator, e.g., phenol red, used to determine hydrogen ion concentration, glucose, and other miscellaneous components necessary for the survival and proliferation of the cells. Minimal medium is serum-free. A variety of minimal media are commercially available from Gibco 0 BRL, Gathersburg, MD, as minimal essential media. However, while growth factors and regulatory factors need not be added to the media, the addition of such factors, or the inoculation of other specialized cells may be used to enhance, alter or modulate proliferation and cell maturation in the cultures. The growth and activity of cells in culture can be affected by a variety of growth factors such as insulin, growth hormone, 5 somatomedins, colony stimulating factors, erythropoietin, epidermal growth factor, hepatic erythropoietic factor (hepatopoietin), and liver-cell growth factor. Other factors which regulate proliferation and/or differentiation include prostaglandins, interleukins, and naturally-occurring negative growth factors, fibroblast growth factors, and members of the transforming growth factor -p family. !0 The micro-organ cultures may be maintained in any suitable culture vessel such as 24 or 0 96 well microplates and may be maintained at 37 C in 5% CO 2 . The cultures may be shaken for improved aeration, the speed of shaking being for example 12 rpm. With respect to the culture vessel in/on which (optionally) the subject micro-organ cultures are provided, it is noted that in the preferred embodiment such vessel may generally be 25 of any material and/or shape. A number of different materials may be used to form the vessel, including but not limited to: nylon (polyamides), dacron (polyesters), polystyrene, polypropylene, polyacrylates, polyvinyl compounds (e.g., polyvinylchloride), polycarbonate (PVC), polytetrafluorethylene (PTFE; teflon), thermanox (TPX), nitrocellulose, cotton, polyglycolic acid (PGA), cat gut sutures, cellulose, gelatin, dextran, etc. Any of these materials 30 may be woven into a mesh. Where the micro-organ culture is itself to be implanted in vivo, it may be preferable to use biodegradable matrices such as poly glycolic acid, catgut suture material, or gelatin, for example. Where the cultures are to be maintained for long periods of time or cryopreserved, non-degradable materials such as nylon, dacron, polystyrene, polyacrylates, polyvinyls, teflons, cotton, etc. may be preferred. A convenient nylon mesh which 35 could be used in accordance with the invention is Nitex, a nylon filtration mesh having an average pore size of 210pm and an average nylon fiber diameter of 901m (#3-210/36, Tetko, Inc., N.Y.). Yet other embodiments are discussed below.

22 In an exemplary embodiment, pancreatic micro-organs containing islets of Langerhans are prepared as cultures of the present invention. The cultures are then provided in encapsulated form so as to avoid immune rejection. Three general (exemplary) approaches for encapsulation might be used. In the first, a tubular membrane is coiled in a housing that contains the micro 5 organ explants. The membrane is connected to a polymer graft that in turn connects the device to blood vessels. By manipulation of the membrane permeability, so as to allow free diffusion of glucose and insulin back and forth through the membrane, yet block passage of antibodies and lymphocytes, normoglycemia can be maintained in pancreatectomized animals treated with this device (Sullivan et al (1991) Science 252:718). 0 In a second approach, hollow fibers containing the pancreatic explants are (optionally) immobilized in the polysaccharide alginate. When the device is placed intraperitoneally in diabetic animals, blood glucose levels can be lowered and good tissue compatibility observed (Lacey et al. (1991) Science 254:1782; see also Example VI). Accordingly, fibers can be pre spun and subsequently loaded with the micro-organexplants (Aebischer et al. U.S. Patent No. 5 4,892,538; Aebischer et al. U.S. Patent No. 5,106,627; Hoffman et al. (1990) Expt. Neurobiol. 110:39-44; Jaeger et al. (1990) Prog. Brain Res. 82:41-46; and Aebischer et al. (1991) J. Biomech Eng. 113:178-183). Third, the micro-organ islet explants can be placed in microcapsules composed of alginate or polyacrylates (see, for example, Lim et al. (1980) Science 210:908; O'Shea et al. !0 (1984) Biochim. Biochys. Acta 840:133; Sugamori et al (1989) Trans Am. Soc. Artif. Intern. Organs 35:791; Levesque et al. (1992) Endocrinology 130:644; and Lim et al. (1992) Transplantation 53:1180). Finally, it is noted that the culture medium in which the micro-organ cultures of the present invention are maintained can be collected as a source of conditioned medium. The term 25 "conditioned media" refers to the supernatant, e.g. free of the cultured cells/tissue, resulting after a period of time in contact with the cultured cells such that the media has been altered to include certain paracrine and/or autocrine factors produced by the cells and secreted into the culture. Examples of such products are insulin, various growth factors, and hormones. This conditioned medium can be used as culture medium for other types of cell and tissue culture. 30 Alternatively, the conditioned medium can be employed as a source of novel cell products such as growth factors. Such products can be fractionated and purified or substantially purified from the conditioned medium. IV Measuring the biological properties of micro-organ culture 35 The micro-organ cultures of the present invention derived from normal tissue have been shown to maintain a state of homeostasis with proliferation of constituent cells without overall growth of the tissue.

23 Methods of measuring cell proliferation are well known in the art and most commonly include determining DNA synthesis characteristic of cell replication. There are numerous methods in the art for measuring DNA synthesis, any of which may be used according to the invention. In an embodiment of the invention, DNA synthesis has been determined using a 5 radioactive label ( 3 H-thymidine) or labeled nucleotide analogues (BrdU) for detection by immunofluorescence. Micro-organ cultures can be formed and maintained not only by the proliferation of mature cells but also by the active participation of precursor cells including in some instances, embryonic cells. The micro-organ cultures have been shown to present a suitable environment 0 for preserving, identifying, isolating and facilitating the natural evolution of these precursor cells. For example, the immature cells of the basal layer have been observed to become mature keratinocytes in skin micro-organ cultures. Similarly, embryonic pancreatic cells can provide a mature pancreatic epithelium in micro-organ cultures. The maturation of precursor cells and their subsequent functioning as adult cells can be monitored by measuring secretion of 5 specialized products such as specific keratins in epidermal cells and insulin, Glut 2 and glucagon in pancreatic epithelia, and albumin and Factor VIII in liver micro-organ cultures. The micro-organ cultures prepared according to the invention preserve the normal tissue architecture that is present in vivo. As set out above, this includes maintenance of hair follicles, sweat glands and sebaceous glands in skin micro-organs in vitro, according to the normal '0 occurrence in vivo and insulin and glucagon secreting cells in pancreatic micro-organs. Because these cultures can be maintained in controlled and uniform conditions and yet they closely resemble the microarchitecture of the organ in vivo, they provide a unique opportunity to observe, measure and control natural phenomena and the perturbation of natural phenomena arising from disease, aging or trauma. Furthermore, the ready availability of techniques to study 25 individual cells at identified sites on the culture, provides insights into the functioning of individual components of the organs and their interact with each other as well as the whole organ. Furthermore, the subject micro-organ cultures are maintainable in culture for extended periods of time. Preferably, the micro-organ cultures are maintainable in culture for at least 30 about twenty-four hours, more preferably for at least about two days, yet more preferably for at least about five days, still more preferably at least about seven days, still further preferably for at least about two weeks or more. The micro-organ cultures of the invention are typically maintained in culture for at least seven days. To illustrate, skin micro-organ cultures from human, mouse, guinea pig, and rat skin have been maintained in culture for at about least 35 twenty-one days. As used herein, the language "maintainable in culture" refers to the population of cells of a tissue explant of which at least about 60%, preferably at least about 70%, more preferably 24 at least about 80%, yet more preferably at least about 90%, most preferably 95% or more of the cells remain viable in culture after a certain period of time. In a preferred embodiment, the ratio of cell proliferation to cell loss, e.g., by death or sloughing, of the cells in the micro-organ cultures is equal to one, i.e., the number of cells 5 proliferating is equal to the number of cells lost. In another embodiment of the present invention, the ratio of cell proliferation to cell loss of the cells in the micro-organ cultures is greater than one, i.e., the cells are proliferating at a greater rate than the cells are being lost. In the instance of the latter, the micro-organ culture is understood to include a population of cells which is being amplified. 0 V. Application of micro-organ cultures Exemplary applications for the micro-organ cultures of the present invention include the following: .5 (a) identification of factors involved in normal homeostasis of tissues and organs; (b) studying the effect on the normal homeostasis of tissues and cells of an organ with respect to changes in the environment including changes in nutrients and the presence of potentially toxic agents; (c) understanding the pathway of changes in the tissues and cells of an organ that are !0 triggered at the beginning and during pathogenesis or trauma; (d) identification of repair mechanisms that reverse the adverse effects in an altered environment associated with pathogenesis or trauma; (e) developmental regulation of cells that differentiate during the normal homeostasis of the tissue. 25 (f) developmental regulation of specialized structures within an organ, such as hair follicles; (g) organ supplementation/transplantation where parts of an individual's organ remain but are insufficient for replacing or regenerating damaged tissue such as occurs in patients with chronic skin ulcers, various forms of diabetes, or chronic liver failure; 30 (h) as a tissue/organ equivalent for drug screening and cytotoxicity studies; (i) as a diagnostic assay for proliferative disorders; (j) as a source of novel growth factors; (k) as a source of stem/progenitor cells; (1) as a source of inducing molecules; 35 (m) as a screen for inducing molecules; To further illustrate, the present method can be used to generate skin equivalents in the form of micro-organ cultures. By way of background, it is noted that numerous attempts have been described for growing epithelial cells in such a way as to mimic human skin for purposes 25 of wound treatment, in particular treatment of bums. The skin consists of two types of tissue. These are: (1) the stroma or dermis which includes fibroblasts that are loosely dispersed within a high density collagen matrix as well as nerves, blood vessels and fat cells; (2) the epidermis which includes an epidermal basal layer of tightly packed, actively proliferating immature 5 epithelial cells. As the cells of the basal layer replicate, some of the young cells remain in the basal layer while others migrate outward, increase in size and eventually develope an envelop resistant to detergents and reducing agents. In humans, a cell born in the basal layer takes about 2 weeks to reach the edge or outer layer after which time the cells die and are shed. The skin contains various structures including hair follicles, sebaceous glands and sweat glands. Hair 0 follicles are formed from differentiating keratinocytes that densely line invaginations of the epidermis. The open ended vesicles that formed from such invaginations collect and concentrate the secreted keratin and a hair filament results. Alternatively, epidermal cells lining an invagination may secrete fluids (sweat gland) or sebum (sebaceous gland). The regulation of formation and proliferation of these structures is unknown. The constant renewal of healthy 5 skin is accomplished by a balanced process in which new cells are being produced and aged cells die. There is a need to understand how this precise regulation comes about in order to counteract abnormal events occurring in aging, and also through disease and trauma that disrupt the balance. In one embodiment of the invention, the microarchitecture of the micro-organ culture '0 mimics or is substantially the same as that of skin in vivo, e.g., it has an epithelial tissue/connective tissue structure. For example, in skin micro-organ cultures, keratinocytes of the epidermis remain associated with the connective tissue and the normal tissue architecture is preserved including the hair follicles. The micro-organ culture which is obtained from a skin tissue section can also include a basal lamina supporting the epidermal cells, an extracellular 25 matrix which includes the dermal cells, and at least one invagination, e.g., at least one hair follicle. The association between skin epithelial tissue and the skin connective tissue facilitates intercellular communication. Moreover, full thickness skin can be grown in a variety of ways allowing an air interface. Exposure of the keratinocytes of the explant to air promotes a more rapid differentiation of keratinocytes and more extensive secretion of keratin layers, which may 30 be very important in skin penetration studies. Finally, it is noted that recent studies have indicated that the skin is an integral and active element of the immune system (Cooper et al., (1987) The nechanobullous disease. In: Dermatology in General Medicine, 3d. Ed., McGraw Hill, NY (pp.610-626). One of the major cell types in the skin which is responsible for various immune activities is the Langerhans cell. 35 These cells may be prepared from fresh skin samples and added to the three-dimensional skin culture to produce an immunologically complete tissue system. Growth of these cells in the culture for long periods of time by conventional tissue culture techniques is difficult. The ability to grow these cells in a three-dimensional system would be of great importance in all aspects of 26 study including engraftment, cytoxicity, and disease mechanisms. This type of skin culture system would have the greatest impact on research involving auto-immune disorders which have direct or indirect cutaneous involvement (lupus erythematosis, bullous pemphigoid, etc.). Accordingly, the micro-organ cultures of the present invention can be used to study 5 proliferative/differentiative disorders under conditions in which immunological aspects of the disease are minimized. An exemplary drug screening assay can be derived using psoriatic skin explants in order to identify agents which can inhibit proliferation of the hyperplastic epithelial cells. The skin is merely an example of a tissue which can be grown as a micro-organ culture 0 having epithelial tissue which is supported by stromal tissue. Other tissues including epithelial tissue can be grown as micro-organ cultures of the present invention. Epithelial tissues are found in every part of the body where an interface between an organ and the environment arise. Epithelial cells cycle continuously in an uninjured body and form the covering tissue for all the free surfaces in the body including the skin. In some cases, such as in the pancreas, the epithelial 5 cells line numerous invaginations and secrete enzymes into open spaces that enable the organ to function. The lung is another example of a highly invaginated organ, each invagination in the lung being lined with epithelial cells through which air diffuses from the environment in to the body. Once again, these epithelial cells have characteristic properties. The lining of the gut is also composed of specialized epithelial cells that not only form a barrier but also contain specialized .0 structures for selectively absorbing food. All the epithelia are supported by connective tissue. Still another organ comprising important cell-stromal interactions is the bone marrow. Thus, in another embodiment of the present invention, microarchitecture of a micro organ pancreas culture mimics or is substantially the same as that of the source pancreas in vivo, e.g., it has an epithelial tissue/connective tissue structure. For example, pancreas micro 25 organ cultures include pancreatic epithelial cells, e.g., islet cells, remain associated with the pancreatic connective tissue. In the pancreas micro-organ culture, therefore, the normal tissue architecture is preserved and the normal pancreatic epithelial cell products, e.g., insulin and glucagon are produced. In another embodiment, the present invention provides for the generation of micro-organ 30 cultures derived from the bone marrow, which cultures preserve the microarchitecture of the in vivo organ. As described in Example XV, bone marrow micro-organs have been isolated in culture to derive a system comparable to physiologic conditions. The bone marrow cultures of the present invention may be used for treating diseases or conditions which destroy healthy bone marrow cells or depress their functional ability. 35 Implantation of the subject micro-organs can be effective in the treatment of hematological malignancies and other neoplasias which involve the bone marrow. This aspect of the invention is also effective in treating patients whose bone marrow has been adversely affected by the environmental factors (e.g., radiation, toxins, etc). While reimplantation of explants derived 27 from the patients own marrow are generally preferable, it is noted that such explants can be allogenic, e.g., from another member of the same species, or xenogenic, e.g., from another organism. An exemplary xenogenic implant could be a micro-organ culture derived from a miniature swine for implantation in a human. 5 Moreover, long-term growth of human hematopoietic progenitors is possible if they are provided with the necessary stromal-derived growth/regulatory factors. Such interactions are provided by the subject micro-organs, rendering these explants as sources of stem and progenitor cells. In general, hematopoietic progenitor cells of the marrow colonize ("seed") the natural packets formed in the stromal matrix of the bone marrow micro-organ. The primary rate 0 limiting factor in the growth of marrow stromal cells is the relatively low mitotic index of the fibroblasts included among the marrow stromal cells. Accordingly, where the growth of these cells and their disposition of extracellular matrix components is desired to be enhanced, the explant can be contacted with such agents as hydrocortisones or other fibroblast growth factors. If the bone marrow is to be cultured in order to treat certain patients with metastatic 5 disease or hematological malignancies, the marrow obtained from the patients should be "purged" of abnormally proliferating cells by physical or chemotherapeutic means prior to culturing. The conditioned medium from a bone marrow micro-organ culture of the present invention can be used as a source of novel or known lymphokines, e.g., as a source of !0 interleukins. The invention contemplates, in one aspect, the use of the subject micro-organ cultures for transplantation in an organism. As used herein the terms "administering"," introducing", and "transplanting" are used interchangeably and refer to the placement of the cell populations of the invention into a subject, e.g., an allogeneic or a xenogeneic subject, by a method or route 25 which results in localization of the cells to a desired site. The cell populations can be administered to a subject by any appropriate route which results in delivery of the cells to a desired location in the subject where at least a portion of the cells remain viable. It is preferred that at least about 5%, preferably at least about 10%, more preferably at least about 20%, yet more preferably at least about 30%, still more preferably at least about 40%, and most 30 preferably at least about 50% or more of the cells remain viable after administration to a subject. The period of viability of the cells after administration to a subject can be as short as a few hours, e.g., twenty-four hours, to a few days, to as long as a few weeks to months. Methods of administering populations of cells of the invention include implantation of cells into the visceral or the parietal peritoneum, for example into a pouch of the omentum, implantation of 35 cells into or onto an organ of the recipient subject, e.g., pancreas, liver, spleen, skin. The micro organs of the invention can also be administered to a subject by implantation under, e.g., a kidney capsule.

28 As used herein, the term "subject" refers to mammals, e.g., primates, e.g., humans. A "xenogeneic subject" as used herein is a subject into which cells of another species are introduced or are to be introduced. An "allogeneic subject" is a subject into which cells of the same species are introduced or are to be introduced. Donor subjects are subjects which provide 5 the cells, tissues, or organs, which are to be placed in culture and/or transplanted to a recipient subject. Recipient subjects can be either xenogeneic or allogeneic subject. Donor subjects can also provide cells, tissues, or organs for reintroduction into themselves, i.e. for autologous transplantation. To facilitate transplantation of the cell populations which may be subject to .0 immunological attack by the host, e.g., where xenogenic grafting is used, such as swine-human transplantations, the micro-organ can be inserted into or encapsulated by rechargeable or biodegradable devices and then transplanted into the recipient subject. Gene products produced by such cells can then be delivered via, for example, polymeric devices designed for the controlled delivery compounds, e.g., drugs, including proteinaceous biopharmaceuticals. A .5 variety of biocompatible polymers (including hydrogels), including both biodegradable and non-degradable polymers, can be used to form an implant for the sustained release of a gene product of the cell populations of the invention at a particular target site. The generation of such implants is generally known in the art. See, for example, Concise Encyclopedia of Medical & Dental Materials, ed. By David Williams (MIT Press: Cambridge, MA, 1990); the Sabel et al. !0 US Patent No. 4,883,666; Aebischer et al. U.S. Patent No. 4,892,538; Aebischer et al. U.S. Patent No. 5,106,627; Lim U.S. Patent No. 4,391,909; and Sefton U.S. Patent No. 4,353,888. Cell populations of the invention can be administered in a pharmaceutically acceptable carrier or diluent, such as sterile saline and aqueous buffer solutions. The use of such carriers and diluents is well known in the art. 25 In one embodiment, the micro-organ cultures of the present invention can be employed for wound healing. Repair of skin lesions is known to be a highly complex process that includes primary epithelial cell migration as well as replication of epidermal cells in response to molecular signals from underlying connective tissue. Skin micro-organ cultures are described herein as a model for wound healing. Under controlled culture conditions, factors controlling 30 healing can be carefully monitored. Furthermore, because the micro-organ culture is isolated from the natural blood supply, analysis of the healing process can be done without the additional complexity of blood borne factors or cells. Normal epidermis has a low mitotic activity with cells cycling every 200-300 hours. When the epidermis is wounded, a burst of mitotic activity takes place so that the cells divide up to 10 times faster depending on the 35 conditions and severity of the wound (Pinkus H.(1951) J. Invest. Dermatol. 16:383-386). As demonstrated in Example II, skin micro-organ cultures show increased proliferation of up to 10 fold for several days. In this example, the edge of a wound is comparable to the micro-organ culture. This increased proliferation mimics the events that are associated with 29 wounding and provides a unique opportunity to study the process of wound healing. Moreover, the appended examples demonstrate in vivo that the epidermal explants of the present invention can be applied to chronic wounds (example IX) and can form a viable implant capable of growing hair (example XI). 5 Moreover, the subject epidermal micro-organs can be used in the treatment of burn patients. The need for a skin replacement for burn patients is evident. Several centers in the United States and Europe have utilized cultured human keratinocyte allografts and autografts to permanently cover the wounds of burns and chronic ulcers (Eisinger et al., (1980) Surgery 88:287-293; Green et al., (1979) Proc. NatL. A cad. Sci. USA 76:5665-5668; Cuono et al., (1987) 0 Plast. Reconstr. Surg. 80:626-635). These methods are often unsuccessful and recent studies have indicated that blistering and/or skin fragility in the healed grafts may exist because of an abnormality in one or more connective tissue components formed under the transplanted epidermal layer (Woodley et al., (1988) JAMA 6:2566-2571). The skin culture system of the present invention provides a skin equivalent of both epidermis and dermis and should overcome 5 problems characteristic of currently used cultured keratinocyte grafts. In yet another embodiment, the micro-organ culture system of the invention may afford a vehicle for introducing genes and gene products in vivo for use in gene therapies. For example, using recombinant DNA techniques, a gene for which a patient is deficient could be placed under the control of a viral or tissue-specific promoter. The recombinant DNA construct !0 can be used to transform or transfect all or certain of the cells in the subject micro-organ culture system. The micro-organ culture which expresses the active gene product could be implanted into an individual who is deficient for that product. The use of the subject micro-organ culture in gene therapy has a number of advantages. Firstly, since the culture comprises eukaryotic cells, the gene product will be properly expressed 25 and processed in culture to form an active product. Secondly, gene therapy techniques are useful only if the number of transfected cells can be substantially enhanced to be of clinical value, relevance, and utility; the subject cultures allow for expansion of the number of transfected cells and amplification. In a further embodiment of the invention, the transgenic micro-organ cultures may be 30 used to facilitate gene transduction. For example, and not by way of limitation, a micro-organ culture comprising a recombinant virus expression vector may be used to transfer the recombinant virus into cells brought into contact with the culture, e.g., by implantation, thereby simulating viral transmission in vivo. Accordingly, this system can be a more efficient way of accomplishing gene transduction than are current techniques for DNA transfection. 35 Accordingly, the cells of the micro-organ cultures of the present invention can be modified to express a gene product. As used herein, the phrase "gene product" refers to proteins, peptides and functional RNA molecules. Generally, the gene product encoded by the nucleic 30 acid molecule is the desired gene product to be supplied to a subject. Examples of such gene products include proteins, peptides, glycoproteins and lipoproteins normally produced by an organ of the recipient subject. For example, gene products which may be supplied by way of gene replacement or addition to defective organs in the pancreas include insulin, amylase, 5 protease, lipase, trypsinogen, chymotrypsinogen, carboxypeptidase, ribonuclease, deoxyribonuclease, triacylglycerol lipase, phospholipase A 2 , elastase, and amylase; gene products normally produced by the liver include blood clotting factors such as blood clotting Factor VIII and Factor IX, UDP glucuronyl transferase, ornithine transcarbanoylase, and cytochrome p450 enzymes, and adenosine deaminase, for the processing of serum adenosine or 0 the endocytosis of low density lipoproteins; gene products produced by the thymus include serum thymic factor, thymic humoral factor, thymopoietin, and thymosin a; gene products produced by the digestive tract cells include gastrin, secretin, cholecystokinin, somatostatin, and substance P, a somatomedin, a colony stimulating factor, erythropoietin, epidermal growth factor, hepatic erythropoietic factor (hepatopoietin), a liver-cell growth factor, an interleukin, a 5 negative growth factor, fibroblast growth factor and transforming growth factor of the P family, Interferon a, Interferon P, Interferon y, human growth hormone, G-CSF, GM-CSF, TNF receptor, PDGF, AAT, VEGF, Super oxide dismutase, Interleukin, TGF-P, NGF, CTNF, PEDF, NMDA, AAT, Actin, Activin beta-A, Activin beta-B, Activin beta-C, Activin beta-E, Adenosine Deaminase, Agarase-Beta, Albumin HAS Albumin, Alcohol Dehydrogenase 20 Aldolase, Alfimeprase, Alpha 1-Antitrypsin, Alpha Galactosidase, Alpha-l-acid Glycoprotein (AGP), Alpha-I -Antichymotrypsin, Alpha-I -microglobulin (AlM), Alpha-2-Macroglobulin (A2M), Alpha-Fetoprotein, Alpha-Galactosidase, Amino Acid Oxidase, D-Amino Acid Oxidase, L-Amino Acid Oxidase, Amylase, Alpha Amylase, Beta Amylase, Angiostatin, Angiotensin, Angiotensin Converting Enzyme, Ankyrin, Apolipoprotein, APO-SAA, Arginase, 25 Asparaginase, Aspartyl Aminotransferase, Atrial Natriuretic factor (ANF), Atrial Natriuretic Peptide (ANP), Avidin, Beta-2-Glycoprotein 1, Beta-2-microglobulin, Beta-N Acetylglucosaminidase B-NAG, beta amyloid, Brain natriuretic protein (BNP), Brain-derived 31 neurotrophic factor (BDNF), Cadherin E, Calc a, Calc b, Calcitonin, Calcyclin, Caldesmon, Calgizzarin, Calgranulin A, Calgranulin C, Calmodulin, Calreticulin, Calvasculin, Carbonic Anhydrase, Carboxypeptidase, Carboxypeptidase A, Carboxypeptidase B, Carboxypeptidase Y, CARDIAC TROPONIN I, CARDIAC TROPONIN T, Alpha Casein, Catalase, Catenins, 5 Cathepsin D, CD95L, CEA, Cellulase, Centromere Protein B, Ceruloplasmin, Ceruplasmin, cholecystokinin, Cholesterol Esterase, Acetyl Cholinesterase, Butyryl Cholinesterase , Chorionic Gonadotrophin (HCG), Chorionic Gonadotrophin Beta CORE (BchCG), Chymotrypsin, Chymotrypsinogen, Creatin kinase, K-BB, CK-MB (Creatine Kinase-MB), CK MM, Clara cell phospholipid binding protein, Clostripain, Clusterin, CNTF, Collagen, 0 Collagenase, Collagens, (type 1-VI), colony stimulating factor, Complement Clq Complement C3, Complement C3a, Complement C3b-alpha, Complement C3b-beta, Complement C4, Complement C5, Complement Factor B, Concanavalin A, Corticoliberin, Corticotrophin releasing hormone, C-Reactive Protein (CRP), C-type natriuretic peptide (Cnp), Cystatin C, D Dimer, Delta 1, Delta-like kinase 1 (Dlkl), Deoxyribonuclease, Deoxyribonuclease I, 5 Deoxyribonuclease II, Deoxyribonucleic Acids, Dersalazine, Dextranase, Diaphorase, T4 DNA Ligase, DNA Polymerase I, T4 DNA Polymerase, EGF, Elastase, Elastin, Endocrine-gland derived vascular endothelial growth factor (EG-VEGF), Endothelin Endothelin I Eotaxin , Epidermal growth factor (EGF), Epithelial Neutrophil Activating Peptide-78 (ENA-78), Erythropoietin (Epo), Estriol, Exodus, Factor IX, Factor VIII, Fatty acid-binding protein, 20 Ferritin , fibroblast growth factor, Fibroblast growth factor 10, Fibroblast growth factor 11, Fibroblast growth factor 12, Fibroblast growth factor 13, Fibroblast growth factor 14, Fibroblast growth factor 15, Fibroblast growth factor 16, Fibroblast growth factor 17, Fibroblast growth factor 18, Fibroblast growth factor 19, Fibroblast growth factor 2, Fibroblast growth factor 20, Fibroblast growth factor 3, Fibroblast growth factor 4, Fibroblast growth factor 5, Fibroblast 25 growth factor 6, Fibroblast growth factor 7, Fibroblast growth factor 8, Fibroblast growth factor 9, Fibronectin, focal-adhesion kinase (FAK), Follitropin alfa, Galactose Oxidase, Galactosidase, Beta, gamalP-10, gastrin, GCP, G-CSF, Glial derived Neurotrophic Factor (GDNF), Glial 32 fibrillary acidic Protein, Glial filament protein (GFP), glial-derived neurotrophic factor family receptor (GFR), globulin, Glucose Oxidase, Glucose-6-Phosphate Dehydrogenase, Glucosidase, Alpha Glucosidase, Beta Glucosidase, Beta Glucuronidase, Glutamate Decarboxylase, Glyceraldehyde-3 -Phosphate Dehydrogenase, Glycerol Dehydrogenase, Glycerol Kinase, 5 Glycogen Phosphorylase ISO BB, Granulocyte Macrophage Colony Stimulating Factor (GM CSF), growth stimulatory protein (GRO), growth hormone, Growth hormone releasing hormone, Hemopexin, hepatic erythropoietic factor (hepatopoietin), Heregulin alpha, Heregulin beta 1, Heregulin beta 2, Heregulin beta 3, Hexokinase, Histone, Human bone morphogenetic protein, Human relaxin H2, Hyaluronidase, Hydroxysteroid Dehydrogenase, Hypoxia-Inducible 0 Factor-I alpha (HIF-1 Alpha), I-309/TCA-3, IFN alpha, IFN beta, IFN gamma, IgA, IgE, IgG, IgM, Insulin, Insulin Like Growth Factor I (IGF-I), Insulin Like Growth Factor II (IGF-II), Interferon, Interferon-inducible T cell alpha chemoattractant (I-TAC), Interleukin, Interleukin 12 beta, Interleukin 18 binding protein, Intestinal trefoil factor, IP 10, Jagged 1, Jagged 2, Kappa light chain, Keratinocyte Growth Factor (KGF), Kissl, La/SS-B, Lactate Dehydrogenase, L 5 Lactate Dehydrogenase, Lactoferrin, Lactoperoxidase, lambda light chain, Laminin alpha 1, Laminin alpha 2, Laminin beta I Laminin beta 2, Laminin beta 3, Laminin gamma 1, Laminin gamma 2, LD78beta, Leptin, leucine Aminopeptidase, Leutenizing Hormone (LH), LIF, Lipase, liver-cell growth factor, liver-expressed chemokine (LEC), LKM Antigen, TNF, TNF beta, Luciferase, Lutenizing hormone releasing hormone, Lymphocyte activation gene-1 protein 20 (LAG-1), Lymphotactin, Lysozyme, Macrophage Inflammatory Protein 1 alpha (MIP-1 Alpha), Macrophage-Derived Chemokine (MDC), Malate Dehydrogenase, Maltase, MCP(macrophage/monocyte chemotactic protein)-l, 2 and 3, 4, M-CSF, MEC (CCL28), Membrane-type frizzled-related protein (Mfrp), Midkine, MIF, MIG (monokine induced by interferon gamma), MIP 2 to 5, MIP-lbeta, Mp40; P40 T-cell and mast cell growth factor, 25 Myelin Basic Protein Myeloperoxidase, Myoglobin, Myostatin Growth Differentiation Factor-8 (GDF-8), Myosin, Myosin LC, Myosin HC, ATPase, NADase, NAP-2, negative growth factor, nerve growth factor (NGF), Neuraminidase, Neuregulin 1, Neuregulin 2, Neuregulin 3, 33 Neuron Specific Enolase, Neuron-Specific Enolase, neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), Neuturin, NGF, NGF-Beta, Nicastrin, Nitrate Reductase, Nitric Oxide Synthetases, Nortestosterone, Notch 1, Notch 2, Notch 3, Notch 4, NP-1, NT-1 to 4, NT-3 Tpo, NT-4, Nuclease, Oncostatin M, Omithine transcarbamoylase, Osteoprotegerin, Ovalbumin, Oxalate 5 Decarboxylase, P16, Papain, PBP, PBSF, PDGF, PDGF-AA, PDGF-AB, PDGF-BB, PEDF, Pepsin, Peroxidase, Persephin, PF-4, P-Glycoprotein, Phosphatase, Acid, Phosphatase, Alkaline, Phosphodiesterase I, Phosphodiesterase II, Phosphoenolpyruvate Carboxylase, Phosphoglucomutase, Phospholipase, Phospholipase A2, Phospholipase A2, Phospholipase C, Phosphotyrosine Kinase, Pituitary adenylate cyclase activating polypeptide, Placental Lactogen, 0 Plakoglobin, Plakophilin, Plasma Amine Oxidase, Plasma retinol binding protein, Plasminogen, Pleiotrophin (PTN), PLGF-1, PLGF-2, Pokeweed Antiviral Toxin, Prealbumin, Pregnancy assoc Plasma Protein A, Pregnancy specific beta I glycoprotein (SPI), Prodynorphin, Proenkephalin, Progesterone Proinsulin, Prolactin, Pro-melanin-concentrating hormone (Pmch), Pro-opiomelanocortin, proorphanin, Prostate Specific Antigen PSA, Prostatic Acid Phosphatase 5 (PAP), Prothrombin, PSA-Al, Pulmonary surfactant protein A, Pyruvate Kinase, Ranpirnase, RANTES, Reelin, Renin, Resistin, Retinol Binding Globulin (RBP), RO SS-A 60 kda, RO/SS A 52 kda, S100 (human brain) (BB/AB), S100 (human) BB homodimer, Saposin, SCF, SCGF alpha, SCGF-Beta, SDF-l alpha, SDF-l Beta, Secreted frizzled related protein I (Sfrpl), Secreted frizzled related protein 2 (Sfrp2), Secreted frizzled related protein 3 (Sfrp3), Secreted 20 frizzled related protein 4 (Sfrp4), Secreted frizzled related protein 5 (Sfrp5), secretin, serum thymic factor, Binding Globulin (SHBG), somatomedin, somatostatin, Somatotropin, s-RankL, substance P, Superoxide Dismutase, TGF alpha, TGF beta, Thioredoxin, Thrombopoietin (TPO), Thrombospondin 1, Thrombospondin 2, Thrombospondin 3, Thrombospondin 4, Thrombospondin 5, Thrombospondin 6, Thrombospondin 7, thymic humoral factor, 25 thymopoietin, thymosin al, Thymosin alpha-1, Thymus and activation regulated chemokine (TARC), Thymus-expressed chemokine (TECK), Thyroglobulin (Tg), Thyroid Microsomal Antigen, Thyroid Peroxidase (TPO), Thyroxine (T4), Thyroxine Binding Globulin (TBG), 34 TNFalpha, TNF receptor, Transferin, Transferrin receptor, transforming growth factor of the b family, Transthyretin, Triacylglycerol lipase, Triiodothyronine (T3), Tropomyosin alpha, tropomyosin-related kinase (trk), Troponin C, Troponin I, Troponin T, Trypsin, Trypsin Inhibitors, Trypsinogen, TSH, Tweak, Tyrosine Decarboxylase, Ubiquitin, UDP glucuronyl 5 transferase, Urease, Uricase, Urine Protein 1, Urocortin 1, Urocortin 2, Urocortin 3, Urotensin II, Vang-like 1 (Vangll), Vang-like 2 (Vangl2), Vascular Endothelial Growth Factor (VEGF), Vasoactive intestinal peptide precursor, Vimentin, Vitamin D binding protein, Von Willebrand factor, Wntl, WntOa, WntlOb, Wntl1, Wntl2, Wnt13, Wnt14, Wntl5, Wntl6, Wnt2, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9, Xanthine Oxidase. 0 Alternatively, the encoded gene product is one which induces the expression of the desired gene product by the cell (e.g., the introduced genetic material encodes a transcription factor which induces the transcription of the gene product to be supplied to the subject). In still another embodiment, the recombinant gene can provide a heterologous protein, e.g., not native to the cell in which it is expressed. For instance, various human MHC components can be 5 provided to non-human micro-organs to support engraftment in a human recipient. Alternatively, the transgene is one which inhibits the expression or action of a donor MHC gene product normally expressed in the micro-organ explant. Additional protein families useable in context of the micro-organs of the present invention, include cytokines, interleukins, bmp, chemokines, growth factors, hormones, enzymes, monoclonal antibodies, single chain fvs 20 antibodies, oxidoreductases, p450, peroxydases, hydrogenases, dehydrogenases, catalase, transferases such as, for example, glycosyltransferases, mannosyltransferases; hydrolases, such as, for example, esterases, glucoamylases, glycosyl hydrolases, transcarbamylases, ribonucleases, atpases, peptidases, phosphodiesterases, lyases; isomerases, such as, for example, topoisomerases; ligases, aminoacyl-trna synthetases, kinases, phosphoproteines, mutator 25 transposons, oxidoreductases, cholinesterases, glucoamylases, glycosyl hydrolases, transcarbamylases, nucleases, meganucleases, ribonucleases, atpases, peptidases, cyclic nucleotide synthetase, phosphodiesterases, phosphoproteins, mutator transposons, dna or ma 35 associated proteins, high mobility group proteins (hmg), pax (paired box) proteins, histones, polymerases, dna repair proteins, ribosomal proteins, electron transport proteins, globins, metallothioneins, membrane transport proteins, structural proteins, receptors, cell surface 5 [text continues on page 36] 36 receptors, nuclear receptors, G-proteins, olfactory receptors, ion channel receptors, channels, tyrosine kinase receptors, cell adhesion molecules and receptors, photoreceptors, active peptides, protease inhibitors, chaperones, chaperonins, stress associated proteins, transcription factors and chimeric proteins. 5 A nucleic acid molecule introduced into a cell is in a form suitable for expression in the cell of the gene product encoded by the nucleic acid. Accordingly, the nucleic acid molecule includes coding and regulatory sequences required for transcription of a gene (or portion thereof) and, when the gene product is a protein or peptide, translation of the nucleic acid molecule include promoters, enhancers and polyadenylation signals, as well as sequences 0 necessary for transport of an encoded protein or peptide, for example N-terminal signal sequences for transport of proteins or peptides to the surface of the cell or secretion. Nucleotide sequences which regulate expression of a gene product (e.g., promoter and enhancer sequences) are selected based upon the type of cell in which the gene product is to be expressed and the desired level of expression of the gene product. For example, a promoter 5 known to confer cell-type specific expression of a gene linked to the promoter can be used. A promoter specific for myoblast gene expression can be linked to a gene of interest to confer muscle-specific expression of that gene product. Muscle-specific regulatory elements which are known in the art include upstream regions from the dystrophin gene (Klamut et al., (1989) Mol. Cell Biol.9:2396), the creatine kinase gene (Buskin and Hauschka, (1989) Mol. Cell Biol. !0 9:2627) and the troponin gene (Mar and Ordahl, (1988) Proc. Natl. Acad. Sci. USA. 85:6404), Negative response elements in keratin genes mediate transcriptional repression (Jho Sh et al, (2001). J Biol Chem). Regulatory elements specific for other cell types are known in the art (e.g., the albumin enhancer for liver-specific expression; insulin regulatory elements for pancreatic islet cell-specific expression; various neural cell-specific regulatory elements, including neural 25 dystrophin, neural enolase and A4 amyloid promoters). Alternatively, a regulatory element which can direct constitutive expression of a gene in a variety of different cell types, such as a viral regulatory element, can be used. Examples of viral promoters commonly used to drive gene expression include those derived from polyoma virus, Adenovirus 2, cytomegalovirus and Simian Virus 40, and retroviral LTRs. Alternatively, a regulatory element which provides 30 inducible expression of a gene linked thereto can be used. The use of an inducible regulatory element (e.g., an inducible promoter) allows for modulation of the production of the gene product in the cell. Examples of potentially useful inducible regulatory systems for use in eukaryotic cells include hormone-regulated elements ( e.g., see Mader, S. and White, J.H. (1993) Proc. Natl. Acad. Sci. USA 90:5603-5607), synthetic ligand-regulated elements (see, 35 e.g., Spencer, D.M. et al 1993) Science 262:1019-1024) and ionizing radiation-regulated elements (e.g., see Manome, Y. Et al. (1993) Biochemistry 32:10607-10613; Datta, R. et al. (1992) Proc. Natl. Acad. Sci. USA89:1014-10153). Additional tissue-specific or inducible regulatory systems which may be developed can also be used in accordance with the invention.

37 There are a number of techniques known in the art for introducing genetic material into a cell that can be applied to modify a cell of the invention. In one embodiment, the nucleic acid is in the form of a naked nucleic acid molecule. In this situation, the nucleic acid molecule introduced into a cell to be modified consists only of the nucleic acid encoding the gene product 5 and the necessary regulatory elements. Alternatively, the nucleic acid encoding the gene product (including the necessary regulatory elements) is contained within a plasmid vector. Examples of plasmid expression vectors include CDM8 (Seed, B. (1987) Nature 329:840) and pMT2PC (Kaufm-an, et al. (1987) EMBO J. 6:187-195). In another embodiment, the nucleic acid molecule to be introduced into a cell is contained within a viral vector. In this situation, the 0 nucleic acid encoding the gene product is inserted into the viral genome (or partial viral genome). The regulatory elements directing the expression of the gene product can be included with the nucleic acid inserted into the viral genome (i.e., linked to the gene inserted into the viral genome) or can be provided by the viral genome itself. Naked nucleic acids can be introduced into cells using calcium-phosphate mediated 5 transfection, DEAE-dextran mediated transfection, electroporation, liposome-mediated transfection, direct injection, and receptor-mediated uptake. Naked nucleic acid, e.g., DNA, can be introduced into cells by forming a precipitate containing the nucleic acid and calcium phosphate. For example, a HEPES-buffered saline solution can be mixed with a solution containing calcium chloride and nucleic acid to form a !0 precipitate and the precipitate is then incubated with cells. A glycerol or dimethyl sulfoxide shock step can be added to increase the amount of nucleic acid taken up by certain cells. CaPO 4 mediated transfection can be used to stably (or transiently) transfect cells and is only applicable to in vitro modification of cells. Protocols for CaPO 4 -mediated transfection can be found in Current Protocols in Molecular Biology, Ausubel, F.M. et al. (eds.) Greene Publishing 25 Associates, (1989), Section 9.1 and in Molecular Cloning: A Laboratory Manual, 2nd Edition, Sambrook et al. Cold Spring Harbor Laboratory Press, (1989), Sections 16.32-16.40 or other standard laboratory manuals. Naked nucleic acid can be introduced into cells by forming a mixture of the nucleic acid and DEAE-dextran and incubating the mixture with the cells. A dimethylsulfoxide or 30 chloroquine shock step can be added to increase the amount of nucleic acid uptake. DEAE dextran transfection is only applicable to in vitro modification of cells and can be used to introduce DNA transiently into cells but is not preferred for creating stably transfected cells. Thus, this method can be used for short term production of a gene product but is not a method of choice for long-term production of a gene product. Protocols for DEAE-dextran-mediated 35 transfection can be found in Current Protocols in Molecular Biology, Ausubel, F.M. et al. (eds.) Greene Publishing Associates (1989), Section 9.2 and in Molecular Cloning: A Laboratory Manual, 2nd Edition, Sambrook et al. Cold Spring Harbor Laboratory Press, (1989), Sections 16.41-16.46 or other standard laboratory manuals.

38 Naked nucleic acid can also be introduced into cells by incubating the cells and the nucleic acid together in an appropriate buffer and subjecting the cells to a high-voltage electric pulse. The efficiency with which nucleic acid is introduced into cells by electroporation is influenced by the strength of the applied field, the length of the electric pulse, the temperature, 5 the conformation and concentration of the DNA and the ionic composition of the media. Electroporation can be used to stably (or transiently) transfect a wide variety of cell types. Protocols for electroporating cells can be found in Current Protocols in Molecular Biology, Ausubel F.M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.3 and in Molecular Cloning: A Laboratory Manual, 2nd Edition, Sambrook et al. Cold Spring Harbor Laboratory 0 Press, (1989), Sections 16.54-16.55 or other standard laboratory manuals. Another method by which naked nucleic acid can be introduced into cells includes liposome-mediated transfection (lipofection). The nucleic acid is mixed with a liposome suspension containing cationic lipids. The DNA/liposome complex is then incubated with cells. Liposome mediated transfection can be used to stably (or transiently) transfect cells in culture in 5 vitro. Protocols can be found in Current Protocols in Molecular Biology, Ausubel F.M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.4 and other standard laboratory manuals. Additionally, gene delivery in vivo has been accomplished using liposomes. See for example Nicolau et al. (1987) Meth. Enz. 149:157-176; Wang and Huang (1987) Proc. NatL. Acad. Sci. USA 84:7851-7855; Brigham et al. (1989) Am. J Med. Sci. 298:278; and Gould-Fogerite et al. .0 (1989) Gene 84:429-438. Naked nucleic acid can also be introduced into cells by directly injecting the nucleic acid into the cells. For an in vitro culture of cells, DNA can be introduced by microinjection. Since each cell is microinjected individually, this approach is very labor intensive when modifying large numbers of cells. However, a situation wherein microinjection is a method of choice is in 25 the production of transgenic animals (discussed in greater detail below). In this situation, the DNA is stably introduced into a fertilized oocyte which is then allowed to develop into an animal. The resultant animal contains cells carrying the DNA introduced into the oocyte. Direct injection has also been used to introduce naked DNA into cells in vivo (see e.g., Acsadi et al. (1991) Nature 332:815-818; Wolff et al. (1990) Science 247:1465-1468). A delivery apparatus 30 (e.g., a "gene gun") for injecting DNA into cells in vivo can be used. Such an apparatus is commercially available (e.g., from BioRad). Naked nucleic acid can be complexed to a cation, such as polylysine, which is coupled to a ligand for a cell-surface receptor to be taken up by receptor-mediated endocytosis (see for example Wu, G. and Wu, C.H. (1988) J. Biol. Chem. 263: 14621; Wilson et al. (1992) J. Biol. 35 Chem. 267:963-967; and U.S. Patent No. 5,166,320). Binding of the nucleic acid-ligand complex to the receptor facilitates uptake of the DNA by receptor-mediated endocytosis. Receptors to which a DNA-ligand complex have targeted include the transferrin receptor and the asialoglycoprotein receptor. A DNA-ligand complex linked to adenovirus capsids which 39 naturally disrupt endosomes, thereby releasing material into the cytoplasm can be used to avoid degradation of the complex by intracellular lysosomes (see for example Curiel et al. (1991) Proc. Natl. Acad. Sci. USA 88:8850; Cristiano et al. (1993) Proc. Nati. Acad. Sci. USA 90:2122 2126). Receptor-mediated DNA uptake can be used to introduce DNA into cells either in vitro 5 or in vivo and, additionally, has the added feature that DNA can be selectively targeted to a particular cell type by use of a ligand which binds to a receptor selectively expressed on a target cell of interest. Generally, when naked DNA is introduced into cells in culture (e.g., by one of the transfection techniques described above) only a small fraction of cells (about 1 out of 105) 0 typically integrate the transfected DNA into their genomes (i.e., the DNA is maintained in the cell episomally). Thus, in order to identify cells which have taken up exogenous DNA, it is advantageous to transfect nucleic acid encoding a selectable marker into the cell along with the nucleic acid(s) of interest. Preferred selectable markers include those which confer resistance to drugs such as G418, hygromycin and methotrexate. Selectable markers may be introduced on 5 the same plasmid as the gene(s) of interest or may be introduced on a separate plasmid. A preferred approach for introducing nucleic acid encoding a gene product into a cell is by use of a viral vector containing nucleic acid, e.g. a cDNA, encoding the gene product. Infection of cells with a viral vector has the advantage that a large proportion of cells receive the nucleic acid which can obviate the need for selection of cells which have received the nucleic !0 acid. Additionally, molecules encoded within the viral vector, e.g. a cDNA contained in the viral vector, are expressed efficiently in cells which have taken up viral vector nucleic acid and viral vector systems can be used either in vitro or in vivo. Defective retroviruses are well characterized for use in gene transfer for gene therapy purposes (for review see Miller, A.D. (1990) Blood 76:271). A recombinant retrovirus can be 25 constructed having a nucleic acid encoding a gene product of interest inserted into the retroviral genome. Additionally, portions of the retroviral genome can be removed to render the retrovirus replication defective. The replication defective retrovirus is then packaged into virions which can be used to infect a target cell through the use of a helper virus by standard techniques. Protocols for producing recombinant retroviruses and for infecting cells in vitro or in vivo with 30 such viruses can be found in Current Protocols in Molecular Biology, Ausubel, F.M. et al. (eds.) Greene Publishing Associates, (1989), Sections 9.10-9.14 and other standard laboratory manuals. Examples of suitable retroviruses include pLJ, pZIP, pWE and pEM which are well known to those skilled in the art. Examples of suitable packaging virus lines include yCrip, y 2 and yAm. Retroviruses have been used to introduce a variety of genes into many different cell 35 types, including epithelial cells endothelial cells, lymphocytes, myoblasts, hepatocytes, bone marrow cells, in vitro and/or in vivo (see for example Eglitis, et al. (1985) Science 230:1395 1398; Danosand Mulligan (1988) Proc. Natl. Acad. Sci. USA 85:6460-6464; Wilson et al. (1988) Proc. Natl. Acad. Sci USA 85:3014-3018; Armentano et al., (1990) Proc. Natl. Acad.

40 Sci. USA 87: 6141-6145; Huber et al. (1991) Proc. NatI. Acad. Sci. USA 88:8039-8043; Feri et al. (1991) Proc. Natl. Acad. Sci. USA 88:8377-8381; Chowdhury et al. (1991) Science 254:1802-1805; van Beusechem et al. (1992) Proc. NatI. Acad. Sci USA 89:7640-7644; Kay et al. (1992) Human Gene Therapy 3:641-647; Dai et al. (1992) Proc. Nati. Acad. Sci. USA 5 89:10892-10895; Hwu et al (1993) J. Immunol. 150:4104-4115; US Patent No. 4,868,116; US Patent No. 4,980,286; PCT Application WO 89/07136; PCT Application WO 89/02468; PCT Application WO 89/05345; and PCT Application WO 92/07573). Retroviral vectors require target cell division in order for the retroviral genome (and foreign nucleic acid inserted into it) to be integrated into the host genome to stably introduce nucleic acid into the cell. Thus, it may 0 be necessary to stimulate replication of the target cell. The genome of an adenovirus can be manipulated such that it encodes and expresses a gene product of interest but is inactivated in terms of its ability to replicate in a normal lytic viral life cycle. See for example Berkner et al. (1988) BioTechniques 6:616; Rosenfeld et al. (1991) Science 252:431-434; and Rosenfeld et al. (1992) Cell 68:143-155. Suitable adenoviral 5 vectors derived from the adenovirus strain Ad type 5 d1324 or other strains of adenovirus (e.g., Ad2, Ad3, Ad7 etc.) are well known to those skilled in the art. Recombinant adenoviruses are advantageous in that they do not require dividing cells to be effective gene delivery vehicles and can be used to infect a wide variety of cell types, including airway epithelium (Rosenfeld et al. (1992) cited supra), endothelial cells (Lemarchand et al. (1992) Proc. Nati. Acad. Sci. USA !0 89:6482-6486), hepatocytes (Herz and Gerard (1993) Proc. Natl. Acad. Sci. USA 90:2812-2816) and muscle cells (Quantin et al. (1992) Proc. Nati. Acad. Sci. USA 89:2581-2584). Additionally, introduced adenoviral DNA (and foreign DNA contained therein) is not integrated into the genome of a host cell but remains episomal, thereby avoiding potential problems that can occur as a result of insertional mutagenesis in situations where introduced DNA becomes 25 integrated into the host genome (e.g., retroviral DNA). Moreover, the carrying capacity of the adenoviral genome for foreign DNA is large (up to 8 kilobases) relative to other gene delivery vectors (Berkner et al. cited supra; Haj-Ahmand and Graham (1986) J. Virol 57:267). Most replication-defective adenoviral vectors currently in use are deleted for all or parts of the viral El and E3 genes but retain as much as 80% of the adenoviral genetic material. 30 Adeno-associated virus (AAV) is a naturally occurring defective virus that requires another virus, such as an adenovirus or a herpes virus, as a helper virus for efficient replication and a productive life cycle. (For a review see Muzyczka et al. Curr. Topics In Micro. And Immunol. (1992) 158:97-129). It is also one of the few viruses that may integrate its DNA into non-dividing cells, and exhibits a high frequency of stable integration (see for example Flotte et 35 al. (1992) Am. J. Respir. Cell. Mol. Biol. 7:349-356; Samulski et al. (1989) J. Virol. 63:3822 3828; and McLaughlin et al (1989) J. Virol. 62:1963-1973). Vectors containing as little as 300 base pairs of AAV can be packaged and can integrate. Space for exogenous DNA is limited to about 4.5 kb. An AAV vector such as that described in Tratschin et al. (1985) Mol. Cell. Biol.

41 5:3251-3260 can be used to introduce DNA into cells. A variety of nucleic acids have been introduced into different cell types using AAV vectors (see for example Hermonat et al. (1984)Proc. Natl. A cad. Sci. USA 81:6466-6470; Tratschin et al. (1985) Mol. Cell Biol. 4:2072 2081; Wondisford et al. (1988) Mol. Endocrinol. 2:32-39; Tratschin et al. (1984) J. Virol. 5 51:611-619; and Flotte et al. (1993) J. Biol. Chem. 268:3781-3790). The efficacy of a particular expression vector system and method of introducing nucleic acid into a cell can be assessed by standard approaches routinely used in the art. For example, DNA introduced into a cell can be detected by a filter hybridization technique (e.g., Southern blotting) and RNA produced by transcription of introduced DNA can be detected, for example, 0 by Northern blotting, RNase protection or reverse transcriptase-polymerase chain reaction (RT PCR). The gene product can be detected by an appropriate assay, for example by immunological detection of a produced protein, such as with a specific antibody, or by a functional assay to detect a functional activity of the gene product, such as an enzymatic assay. If the gene product of interest to be interest to be expressed by a cell is not readily assayable, an expression system 5 can first be optimized using a reporter gene linked to the regulatory elements and vector to be used. The reporter gene encodes a gene product which is easily detectable and, thus, can be used to evaluate efficacy of the system. Standard reporter genes used in the art include genes encoding p-galactosidase, chloramphenicol acetyl transferase, luciferase, GFP/EGFP and human growth hormone. 0 When the method used to introduce nucleic acid into a population of cells results in modification of a large proportion of the cells and efficient expression of the gene product by the cells (e.g., as is often the case when using a viral expression vector), the modified population of cells may be used without further isolation or subcloning of individual cells within the population. That is, there may be sufficient production of the gene product by the 25 population of cells such that no further cell isolation is needed. Alternatively, it may be desirable to grow a homogenous population of identically modified cells from a single modified cell to isolate cells which efficiently express the gene product. Such a population of uniform cells can be prepared by isolating a single modified cell by limiting dilution cloning followed by expanding the single cell in culture into a clonal population of cells by standard techniques. 30 As used herein, the phrase "transgenic cell" referred to a cell into which a nucleic acid sequence which is partially or entirely heterologous, i.e., foreign, to the cell in which it has been inserted or introduced. A transgenic cell can also be a cell into which an nucleic acid which is homologous to an endogenous gene of the cell has been inserted. In this case, however, the homologous nucleic acid is designed to be inserted, or is inserted, into the cell's genome in such 35 a way as to alter the genome of the cell into which it is inserted. For example, the homologous nucleic acid is inserted at a location which differs from that of the natural gene or the insertion of the homologous nucleic acid results in a knockout of a particular phenotype. The nucleic acid inserted into the cells can include one or more transcriptional regulatory sequences and any 42 other nucleic acid, such as an intron, that may be necessary for optimal expression of a selected nucleic acid. In yet another aspect of the present invention, the subject micro-organ cultures may be used to aid in the diagnosis and treatment of malignancies and diseases. For example, a biopsy 5 of an organ (e.g. skin, kidney, liver, etc.) may be taken from a patient suspected of having a hyperproliferative or neoproliferative disorder. If the biopsy explant is cultured according to the present method, proliferative cells of the explant will be clonally expanded during culturing. This will increase the chances of detecting such disorders, and, therefore, increase the accuracy of the diagnosis. Moreover, the patient's micro-organ culture could be used in vitro to screen .0 cytotoxic and/or pharmaceutical compounds in order to identify those that are most efficacious; i.e. those that kill malignant or diseased cells, yet spare the normal cells. These agents could then be used to therapeutically treat the patient. A further aspect of the invention pertains to a method of using the subject micro-organ cultures to screen a wide variety of compounds, such as cytotoxic compounds growth/regulatory 5 factors, pharmaceutical agents, etc. For example, the need for thorough testing of chemicals of potentially toxic nature is generally recognized and the need to develop sensitive and reproducible short-term in vitro assays for the evaluation of drugs, cosmetics, food additives and pesticides is apparent. The micro-organ cultures described herein permits the use of a tissue-equivalent as an assay substrate and offers the advantages of normal cell interactions in a !0 system that closely resembles the in vivo state. To this end, the cultures are maintained in vitro and exposed to the compound to be tested. The activity of a cytotoxic compound can be measured by its ability to modulate the phenotype (including killing) of cells in the explant. This may readily be assessed by vital staining techniques, expression of markers, etc. For instance, the effect of growth/regulatory 25 factors may be assessed by analyzing the cellular content of the culture, e.g., by total cell counts, and differential cell counts. This may be accomplished using standard cytological and/or histological techniques including the use of immunocytochemical techniques employing antibodies that define type-specific cellular antigens. The effect of various drugs on normal cells cultured in the present system may be assessed. For example, drugs that decrease proliferation 30 of psoriatic tissue can be identified. In an exemplary embodiment of this method, derived for detecting agents which stimulate proliferation of a cell in the explant, the method includes isolating a tissue explant from a subject, wherein the population of cells of the explant retains a microarchitecture of the organ or tissue from which the explant was isolated, e.g., the explant is characterized by Aleph 35 of at least about 1.5 mm ', and includes at least one cell which has the ability to proliferate. The explant is cultured and contacted with a candidate compound. The level of cell proliferation in the presence of the candidate compound is then measured and compared with the level of cell proliferation in the absence of the candidate compound. A statistically significant increase in the 43 level of cell proliferation in the presence of the candidate compound is indicative of a cell proliferative agent. The phrase "candidate compound" or "candidate agent" as used herein refers to an agent which is tested or to be tested for proliferative, anti-proliferative, differentiating, anti 5 differentiating, or anti-viral activity. Such agents can be, for example, small organic molecules, biological extracts, and recombinant products or compositions. Methods of measuring cell proliferation are well known in the art and most commonly include determining DNA synthesis characteristic of cell replication. There are numerous methods in the art for measuring DNA synthesis, any of which may be used according to the 0 invention. In one embodiment of the invention, DNA synthesis has been determined using a radioactive label ( 3 H-thymidine) or labeled nucleotide analogues (BrdU) for detection by immunofluorescence. Yet another embodiment provides a method for identifying an inhibitor of cell proliferation. This method includes providing a tissue explant as above, contacting that explant 5 with a candidate compound, and measuring the level of cell proliferation in the presence of the candidate compound. A statistically significant decrease in the level of cell proliferation in the presence of the candidate compound is indicative of an inhibitor of cell proliferation. In an illustrative embodiment, both potentiators and inhibitors of cell proliferation (also referred to herein as anti-proliferative agents) can be used, for example to control hair growth .0 depending on the desired effect. The growth of hard keratin fibers such as wool and hair is dependent on the proliferation of dermal sheath cells. Hair follicle stem cells of the sheath are highly active, and give rise to hair fibers through rapid proliferation and complex differentiation. The hair cycle involves three distinct phases: anagen (growing), catagen (regressing), and telogen (resting). The epidermal 25 stem cells of the hair follicle are activated by dermal papilla during late telogen. This is termed "bulge activation". Moreover, such stem cells are thought to be pluripotent stem cells, giving rise not only to hair and hair follicle structures, but also the sebaceous gland and epidermis. Cell proliferative agents and inhibitors of cell proliferation provide means for altering the dynamics of the hair growth cycle to, for example, induce quiescence of proliferation of hair follicle cells, 30 particularly stem cells of the hair follicle. Inhibitors of hair follicle cell proliferation can be employed as a way of reducing the growth of human hair as opposed to its convention removal by cutting, shaving, or depilation. For example, inhibitors of hair follicle cells identified using the method of the present invention can be used in the treatment of trichosis characterized by abnormally rapid or dense growth of 35 hair, e.g., hypertrichosis. In an illustrative embodiment, such inhibitors can be used to manage hirsutism, a disorder marked by abnormal hairiness. Use of such inhibitors can also provide a process for extending the duration of depilation.

44 Inhibitors of hair follicle cell proliferation can also be used to protect hair follicle cells from cytotoxic agents which require progression into S-phase of the cell-cycle for efficacy, e.g. radiation-induced death. Treatment with such inhibitors provides protection by causing the hair follicle cells to become quiescent, e.g., by inhibiting the cells from entering S phase, and 5 thereby preventing the follicle cells from undergoing mitotic catastrophe or programmed cell death. For example, inhibitors of hair follicle cell proliferation can be used for patients undergoing chemo- or radiation-therapies which ordinarily result in hair loss. By inhibiting cell cycle progression during such therapies, the inhibitor treatment can protect hair follicle cells from death which might otherwise result from activation of cell death programs. After the 0 therapy has concluded, inhibitor treatment can also be removed with concomitant relief of the inhibition of follicle cell proliferation. However, in order to start characterizing the molecular mechanisms underlying hair growth control, as well as to test potential hair affecting drugs, appropriate in vitro models for hair growth are required. In one aspect of the present invention, the subject method is used to 5 generate hair follicle micro-organ explants which retain the microarchitecture of the follicle, e.g., the interaction between the hair follicle epithelial layer and stromal components (the dermal papilla) of the hair follicle, e.g., one or more of the stem cells, outer root sheath cells, matrix cells, and inner root sheath cells. As demonstrated in the appended examples, hair growth can be observed in these micro-organ cultures even in the absence of serum, e.g., in a 0 minimal media. Importantly, the present invention also provides a hair follicle culture which provide the hair follicles in a substantially telogenic phase, e.g., resting. As demonstrated below, the telogenic hair follicle explants can be activated in the in vitro culture to growing anagen follicles, and in a certain embodiment, in a synchronized manner. The early transient proliferation of the epidermal stem cells of the follicle provide a unique opportunity to 25 understand the activation of anagenic phase as mediated, for example, by paracrine and/or autocrine factors produced by the various tissues of the hair follicle organ. Moreover, the subject micro-organ cultures supply a system for identifying agents which modulate the activation or inactivation of the hair follicles, e.g., to identify agents which can either promote or inhibit hair growth. In one embodiment, telogenic (resting) hair follicle 30 explants, such as described in Example XVIII below, are contacted with various test agents, and the level of stimulation of the hair follicles is detected. For example, transition of the hair follicle stem cells from telogen to anagen can be monitored by observing the mitotic index of the cells of the follicle, or some other similar method of detecting proliferation. To illustrate, Figure 17 shows that thymidine incorporation can be used to measure the relative levels of stem 35 cell activation in the explant in the absence or presence of the test compound (FGF in the figure) with increased proliferation indicative of a test agent having hair growth promoting activity.

45 In the reverse assay, anagenic micro-organ explants are provided in culture, e.g., such as the activated Sencar explants described in the appended examples, or growth factor stimulated explants (e.g., FGF stimulated). Test agents which inhibit proliferation of the hair follicle stem cells, e.g., relative to the untreated anagen explants, could be considered further for use as 5 telogenic agents that prevent hair growth. In still other embodiments, inhibitors of cell proliferation identified by the subject assay can be employed to inhibit growth of neoplastic or hyperplastic cells, e.g., tumor formation and growth. A preferred embodiment of the invention is directed to inhibition of epithelial tumor formation and growth. For a detailed description of skin epithelial tumor formation, see United 0 States Patent Application Serial Number 08/385,185, filed February 7, 1995. Tumor formation arises as a consequences of alterations in the control of cell proliferation and disorders in the interactions between cells and their surroundings that result in invasion and metastasis. A breakdown in the relationship between increase in cell number resulting from cell division and withdrawal from the cell cycle due to differentiation or cell death lead to disturbances in the 5 control of cell proliferation. In normal tissues, homeostasis is maintained by ensuring that as each stem cell divides only one of the two daughters remains in the stem cell compartment, while the other is committed to a pathway of differentiation (Cairns, J.(1975)Nature 255: 197 200). The control of cell multiplication will therefore be the consequence of signals affecting these processes. These signals may be either positive or negative, and the acquisition of .0 tumorigencity results from genetic changes that affect these control points. As described in Example IX and illustrated in Figure 12, skin micro-organ cultures of the present invention have been used for identifying cell proliferative agents and inhibitors of cell proliferation. As described in Example IX, TGF- p was tested and found to act as an inhibitor of cell proliferation. Activin, a protein which is a member of the TGF-p superfamily, 25 has also been shown to inhibit proliferation of epidermal cells. These results indicate there may be other members of the TGF-p family that play a role in inhibition of proliferation of epithelial cells. The data suggests a role for proteins in the TGF-p family as significant regulators of epidermal homeostasis and in inhibiting epithelial tumor formation and growth in vivo. 30 Another aspect of the present invention pertains to a method for identifying a cell differentiating agent, i.e., a compound which causes cell differentiation. This method includes isolating a population of cells from a subject wherein the population of cells having a microarchitecture of an organ or tissue from which the cells are isolated, a surface area to volume index of at least about 1.5mm , and includes at least one cell which has the ability to 35 differentiate. The cells are then placed in culture for at least about twenty-four hours and contacted with a candidate compound. The level of cell differentiation in the presence of the candidate compound is then measured and compared with the level of cell differentiation in the absence of the candidate compound. A statistically significant increase in the level of cell 46 differentiation in the presence of the candidate compound is indicative of a cell differentiating agent. Differentiation, as used herein, refers to cells which have acquired morphologies and/or functions different from and/or in addition to those that the cells originally possessed. Typically, 5 these morphologies and functions are characteristic of mature cells. The differentiation of populations of cells of the present invention can be monitored by measuring production and/or secretion of specialized cell products. In similar fashion, the present invention also pertains to a method for identifying an inhibitor of cell differentiation. Following the same protocol as above, the level of cell [0 differentiation in the presence of the candidate compound is measured and compared with the level of cell differentiation in the absence of the candidate compound. A statistically significant decrease in the level of cell differentiation in the presence of the candidate compound is indicative of an inhibitor of cell differentiation. In yet another embodiment, the subject cultures permit the generation of in vitro models 15 for viral infection. For example, epidermal or squamous tissue can be isolated, and infected with such viruses as herpes viruses, e.g., herpes simplex virus 1, herpes simplex virus 2; varicella-zoster virus; or human papilloma viruses, e.g., any of human papilloma viruses 1-58, e.g., HPV-6 or HPV-8. Similarly, hepatic models can be provided for hepatitis infection, e.g., an explant infected with hepatitis viruses, e.g., hepatitis A virus, hepatitis B virus, or hepatitis C !0 virus. The virally-infected tissue explants can be used to identify inhibitors of viral infectivity by method of the present invention. As above, the particular micro-organ culture is provided, and contacted (optionally) with a virus which infects the cells to produce a population of virus infected cells. The virus-infected cells can then be contacted with a candidate compound and the level of infectivity of the virus in the presence of the candidate compound measured. The 25 measured level of viral infectivity in the presence of the candidate compound is then compared to the level of viral infectivity in the absence of the candidate compound. A statistically significant decrease in the level of infectivity of the virus in the presence of the candidate compound is indicative of an inhibitor of viral infectivity. Methods of measuring viral infectivity are known in the art and vary depending on the 30 type of virus used. For example, one method which can be used to measure the level of viral infectivity is by measuring the level of production in the infected cells of the micro-organ culture or in the micro-organ culture medium of gene products specific for the particular virus being tested. For example, to measure the level of infectivity of hepatitis virus, e.g., hepatitis B virus, of cells in a micro-organ culture, hepatitis protein production and hepatitis DNA can be 35 quantitated. In general, micro-organ culture medium can be incubated with antibodies against a selected viral protein and the immunoreactive proteins analyzed by a variety of methods known in the art, e.g., on SDS-polyacrylamide gels, ELISA. For example, to measure production of hepatitis B surface antigen, micro-organ culture medium from micro-organs previously 47 incubated with hepatitis B virus can be sampled at daily intervals and assayed for the surface antigen by an ELISA (Abbott) method as described by the manufacturer. This method can be modified for quantitation using serially diluted standard surface antigen (CalBiochem). A statistically significant decrease in the accumulation of hepatitis B surface antigen in the culture 5 medium indicates that the candidate compound tested is an inhibitor of hepatitis virus infectivity. In addition to measuring levels of HBsAg in the micro-organ culture medium, newly synthesized hepatitis B virus DNA from cell extracts from the micro-organ culture can be detected and quantitated by PCR amplification of the DNA, followed by Southern blot analysis 0 using labeled primer pairs in the HBV pre-S (HBsAg encoding) region as probes (see e.g., Sambrook, J. Et al. (1989) Molecular Cloning - A Laboratory Manual, Cold Spring Harbor Laboratory, 2nd ed., vol. 2, pp. 10.14-10.15). Relative quantitation can be achieved by densitometry and confirmed by scintillation counting of corresponding bands. Reduction in levels of newly synthesized viral DNA indicate that the candidate compound tested is an 5 inhibitor of hepatitis virus infectivity. In another example, the gag, pol, and env protein products of retroviruses, e.g., human immunodeficiency virus (HIV), can also be measured using the above-described and other standard techniques known in the art. For example, pol protein expression in cells of micro organ cultures infected with HIV can be measured by incubating cell extracts with anti pol 0 antibodies or pooled AIDS patients sera and immunoreactive proteins analyzed on SDS/polyacrylamide gels. To measure infectivity of herpes virus, e.g., epstein/barr virus (EBV), in the micro-organ cultures of the present invention, EBV DNA and EBV -induced nuclear antigen production can be analyzed using the methods described herein. The micro-organ cultures of the present invention can also be used to promote wound 25 healing in a subject. Thus, the present invention further pertains to a method for promoting wound healing in a recipient subject. This method includes isolating, from a donor subject, a population of cells having a surface area to volume index is at least approximately 1.5mm". Typically, the population of cells is placed in culture for at least about twenty-four hours. The population of cells can then be applied to a wound of the recipient subject. In one embodiment, 30 the wound or lesion, is slow-healing or chronic, e.g., a wound associated with diabetes, e.g., a burn, e.g., an ulcer. As demonstrated in Examples X and XI, skin micro-organ cultures of the present invention can be used as micro-explants to be applied to chronic wounds (Example X) and can form a viable implant capable of growing hair (Example XI). In still another embodiment, the subject micro-organ explants are provided in an assay 35 to test for cytotoxicity or for irritation. In an exemplary embodiment, the subject method provides a technique for in vitro testing of ocular and dermal irritants. The process, much like above, involves the topical application of liquid, solid granular or gel-like materials (e.g., 48 cosmetics) to the micro-organ cultures of the present invention, followed by detection of the effects produced in the culture. Currently, potential eye and skin irritation of many chemicals, household cleaning products, cosmetics, paints and other materials are evaluated through direct application to 5 animals or human subjects. However, as is appreciated by most in the industry, such approaches are not met with overwhelming public support. The present method provides an alternative assay which does not require sacrifice or permanent maiming of an animal and also provides data in an objective format. In an illustrative embodiment, skin micro-organ cultures are derived according to the present invention. The cultured explants are contacted with a test agent, such as 0 a cosmetic preparation, and the cell viability is assessed at some time after the exposure. In a preferred embodiment, an MTT assay (based on the reduction of a tetrazolium dye by functional mitochondria) is used to score for viability. The micro-organ cultures of the present application can additionally be used to identify factors involved in normal homeostasis of tissues and cells, study the effect on the normal 5 homeostatis of tissues and cells of changes in the environment of the cells including changes in nutrients and the presence of potentially toxic agents, study the pathway of changes in the tissues and cells that are triggered at the beginning and during pathogenesis or trauma; identify repair mechanisms that reverse the adverse effects in an altered environment associated with pathogenesis or trauma; study developmental regulation of cells that differentiate during the .0 normal homeostasis of the tissue and developmental regulation of specialized structures (e.g., hair follicles) within the tissue; and for organ supplementation where pieces of an individual's organ remain but are insufficient for replacing or regenerating damaged tissue such as occurs in patients which chronic skin ulcers, which have healing deficiencies caused by inappropriate blood supply, or where the local skin is unable to heal such as in the conditions known as type I 25 or type II diabetes. In yet other embodiments of the invention, a method of utilizing micro-organs, extracts derived therefrom or pharmaceutical compositions including said extracts for inducing angiogenesis in mammalian tissues are provided. Specifically, the present invention can be used to induce angiogenesis in mammalian tissue via either implantation of one or more micro 30 organs, or via local administration of a soluble extract derived therefrom which is preferably included within a pharmaceutical composition. Complex multicellular organisms rely on a vascular network to support the needs of each and every cell for oxygen, nutrients and waste removal. This complex network of blood vessels is created and sustained through the process of angiogenesis. In humans, the 35 deterioration of the vascular network leads to occlusive arterial disease, which is the leading cause for morbidity and mortality in the Western world. Most currently available therapeutic options are based on surgical or other invasive procedures, such as vascular bypass or angioplasty. These solutions are for the most part successful but may be short lived or not 49 applicable to all patients. Since angiogenesis is a fundamental component of tissue and organ genesis, most tissues retain the capacity to induce new vessel formation during regeneration. Thus, the inventors of the present invention postulate that tissue which is removed from the body is in essence at least attempting to undergo regeneration and thus can be utilized as an 5 angiogenic stimulant. The present invention provides a new approach to induce angiogenesis, which approach is based on the use of the micro organs. Such micro-organs retain the basic micro-architecture of the tissues of origin while at the same time are prepared such that cells of an organ explant are not more than 100-450 microns away from a source of nutrients and gases. Such micro 0 organs function autonomously and remain viable for extended period of time both as ex-vivo cultures and in the implanted state. Thus, according to the subject aspect of the invention there is provided a method of inducing angiogenesis in a tissue of a mammal, such as, for example a human being. The method is effected by implanting at least one micro-organ within the tissue of the mammal. 5 Examples of tissue suitable for micro-organ implantation include but are not limited to, organ tissue or muscle tissue. Such implantation can be effected via standard surgical techniques or via injecting micro-organ preparations into the intended tissue regions of the mammal utilizing specially adapted syringe employing a needle of a gauge suitable for the administration of micro-organs. !0 The micro-organs utilized for implantation are preferably prepared from an organ tissue of the implanted mammal or a syngeneic mammal, although xenogeneic tissue can also be utilized for the preparation of the micro-organs providing measures are taken prior to, or during implantation, so as to avoid graft rejection and/or graft versus host disease (GVHD). Numerous methods for preventing or alleviating graft rejection or GVHD are known in the art and as such 25 no further detail is given herein. It will be appreciated that the angiogenic factors expressed in micro-organs can be extracted therefrom as a crude or refined extract in a soluble phase and utilized directly, or as part of a pharmaceutical composition for local administration into host tissues, e.g., via in, in order to induce angiogenesis. It will further be appreciated that 30 since micro-organs express different levels of the various angiogenic factors at different time points following implantation or during culturing, one can extract soluble molecules from different micro-organ cultures at different time points, which when locally administered in a series mimic the temporal expression of an implanted or cultured micro-organ. 35 Thus, according to another aspect of the present invention, there is provided another method of inducing angiogenesis in a tissue of a mammal. This method is effected by extracting soluble molecules from micro-organs and locally administering at least one predetermined dose of the soluble molecules extracted into the tissue of the 50 mammal. Numerous methods of administering are known in the art. Detailed description of some of these methods is given hereinbelow with regards to pharmaceutical compositions. As mentioned above and according to another preferred embodiment of the 5 present invention the soluble extracts are included in a pharmaceutical composition which also includes a pharmaceutically acceptable carrier which serves for stabilizing and/or enhancing the accessibility or targeting of the soluble extract to target body tissues. Examples of a pharmaceutically acceptable carrier include but are not limited to, 0 a physiological solution, a viral capsid carrier, a liposome carrier, a micelle carrier, a complex cationic reagent carrier, a polycathion carrier such as poly-lysine and a cellular carrier. The soluble extract which constitute the "active ingredient" of the pharmaceutical composition can be administered to the individual via various administration modes. 5 Suitable routes of administration may, for example, include transmucosal or parenteral delivery, including intramuscular, subcutaneous and intramedullary injections as well as intrathecal, direct intraventricular, intravenous, inrtaperitoneal, intranasal, or intraocular injections. Preferably, the composition or extract is administered in a local rather than a .0 systemic manner, for example, via injection directly into an ischemic tissue region of the individual. Pharmaceutical compositions of the present invention may be manufactured by processes well known in the art, e.g., by means of conventional mixing, dissolving, granulating, dragee-making, levigating, emulsifying, encapsulating, entrapping or 25 lyophilizing processes. Pharmaceutical compositions for use in accordance with the present invention thus may be formulated in conventional manner using one or more physiologically acceptable carriers comprising excipients and auxiliaries, which facilitate processing of the active ingredients into preparations which, can be used pharmaceutically. Proper 30 formulation is dependent upon the route of administration chosen. For injection, the active ingredient may be formulated in aqueous solutions, preferably in physiologically compatible buffers such as Hank's solution, Ringer's solution, or physiological salt buffer. For transmucosal administration, penetrants appropriate to the barrier to be permeated are used in the formulation. Such penetrants 35 are generally known in the art. The composition described herein may be formulated for parenteral administration, e.g., by bolus injection or continuos infusion. Formulations for injection may be presented in unit dosage form, e.g., in ampoules or in multidose containers with 51 optionally, an added preservative. The compositions may be suspensions, solutions or emulsions in oily or aqueous vehicles, and may contain formulatory agents such as suspending, stabilizing and/or dispersing agents. Pharmaceutical compositions for parenteral administration include aqueous 5 solutions of the active ingredient in water-soluble form. Additionally, suspensions of the active ingredients may be prepared as appropriate oily or water based injection suspensions. Suitable lipophilic solvents or vehicles include fatty oils such as sesame oil, or synthetic fatty acids esters such as ethyl oleate, triglycerides or liposomes. Aqueous injection suspensions may contain substances, which increase the viscosity of 0 the suspension, such as sodium carboxymethyl cellulose, sorbitol or dextran. Optionally, the suspension may also contain suitable stabilizers or agents which increase the solubility of the active ingredients to allow for the preparation of highly concentrated solutions. In addition, the composition of the present invention may be delivered via 5 localized pumps, or time release reservoirs which can be implanted within ischemic tissues of the individual. Since angiogenic factors are typically secreted from producing cells, micro organs can also be cultured in suitable media and the conditioned media which includes the secreted angiogenic factors can be collected at predetermined time points and .0 utilized as described hereinabove with respect to the soluble extract. Thus, according to yet another aspect of the present invention there is provided a method of inducing angiogenesis in a tissue of a first mammal. The method according to this aspect of the present invention is effected by culturing at least one micro-organ in a growth medium to thereby generate a conditioned medium, collecting the conditioned medium following at least 25 one predetermined time period of culturing and administering at least one predetermined dose of the conditioned medium into the tissue of the first mammal to thereby induce angiogenesis in the tissue. Preferably, the growth medium a minimal essential medium (described hereinabove) which does not contain undefined proteins or other growth factors which may interfere with the 30 intended function of the conditioned media or which may cause undesired reactions in the administered mammal. It will be appreciated that the collected conditioned media can be processed using chromatographic techniques, such as affinity columns and the like, so as to yield a substantially pure preparation which includes an array of angiogenic factors suitable for inducing 35 angiogenesis when administered to a mammal. It will further be appreciated that the conditioned medium and the soluble extract described herein can also be derived from micro-organs which include exogenous polynucleotides as described hereinabove. In such cases, if the exogenous polynucleotides 52 utilized encode angiogenic factors, the sequence of such exogenous polynucleotides is selected suitable for the intended administered mammal. For example, in cases where the soluble extract or conditioned medium is administered to humans recipients, human or humanized exogenous polynucleotides are preferably utilized. 5 The micro-organs according to the teachings of the present invention can be utilized following preparation, or alternatively they can be cryopreserved and stored at -160 0C until use. For example, micro-organs can be cryopreserved by gradual freezing in the presence of 10% DMSO (Dimethyl Sulfoxide) and 20% serum. This can be effected, for example, by encapsulating the micro-organs within planar 0 sheets, (e.g., a semi-permeable matrix such as alginate) and inserting these encapsulated micro organs into a sealable sterile synthetic plastic bag of dimensions closely similar to that of the encapsulated micro-organs. The bag would contain one plastic tubing input at one end and one plastic tubing output at the opposite end of the bag. The sealed plastic bag containing the planar sheet with the micro-organs could then be perfused with standard culture medium such as 5 Ham's F12 with 10% DMSO and 20% serum and gradually frozen and stored at -1600C. An important goal in cardiovascular medicine would be to replace surgical bypasses with therapeutic angiogenesis. Yet, in spite of the considerable efficacy observed when angiogenic factors were used in animal models of coronary or limb ischemia, the clinical results have been disappointing. Recently, it has been suggested that clinical failure may be due to the t0 application of the angiogenic factor or the combination of factors utilized. The angiogenesis method of the present invention overcome such limitations of prior art methods. The present invention brings forth a novel approach which recognizes that angiogenesis is a complex, highly regulated and sustained process, mediated by several regulatory factors. The results presented by the present invention provide a model which allows to study induction 25 of angiogenesis both in and out of the body and as such allows to establish the pattern of expression of key regulatory factors. The results presented herein show that implanted micro organs express several key angiogenic factors in a coordinated manner both in and out of the body. Furthermore, as shown by the in-vivo experiments, micro-organs function as genuine angiopumps not only by transcribing angiogenic factors, but also by inducing the formation of 30 new blood vessels. Furthermore, the magnitude of the induction is such that the vessels formed are sufficient to irrigate the surrounding area and rescue artificially induced hypoxic tissue regions in mice and rats. The model for ischemia in rats presented hereinbelow in the examples section appears to mimic chronic ischemia since no irreversible damage has occurred. In untreated animals, the 35 ischemia was apparent only after exertion. Presumably, there is enough collateral circulation to keep the limb viable but not enough to allow normal function when faced with an additional challenge. The implantation of micro-organs appears to have reversed this condition by increasing blood supply to ischemic regions. The results show a significant difference between 53 the micro-organ-treated and the control groups which difference is undoubtedly due to induction of angiogenesis by the micro-organs. In the mice series of in-vivo rescue experiments presented herein the ischemic insult was increased. Mice have inferior collateral circulation to the hindlimbs due to less developed 5 tail arteries as compared to rats. In this group, signs of acute irreversible ischemic damage such as gangrene and autoamputation, were detected in the control group. This finding suggests that the present invention may also be useful for salvage procedures, but this issue has to be further tested. In an additional series of trials presented below the ischemic challenge was increased 0 even further by inducing ischemia in previously diseased animals. Again, irreversible ischemic damage occurred only in the control animals. The damage to the control animals was so severe that there was no point in attempting a stress test. The number of mice was small but the differences were marked. These results are particularly important since they illustrate that micro-organs are capable of inducing angiogenesis even in tissues affected by certain types of 5 peripheral vascular disease. Thus, the present invention provides methods and compositions for inducing and maintaining blood vessel formation within host tissues for the purposes of rescuing ischemic tissues or generating natural bypasses around blocked blood vessels. !0 Exemplification The invention now being generally described, it will be more readily understood by reference to the following examples which are included merely for purposes of illustration of certain aspects and embodiments of the present invention, and are not intended to limit the 25 invention. Micro-organ cultures from animals including adult human skin, mouse, guinea pig and rat skin have been isolated and grown for up to 21 days in culture. However, it is within the scope of the invention to maintain cultures periods of time beyond 21 days. Furthermore, it is within the scope of the invention to form micro-organ cultures from a 30 wide range of animals. The range of animals is merely exemplified but is not limited to the samples provided below. As described in the appended examples, micro-organ cultures were prepared from skin and also from organs including the mammalian pancreas, liver, kidney, duodenum, esophagus and bladder. Similarly, micro-organ cultures of epithelia from mammalian cornea, kidney, 35 breast tissue and various gut derived tissues in addition to the esophagus such as intestine and colon may also be prepared using the methods of the invention. Indeed, it is within the scope of the invention to isolate and maintain micro-organ cultures from any site which contains an epithelial/stromal architecture within the body.

54 The above notwithstanding, the subject micro-organ culture technique has been used to preserve tissue explants in long-term culture from tissue not having epithelial/stromal architecture, such as certain lymphoid tissue, e.g., thymus and spleen explants. 5 Example I Preparation of Micro-Organ Cultures of Epidermis Fresh skin was obtained after surgery, cleaned from underlying fat tissue and cut into 0.4 x 5 cm flaps, which are then transversely sectioned, using a tissue chopper or other suitable 0 cutting means into 300ptm sections under sterile conditions so that the final tissue segments had dimensions of 4 mm in width and 0.3 mm in thickness (see Figure 1). These micrograns were placed in a 24-well microplate in 400ptl of DMEM in the absence of serum under 5% CO 2 at 37 0 C, under constant shaking at 12 rpm for periods of one to eight days. Twenty micro-explants were grown per well. 5 Example II Measurement Of The Proliferation of mouse, Guinea Pig and Human Epidermal Micro Organ Cultures t0 Micro-organ cultures were prepared according to Example I and proliferation of the cells was measured by analyzing the amount of DNA synthesis as follows. Mouse skin and guinea pig skin were grown for two days and human skin grown for four days after which BrdU was added to the medium at a final concentration of I 00ptM for three hours, followed by fixation of the cells in 4% formaldehyde. After fixation, the cultures were stained with goat anti-BrdU 25 antibodies followed by anti-goat-FICT labeled IgG. Histological preparations were embedded in following fixation in 4% formaldehyde and cut into 3 ptm slices and stained with methylene blue. It was found that the fraction of cells synthesizing DNA in vitro after two to four days in culture increased up to 10 fold compared with the values observed in vivo, after which the rate 30 of DNA synthesis gradually decreased but remained high for up to 10 days in culture(see Figures 2, 3 and 4A-4D). Even at six days in culture, the cells maintained a steady state of proliferation and differentiation so that the tissue architecture was preserved (Figures 5A-5C).

55 Example III Proliferation of Cells in Micro-Organ Cultures of Various Sizes Guinea pig micro-organs were prepared as in Example I. Whole thickness skin strips 4 5 mm in width were sectioned into explants of varying thickness including slices of 300, 450, 600, 700, 900, 1200 and 3000ptm thickness. These slices were placed individually into wells containing serum free medium for two days. BrdU was added for four hours before termination at a final concentration of 100tM. The explants were then fixed in 4% formaldehyde and stained with goat antibodies to BrdU followed by an anti-goat IgG FITC labeled secondary 0 antibody preparation. The results of this experiment are illustrated in Figure 6. The amount of BrdU incorporation as a function of the number of cells/unit tissue is significantly reduced as the thickness of the explants increases. Example IV 5 Preparation of Pancreatic Micro-Organ Cultures and Measurement of Cell Proliferation Guinea-pig pancreas was removed and then cut into sections of 300tm in thickness, 4 mm in width and 2mm in depth using an appropriate tissue chopper and in such a way that the pancreas microarchitecture was maintained. The micro-explants were grown in culture for !0 several time periods from two to eighteen days. Seven micro-organs were placed in each of 96 wells of a plate in 150pl of serum-free DMEM under 5% CO 2 at 370C under constant shaking at 12 rpm. BrdU was added three hours before termination at a final concentration of 100pM and the explants were then fixed in 4% formaldehyde and stained with goat antibodies to BrdU followed by anti-goat-FITC labeled IgG. Figures 7A-7B illustrate that cells in the pancreas 25 derived micro-organs were actively proliferating. Example V Preparation of Pancreatic Micro-Organ Cultures and Measurement of Insulin Secretion into the Culture Medium 30 Adult pig pancreas micro-organ cultures were prepared as in the previous examples for skin. Pancreases were removed, cut with scissors to an approximate depth of 2 mm and sliced into sections 300ptm thick having a width of 4mm. The micro-organ cultures were grown for 14 days in serum free medium. Every two days, the medium was removed and fresh medium 35 added. Collected media was assayed for insulin content using standard radioimmunoassay methods.

56 Example VI Transplantation of Pig Pancreatic Micro-Organs into a Xebogeneic Subject Adult pig pancreas micro-organ cultures were prepared as in the previous examples for 5 skin. Pancreases were removed, cut with scissors to an approximate depth of 2 mm and sliced into sections 300ptm thick having a width of 4 mm. The micro-organ cultures were then grown for different time periods of 0 to 5 days in serum-free medium, and after culturing, the pancreatic micro-organs were removed from the culture and transplanted into both the visceral and parietal mesoderm of rat hosts. The micro-organs survived for at least one month in vivo 0 and became well vascularized. After three, five, seven and fourteen days in vivo, extensive cell proliferation could be detected. Moreover, positive insulin staining was observed in vivo after four, seven, and thirty days post transplantation. Example VII 5 Preparation of Liver, Kidney, Duodenum, Esophagus and Bladder Micro-Organ Cultures and Measurement of Cell Proliferation in the Micro-Organ Cultures Guinea-pig micro-organ cultures from several epithelial tissue containing organs were prepared as in previous examples for skin. Organs were removed and with scissors, were cut to .0 an appropriate width of 2 mm, length of 3 mm, and sliced into sections of 300plm thick. The microcultures were incubated for three, four and six days in serum-free medium. Twelve hours before termination of the experiment, 3 H-thymidine was added to the cultures of explants. At termination, the tissue was fixed, rinsed several times and counted in a scintillation counter. The results of this experiment are illustrated in Figure 9. As shown in Figure 9, all tissues exhibited 25 active proliferation which continued for six days as determined by uptake of 3 H-thymidine. Example VIII Proliferation of Hair Follicles in Micro-Organ Cultures Skin micro-organ cultures were prepared according to Example I and incubated for two days. 30 BrdU was added three hours before termination of incubation. Cells were fixed in 4% formaldehyde and stained with goat anti-BrdU antibodies followed by anti-goat-FITC labeled IgG. Intact hair follicles that were present in vivo in their normal surroundings could be maintained under precisely controlled culture conditions, without the need of adding serum or any other exogenous factor. Hair follicle cells in these micro-organs were found to proliferate 35 vigorously for several days under the conditions of the present method as indicated by the large number of hair follicles cells that incorporated BrdU (Figures 10A-10C). The size distribution of hair shafts at time zero of a micro-organ guinea pig culture and after two weeks is shown in Figure 11. The medium was exchanged every two days. Hair shaft sizehas been arbitrarily 57 classified as small, medium and large. After nine days in culture, there was a clear shift in size distribution so that the percentage of small hairs decreased from 64% to 28%, while large shafts which were not present at the beginning of the culture represented 30% of the shaft population. 5 Example IX Preparation of Assay for Measuring the Effect of a Candidate Compound On Cell Proliferation The cultures were prepared and maintained in defined medium in similar growth 0 conditions as described in Example I. Control samples were analyzed by immunocytochemistry to determine that the micro-organ culture was maintained in a manner that was similar to that occurring in vivo. Duplicated samples of skin micro-cultures were treated with TGF-p at 2.5 ng/mI. A 5 quantitative analysis of the number of BrdU labeled cells/explant was performed according to Example II. Greater than 90% inhibition of DNA synthesis was observed in the presence of TGF-p compared with controls (Figure 12). Example X !0 A Method for PromotingHealing Chronic Non-Healing Skin Ulcers According to this method, a small-area of normal, uninvolved skin graft is removed from the patient and full thickness micro explants of 4 mm in width and 0.3 mm thick are prepared as described in Example I. The preparation however differs from Example I in that the 25 sectioning into 0.3 mm slices is deliberately incomplete so that a series of sections are held together as indicated in Figure 13, the upper epidermal layers including the stratum corneum. The design of this implant is directed to permitting the nutrients to reach all the cells but maintaining the tissue slices in a manipulatable format. The patient's wound is cleaned and surrounding skin edges are removed. The area devoid of skin is then carefully covered by the 30 micro-explants, which are placed on the wound such that the non-sectioned edge is facing outward and the opposing sectioned pieces are suspended in the fluid within the wound. Sufficient micro-explants are prepared to substantially cover the wounded area. The treated region is then covered with a suitable dressing and allowed to heal.

58 Example XI Proliferation of Hair Follicles In Vivo An in vivo animal experiment was performed where a 1 cm2 area of skin was removed 5 from a mouse and incompletely microsectioned so that the stratum comeum of the whole skin area was left intact as described above. The micro-organ was reimplanted into its original position in the mouse stitched and allowed to heal. The implant remained viable, became incorporated into the animal tissue and new hair shafts grew from the implant after one to two weeks in culture. (See Figure 14). 0 Example XII Human Psoriatic Skin Micro-Organ Cultures Split-thickness psoriatic skin from an 82 year old patient was obtained after autopsy 5 using a dermatome. The skin was then sectioned into 0.5 x 5 cm flaps which were then transversely sectioned using a tissue chopper or other suitable cutting device into 300 pm sections. These micro-organ sections were placed in microplates in serum-free DMEM under 5% CO 2 at 37 0 C under constant shaking for periods of one to fourteen days. In some instances, growth factors were added to the culture medium. The medium was changed every two days. .0 The human psoriatic skin proliferated extensively as micro-organ culture. Example XIII Liver Micro-Organ Cultures Infected with Hepatitis Virus 25 Human, rat, mouse, and guinea pig liver was sectioned and cultured as micro-organ cultures as described in Example VII. Active proliferation in these micro-organ cultures was detected using BrdU incorporation as described herein. The hepatocytes in these micro-organ cultures were determined to be functional as measured by assay of urea (Sigma Chemical, urea detection kit) and albumin production (ELISA) after at least 14 days in culture. 30 Human liver micro-organ cultures prepared above were incubated with sera from patients positive for hepatitis B and hepatitis C virus. After 24 hours, the medium was removed and fresh DMEM with and without 10% normal fetal calf serum (FCS) was added. Every two days, the culture medium was exchanged with fresh medium and the conditioned medium tested for viral particles using antibodies against the viral protein HBs. A significant increase in 35 number of viral particles was detected after 4 days in those micro-organ culture that were cultured in the presence of FCS.

59 Example XIV Thymus and Spleen Micro-Organ Cultures Mouse and rat micro-organ cultures from thymus and spleen were prepared essentially as in the 5 previous examples for skin. Organs were removed and cut with scissors to an approximate width of 2mm and length of 3mm. These samples were then spliced into explants of approximately 300pim thick using an appropriate tissue chopper in such a way as to preserve the essential microarchitecture of the organ. The micro-organs were then incubated for 1, 3, 5 and 10 days in serum free medium. Active proliferation in these micro-organ cultures was detected 0 using BrdU incorporation as described herein. Example XV Bone marrow Micro-Organ Cultures .5 Micro-organ cultures from bone marrow were prepared by carefully removing the bone marrow intact from femurs of rats and mice. Since the diameter of the marrow in such explants is only about 1-2 mm, the marrow was directly sliced into micro-organ explants using 300pm thick using a tissue chopper. This method ensured the microarchitecture of the marrow was preserved while at the same time retaining a surface/volume index amenable to long-term !0 culture. The micro-organs were incubated for 3 days in serum free medium. Active proliferation of marrow cells in these micro-organ cultures was detected using BrdU incorporation as described herein. Example XVI 25 Delivery of Gene Products to Skin Micro-Organ Cultures The high surface area to volume inherent to the micro-organ cultures of the present invention allows easy access to tissues for a variety of gene transfer techniques. In this example, micro-organ cultures are transfected with foreign genes using electroporation and lipofection. 30 The micro-organ cultures can be transplanted into animals and survive for at least about thirty days in vivo and become vascularized. This demonstrates the feasibility of using MC cultures of tissues in ex vivo gene therapy protocols. A further advantage of the MC culture is that it can be transplanted to a defined position in the body, so that if necessary it could be readily removed in the future. This contrasts with cell suspension transplantation into the body in which the cells 35 can migrate or become "lost" in normal tissue. Guinea pig skin was dissected and sliced into sections with a width of 2 mm and a thickness of 300pim. The skin was cultured as a micro-organ in serum-free Dulbecco's minimum essential media with penicillin and streptomycin at the concentrations recommended by the 60 manufacturer. After one day in culture at 37*C and 5% C0 2 , the skin micro-organ cultures were rinsed with DMEM without antibiotics and added to a 0.4 cm gap disposable electroporation cuvette with 500p1l of media on ice. Ten micrograms of the plasmid DNA containing the indicated reporter genes were added 5 as shown (each plasmid had a cytomegalovirus promoter driving the expression of either a p galactosidase (control) or luciferase reporter gene. The luciferase plasmid backbone was pRC CMV (Invitrogen) fused in frame with the firefly luciferase gene. The samples were electroporated at 220 mV and the capacitance varied as shown in Figure 15 (Hi=900pF, medium=500pF, low=250pF). NIH3T3 cells were treated at 250pF) with a Bio .0 Rad electroporation device. The samples were then further incubated with DMEM containing 10% bovine calf serum, penicillin, streptomycin, and glutamic acid for 2 days in a 24 well culture plate. The media was removed, and the samples were suspended in about 700pl of cell culture lysis reagent (Promega). The tissue pieces were homogenized, and then 20pl was added to 100pl of luciferase assay reagent (Promega), and luminescence was detected in triplicate with .5 the Packard TopCount. As a positive control, NIH3T3 cells from a 75cm culture flask were trypsinized, and treated identically to the micro-organ cultures. As illustrated in Figure 15, at the medium (500pF) and low (250pF) capacitance settings, significant luciferase activity was detected. For comparison, similar amount of NIH3T3 immortal cultured cells were electroporated with the same plasmids at 250pF. !0 In another experiment, the transfection of the micro-organ explants was accomplished by lipofection, which was observed to be more efficient than electroporation. In particular, micro-organ cultures from guinea pig skin, newbom mouse skin, and rat lung were transfected with a plasmid containing a luciferase reporter. Briefly, the micro-organ cultures were grown at 37*C in 5.5% CO 2 in DMEN with 1% penicillin/streptomycin and 1% L-glutamine for one day 25 before transfection. The explants were plated on 24 well plates with 20 explants and 400pl of media per well. For transfection, the micro-organ cultures were rinsed twice with Optimem, and 10pl of Lipofectin (Gibco BRL) +2pg of DNA + Optimem was added to each well with the final volume being 500 pl. The Optimem/Lipofectin/DNA solution was made according to the Lipofectin manufacturer's directions. The cultures were then incubated for 5-6 hours at 37*C in 30 5.5% CO 2 . The Optimem/Lipofectin/DNA media was then replaced with 400 11 of DMEN with 1% penicillin/streptomycin, 1% L-glutamine and 10% FCS, and the cultures incubated overnight at 37*C in 5.5% CO 2 . The following morning, the micro-organ cultures were removed, washed twice with IX PBS, and ground in a hand-operated glass tissue grinder in 750 pl of IX cell culture lysis buffer (Promega). Luciferase activity from the transgene was detected 35 using Luciferase Assay System (Promega), with the results reported in Figure 18.

61 Example XVII Delivery of Gene Products to Micro-Organ Cultures Lung and thymus from an eight week old female Lewis rat were dissected and processed for 5 micro-organ culturing as described in Example XV. The micro-organ cultures were placed in culture wells and transfected with cationic lipid/luciferase encoding plasmid DNA complexes for five to six hours while incubating at 37*C. The cationic lipid/plasmid DNA solution was aspirated, and the cultures were then incubated in medium plus 10% serum for two days, and then assayed for luciferase reporter gene expression (expressed in arbitrary light units). The 0 results of this experiment are illustrated in Figure 16. As demonstrated in Figure 16, the lung, but not the thymus expresses the transfected luciferase gene under these conditions. As expected, the negative control p-galactosidase transfected lung micro-organ culture (10 p1l cationic lipid concentrate) was near machine background for light production (23 light units). 5 Example XVIII Hair Shaft Growth in vitro New born mouse skin was obtained after surgery, cleaned of underlying fat tissue and cut into 0.4 x 5 cm flaps, which were then transversely sectioned, using a tissue chopper or 0 other suitable cutting devise into 300tm sections. The micro-organs were placed in microplates in DMEM in the absence of serum under 5% CO 2 at 37 0 C under constant shaking for periods of I to 14 days. Certain of the micro-organ explants were contacted with a growth factor, e.g., FGF, which was added to the culture media. The medium was changed every 2 days. New born "hairless" skin can be induced to produce hair shafts when grown in MC ,5 cultures. Micrographs of skin from 30 hr-old mouse, grown in micro-organ cultures for 3 days in the presence of 1 ng/ml EGF indicated the development of hair shafts in the explants, which growths were not present at the beginning of the culture period. In another set of experiments, activation of telogen follicles was observed. The Sencar mouse provides a useful model to study hair follicle activation because the follicles are well 30 synchronized and the cycle stages have been well characterized. Sencar mice provide an in vivo model for anagen activation. The removal of the club from a telogen follicle can induce new hair formation, the first signs of which, are well characterized. Skin from adult Sencar mice was obtained after surgery, cleaned from underlying fat tissue and cut into 0.4 x 5 cm flaps, which were then transversely sectioned, using a tissue chopper or other suitable cutting device into 300 35 ptm sections. The micro-organs were placed in microplates in DMEM in the absence of serum 5% CO 2 at 37*C under constant shaking for periods of I to 14 days. Activation of telogenic follicles, whether induced by club removal or growth factor treatment, was manifested by the 62 proliferation of follicle stem cells. Figure 17 illustrates the activation of a telogenic explant, as detected by thymidine incorporation. Example IXX 5 Preparation of Pancreatic Islets For Transplantation Several techniques have been developed to prepare islet cells from various mammalian sources, in large quantities since they constitute a potentially transplantable beta cell mass with which to treat established type I diabetes. Two main drawbacks have been encountered so far. It 0 has proven difficult to obtain a reproducible reliable way of preparing beta cells. Second, the viability of these cells both in vitro and in vivo is largely variable . In part due to the first reason and in part due to the fact that the p-cells most likely require support from the stroma that underlies the islets in the normal pancreas. Attempts of course at maintaining pancreatic organs ex vivo have so far been unsuccessful. Using the MC culture technology described herein, 5 success has been achieved for establishing micro-organ cultures of mouse, rat, guinea pig and pig pancreas in vitro in defined culture medium Pancreas micro-organ cultures have now been grown in vitro for periods of up to one month. Within the cultures, explants maintain their tissue microarchitecture and certain cell subpopulations proliferate actively as determined by BrdU incorporation and labeling. O Furthermore the islet cells secrete insulin into the medium even after one month of in vitro culture. Transplantation experiments have been performed in which pig micro-organ pancreas cultures have been implanted into both the visceral and parietal mesoderm of rat hosts. Explants have been kept for periods varying from a few days up to one month in vivo. The explants z5 become well vascularized and incorporate into the tissue host. Example XX Preparation of Human Psoriatic Skin Micro-organ Cultures 30 Split-thickness psoriatic skin from a patient was obtained after autopsy, using a dermatome. The skin was cut into 0.4 X 5 cm flaps, which were then transversely sectioned with a tissue chopper into 300pm thick sections. These micro-organ explants were cultured in DMEM (no serum) in microplates at 37*C and 5% CO 2 for periods of 1 to 14 days. Inspection of the micro-organ explants at various time points indicated that the cells of the explant had 35 remained viable, and proliferation was occurring. The nomenclature used and the laboratory procedures utilized in the following examples and the invention per se include molecular, biochemical, microbiological and recombinant DNA techniques. Such techniques are thoroughly explained in the literature. See, for example, 63 "Molecular Cloning: A laboratory Manual" Sambrook et al., (1989); "Current Protocols in Molecular Biology" Volumes I-III Ausubel, R. M., ed. (1994); Ausubel et al., "Current Protocols in Molecular Biology", John Wiley and Sons, Baltimore, Maryland (1989); Perbal, "A Practical Guide to Molecular Cloning", John Wiley & Sons, New York (1988); Watson et al., 5 "Recombinant DNA", Scientific American Books, New York; Birren et al. (eds) "Genome Analysis: A Laboratory Manual Series", Vols. 1-4, Cold Spring Harbor Laboratory Press, New York (1998); methodologies as set forth in U.S. Pat. Nos. 4,666,828; 4,683,202; 4,801,531; 5,192,659 and 5,272,057; "Cell Biology: A Laboratory Handbook", Volumes I-III Cellis, J. E., ed. (1994); "Current Protocols in Immunology" Volumes I-III Coligan J. E., ed. (1994); Stites et 0 al. (eds), "Basic and Clinical Immunology" (8th Edition), Appleton & Lange, Norwalk, CT (1994); Mishell and Shiigi (eds), "Selected Methods in Cellular Immunology", W. H. Freeman and Co., New York (1980); available immunoassays are extensively described in the patent and scientific literature, see, for example, U.S. Pat. Nos. 3,791,932; 3,839,153; 3,850,752; 3,850,578; 3,853,987; 3,867,517; 3,879,262; 3,901,654; 3,935,074; 3,984,533; 3,996,345; 5 4,034,074; 4,098,876; 4,879,219; 5,011,771 and 5,281,521; "Oligonucleotide Synthesis" Gait, M. J., ed. (1984); "Nucleic Acid Hybridization" Hames, B. D., and Higgins S. J., eds. (1985); "Transcription and Translation" Hames, B. D., and Higgins S. J., eds. (1984); "Animal Cell Culture" Freshney, R. I., ed. (1986); "Immobilized Cells and Enzymes" IRL Press, (1986); "A Practical Guide to Molecular Cloning" Perbal, B., (1984) and "Methods in Enzymology" Vol. 1 .0 317, Academic Press; "PCR Protocols: A Guide To Methods And Applications", Academic Press, San Diego, CA (1990); Marshak et al., "Strategies for Protein Purification and Characterization - A Laboratory Course Manual" CSHL Press (1996); all of which are incorporated by reference as if fully set forth herein. Other general references are provided throughout this document. The procedures therein are believed to be well known in the art and 25 are provided for the convenience of the reader. All the information contained therein is incorporated herein by reference. Example XXI Materials and Experimental Methods 30 Micro-organ preparation: Adult animals (C-57 mice or Sprague Dawley rats) were sacrificed by asphyxiation with C02 and the lungs were removed under sterile conditions. The lungs were kept on ice and rinsed once with Ringer solution or DMEM including 4.5 gm/l D-glucose. micro-organs were 35 prepared by chopping the lungs with a Sorvall tissue chopper into pieces approximately 300 pm in width. micro-organs were rinsed twice with DMEM containing 100 units/ml Penicillin, I mg/ml Streptomycin and 2 mM L-Glutamine (Biological Industries) and kept on ice until use. Micro-organ implantation: 64 Adult C-57 mice were anesthetized using 0.6 mg Sodium Pentobarbitol per gram body weight. The mice were shaved, and an incision about 2 cm long was made in the skin at an area above the stomach. A hemostat was used to create subcutaneous "pockets" on both sides of the incision, and 8-9 micro-organs were implanted in each pocket; implantation was done by simply 5 layering the micro-organs over the muscle layer. The incision was sutured and the animals were kept in a warm, lit room for several hours following which they were transferred to the animal house. Four animals were sacrificed at a time interval of either 4 hours, 24 hours, 72 hours or 7 days following implantation and the implanted micro-organs were dissected from surrounding tissues under a surgical microscope and utilized for RNA extraction. The extracted RNA was .0 reverse transcribed and the resulting cDNA was used as a template for PCR analysis using standard methodology. The oligonucleotide primer sequences utilized in the PCR reaction, the expected product size and references are given in Table 2, below. Table 2 .5 R T-PCR primer sequence and source Name Genebank # Sequence Product Ang2 AF004326.1 F: 5'-CGTGGGTGGAGGAGGGTGGAC-3' 400 bp R: 5'-TGCGTCAAACCACCAGCCTCC-3' p-Actin F: 5'-TACCACAGGCATTGTGATGG-3' 310 bp**** R: 5'-AATAGTGATGACCTGGCCGT-3' Angl U83509 F: 5'-GGTC' (SEQ ID NO:5) 273 bp*/** R: 5'-CCAAGGGCCGGATCAGCATGG-3' VEGF U41383 F: 5'-ACTTTCTGCTCTCTTGGGT-3' 444, 573***/** R: 5'-CCGCCTTGGCTTGTCACA-3' * Another discrete band is often detected at approximately 320 bp - the origin of this band is unknown. ** Primers (Angl) or primer sequence (VEGF) was kindly supplied by professor Eli Keshet, Israel. ***VEGF mRNA 20 undergoes alternative splicing. PCR product sizes are 444 bp for VEGF121, 573 bp for VEGF 1 6 5 , and 645 bp for

VEGF

1 8 9 . **** Ibrahim et al. 1998 Biochimica et Biophysica Acta 1403, 254-264. Densitometric analysis and quantification: A 10 pl aliquot of each PCR reaction was electrophoresed in a 1.5 % agarose gel stained 25 with ethidium bromide. Gels were imaged utilizing a Macintosh Centris 660 AV computer and a Fujifilm Thermal Imaging System with a Toyo Optics TV zoom lens (75-125 mm, F=1.8, with a Colkin orange 02 filter). Densitometric analysis was performed using the public domain NIH 1.61 analysis software. Quantitation was done by normalizing the expression level of each PCR product to those obtained for p-Actin. All PCR reactions were performed in duplicate.

65 Statistical analysis of the relative expression levels of the various VEGF isoforms was performed by comparison of the means, using Welsh's t-test on nonpaired samples before and after implantation. Ischemic tissue rescue experiments: 5 Thirty two Sprague Dawley rats aged 1-4 months and weighing 200-300 grams were utilized. The left common Iliac of each rat was ligated and excised from rats anesthetized using 0.9-1.1 mg/gram body weight of Pentothal at the aortic bifurcation just proximal to the Iliac bifurcation. Sixteen rats were implanted with 3-4 micro-organs each 24 hours following the induction of ischemia. The micro-organs were implanted intramuscularly and subcutaneously 0 along the Femoral artery (medially) and along the sciatic nerve (laterally). The remaining sixteen rats underwent sham implantation 24 hours following the induction of ischemia. Twenty six C57/B mice aged 1-3 months and weighing 19 to 27 grams were also tested. The left Common Iliac artery of anesthetized mice was ligated and excised at the aortic bifurcation just proximal to the Iliac bifurcation. 3-4 micro-organs were implanted in each 5 mouse at 24 hours following the induction of ischemia. Nine mice were implanted intramuscularly and subcutaneously along the Femoral artery (medially) and along the sciatic nerve (laterally) in the proximal left hindlimb. Seventeen control mice were prepared for implantation following ischemia induction but no implantation was performed. Animals that had venous or nervous damage during the operation as well as those that suffered from !0 significant bleeding were excluded from the trial. Seven old C57/B mice aged 22 months and weighing 24 to 28 grams also underwent ligation and excision of the left Common Iliac artery as described above. Three were immediately implanted with micro-organs derived from normal healthy syngeneic mice. Four had immediate sham implantation. None suffered from venous or nervous damage or had 25 significant bleeding during the operation. Functional assay: The animals were tested on the first and second days following implantation to rule out nerve damage. The test consisted of swimming in a lukewarm water bath which was set at a water level such that the animal needed to constantly exert all four limbs in order to stay afloat. 30 The time limits for exercise were gradually increased. During the first week the time limit was 3 minutes or until efforts to remain afloat ceased. During the second week the limit was raised to 5 minutes, while from the third week onwards the time limit was 6 minutes. A scale from 0 to 10 was created to assess the degree of claudication. A score of 0-1 indicated normal or near normal gait. A score of 2-3 meant slight to moderate claudication with normal weight bearing. 35 A score of 4-5 indicated moderate claudication with disturbance in weight bearing. A score of 6-7 indicated severe claudication. A score of 8-9 indicated a non functioning limb, atrophy or contracture and a score of 10 meant gangrene or autoamputation. The scores were assigned by an independent observer not involved in the experiment and having no knowledge of previous animal treatments. Angiography: Angiography was performed on several rats at days 4, 14, 26 and 31 following 5 implantation. The rats were anesthetized as previously described and a P10 catheter was introduced through the right superficial femoral artery and placed in the aorta. A bolus injection of I cc Telebrix was injected and the animal was photographed every 0.5 seconds. Animals undergoing angiography were subsequently excluded from the trial groups. 0 Experimental Results Implanted micro-organs induce angiogenesis: Figure 19 illustrates the response of surrounding tissues to implanted micro-organs. When an micro-organ is implanted subcutaneously into a syngeneic animal, it induces an angiogenic response towards the micro-organ (arrow, Figure 19). A major blood vessel forms 5 and branches into smaller vessels which branch into a net of capillaries which surround the implanted micro-organ. Micro-organs transcribe a sustained and dynamic array of angiogenic growth factors when implanted subcutaneously into syngeneic mice. Figure 20 illustrate a representative semi quantitative analysis of several known angiogenic growth factors as determined from the RT !0 PCR analysis performed on RNA extracted from the micro-organs. As seen from the results, a strong induction of angiogenic factor expression occurs at 4 hours post implantation (PI). Following this initial induction, each individual growth factor follows a different expression pattern as is further detailed below. VEGF: VEGF transcription level continued to rise at 24 hours PI. At three days PI, 25 transcription levels of VEGF decreased. In the days following, lower mRNA levels of this angiogenic factor were detected, which levels were probably necessary in order to maintain the neo-angiogenic state thus formed. At seven days PI, VEGF mRNA returned to a level similar to that detected in micro-organs at the time of implantation (to). Angiopoletin 1: The level of Angl mRNA increased for the first 4 hours PI, although 30 variation was high. At one to three days PI, transcription dropped to levels which are even lower than that detected for micro-organs at the time of implantation (to) (see Maisonpierre et al., 1997, Science 277, 55-60, Gale and Yancopolous, 1999 Ibid.). At seven days PI, Angi mRNA returned to a level similar to that detected at to. Angiopoletin 2: Ang2, the antagonist of Angl, was transcribed at high levels at 24 35 hours Pl. mRNA levels dropped at 3 and 7 days PI, although these levels were still higher than the levels detected at to, possibly due to ongoing vascular remodeling in and around the implanted micro-organ.

67 Thus, as is evident from these results, implanted micro-organs transcribe a dynamic array of factors, both stimulators and inhibitors, which participate in the regulation of angiogenesis. This transcription pattern which is responsible for the generation of new blood vessels around the micro-organs is sustained over a period of at least one week Pl. 5 Micro-organs transcribe a sustained and dynamic array of angiogenic growth factors when cultured: In order to determine the capacity of micro-organs to transcribe angiogenic factors when cultured ex-vivo, micro-organs prepared as described above, were grown in the absence of serum for periods of over one month. Samples were removed at various time points and 0 assayed for the mRNA levels of the several factors. Figure 22 illustrate a representative semi quantitative analysis of several known angiogenic growth factors as determined from RT-PCR performed on RNA extracted from cultured micro-organs (Figure 21). As shown in both Figures a strong induction of angiogenic factor expression occurs 4 hours following culturing. Following this initial induction, each different growth factor follows a different expression 5 pattern as is described in detail below. VEGF: VEGF expression levels continued to rise 24 hours after culturing. Three days after culturing, the expression level of VEGF decreased only to increase again at 7 days PI. In the following days expression levels drop and VEGF expression returns to a level comparable to that expressed by micro-organs at the time of culturing. o Angiopoletin 1: The level of Angl expression increased for the first 4 hours following culturing although variation was high. Expression dropped 1 to 3 days after culturing to levels even lower than that detected at time of culturing. At seven days after culturing Angi expression returned to a level comparable to the level at time of culturing. Angiopoietin 2: Ang2, the antagonist of Angl, was expressed at a high level during the z5 first day after culturing. The expression levels were lower 3 and 7 days after culturing, although they are still higher than the expression level at time of culturing. As is evident from these results, micro-organs which are cultured outside the body remain viable and functional for over a month in vitro and express a dynamic array of angiogenic factors, including both stimulators and inhibitors, which participate in the regulation 30 of angiogenesis. Implantation of micro-organs reverse ischemia in limbs of rats and mice: Series 1: The left common iliac artery of thirty two rats was ligated and excised as described above. micro-organs implantation was conducted in sixteen of these rats while the sixteen remaining rats served as the control group (sham operations). All 32 rats survived the 35 operation. No significant difference was detected between the two groups prior to exertion (Figure 23). Following exertion, a significant difference was detected; the cumulated average claudication score for the control group was 4.8 whereas in the micro-organ implanted group the score was 1.6 (Figure 24). Similar results were recorded throughout the study period. The 68 control group scored 5 on days 6-10 post operation (PO), 5 at 11-15 days PO and 4 at day 17 PO. The scores for the micro-organ implanted group were 1.67, 1.5 and 1.7, respectively. It should be noted that the micro-organ implanted group included one rat with an average score of 6.5. A histological examination revealed necrotic micro-organ implants in this rat. 5 Series 2: Twenty six young C57/B mice were operated as described above without operative damage or preoperative mortality. Micro-organ implantation was conducted in nine of these mice while seventeen served as control (sham operations). Of the 17 control mice, 4 developed gangrene on the ischemic-induced limb and died 2-3 days PO (23.5 %). Another mouse from this group had autoamputation of an atrophied limb 8 days after operation (5.9 %). o None of the micro-organ implanted mice developed gangrene, autoamputation or postoperative death (0 %). The average cumulated post exertion claudication score for the control group was 6 with scores of 7.7 on days 5-9 PO, 6.2 on days 13-19 PO and 4.1 on days 21-25 PO. The average cumulated claudication score for the micro-organ implanted group was 2.4, with scores of 1.8 on days 5-9 PO, 2.2 on days 13-19 PO and 3.1 on days 21-25 PO (Figure 25). 5 Micro-organ implantation rescues ischemic limbs in old mice: Series 3: Seven aged C57/B mice were operated upon with no operative damage or death. Three mice received micro-organ implants and 4 served as control. Of the control group, I developed gangrene and died 3 days PO (25%) and one had autoamputation of an atrophied limb 5 days PO (25%). The remaining two mice had non functioning limbs at rest (a score of 8 0 on the claudication index). None of the micro-organ implanted mice developed gangrene or autoamputation (0%) and their average claudication score at one week was 5.7 . Implanted micro-organs are viable, and vascularized: In sampled rat specimens the micro-organ implants were viable, with preserved architecture and no evidence of rejection. The micro-organs and surrounding muscle tissue was .5 vascularized via macroscopically visible blood vessels. Angiography reveals angiogenic activity in micro-organ-implanted rats: Angiography was performed on days 4, 14, 26 and 31 PO. There were subtle but detectable differences between the micro-organ-treated groups and the control groups. Evidence of increased angiogenic activity in the implanted limb was detected as early as day 4 30 PO. New, medium sized blood vessels were visible in the implanted limb sixteen days PO. Example XXI Spleen micro-organs Mouse Spleen micro-organs were prepared from as described hereinabove and 35 implanted into syngeneic mice. Figure 26 illustrates an micro-organ (arrow) which was implanted subcutaneously into the syngeneic mouse and examined at six months following implantation. As is clearly demonstrated in Figure 26, the micro-organ induced angiogenesis. In 69 fact, the pattern of blood vessels formed, gives the impression that the micro-organ is micro organ was an inherent organ of the host. Example X XII 5 Cornea implantation of micro-organs The cornea is the only tissue of the body which is devoid of blood vessels. As such, the cornea is an excellent model tissue for studying angiogenesis. Rat lung micro-organs were implanted in the corneas of syngencic rats. As shown in Figure 27, a most remarkable angiogenic pattern was also induced in the cornea. These remarkable results again verify that 0 micro-organs are effective in inducing and promoting angiogenesis. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific assay and reagents described herein. Such equivalents are considered to be within the scope of this invention and are covered by the following claims. 5 Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, references, patents, patent applications mentioned in this specification are 10 herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, reference, patent, patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention, or is not an admission that the 25 disclosures constitute common general knowledge in Australia. The term "comprise" and variants of the term such as "comprises" or "comprising" are used herein to denote the inclusion of a stated integer or stated integers but not to exclude any other integer or any other integers, unless in the context or usage an exclusive interpretation of the term is required. 30

Claims (17)

1. A genetically modified micro-organ explant expressing at least one recombinant gene product, wherein said micro-organ explant is a section of tissue comprising a population of cells maintaining the microarchitecture and the three-dimensional structure of an organ from which it is obtained and at the same time having dimensions selected so as to allow diffusion of adequate nutrients and gases to cells in the micro-organ explant and diffusion of cellular waste out of the micro-organ explant so as to minimize cellular toxicity and concomitant death, wherein said dimensions provide for a micro-organ explant with a surface area to volume index characterized by the formula l/x + 1/a > 1.5 mm'; wherein 'x' is a tissue thickness and 'a' is a width of said tissue in millimeters, and wherein at least some of the cells of said population of cells of said micro-organ explant comprise a recombinant gene and express at least one recombinant gene product, wherein said recombinant gene product is Factor VIII.

2. The genetically modified micro-organ explant of claim 1, wherein at least some of the cells of said population of cells of the micro-organ explant further express a reporter gene.

3. The genetically modified micro-organ explant of claim 1 or claim 2, wherein said recombinant gene produces a gene product that is native to the organ from which said micro organ explant is obtained.

4. The genetically modified micro-organ explant of any one of claims 1 to 3, wherein said recombinant gene produces a gene product that is heterologous to the organ from which said micro-organ explant is obtained.

5. The genetically modified micro-organ explant of any one of claims I to 4, wherein said organ is skin.

6. The genetically modified micro-organ explant of any one of claims I to 5, wherein at least a portion of the population of cells is transduced, transformed or transfected with a recombinant construct carrying a recombinant gene encoding said recombinant protein.

7. The genetically modified micro-organ explant of claim 6, wherein said recombinant construct is a recombinant adeno virus, a recombinant adeno-associated virus, or a recombinant plasmid. 71

8. A pharmaceutical preparation comprising the genetically modified micro-organ explant of any one of claims I to 7.

9. A method of delivering a gene product to a subject, the method comprising the steps of: (a) providing a micro-organ explant obtained from said subject, wherein said micro organ explant is a section of tissue comprising a population of cells, which maintains a microarchitecture and a three dimensional structure of an organ from which it is obtained and at the same time having dimensions selected so as to allow diffusion of adequate nutrients and gases to cells in the micro-organ explant and diffusion of cellular waste out of the micro-organ explant wherein said dimensions provide for a micro-organ explant with a surface area to volume index characterized by the formula 1/x + 1/a > 1.5 mmf; wherein 'x' is a tissue thickness and 'a' is a width of said tissue in millimeters; (b) genetically modifying ex-vivo at least some cells of the micro-organ explant with a recombinant gene to express at least one recombinant gene product; and (c) implanting the genetically modified micro-organ explant in said subject, wherein said subject is a human being.

10. The method of claim 9, wherein said at least one recombinant gene product is selected from the group consisting of a recombinant protein, a recombinant peptide, and a recombinant functional RNA molecule.

11. The method of claim 9 or claim 10, wherein at least some of the cells of said population of cells of the micro-organ explant further express a reporter gene.

12. The method of any one of claims 9 to 11, wherein said recombinant gene produces a gene product that is native to the organ from which said micro-organ explant is obtained.

13. The method of any one of claims 9 to 12, wherein said recombinant gene produces a gene product that is heterologous to the organ from which said micro-organ explant is obtained.

14. The method of any one of claims 9 to 13, wherein said recombinant gene product is Factor VIII.

15. The method of any one of claims 9 to 14, wherein said organ is skin. 72

16. The method of any one of claims 9 to 15, wherein at least a portion of the population of cells is transduced, transformed or transfected with a recombinant construct carrying a recombinant gene encoding said recombinant protein.

17. The method of claim 16, wherein said recombinant construct is a recombinant adeno virus, a recombinant adeno-associated virus or a recombinant plasmid vector. Date: 23 December 2010