THE BEHAVIOR OF HUMAN SKIN, ITS APPENDAGES AND TUMORS IN HETEROLOGOUS HOSTS* GEORGE W. HAMBRICK, Ja., M.D. AND ROSALYN BLOOMBERG, B.S.

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1 THE BEHAVIOR OF HUMAN SKIN, ITS APPENDAGES AND TUMORS IN HETEROLOGOUS HOSTS* GEORGE W. HAMBRICK, Ja., M.D. AND ROSALYN BLOOMBERG, B.S. Several workers, notably Greene and Toolan, have grown human tissues, both normal and malignant, in heterologous hosts with or without prior radiation or treatment with cortisone (9, 6, 15, 8). Highly malignant tumors have been maintained for several years in hamsters (17). According to Greene, the degree of transplantability of a given tumor parallels the degree of malignancy within the patient and is of prognostic significance (6). Toolan (14) maintains, however, that even adult non-malignant tissues live for several weeks within heterologous hosts. There are few reports of transplantations into small animals of basal cell carcinoma or squamous cell carcinoma of epidermal origin (12). Several squamous cell carcinomas arising from other epithelial tissues have been readily transplanted (9, 15, 17, 18). Embryonic skin has been easily transplanted and maintained for several months by various workers (5, 16). In an effort to develop a transplantable strain of epidermal tissue, specimens of basal cell carcinoma, squamous cell carcinoma, normal adult skin and its appendages have been transplanted into heterologous hosts. In addition, the behavior of the foregoing tissues has been compared with that of embryonic skin under the same conditions. It has been proposed that basal cell carcinoma originates from the primary epithelial germ of embryonic skin (10). MATERIALS AND METHODS The hamster pouch technic (11) and a modified diffusion chamber technic (1, 2) were used in this study. 1. Hamster pouch technic. A minced suspension of tissue was injected through a trocar into the submucosal connective tissue of the everted pouch of the hamster. Female weanling hamsters weighing less than 45 grams were used. Surgical specimens were implanted within an hour following removal; some of the fetal skin specimens were implanted as late as 8 hours after obtaining them. All specimens of tissue were minced or cut into small pieces by means of surgical blades or scissors and suspended in physiological saline containing approximately 50 units of aqueous crystalline peniclllin G per c.c. The suspension of tissue in one half cubic centimeter or less was injected into the connective tissue of the pouch with a No. 16 needle or trocar attached to a one cc. tuberculin syringe. Intraperitoneal pentobarbital sodium (Nembutal), 2) to 4 mgms., was employed to produce anesthesia in the hamster. Cortisone acetate, 3 mgms., was given at the time of transplantation and repeated once weekly. Inspection of the site of implantation was carried out frequently. The animals were killed at varying intervals and specimens were either subjected to histological examination by serial section or also transplanted into another animal. * From the Department of Dermatology (Donald M. Pillsbury, M.D., Director), School of Medicine, University of Pennsylvania, Philadelphia. This study was supported by a grant from the National Institutes of Health, U.S.P.H.S. Grant No. C-2912 (Path.). Presented at the Eighteenth Annual Meeting of The Society for Investigative Dermatology, Inc., New York, N. Y., June 1,

2 354 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY The following tissues were implanted: 24 basal cell carcinomas, five squamous cell carcinomas, one keratoacanthoma, one metastatic squamous carcinoma, originating in the oropharynx, one cylindroma and skin of the palms, soles and trunk of six fetuses, varying in age from 3 to 5 months. The fetuses were obtained following spontaneous or induced abortions. In addition, several normal adult skin specimens of the face, palm, axilla and trunk were implanted. In many specimens of tumor adjacent normal, uninvolved skin was necessarily implanted. 2. Modified diffusion chamber technic. This technic was employed to a lesser extent than the hamster-pouch technic. The walls of the diffusion chamber (1, 2) were composed of a Millipore Filter paper5 150 microns thick with a pore size of 0.45 microns; one per cent mcthacrylate in acetone was the sealing agent. The specimens were prepared in the same manner as for the hamster-pouch and small pieces were placed within the chambers. Several specimens of basal cell carcinoma, fetal skin and normal skin were used. After sealing, the chambers were placed in the peritoneal cavity of rats or in the subcutaneous tissue of the rabbit. Five milligrams of cortisone acetate were given initially to the rats. At varying intervals, the animals were sacrificed and the chamber with its contents was submitted for routine histological study. All specimens were sectioned serially and stained with hematoxylin and eosin. The periodic acid-schiff technic was carried out on every fifth slide following digestion of a part of the slide sections by saliva. The neotetrazolium method for succinic dehydrogenase was carried out on frozen sections of several specimens of fetal skin and basal cell carcinoma. RESULTS If an implant on routine staining revealed normal histology and no aberration in its staining characteristics, it was considered viable. Subsequent successful transplantation of a part of the specimen confirmed this. The heterograft reaction as observed in numerous implants consisted of early invasion by leukocytes and vascularization of the implant; later hemorrhage and necrosis of the tissue occurred. Finally, a foreign-body type of granulomatous reaction was evident with innumerable macrophages filled with a foamy, granular material. Normal Skin. Normal-appearing epidermis was present in 17 specimens removed from hamsters on the 7th through the 56th day. Twelve of these were living at three to eight weeks. One specimen of epidermis was living at 98 days. Three specimens of viable epidermis were present within second-generation animals at 28, 56, and 98 days. In young implants, the epidermis was forming small cysts centrally enclosing moderate amounts of keratin (Fig. 1A, B). In the older implants most of the epidermis lined well-formed cysts. Small amounts of glycogen were present in the squamous cells. Within the wall of three cysts, sebaeeous glands were present. An abnormal keratinization as evidenced by retention of nuclei was demonstrable in numerous specimens. No true proliferative activity of the epidermis into the surrounding connective tissue was evident. Usually small parts of the original dermis were present, often containing appendages of the skin. In others, the original epidermis was represented by dead, calcified masses. Becrine Sweat Gland and Duct, Eecrine sweat glands and ducts were present in ten specimens removed at 11 through 56 days (Fig. 2A, B, U). In a single specimen at 100 days, in a third-generation animal, a normal-staining sweat gland * Millipore Filter Corporation, Watertown, Massachusetts.

3 THE BEHAVIOR OF HUMAN SKIN 355 FIG. 1. Human skin and basal cell carcinoma transplated to the hamster. A. Section through hamster pouch lined by squamous epithelium (s.e.) showing human epidermis (e) which has formed a cyst (c) at the end of 7 days. Basal cell tumor (b.c.e.) and degenerating sebaceous gland (s.g.) are also present. Heterograft reaction is beginning. H. & E., X24. B. Hair follicle (hf.) and sebaceous gland (s.g.) at 9 days after implanting. H. & E., X116. C. Section showing an implant of basal cell carcinoma (b.c.e.) and overlying epidermis (e) at 9 days. H. & E., X1l6. D. Section adjacent to that of 1C showing glycogen within the epidermis and parts of the tumor. Periodic acid-schiff, X116. was viable. The eccrine sweat gland tubules did not show proliferation and were usually located within the original dermis or fat tissue; with one exception, all of the specimens examined contained moderate amounts of glycogen within their cells (Fig. 2B). Several eccrine ducts showed local, organized proliferation with incomplete lumen formation. In addition, casts of eosinophilic material were present in several ducts. Often heterograft reaction was occurring in adjacent transplanted tissues without involving the sweat gland. Hair Follicle. In S Specimens definite hair follicle structures were viable when removed at 7 to 39 days (Fig. 1B). Specimens removed later than this showed no evidence of these structures. Sebaceous glands were present in six specimens. In one specimen "ghost" sebaceous gland cells were contained within a small cyst (Fig. 1A). Small amounts of glycogen were demonstrable within the hair follicle structures. Apocrine Gland. Although three specimens of axillary skin have been implanted,

4 356 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY FIG. 2. Ecerine and apocrine glands transplanted to the hamster. A. Eccrine sweat gland and duct 20 days after implanting. H. & E., X560. B. Same as 2A stained to show the presence of glycogen in cells of the eccrine gland. Periodic acid-sehiff, X560. C. Ecerine gland 56 days after transplantation. Glycogen is present in this gland. H. & E., X230. D. Apoerine gland 42 days after transplantation. Dark-staining mass represents dead, calcified structure. H. & E., X235. in only one was apocrine gland structure present at 42 days (Fig. 2D). Mitoses were not seen in this gland on serial sectioning. Basal Cell Carcinoma. The basal cell carcinoma specimens were mainly of the undifferentiated cellular type, several of which contained moderate amounts of

5 THE BEHAVIOR OF HUMAN SKIN 357 pigment. One cystic and one scierosing type were included. Of the 24 specimens transplanted, tumor tissue was demonstrable in the implants from 10 different tumors at seven to 57 days with seven tumors living from three to eight weeks (Fig. 1A, C, 3A, B, C). One specimen was living at the end of 100 days. All of these viable transplants were relatively undifferentiated basal cell carcinomas. The original dermis tissue was often times detectable. In only two of the viable tumors was glycogen demonstrable in both the original and the implant specimen (Fig. 1C, D). One other basal cell tumor containing small amounts of glycogen failed to grow. Mitoses were infrequent. In several of these transplants, areas of necrosis and heterograft reaction were present in the surrounding tissue. Repeated attempts to increase the proliferative abili1y of these tumors by serial passage through 3 or 4 animals failed to achieve any such effect. Although viable tumor was present in specimens from the original animal, no evidence of increased growth or further growth occurred in subsequent animals with the exception of the one specimen living at one hundred days in a third-generation enimal. Other Tumors. Of the five squamous cell carcinomas investigated, two were Grade I arising in senile keratoses; three were clinically and histopathologically of a moderate grade of malignancy. These latter three were living at 13, 19 and 52 days, while the Grade I tumors did not grow in any of the animals (Fig. 3D). One of the three more malignant tumors contained glycogen, but was viable for only 13 days. It is interesting to note that in this tumor implant the accompanyrng normal epidermis was living at 56 days. Mitotic figures were abundant in all of the squamous cell carcinomas. A metastatic undifferentiated squamous cell carcinoma arising from the oropharynx grew slowly during a seven weeks period, but frank proliferation of this tumor did not occur. The patient died within a few days after our obtaining the specimen. This tumor did not contain any demonstrable glycogen. The keratoacanthoma failed to grow although moderate amounts of glycogen were present within its cells. Normal-appearing epidermis was growing in this implant at 32 days. Specimens from a cylindroma failed to live beyond the first week of implantation although remnants of the hyaline membrane were present in all implants at two months. Fetal Skin. Skin from two of the six fetuses did not have primary epithelial germs. The embryonic or fetal skin varied from two to five cells in thickness and always contained large amounts of glycogen. In the outermost layers of the fetal skin, numerous large bladder-like cells containing large basophilic vesicular nuclei and clear cytoplasm were present (5A, B); in several, larger similar cells with septa or partitions in the cytoplasm and basophilic nuclei were evident (Fig. 6A, B). Both types of cells contained large amounts of glycogen. They represented periderm cells, being shed as development progressed. Transplants removed from hamsters through 92 days at the present writing revealed viable epidermis and appendages in most implants. In several, evidences of the heterograft reaction were present. The transition from fetal or embryonic skin to adult skin was usually evident within 2 weeks after transplanting. Intermediate-type cells and the large bladder-like fetal cells were present in some

6 358 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY Flu. 3. Four transplanted human carcinomas; none of these tumors contained glycogen in the original or the transplant. A. Basal cell carcinoma 10 days after transplanting. A mild reaction consisting of infiltration of polymorphonuclear leukocytes, lymphocytes and fibroblastic proliferation is occurring in the surrounding stroma. H. & E., X130. B. Another basal cell carcinoma at 51 days without surrounding reaction. Original dermis (d) is evident. H. & E., X230. C. Basal cell carcinoma at 57 days. H. & E., X230. D. Squamous cell carcinoma at 52 days. Mitoses are present; tumor does not show invasive properties within the hamster, although the patient died with metastatic carcinoma. H. & E., X235.

7 S S S FIG. 4. Skin of a 3 months fetus without primary epithelial germs. A. Skin 10 days after transplantation to the hamster. The epidermis has not become adult keratinizing epithelium, but is still forming bladder-like periderm cells. H. & E., X500. B. Same skin as A forming two cysts at 23 days. No appendageal structures are present. Glycogen is present within the cells of the cyst wall and in the contents of the cysts amidst the keratinized, desquamated cells. Periodic acid-schiff, X

8 I.4 4 Fin. 5. Skin from the trunk of a 4 months fetus. A. Basal cell layer (b) is clearly delineated. Periderm (p) layer with bladder-like cells is present. Primary epithelial germs (peg.) are present. H. & E., X180. B. Same as A showing glycogen in the epidermis and periderm cells. Periodic acid-schiff, X420. C. Same skin 32 days after transplanting into the hamster; two cysts have formed; the epidermis has begun to form keratin. Differentiation of hair follicles with early sebaceous gland formation (s.g.) has occurred. H. & E., X82. D. Same as SC showing a cyst lined by epithelial cells containing large amounts of glycogen. Periodic acid-schiff, X175. E. Same skin at 67 days; large sebaceous glands (s.g.) and lanugo type hairs (h) are numerous. H. & E., X118. F. Same as SE showing glycogen in this adult-appearing skin and appendages. Periodic acid-schiff, X

9 c / Iv I '< 4. i.b p S FIG. 6. Skin from the sole of 3 months fetus. A. Original specimen showing early basal cell layer and primary epithelial germs (p.e.g.). Periderm cells (p) are present and becoming detached from the epidermis. H. & E., X560. B. Same as A. Basal cell layer and primary epithelial germs (p.e.g.) contain small amounts of glycogen in contrast to the other layers. Periodic acid-schiff, X560. C. At 32 days after transplantation, primary epithelial germs have developed into eccrine sweat ducts (d) and glands (g). Epidermis (e) lines a well formed cyst (not shown). H. & E., X82. D. Same as 6C; glycogen present in epidermis and the appendageal structures. Periodic acid-schiff, X82. E. Skin at 82 days showing a part of a cyst (c) formed by fetal skin; ecerine duct and gland resemble normal adult structures. H. & E., X150. F. Higher magnification of the eccrine gland shown in 6E. Tubular structure and proximal eccrine ducts are distinct; lumina have formed in both. H. & E., X F

10 362 THE JOTJRNAL OF INVESTIGATIVE DERMATOLOGY specimens as long as 3 weeks (Fig. 4A). In several specimens, as early as 4 days, epidermal cysts were forming. The epidermis gradually assumed that of adult normal skin producing large amounts of keratin within the center of the epidermal cysts (Fig. 4B, 5C, D). Skin from the two fetuses without primary epithelial germs did not develop appendageal structures. In the older specimens with primary epithelial germs present at the time of implantation, differentiation and development into normal-appearing appendages progressed rapidly. In the older specimens, normal-appearing lanugo hairs, sebaceous glands, and small eccrine sweat glands were evident (Fig. 5E, F, GE, F). All the fetal or embryonic skin specimens observed contained large amounts of glycogen within the epidermis and appendages throughout their stay in the hamsters (Fig. 4B, FIG. 7. Specimens of skin and basal cell carcinoma in modified diffusion chambers. A. Cross-section of a chamber showing walls (w) enclosing epidermis (e) and dermis (d). Most of the epidermal cells are living at 7 days but retention of nuclei is occurring in the keratinized layer. H. & E., X80. B. Specimen removed from a chamber (w) at 66 days; a cyst-like cavity (c) containing keratin without a lining of living epidermal cells is present. Fibroblastic proliferation (f) has occurred; original dermal tissue (d) is evident also. H. & E., X48. C. At 9 days epidermis has formed a cyst (c) within chamber. Two islands of basal cell carcinoma (b.c.c.) are present. H. & E., X150. D. At 7 days cells arising from a basal cell carcinoma (b.c.c.) are proliferating within chamber; part of the specimen is dead. H. & E., X156.

11 THE BEHAVIOR OF HUMAN SKIN 363 SD, F, 6D). Succinic dehydrogenase was demonstrated in the epidermis and appendages of several transplants at two weeks. Within the lumen of the cysts definite glycogen containing materials were present originating from the epidermal wall of the cyst. In aged, transplanted embryonic skin melanocytes were often present within the epidermis and hair matrix; in the original specimens, few were present. It is likely that many of these were originally in the adjacent embryonic dermis at the time of transplanting. The dermis dil not differentiate into adult type structure to any great degree as evidenced by the persistence of young fibroblasts (Fig. 6E, F). Modified Diffusion Chamber. Experiments with the modified diffusion chamber technic were not as rewarding as those with the hamster pouch technic. Abscess formation about the chambers was frequent. In two specimens basal cell carcinoma was living (Fig. 7C, D) in one proliferation was occurring at the 7th day, in the other only islands of carcinoma were evident. In another, living epidermis was present but it was not proliferating at 7 days (Fig. 7A). At 66 days a cyst-like structure containing keratinized cells was present but living squamous cells were not present in the wall of the cyst (Fig. 7B). In numerous specimens, the original dermis of the implant was detected; in several fibrocytic proliferation was occurring (Fig. 7A, B). COMMENT Our findings with normal skin substantiate and extend Toolan's findings (14); this author has stated that the degree of malignancy was not a determining factor as to whether or not transplantation of a tissue is possible (18). Approximately 43 per cent of our carcinomas showed some degree of viability over a span of 7 to 100 days. Toolan stated that approximately 90 per cent of tumors tested had survived and proliferated for 12 to 20 days in rats, either irradiated or cortisone-treated (15). Furthermore, she has stated that approximately two or three of every 100 tumors will proliferate sufficiently to make serial transplantation possible (17). Human epidermoid carcinoma from buccal mucosa, human sarcoma, human epidermoid carcinoma from the cervix, and human embryonal rhabdomyosarcoma have been transplanted for several years by her within small animal hosts (17). Other reports indicate a lower percentage of transplantability of tumors such as 70 per cent survival through 12 days (12). Other workers (7) screened over 200 tumors without finding a single tumor with proliferative qualities. A previous report indicated that a basal cell carcinoma survived as long as 12 days on implantation into the hamster (12). Of our 24 tumors, eight have survived for considerably longer periods, with one viable at 100 days. The five squamous cell carcinomas arising from skin have not behaved like squamous cell carcinoma arising from other sites (17). However, the number is insufficient for comparative purposes. Adult skin and its appendages, particularly eccrine sweat glands, survive heterotransplantation up to 98 days. Thus, normal skin and basal cell carcinoma parallel each other in their ability to survive and proliferate. Neither has shown any degree of proliferation.

12 364 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY In contrast to normal skin and its appendages, embryonic or fetal epidermis shows definite proliferative qualities. No wild or abnormal transformation has been reported by others (5, 16) or observed by us; rather, it appears that such tissue undergoes a normal "life cycle" at an accelerated rate. Fetal melanocytes (3) also participate in the differentiation of fetal tissue toward adult tissue. Fetal skin with or without primary epithelial germs demonstrated its inherent property to form epithelial cysts. This finding parallels those of previous reports in which buried autotranspiants of adult skin subsequently developed epithelial cysts (4, 13). Likewise, our finding that the eccrine sweat gland is a good survivor upon transplantation into hamsters recalls similar findings in autotransplant experiments with adult skin (13). The possible role of glycogen in the viability of heterotranspianted tissues remains unsettled. Viable, transplanted adult epidermis, eccrine gland and embryonic epidermis, contained moderate to large amounts of glycogen; however, the tumors with the exception of two basal cell carcinomas and one squamous cell carcinoma did not have any. Thus, one can not conclude that survival of malignant and benign tissues is dependent upon the presence of glycogen. Furthermore, at the time of transplantation several specimens of normal epidermis did not have any glycogen but subsequently did contain it in the transplant. The possibility must be considered that fetal skin eliminates or "excretes" glycogen into the amniotic fluid; this property may be maintained in the transplanted specimens forming cysts. Our inconclusive results with the modified diffusion chamber may be caused by several factors such as infection and trauma. Although connective tissue proliferation with large, stellate fibroblastic cells was appreciable within the diffusion chamber, in a few specimens only were viable epidermal cells observed. Algire (2) observed that human skin proliferates for two weeks in similar duff u- sion chambers within the peritoneal cavity of mice. SUMMARY AND CONCLUSIONS 1. Following heterotransplantation into hamsters, human adult skin, eccrine sweat gland and apocrine gland survived for two months or longer. Hair follicles survived as long as five weeks. No marked proliferative qualities occurred in the epidermis or appendages. Characteristically the epidermis formed cysts. 2. Basal cell carcinomas remained viable for eight weeks or longer following implantation into the hamster. No malignant proliferation resulted. Implants of five epidermal squamous cell carcinomas have likewise demonstrated only slight proliferative tendencies. No definite comparison is possible due to the small number of the latter tumors. 3. Embryonic or fetal skin showed definite proliferation with differentiation into adult type epidermis and appendages within a three weeks' period following heterotransplantation. Many specimens remained alive for 90 days or longer. 4. The foregoing findings suggest that basal cell carcinoma reacts as adult skin in this experimental system. It showed none of the characteristics of malignancy or of embryonic skin under the same conditions.

13 THE BEHAVIOR OF HUMAN SKIN The possible correlation of the transplantability of adult and embryonic skin or of epidermal tumors with their histochemically-demonstrated glycogen content has been investigated. Some adult skin and appendages, notably the eccrine sweat gland, and embryonic or fetal skin contained large amounts of glycogen prior to and following heterotransplantation. On the other hand, with few exceptions, basal cell and squamous cell carcinomas did not contain glycogen. The proliferative properties of embryonic skin probably indicated the metabolic significance of its glycogen. Apparently, this was not the case for transplanted eccrine sweat glands. REFERENCES 1. ALGIRE, G. H., WEAVER, J. M. AND PREEN, R. T.: Growth of cells in ViVO in diffusion chambers. I. Survival of homografts in immunized mice. J. Nat. Cancer Inst., 15: 493, ALGIRE, G. H., BORDERS, M. L. AND EVANS, V. J.: Studies of human cells in diffusion chambers in mice. Proc. Amer. Assoe. for Cancer Res., 2: 90, BECKER, S. W., JR. AND ZIMMRRMANN, A. A.: Further studies on melanoeytes and melanogenesis in the human fetus and newborn. J. Invest. Dermat., 25: 103, EPSTEIN, W. L. AND KLIOMAN, A. M.: Epithelial cysts in buried human skin. Arch. Dermat. & Syph., in press, GREENE, H. S. N.: The heterologous transplantationof embryonic mammalian tissues. Cancer Research, 3: 809, GREENE, H. S. N.: The significance of transplantability. Tr. & Stud., Coll. Physicians, Philadelphia, 24: 101, HRREUT, P. A. AND KRAEMRR, W. H.: Heterologous transplantation of human tumors. Cancer Research, 16: 408, HANDLER, A. H.: Growth of tumors from children in the cheek pouch of the golden hamster. Proc. Amer. Assoc. Cancer Res., 2: 114, LEMON, H. M., LUTZ, B. R., POPE, R., PARSONS, L., HANDLER, A. H. AND PATT, D. I.: Survival and growth of human tissues transplanted to hamster cheek pouch. Science, 115: 461, LEVER, W. F.: Pathogenesis of benign tumors of cutaneous appendages and of basal cell epithelioma. Arch. Dermat. & Syph., 57: 679, LUTZ, B. R., FULTON, G. P., PATT, D. E., HANDLER, A. H. AND STEVENS, D. F.: The cheek pouch of the hamster as a site for the transplantation of a methylcholanthreneinduced sarcoma. Cancer Research, 11: 64, PATTERSON, W. B., CHUTE, H. N. AND SOMMRR5, S. C.: Transplantation of human tumors into cortisone-treated hamsters. Cancer Research, 14: 656, PEER, L. A.: Fate of buried skin grafts in man. Arch. Surg., 39: 131, TOOLAN, H. W.: Proliferation and vaseularization of adult human epithelium in subcutaneous tissues of x-irradiated heterologous hosts. Proc. Soc. Exper. Biol. & Med., 78: 540, TOOLAN, H. W.: Growth of human tumors in cortisone-treated laboratory animals: The possibility of obtaining permanently transplantable human tumors. Cancer Research, 13: 389, TOOLAN, H. W.: Growth of human embryonic tissues in cortisone-treated laboratory animals. Proc. Soc. Exper. Biol. & Med., 86: 607, TOOLAN, H. W.: Transplantable human neoplasms maintained in cortisone-treated laboratory animals: H.S. #1; H. Ep. #1; H. Ep. #2; H. Ep. #3; and H. Emb. Rh. #1. Cancer Research, 14: 660, TOOLAN, H. W.: Subcutaneous growth of normal and malignant human tissues in heterologous hosts. Tr. New York Acad. Se., 17: 589, 1955.