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Transplantation of bovine germinal cells into mouse testes.

To develop techniques for spermatogonial transplantation in bulls, it is essential to have an effective bioassay procedure to evaluate the transplantation efficiency of spermatogonial stem cell collection, purification, and culture techniques. The objective of the present study was to develop a mouse bioassay model to evaluate transplantation efficiency of fresh and cultured bovine germ cells. Bull calves of four ages (1, 2, 3, and 4 mo) were used as a source of donor testes cells. Two calves were used for each age point, one calf was experimentally made cryptorchidistic at 1 wk of age and the other left normal. A STO (mouse fibroblast) feeder cell line was used to culture bovine testes cells for 2 wk preceding transfer into recipient testes. Immunodeficient nude mice (nu/nu) in which endogenous spermatogenesis had been abolished by busulfan treatment served as recipient animals for transplantation. Donor bovine germ cells were microinjected into mouse seminiferous tubules. Mouse testes were analyzed 2 wk after transplant with the use of a bovine-specific antibody and whole-mount immunohistochemistry for the presence of bovine donor germ cells. Bovine testis cells were present in all recipient mouse testes analyzed. Fresh bovine testes cells were observed as colonies of round cells within mouse seminiferous tubules, indicating spermatogonial expansion and colonization; however, cultured bovine testes cells appeared as fibrous tissue and not as spermatogenic colonies. The average number of colonies resulting from donor cryptorchid testes was not different (P > 0.05) from noncryptorchid, 56+/-4 and 78+/-7, respectively. Fresh donor cells from calves older than 1 mo gave rise to a greater average number of colonies within recipient testes (P <0.05) (1 mo, 33+/-4; 2 mo, 70+/-8; 3 mo, 63+/-6; 4 mo, 87+/-9). Fresh bovine germ cells are capable of colonization in the busulfan-treated nude mouse testis, making it a suitable model for evaluation and development of spermatogonial transplant techniques in bulls.