267 TESTIS TISSUE XENOGRAFTING AS A BIOASSAY FOR GERM CELL DEVELOPMENTAL POTENTIAL IN EQUINE CRYPTORCHID TESTES

Abstract

In domestic animals, spermatogenic differentiation is blocked in abdominally retained testes exposed to core body temperature. It is not known if undifferentiated germ cells are retained in long-term cryptorchid equine testes, nor is it known whether any surviving germ cells retain their ability to progress through spermatogenesis. If functional germ cells do persist in equine abdominal testes, then the possibility exists that offspring could be derived even from bilaterally cryptorchid individuals. Previously, we reported an in vivo model where completion of spermatogenesis with production of spermatozoa capable of fertilization occurred in fragments of testicular tissue from immature mice, domestic animals, and monkeys grafted under the skin of immunodeficient mice. Therefore, spermatogenic development in testis tissue xenografts can serve as an in vivo assay system for the developmental potential of germ cells. The objective of this study was to investigate if cryptorchid horse testes that had been exposed to core body temperature for 1–3 years had retained developmentally competent germ cells. Small fragments of abdominally cryptorchid testis tissue (about 1 mm³) from three donor horses (1-, 2-, and 3-year-old Quarterhorse) were grafted under the back skin of castrated male immunodeficient mice (n = 8, 6, and 3 recipient mice, respectively). At the time of grafting, donor tissue did not contain differentiated germ cells. Histological examination of the testis xenografts was performed between 5 and 45 weeks post-transplantation. Weight of the seminal vesicles in the host mouse was recorded as an indicator of bioactive testosterone produced by the xenografts. By 28 weeks after grafting, pachytene spermatocytes were observed in xenografts from all cryptorchid donor testes. While haploid gametes would be expected to be present in xenografted testis tissue from descended equine testes by 35 weeks after grafting, spermatogenesis did not progress through meiosis in the cryptorchid grafts. In all recipient animals where spermatogenic differentiation occurred, the weight of the seminal vesicles in the castrated host mice was restored to pre-castration values, indicating that xenografts were capable of releasing biologically active testosterone. These results indicate that even after 3 years of exposure to core body temperature, equine cryptorchid testes contain germ cells capable of differentiation. It remains to be investigated if supplementation of exogenous gonadotropins might support post-meiotic differentiation of germ cells in cryptorchid equine testes xenografts.

This work was supported by USDA 03-35203-13486.