Development of bovine fetal testis tissue after ectopic xenografting in mice

Testis tissue xenografting represents a versatile model to study testis biology, and to preserve fertility in immature animals. To evaluate whether bovine fetal testes can mature when grafted into mouse hosts, small fragments of testes from midgestation (125 to 145 days of gestation) bovine fetuses were grafted ectopically into immunodeficient castrated male mice. At grafting, donor tissue displayed the typical seminiferous cords composed of gonocytes and primitive Sertoli cells. At 5 or 10 months after grafting, weight of the seminal vesicles in recipient mice was indicative of production of bioactive testosterone by xenografts. Xenografts showed similar development regardless of donor age. At 5 months, tubule formation occurred but germ cell differentiation had not proceeded beyond the spermatogonia stage. At 10 months, an increase in tubule size was evident and pachytene spermatocytes were observed as the most advanced type of germ cells in the xenografts of 2 donors. The number of tubules with germ cells was reduced in xenografts compared to donor tissue, but at 10 months the number of germ cells per tubule was higher than in donors. Germ cell proliferation was similar in donor tissue and xenografts. However, Sertoli cells showed a higher proliferation rate in xenografts collected at 5 months than in donor fetal testes and xenografts collected at 10 months. Sertoli cells in xenografts showed a progressive but incomplete loss of expression of Müllerian inhibiting substance and weak androgen receptor expression, indicating an incomplete Sertoli cell maturation. In conclusion, fetal testis tissue developed partially, qualitatively similar to pubertal testes in situ.