Sertoli Cell Proliferation and Thyroid Hormone in Prepubertal Ram Lambs.

Studies in rodents have suggested that serum Thyroid Hormone (TH) contributes to stopping Sertoli cell proliferation during the first wave of spermatogenesis, and thus affects Sertoli cell numbers within testes, testicular size and maximum sperm output of testes. It is not clear that Thyroid Hormone plays a similar role in all species under normal conditions. The purpose of this research was to compare serum Thyroid Hormone (TH) and Follicle Stimulating Hormone (FSH) levels to Sertoli cell proliferation, and to examine expression of TH receptor in ram testes during first wave of spermatogenesis in normal (euthyroid) ram lambs. The design of these experiments was novel in that weekly biopsies and blood were collected from 12 ram lambs (from 10 to 19 weeks of age) to directly compare hormone levels (radioimmunoassay) to number of Sertoli cell divisions (antibody against PCNA) and differentiation (seminiferous tubule lumen formation and spermatogenesis) in the same animals over the course of the first wave of spermatogenesis. We found that the number of dividing Sertoli cells was already decreasing at the beginning of the first wave of spermatogenesis (true for 12/12 rams) and continued until the point at which at least 90% of tubules contained spermatocytes; serum T3 and T4 decreased after the decrease in Sertoli cell divisions (true for 11/12 rams); and the decrease in T3 and T4 occurred after lumens had begun to form in the seminiferous tubules (one sign of the final differentiation of Sertoli cells). TH receptor was expressed throughout first wave of spermatogenesis, but early in first wave of spermatogenesis 2 isoforms were expressed while only 1 isoform was expressed by the end. There was no significant change in serum FSH during first wave of spermatogenesis. Overall, higher levels of serum TH are generally present as Sertoli cells stop dividing and differentiate, but there was 1 animal in which Sertoli cells stopped dividing without higher TH levels, suggesting additional mechanisms. (poster)