Suppression of the intratesticular testosterone is associated with quantitative changes in spermatogonial populations in intact adult rats.

To investigate the interactions between testosterone and FSH on quantitative aspects of spermatogenesis, intact adult rats were implanted sc with 0.5- to 50-cm long testosterone-filled silastic capsules (TC) for 8 weeks. Serum testosterone levels were suppressed slightly in rats bearing 1-cm TC implants but were elevated 6- to 30-fold when 5 cm or longer TC implants were used. These changes in serum testosterone were associated with a biphasic, dose-dependent response in testicular testosterone concentrations, reaching a minimum of 10-15% of the control values in rats bearing the 1- or 5-cm TC implants, but rebounding to 80% when 50-cm long TC implants were used. Meanwhile, serum FSH was comparably suppressed by 40-70%. Complete spermatogenesis was observed in all experimental animals with the exception of three rats with 1-cm TC implants in which elongated spermatids were absent or reduced in number. Enumeration of various cell types in the basal compartment of whole mounted seminiferous tubules revealed a 20% increase in type A1 spermatogonia in rats with the 1-cm TC implants. On the other hand, a 15-20% reduction in type B spermatogonia and preleptotene spermatocytes was noted in rats receiving 5-cm or longer TC implants. These results demonstrate that complete spermatogenesis can be maintained in intact animals in the presence of 10-15% of the normal testicular testosterone concentration. Failure to maintain normal testis weight, testicular testosterone concentration, or germ cell number at a 30-fold increase in serum testosterone demonstrates that high levels of serum testosterone may not be beneficial for quantitative spermatogenesis. Furthermore, both quantitative and qualitative differences were observed in different phases of spermatogenesis in the presence of comparable serum FSH and testicular testosterone concentrations. These results suggest that factors other than these two hormones are also involved in the regulation of spermatogenesis.