Spermatogenic onset. II. FSH modulates mitotic activity of germ and Sertoli cells in immature rats

By using high doses of testosterone propionate (TP) endogenous FSH was lowered to non-detectable levels in immature rats of different ages. Combined administration of TP and human menopausal gonadotrophin (hMG) or purified human FSH (hFSH) restored circulating FSH to normal or supranormal levels. This experimental model was used to investigate the influences of hormones on the proliferation of Sertoli and germ cells. Mitoses in seminiferous tubules were counted after being blocked by administration of colchicine. The mitotic index so determined showed a reduction with FSH withdrawal and a significant increase when the hormone levels were restored. Testicular DNA content was determined in testicular homogenates and showed similar changes. Autoradiographs were prepared and 3H-thymidine incorporation into germ and Sertoli cells was quantified. It was found that hFSH induced a significant increase in the labelling indices of gonocytes, type A-spermatogonia and Sertoli cells. All of these changes were effective in rats younger than 10 days but no modifications in any of the parameters under study were observed after 20 days. It is concluded that FSH exerts a stimulatory effect on the proliferation of spermatogonia, Sertoli cells and testicular DNA content during the first 10 days of life.