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.

The hormonal regulation of premeiotic steps of spermatogenesis in the newborn rat.

.he effect of high doses of testosterone propionate (TP) on the development of the first spermatogenic wave was systematically analyzed to determine the period of maximal susceptibility to testosterone. Two mg of TP were administered daily to groups of immature male rats, starting from birth, or days 5, 10, 15, and 20 until day 35 of life. Animals injected from birth or day 5 showed severe testicular atrophy, with reductions of more than 70% of testicular weight, diminished tubular diameters, and spermatogenic arrest during the meiotic prophase. Groups treated from days 10 or 15 showed increasing testicular weights with qualitatively normal spermatogenesis. When treatment started at 20 days, completely normal testicular development was achieved. To test the responsiveness of the neonatal hypothalamo-pituitary axis to TP administration, groups of 5-day-old male rats received daily injections of TP, and plasma FSH was determined at ten, 20, and 35 days. FSH levels were not detectable at ten and 20 days, and extremely depressed at 35. A group of 5-day-old male rats was injected simultaneously with 2 mg of TP and 14 IU of FSH (human menopausal gonadotropin: 71 IU of FSH and 80 IU of LH/ml) until day 35. Testicular weights and tubular diameters were increased compared to controls, and spermatid differentiation had proceeded to more mature steps than those seen in control animals. Inhibition of testicular development by neonatal TP administration was paralleled by a sharp decrease in circulating FSH levels and reversed by FSH replacement.(ABSTRACT TRUNCATED AT 250 WORDS)