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)