Prepubertal Follicular Development and Ovarian Activity in Adulthood after Suppression of Gonadotropin Levels in Immature Female Rats

To investigate the significance for ovarian maturation of the high FSH levels present in immature female rats before Day 20 after birth, the effect of suppression of these high FSH levels by repeated injections of 5cs-dihydrotestosterone propionate (DHTP) on Days 5, 10, 15 and 20 was studied. Numbers of developing follicles at Days 10, 20 and 35, time of first ovulation and ovarian activity in adulthood were examined. Injections of 100 �ig DHTP resulted in reduced FSH and LH concentrations between Days 10 and 30 and subnormal numbers of medium and large follicles at Day 35. Time of first ovulation (Day 39) and ovarian cyclicity were not affected. Injections of 1 mg D1-ITP resulted in nearly undetectable serum FSH and LH concentrations between Days 10 and 35 and in a larger effect on numbers of medium and large follicles at Days 20 and 35. First ovulation was delayed by 4 days. Normal ovarian cyclicity was observed thereafter. These findings confirm that the high gonadotropin concentrations in immature female rats are of importance for normal follicular development. However, the onset of first ovulation and subsequent ovarian function seem to occur as events independent of the presence of these high serum FSH concentrations on Days 5-20.