SIGNIFICANCE OF ESTRADIOL FOR FOLLICULAR DEVELOPMENT IN HYPOGONADOTROPIC IMMATURE RATS TREATED WITH FSH AND HCG

Administration of hCG (0.5 i.u.) to immature female rats that were mad e hypogonadotrophic by injections of an LHRH-antagonist at 22, 24, 26 and 28 days of age and in which follicular development was induced by FSH (10 i.u., given from day 26 until day 29) stimulates follicles to grow to their preovulatory size with a low amount of atresia. To inves tigate whether this effect of hCG on follicular growth is due to stimu lation of oestradiol production, intraovarian concentrations of oestra diol were suppressed by an aromatase inhibitor and oestradiol action w as blocked by an oestrogen antagonist; both of these were administered from day 25 until day 29. By day 30 this treatment had resulted in an increase in the percentage of atretic follicles. In rum, oestradiol b enzoate (100 mu g or 1000 mu g day(-1)), given from day 26 until day 2 9 in rats treated with 10 i.u. FSH, resulted in (1) an increase in the total number of antral follicles, (2) an increase in follicular size and (3) a decrease in the percentage of atretic follicles. Although th e administration of 1000 big oestradiol benzoate can mimic the effects of 0.5 i.u. hCG on follicular growth to a large extent, the number of follicles with a diameter > 575 mu m in rats treated with FSH plus oe strogen was smaller than that in rats treated with FSH plus hCG (10 +/ - 2 and 24 +/- 2, respectively). Furthermore, a single injection of 10 i.u. hCG given on day 29 was used to induce ovulation: fewer ova were found in rats treated with FSH plus 1000 mu g oestrogen than in rats treated with FSH plus hCG (15.2 +/- 2.1 and 46.5 +/- 1.6, respectively ). From these studies, it is concluded that the effects of hCG on foll icular growth and atresia can be largely attributed to the mitotic and anti-atretic effect of oestradiol. However, part of the effect of hCG cannot be explained on the basis of oestradiol action.