Meiosis-inhibiting effects in vivo of antiserum to progesterone on follicular ova in immature rats treated with gonadotropins.

Effects of the neutralization of endogenous progesterone with rabbit antiserum to progesterone (anti-progesterone) on germinal vesicle breakdown of ova in follicles of small (less than 125 micrometers), intermediate (125-250 micrometers) and large (greater than 250 micrometers) diameter were examined by a quantitative histological technique. Immature rats were treated with 5 IU pregnant mare's serum gonadotropin (PMS) then with 10 IU human chorionic gonadotropin (hCG). Administration of anti-progesterone together with hCG 6 h later significantly decreased the incidence of germinal vesicle breakdown of ova in the large follicles, but not in the intermediate ones. This treatment did not affect the proportion of intermediate to large follicles in the population. Replacement with progesterone 1 h after the simultaneous injection of hCG and anti-progesterone partly reversed the reduced incidence of meiosis. An injection of rabbit antiserum to estrone, in addition to the replacement with progesterone 1 h after the simultaneous injections of hCG and anti-progesterone, restored the incidence of meiosis to a value comparable to the values found for control rats treated sequentially with PMS and hCG. We concluded that the hCG-induced preovulatory rise in progesterone has a limited but definite stimulatory effect on the resumption of meiosis in the ova of large follicles and that it mediates the meiosis-inducing action of hCG.