Progesterone protects oocytes from premature degeneration within the follicle.

The present study was designed to determine the effects of gestrinone (R2323) in the process of follicle rupture and oocyte maturation and degeneration in an in vitro perfused rabbit ovary model. In the first experiment, R2323 at 10(2), 10(3), or 10(4) ng/ml was added to the perfusate of one ovary. The contralateral control ovary was perfused simultaneously with medium alone. Thirty minutes after the onset of perfusion, 50IU of human chorionic gonadotropin (hCG) was added to the perfusate of both ovaries. All ovaries exposed to R2323 plus hCG or hCG alone ovulated. The addition of R2323 to the perfusate did not affect the ovulatory efficiency of ovaries treated with hCG. No significant difference in the percentage of ovulated ova or follicular oocytes demonstrating germinal vesicle breakdown was seen with R2323 treatment. R2323 increased the degeneration rate of ovulated ova in a dose-dependent fashion. In the second experiment, in which experimental ovaries were perfused with R2323 (10(4) ng/ml) plus progesterone (10(3) ng/ml) and the control ovaries with R2323 (10(4) ng/ml) alone ovulation occurred in response to hCG. However, the addition of progesterone to the perfusate reduced the degeneration-inducing effect of R2323 on both ovulated ova and follicular oocytes. In conclusion, R2323 appears to act as an antiprogesterone, thereby promoting the degeneration of oocytes. The increased production of progesterone in the preovulatory follicle following the gonadotropin surge protects oocytes from premature degeneration within the follicles.