Advancement of meiotic resumption in graafian follicles reduces fertility in the rat.

Fertile life of oocytes is usually considered to be related to ovulation time. In the present study, fertile life of rat oocytes was studied in relation to resumption of meiosis. In pro-oestrus, meiotic resumption without concomitant ovulation was induced in most graafian follicles by injection of a small amount of LH or FSH followed by Nembutal. These follicles either ovulated or developed into luteinized unruptured follicles if Ovalyse (a GnRH analogue) was given 8 h after LH or FSH. In subsequent experiments, rats were injected with FSH or saline, and Nembutal; 4 or 8 h later, Ovalyse was given to induce ovulation; the rats were mated 14 h after Ovalyse. At day 20 of pregnancy, fetal survival was 30% in rats with meiosis advanced by 8 h, against 91% and 70% in rats advanced by 0 or 4 h, respectively. Mortality occurred mainly during pre-implantation and early post-implantation. Advanced resumption of meiosis may cause pre-ovulatory ageing of oocytes; consequently, viability of these oocytes after ovulation is reduced.