ADVANCEMENT OF MEIOTIC RESUMPTION IN GRAAFIAN-FOLLICLES BY LH IN RELATION TO PREOVULATORY AGING OF RAT OOCYTES

After ovulation, the fertile life of oocytes is short. In the present study, the fertile life of oocytes was studied in relation to the resu mption of meiosis. Early on the day of pro-oestrus, meiotic resumption was advanced in rats by a brief infusion of LH; ovulation was induced 8 h later by Ovalyse, a GnRH analogue; rats were mated 13 h after rec eiving Ovalyse, that is at about the time of ovulation. Rats in which meiosis was not advanced were injected with Ovalyse and mated at vario us intervals after ovulation. Rats were killed 13 h after mating or on day 20 of pregnancy. In rats in which meiosis was not advanced that w ere mated around ovulation, fetal survival was about 90%. In rats with meiosis advanced by 8 h and mated around ovulation, only 44% of the o vulated oocytes with advanced meiosis developed into healthy fetuses; mortality before and after implantation was 37 and 19%, respectively, Rats in which meiosis was not advanced that were mated 8-9 h after ovu lation had similar fetal survival and similar mortality before and aft er implantation. Thus' ageing of the oocyte may occur either before, o r after, ovulation. Preovulatory ageing is related to the resumption o f meiosis.