POSTOVULATORY AGING OF MOUSE OOCYTES IN-VIVO AND PREMATURE CENTROMERESEPARATION AND ANEUPLOIDY

Two paramount observations exist regarding aneuploidy in human oocytes : its association with maternal age and its more frequent occurrence d uring meiosis I. Numerous experimental studies have shown that fertili zation of postovulatory aged oocytes is coupled with reproductive fail ure and cytogenetic aberrations in embryos. However, the basic cytogen etic defect(s) of aged oocytes that causes these abnormalities has not been adequately described. The objective of this study was to test th e hypothesis that postovulatory oocyte ageing results in increased fre quencies of premature centromere separation (PCS) in metaphase II (MII ) oocytes and aneuploidy in zygotes. MII oocytes and one-cell zygotes were collected from superovulated mice at different times after ovulat ion and fertilization. Chromosomes were C-banded and analyzed for stru ctural and numerical aberrations. The frequencies of PCS in oocytes si gnificantly (p < 0.01) increased with time postovulation: 15 h (15 of 529, 2.8%), 20 h (82 of 627, 13.1%), and 25 h (118 of 502, 23.5%). In zygotes, the frequencies of hyperploidy significantly (p < 0.01) incre ased with time post-fertilization: 0-4 h (0 of 260), 4-8 h (5 of 212, 2.4%), and 8-12 h (8 of 262, 3.1%). These data support the hypothesis that postovulatory ageing results in elevated levels of PCS in oocytes and of aneuploidy in zygotes. The link between PCS and aneuploidy may be random segregation of sister chromatids during anaphase II.