Second meiotic nondisjunction is not increased in postovulatory aged murine oocytes fertilized in vitro

We used an in vitro fertilization system to examine the effects of postovulatory oocyte age on nondisjunction at the second meiotic division. After ovulatory-inducing injections of hormone, we recovered mouse oocytes either at the estimated time of ovulation (controls) or 2, 4, 5, 10, or 14 h later. Oocytes were subjected to an in vitro fertilization procedure, and chromosomal preparations were made from first cleavage metaphase eggs. The first cleavage assay reveals morphologically distinguishable paternal and maternal chromosomes. Many of the aged oocytes were activated rather than fertilized by the in vitro procedure, but could still be analyzed for nondisjunction. We found a tendency toward retention of the second polar body after 10 and 14 h aging. A total of 488 maternal genomes, 290 of which were in the control group, were analyzable for nondisjunction. Seven hyperhaploid genomes (2.4%) were observed in the controls and 6 (3.0%) in the combined aged group. The difference between these two frequencies is not significant (Gadj = 0.164, P greater than 0.50). In the aged group, one hyperhaploid genome was in the 2-h population, three in the 5-h population, and two in the 10-h population. We were unable to find any significant increase in the frequency of nondisjunction after postovulatory oocyte aging.