In mammalian oocytes, homologous chromosomes (also known as bivalents) are prone to mis-segregation during the first meiotic division (MI) which may generate aneuploid embryos, leading to birth defects and infertility. Increased maternal age has been well established as a major contributor to high aneuploidy in eggs, both in human and mouse. Current models suggest that age-related aneuploidy is primarily caused by chromosome cohesion deterioration, resulting in sister chromatid pre-division during MI as confirmed by the prevalence of single chromatids in metaphase II arrested (metII) eggs.
Here we examined the dynamics of chromosomes and their kinetochores from the time of GV breakdown until metII arrest in oocytes from aged (>12-months) and young (1-month) mice. We employed both traditional immunohistochemical techniques on fixed oocytes as well as chromosome-tracking approaches with kinetochore labeling of live oocytes. In fixed oocytes, consistent with previous studies, we observed chromosome cohesion loss, measured by an increase in inter-kinetochore distances, and reduction in kinetochore Sgo2 signal with increasing maternal age. Live cell imaging during MI showed no difference in chromosome congression with age, however, weakly-attached bivalents were commonly seen in these oocytes during prometaphase. At anaphase-onset there was an increase in numbers of lagging chromosomes but premature separation of bivalents was rarely observed. Instead, we found that pre-division during MII was the major segregation defect observed in mice of this age. In many live MII eggs from aged mice the segregation of a sister chromatid pair was observed (n=11/24), and this event was concurrent with metaphase II spindle assembly, occurring at 133 ± 24 min (n=11) after anaphase I. These data show that although considerable loss of cohesion is evident during MI, the consequences of such loss are often observed only in MII when centromeric cohesion is needed to prevent premature segregation of the sister chromatid pair.