Chromatin organization and meiotic non-disjunction in mouse oocytes

Abstract

Two forms of oocytes termed SN (Surrounded Nucleolus) and NSN (Non Surrounded Nucleolus) differing for the spatial distribution of nuclear and nucleolar-associated chromatin have been described within the antral compartment of the ovary of a number of mammals. The biological significance of these two kinds of oocytes is as yet not completely clear. In previous studies we have shown that prior to ovulation, SN oocytes isolated from the antral compartment, cultured and fertilizedin vitro (IVM-IVF) have a far better meiotic and developmental competence than NSN oocytes. Following ovulation the proportion of SN antral oocytes diminishes and those SN and NSN oocytes that remain in the antral compartment are uncapable of embryonic development beyond the 2-cell stage. To further examine the correlation between chromatin distribution and meiotic competence of mouse antral oocytes, in the present study we have analyzed chromosome segregation at the time of first meiotic division in SN and NSN antral oocytes and in ovulated oocytes. SN and NSN antral oocytes were isolated before (48 hr post PMSG injection) or after (15 hr post hCG injection) ovulation from ovaries of females of increasing age, they were culturedin vitro to metaphase II, and their aneuploidy rate was examined. Comparison of data obtained before and after ovulation highlights two main points: 1) following ovulation a statistically significant increase of aneuploidy is observed in antral oocytes in most age groups. This increase is mainly attributable to SN oocytes, whereas the aneuploidy rate in the group of NSN oocytes does not significantly change before and after ovulation. 2) The aneuploidy rate of MII ovulated oocytes has a decreasing trend during female aging. These results suggest that in the ovarian dynamics, SN antral oocytes may be favoured in the selection for ovulation.