ABSTRACTAneuploidy is the leading genetic cause of miscarriage and infertility in women and occurs frequently in oocytes. Spindle formation and positioning are two critical events that must be regulated tightly to avoid erroneous chromosome segregation. Following nuclear envelope breakdown (NEBD), the spindle is assembled centrally before migrating towards the cortex to allow the first asymmetric division. The biological significance of the primary central positioning of the spindle is unknown. Because the spindle forms where NEBD occurs, whether positioned at the center or at the cortex of the cell, full-grown GV (prophase I) oocytes were collected from CF-1 mice (6-8 weeks old) and sorted according to the position of the GV into three groups: central, intermediate, and peripheral. Approximately 50% of the cells exhibited a central GV position, while 25% of cells showed a peripheral GV position. These proportions were similar to those obtained by histological evaluation of ovarian oocytes in 6-8 week mice, but not to those of mice aged less than 5 weeks, which tended to have a majority of peripherally positioned GVs. When peripheral GV oocytes were matured in vitro, GVs (88%) migrated towards the center and NEBD (and spindle assembly) occurred either at the center of the cell or during migration. The percentages of NEBD and polar body (PB) extrusion did not vary significantly among groups. Importantly, peripheral GV oocytes had a significant increase of abnormal K-MT attachments at metaphase I (Met I) and showed a significant increase of aneuploidy at metaphase II (Met II) when compared to central GV oocytes. Interestingly, peripheral GV oocytes that were able to achieve full GV relocation to the center were half as likely to experience aneuploidy when compared to peripheral GV oocytes unable to achieve relocation. These results indicate that preferential central spindle formation is an insurance mechanism to protect against incorrect K-MT attachments and aneuploidy.

Spindle positioning within the oocyte must be tightly regulated. In mice, the spindle is predominantly assembled at the oocyte center before its migration toward the cortex to achieve the highly asymmetric division, a characteristic of female meiosis. The significance of the initial central positioning of the spindle is largely unknown. We show that initial spindle positioning at the oocyte center is an insurance mechanism to avoid the premature exposure of the spindle to cortical CDC42 signaling, which perturbs proper kinetochore-microtubule attachments, leading to the formation of aneuploid gametes. These findings contribute to understanding why female gametes are notoriously associated with high rates of aneuploidy, the leading genetic cause of miscarriage and congenital abnormalities.

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