Oocyte checkpoint response to the spindle poison depends on the cytoplasm volume

The embryonic aneuploidy in mammals may arise from impaired Spindle Assembly Checkpoint (SAC) function, a mechanism which prevents errors in chromosome segregation by blocking anaphase in response to spindle anomalies. Mammalian oocytes are particularly susceptible to these errors, possibly because the large oocyte volume favors dilution of the checkpoint signal, preventing its efficient function. This study aimed to investigate hypothesis that oocyte cytoplasmic volume affects SAC functionality. Oocyte size was manipulated in prophase oocytes (before nuclear envelope breakdown, NEBD) or in M-phase oocytes (after NEBD) by either reducing or increasing cytoplasmic volume by half. These oocytes were then cultured in the presence of nocodazole which activated the SAC by arresting oocytes in metaphase I of the first meiotic division. The functionality of SAC was assessed by measuring the proportion of oocytes escaping SAC-induced metaphase I arrest and completing the first meiotic division i.e., extruding the first polar body and entering the metaphase II of the second meiotic division. Reduction of the cytoplasmic volume in the prophase stage resulted in stronger checkpoint function, with only 4% of oocytes escaping SAC arrest compared to 36% of control normal-sized oocytes. Conversely, enlarged oocytes showed diminished checkpoint efficiency, with 54% bypassing checkpoint-induced arrest compared to 20% of control normal-sized oocytes. Importantly, no such relationship was observed when cytoplasmic volume was altered in oocytes after NEBD. This may suggest that the SAC depends on some nucleus-associated factors that are released into the cytoplasm after NEBD, since such factors would be twice as concentrated in oocytes undergoing volume reduction before NEBD compared to those undergoing reduction after NEBD. These results prove that SAC efficiency in mouse oocytes is influenced by cytoplasmic volume, with larger volumes impairing its function.