Meiotic purification of dysfunctional mitochondria in mouse oocytes

Mitochondrial purification, including mitophagy, during oogenesis is critical for ensuring accurate mitochondrial DNA transmission, but it remains unclear whether meiosis plays a role in eliminating defective mitochondria. Here, we show that mitochondria in the first polar body (PB1) exhibit reduced membrane potential compared with those retained in oocytes. TUNEL and cytochrome c assays suggest that mitochondrial dysfunction in PB1 precedes nuclear fragmentation. Notably, the second polar body (PB2) exhibits heterogeneous membrane potentials, with both functional and dysfunctional populations present. By injecting dysfunctional and functional mitochondria, isolated respectively from the cytoplasts of the PB1and the oocyte, into germinal vesicle-stage mouse oocytes, we found that dysfunctional mitochondria were preferentially extruded into PB1, while functional mitochondria were retained after meiosis I. Interestingly, mitochondria from PB1 exhibit low membrane potential even after being transferred into a new, healthy oocyte, whereas oocyte-derived mitochondria maintain normal membrane potential following the same procedure. Immunofluorescence analysis further shows that PB1-derived mitochondria lack colocalization with motor protein Rho T1, in contrast to their oocyte-derived counterparts. Furthermore, PB1 transfer combined with mitochondrial probe and fast-NGS demonstrated that meiosis II also contributes to the extrusion of dysfunctional mitochondria into PB2. By comparison, spindle transfer revealed that most functional mitochondria were retained in the oocyte, with only minimal amounts detected in PB2. Predictably, PB2 and pronuclear transfer failed to extrude foreign mitochondria. Collectively, these findings identify meiosis as a distinctive safeguard for mitochondrial quality during oogenesis, with implications that warrant further investigation in humans.