Mechanism of the chromosome-induced polar body extrusion in mouse eggs

Chromosomes independently induce spindle formation and cortical actomyosin assembly into special cap and ring structures in the cortex of the oocyte. The spindle and the cortical cap/ring interact to generate mechanical forces, leading to polar body extrusion. Two distinct force-driven membrane changes were observed during 2nd polar body extrusion: a protrusion of the cortical cap and a membrane invagination induced by an anaphase spindle midzone. The cortical cap protrusion and invagination help rotate the spindle perpendicularly so that the spindle midzone can induce bilateral furrows at the shoulder of the protruding cap, leading to an abscission of the polar body. It is interesting to note that while the mitotic spindle midzone induces bilateral furrowing, leading to efficient symmetric division in the zygote, the meiotic spindle midzone induced cytokinetic furrowing only locally.Distinct forces driving cortical cap protrusion and membrane invagination are involved in spindle rotation and polar body extrusion during meiosis II in mouse oocytes.Female meiosis in most animals is characterized by two sequential asymmetric meiotic divisions following one round of DNA replication, which results in formation of a haploid egg and extrusion of two small polar bodies destined for degeneration. Oocyte haploidization by means of discarding half of the chromosomes into the polar bodies represents a special mechanism for female gamete formation. To accomplish asymmetric cell division, a cell needs to establish a cortical polarity, according to which the mitotic/meiotic spindle is asymmetrically positioned [1-3]. The highly asymmetric cell divisions during female meiosis ensure that the produced haploid gametes maximally inherit maternal components, which are beneficial for embryo development.Although polar body extrusion during female meiosis has been recognized for many years, the mechanism by which oocytes accomplish the special asymmetric divisions is still poorly underst