Migration of meiosis-I spindle from the cell center to a sub-cortical location is a critical step for mouse oocytes to undergo asymmetric meiotic cell division. In this study, we investigate the mechanism by which formin-2 orchestrates the initial movement of meiosis-I spindle. A model based on polymerizing actin filaments pushing against mitochondria, thus generating a counter force on the spindle, demonstrated an inherent ability of this system to break symmetry and evolve directional spindle motion.
In mitotic cells, spindle orientation is largely controlled by microtubule-based mechanisms, but meiotic spindles in mammalian oocytes lack astral microtubules required for such mechanisms. In this study, we show that spindle rotation occurs at the completion of chromosome segregation, whereby the separated chromosome clusters each defines a cortical actomyosin domain that produces cytoplasmic streaming, resulting in hydrodynamic forces on the spindle.