Ca2+-driven cytoplasmic backflow secures spindle position in fertilized mouse eggs

Abstract Fertilization triggers hours-long Ca 2+ oscillations in mammalian eggs, but the effects of repeated Ca 2+ surges remain unclear. Here, we investigate spindle dynamics and its relationship with cytoplasmic streaming in fertilized mouse eggs. The spindle, initially parallel to the plasma membrane, rotates vertically, in accordance with previously reported results using artificially activated eggs. Intriguingly, it transiently reverses its rotation direction in synchrony with Ca 2+ oscillations, regardless of artificially altered frequency. This effect results from cytoplasmic streaming, initially moving from spindle to egg center, displaying a Ca 2+ -dependent backflow. Streaming also impacts spindle positioning, balancing spindle rotation and cortical localization maintenance. We provide evidence that Ca 2+ -dependent cortical myosin II activation causes actomyosin contraction, leading to transient streaming towards non-contracting actin cap regions overlaying chromosomes. Our findings underscore the role of Ca 2+ oscillations in maintaining spindle position in fertilized eggs, thereby ensuring highly asymmetric division and preservation of maternal stores in zygotes.