Ca²⁺-driven cytoplasmic backflow ensures spindle anchoring in fertilized mouse eggs

Female meiosis is highly asymmetric, producing a large egg and a small polar body to preserve maternal storage essential for embryogenesis. To achieve asymmetric division, the egg spindle must maintain its cortical position until fertilization completes meiosis. In mice, fertilization triggers chromosome segregation, followed by spindle rotation to achieve the perpendicular orientation relative to the cortex, leading to the extrusion of one set of chromosomes. However, it was unknown how the spindle maintains its cortical position while rotating. Here, we developed a high-resolution live-imaging method to investigate spindle dynamics during fertilization. Our results indicate that Ca²⁺ oscillations put the brakes on spindle rotation by transiently reversing cytoplasmic streaming and that this cytoplasmic backflow secures the spindle localization at the cortex. Mechanistically, Ca²⁺ oscillations drive cortical actomyosin contraction to induce the cytoplasmic backflow. Altogether, this work revealed a previously unknown role of Ca²⁺ oscillations in maintaining spindle position, ensuring the highly asymmetric divisions inherent to female meiosis.