Live imaging captures the events immediately following fertilization.

Abstract

Fertilization involves dynamic sperm-egg interactions, yet has been primarily studied in static samples. Here, we use high-resolution live imaging to capture fertilization from the moment of sperm binding in zona-intact mouse oocytes. We identify two phases of sperm remodeling: a static phase, during which sperm remain beneath the oocyte cortex as DNA decondensation and histone loading occur, and a mobile phase characterized by stereotyped sperm movement. Initial displacement away from the spindle is driven by cytoplasmic streaming, with manipulations in mouse indicating that sperm movement requires chromatin decondensation and oocyte polarization. Subsequently, polar body cytokinesis generates convergent cortical flows that draw sperm toward the emerging female pronucleus. Finally, we capture sperm-egg fusion in human oocytes and characterize post-fusion events including meiotic resumption and sperm movement, offering a live-imaging description of human fertilization dynamics. Together, these findings provide a continuous spatiotemporal framework for mammalian fertilization, extended by initial observations in human oocytes.