Practice question

After sperm-egg plasma membrane fusion, egg cortex reorganizes beneath fusion site into funnel-like protrusion called fertilization cone that engulfs sperm head and midpiece drawing nucleus inward. This structure depends on rapid polymerization of cortical actin microfilaments orchestrated by small GTPases RhoA, Rac, and Arp2/3 complex nucleating branched filaments. Actin mesh pushes membrane around sperm and provides traction. Myosin II later contracts cone, tubulin microvilli elongate slightly, and dynein remains sperm flagellar motor. Inhibition of actin polymerization with cytochalasin D prevents cone formation leaving sperm attached externally, proving actin assembly essential for incorporation of male pronucleus.