Practice question

Immediately after sperm-egg plasma membrane fusion, egg cortex locally remodels beneath bound spermatozoon to form conspicuous fertilization cone. Process begins with transient depolymerization of cortical actin followed by rapid GTPase-dependent actin polymerization involving Arp2/3 complex and formins, pushing plasma membrane outward into blunt protrusion engulfing sperm head and midpiece while drawing sperm nucleus into ooplasm. Myosin II contractility and microtubule-based transport assist later incorporation and pronuclear migration but initial protrusion itself driven by localized actin polymerization triggered by calcium and small GTPases. Cytochalasin B inhibition blocks cone formation preventing sperm internalization, demonstrating dependence on actin dynamics.