Practice question

Mammalian eggs prevent polyspermy through rapid membrane depolarization and slower zona hardening. A critical chemical event is fertilization-induced zinc spark, where cortical vesicle-like zinc stores expel billions of zinc ions within minutes of sperm-egg fusion. Extracellular zinc binds zona pellucida proteins, induces conformational changes and crosslinking involving ovastacin-mediated ZP2 cleavage, increasing matrix rigidity and preventing additional sperm penetration. This zinc-dependent hardening works alongside calcium wave-triggered cortical granule exocytosis. Sodium, potassium or calcium alone do not mediate this structural zona modification; zinc provides specific physicochemical block essential for monospermic fertilization.