Practice question

Prevention of polyspermy in mammals employs fast electrical depolarization and slow zona block. Upon gamete fusion, cortical zinc stores release massive burst called zinc spark into extracellular milieu. Zinc binds zona pellucida, increasing stiffness and inducing structural remodeling that blocks additional sperm binding, complementing ovastacin-mediated ZP2 cleavage. Calcium influx triggers exocytosis but zinc directly hardens matrix and reduces sperm motility in perivitelline space. Sodium influx mediates fast block in some non-mammalian species, potassium efflux not implicated. Thus zinc spark is recognized as essential chemical signal coupling egg activation to physicochemical hardening of zona ensuring monospermy and viability.