Practice question

Upon fertilization, oocytes release massive stores of chelatable zinc in discrete spark events concomitant with cortical granule exocytosis. Each spark liberates billions of zinc ions into perivitelline space, zinc concentration in zona increases threefold. Zinc binding alters supramolecular architecture of ZP filaments increasing density and rigidity observed by electron microscopy, phenomenon termed zona hardening. Additionally extracellular zinc transiently inhibits sperm motility and CatSper activity. Collectively these physiochemical changes erect rapid structural barrier blocking supernumerary sperm penetration, complementing ZP2 cleavage for durable polyspermy prevention and embryo protection during early development.