Practice question

Zinc spark released upon fertilization delivers concentrated zinc into perivitelline space and zona matrix. Zinc binding causes physicochemical alterations of ZP glycoproteins, facilitating ovastacin-mediated cleavage of ZP2 and enhancing disulfide crosslinking, reducing zona solubility and sperm binding sites. This zona hardening provides sustained block to polyspermy complementing fast electrical block. Zinc does not lyse sperm nor alter egg membrane potential directly, and does not regulate capacitation. Experimental chelation of zinc preserves zona softness allowing supernumerary sperm binding, while exogenous zinc induces hardening even without fertilization, demonstrating zinc as key regulator of zona-based polyspermy defense.