ZP3 glycoprotein. Computer artwork of a molecule of ZP3 (zona pellucida glycoprotein 3), a sperm receptor in the external membrane (zona pellucida) of a female germ cell (egg). The receptor binds with the plasma membrane of the head of the sperm (the acrosome) as it approaches the zona pellucida during human sexual reproduction. This exposes the contents of the acrosome and renders the sperm capable of fusing with the egg.