Sperm receptors and fertilization in mammals.

During fertilization in mammals, sperm must first bind in a species-specific manner to the egg s thick extracellular coat, the zona pellucida. They then undergo a form of cellular exocytosis, the acrosome reaction. ZP3, one of three zona pellucida glycoproteins, serves as a structural glycoprotein, a sperm receptor, and an acrosome reaction-inducer. The latter two functions are mediated, at least in part, by ZP3 oligosaccharides. ZP3 is unique to mammalian eggs, from mice to humans, although related glycoproteins are found in vitelline envelopes of virtually all non- mammalian eggs, from fish to birds. Furthermore, the "zona domain" of zona pellucida glycoproteins, a characteristic ~260 amino acid-long region, is present in many (glyco)proteins with various functions in both vertebrates and invertebrates. It is tempting to suggest that egg- coat glycoproteins have evolved from solely structural components of certain non-mammalian egg vitelline envelopes into sperm receptors and acrosome reaction-inducers of the mammalian egg zona pellucida.