The role of carbohydrate residues in mammalian fertilization

The fertilization process in mammals involves binding and fusion of free-swimming sperm and ovulated eggs. This review focuses on the role of carbohydrate residues in the process of sperm-egg interaction in mammals. The zona pellucida (ZP), the acellular glycoprotein coat surrounding the egg is highly glycosylated and posssess both Ash- (N-) linked and Ser/Thr- (O- )linked oligosaccharides, with an extreme structural heterogeneity between the different species. Different carbohydrates on ZP3, such as Galactose in α-linkage, N-acetylglucosamine in β-linkage, were suggested as the complementary sperm receptors, mediating the primary binding between the spermatozoon and the ZP. Several suggested complementary ZP3 binding proteins on the sperm are sp56, β-1,4-galactosyltransferase and p95. Some carbohydrate residues of the ZP undergo postfertilization modifications that might alter the sperm receptor, thus assisting in the establishment of the block to polyspermy. The studies summarized in this review imply a main role for the carbohydrate residues in the process of sperm egg interaction.