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Summary
All mammalian eggs are surrounded by a thick extracellular coat called the zona pellucida (ZP). To fertilize an egg, sperm must bind to the ZP, penetrate through it, and fuse with the egg's plasma membrane (1, 2). Binding of sperm to this outer coat occurs in a species-restricted manner, suggesting that ZP elements to which sperm bind may differ among mammalian species, and may be attributable in part to oligosaccharides (1, 2). On page 1761 of this issue, Pang et al. (3) report that binding of human sperm to eggs is attributable to an abundance of a sequence of sugar molecules called sialyl-Lewisx (SLeX) at the ends of oligosaccharides of ZP glycoproteins. These terminal sequences are known to function in the adhesion of other cells, including blood and tumor cells (4).
