Carbohydrate analysis of the zona pellucida and cortical granules of human oocytes by means of ultrastructural cytochemistry.

BACKGROUND The zona pellucida (ZP), the mammalian oocyte coat, consists of a restricted number of highly glycosylated proteins. In vitro sperm binding studies suggest a higher binding affinity for the outer region of the ZP compared to its inner region in different species. However, the reason for this difference in binding distribution remains unresolved. Many studies suggest that the carbohydrate sequences linked to ZP glycoproteins act as ligands for sperm binding to this matrix. METHODS Lectins and antibodies that recognize different carbohydrates were employed to perform an ultrastructural analysis of human ZP and cortical granule glycosylation. RESULTS This study reveals variable glycosylation of the human ZP throughout its thickness, with pronounced differences between the most external and internal regions of this matrix. The binding studies also indicate that ZP glycoproteins express some carbohydrate sequences not previously detected in other species. Finally, cytochemical analysis of human cortical granules suggests similarities in glycosylation to ZP glycoproteins but not to cortical granules from other mammalian species. CONCLUSION A heterogeneous carbohydrate composition was observed in the thickness of the human ZP that could be responsible for the different sperm binding affinity detected between the outer and inner regions of the ZP.