Heterogeneity of the zona pellucida carbohydrate distribution in human oocytes failing to fertilize in vitro.

The mammalian zona pellucida contains several glycoproteins whose oligosaccharide moieties are known to play a key role in the interaction with spermatozoa. Since zona pellucida defects may represent one of the most likely causes of failed fertilization in human in-vitro reproduction, we have studied the carbohydrate composition and distribution over the human zona pellucida by means of lectins. Donated, not inseminated cumulus-oocyte complexes, from cohorts with high fertilization rates, and fertilization-failed oocytes from cohorts inseminated with proven fertile donor semen, were analysed using 11 fluorescein-labelled lectins, on deplasticized semi-thin epoxy sections. Results showed that wheat germ agglutinin (WGA), Maclura pomifera (MPA) and Pisum sativum (PSA) bound to the extracellular matrix bordering the zona pellucida-corona radiata interface of cumulus-oocytes complexes, while the zona pellucida was labelled by WGA, Concanavalin A (ConA) and PSA. WGA labelling and correlative electron microscopy on the cumulus-oocyte complexes demonstrated that this lectin is a useful tool to trace the cortical granule distribution in the human oocyte. Surprisingly, in the failed-fertilized oocytes the zona pellucida was also labelled by MPA and showed three different patterns: (i) labelling of the zona pellucida outer surface; (ii) uniform labelling; (iii) labelling of an outer zona pellucida layer with variable thickness. Comparative analysis of WGA and MPA labelling on single failed-fertilized oocytes demonstrated that MPA zona pellucida patterns are not related to the cortical reaction. The nature and meaning of the MPA pattern of failed-fertilized oocytes were discussed in the light of zona pellucida defects impairing sperm receptivity.