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Normozoospermic men who do not fertilize human eggs in vitro constitute a clearly identifiable class of occult male factor infertility. We have studied the relationships between in-vitro fertilization (IVF) outcomes and the appearance of sperm surface mannose lectin/zona pellucida ligand activity measured by the binding of fluorescein isothiocyanate (FITC)-labelled mannosylated albumin. Of 338 semen specimens with normal profiles, 10 (2.96%) failed to fertilize in vitro (IVF-). Motile spermatozoa from six of these were analysed retrospectively for zona ligand binding activity on the sperm head in relation to acrosome status and the free cholesterol content of their plasma membrane. IVF+ males and fertile donors served as controls. The six IVF- occult male factor cases were distinguished from controls by their failure to show time-dependent increases in the percentage of spermatozoa exhibiting head-directed mannose ligand activity, by the slow loss or apparent increase in membrane cholesterol and by the absence of spontaneous and/or mannose-induced acrosome exocytosis.

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