Morphological observations on the binding of boar sperm to porcine zona pellucida

Scanning electron microscopy demonstrated that freshly ejaculated boar sperm bind in large numbers to the cumulus-free zona pellucida of porcine oocytes. Sperm were bound head-first although some portions of the flagellum occasionally made contact with the zona. Scanning microscopy also showed that sperm did not undergo an acrosome reaction or penetrate the zona after binding. Binding of boar sperm was visibly inhibited by pretreatment of the sperm by Fab antibodies to boar sperm plasma membranes. The forgoing indicate that specific receptors for the zona pellucida are present and exposed on the surface of non-capacitated boar sperm. In addition, when soluble components which were extracted from sperm by sedimentation at high speed, were preincubated with oocytes, sperm did not bind. Minor surface changes were noted after these treatments, but the zona remained intact and was not penetrated by sperm. Either this treatment inactivated sperm receptors or the outer layer of the zona, which may be the only portion of the zona containing receptors, was removed. Since binding of the sperm to the zona pellucida involved the sperm plasma membrane, we suggest that this membrane is intact when the sperm reaches the zona. Surface changes accompanying capacitation are apparently not necessary for species-specific sperm-egg binding in vitro in the pig.