Electron microscope studies of sperm incorporation into the golden hamster egg

Abstract Hamster eggs freed from the cumulus oophorus and zona pellucida were inseminated in vitro with capacitated spermatozoa. As soon as the spermatozoon reaches the surface of the egg cytoplasm, numerous microvilli of the egg surround the spermatozoon. Those microvilli attached to the acrosomal collar region of the spermatozoon quickly spread over the inner acrosomal membrane of this region, dislodging the outer acrosomal membrane and the overlying sperm plasma membrane. At the same time, microvilli fuse with the plasma membrane of the postnuclear cap region of the spermatozoon, and the egg cytoplasm quickly flows around the nucleus of that region of the spermatozoon. The inner acrosomal membrane persisting on the sperm head never fuses with the egg (microvillar) plasma membrane. This membrane is destined to disintegrate and be absorbed into the egg cytoplasm. Incorporation of the sperm flagellum into the egg cytoplasm takes place by fusion between plasma membranes of the spermatozoon and egg.