Surface Architecture of Sperm Tail Entry into the Hamster Oocyte

At various times after artificial insemination in vivo, fertilized eggs were flushed from the ampulla of the oviduct of the hamster. The processes of sperm tail entry into the oocyte were studied with phase-contrast and electron microscopes. At 6-7 hr after insemination, the sperm head was incorporated completely into the ooplasm, but the entire length of the sperm tail still projected freely over the oocyte surface. The region on the oocyte surface where the second polar body was extruded was different from where the first polar body emerged. At 8-9 hr after insemination, the sperm tail was attached in a wave-like fashion to the oocyte surface. Where some portions of the tail were attached, they were trapped by the microvilli of the oocyte and had begun to sink into the ooplasm. Thus, the entire length of the sperm tail was incorporated into the ooplasm successively but almost synchronously. From the present observations, we have proposed a model for the mechanism of sperm tail entry into the vitellus in vivo.