[Sequential observation of human sperm penetration into zona pellucida-free hamster oocytes by scanning electron...

Yanagimachi et al. have shown that zona pellucida-free hamster oocytes can be penetrated by capacitated and acrosome-reacted human spermatozoa, and both pronuclei be formed. Recently, it has been said that this technique may be used to test the fertilizing ability of human sperm for clinical use. We have made scanning electron microscopic studies of the interaction of these heterologous gametes, especially the mechanisms of sperm entry into the oocytes. Our results are as follows: The surface of the oocyte is covered with numerous microvilli, which are found to be evenly and densely distributed on the vitellus surface. The surface of the first polar body already released is smooth and microvilli-free. Most spermatozoa lie flat on the vitellus surface, but a few are oriented perpendicularly to the vitellus surface. Most bound sperm had lost their acrosomal caps, because a ridge exists at the leading edge of the equatorial segment. Many microvilli are shown to participate in sperm-egg contact. Initially most microvilli appeared to grasp and immobilize the anterior tip of the sperm head. But as gamete interaction proceeded, microvilli were overlying the postacrosomal region, and observed adjacent to the plasma membrane of the postacrosomal region. The postacrosomal region is first incorporated into the ooplasma, the anterior tip of sperm head being the last portion to be incorporated. The microvilli of the oolenmal surface where sperm penetrated did not show major changes in size or appearance, and the so-called "incorporation cone" was not observed. Our scanning electron micrographs show that the microvillar portion of the oolenma greatly participates in this heterologous gamate interaction.