Ultrastructural observations on binding and membrane fusion between human sperm and zona pellucida-free hamster oocytes.

It has been shown previously by others that capacitated human sperm will penetrate zona pellucida-free (ZPF) hamster oocytes. We have made ultrastructural observations on the binding and fusion of sperm to the oolemma in this cross. Our results show that only human sperm without acrosomes bind to ZPF hamster oocytes. Bound sperm had undergone either a normal or degenerative acrosome reaction and in some cases were morphologically abnormal. Initial binding of oocyte microvilli occurred over the sperm's inner acrosomal membrane. In later stages, microvilli were observed closely apposed to the sperm's plasma membrane overlying the equatorial segment of the acrosome and the postacrosomal region. Gamete membrane fusion occurred between the plasma membrane of the oocyte's microvilli and the sperm's plasma membrane overlying the equatorial segment and the anterior third of the postacrosomal sheath. In most experiments, sperm nuclear envelope breakdown and nuclear decondensation began soon after membrane fusion. As the sperm's chromatin dispersed, a zone devoid of organelles appeared in the cortex of the oocyte. The significance of these results as they relate to the use of the ZPF hamster oocyte in fertility testing and to the general topic of fertilization is discussed.