Ultrastructural study of the interactions and fusion of ram spermatozoa with zona-free hamster oocytes.

The interaction of preincubated ram sperm with zona-free hamster oocytes was studied by scanning and transmission electron microscopy. The behaviour of the sperm cells was quite different, depending on whether the acrosomal reaction had taken place or not. The apical ridge of intact spermatozoa contacted the oocyte surface, and egg microvilli spread onto the anterior segment but no fusion ensued. When the acrosomal cap was fenestrated, microvilli were also found on its surface but were then spread over the surface of the postacrosomal region and the equatorial segment of the sperm head lying flat on the egg surface; fusion with the oocyte occurred in the equatorial segment and extended to the postacrosomal region. Contacts between the microvilli and the inner acrosomal membrane were infrequent and no fusion occurred in the anterior segment. These observations confirm that local changes in the adhesiveness and fusibility of the sperm plasma surface occurred during the acrosome reaction.