Significance of the equatorial segment of the acrosome of the spermatozoon in eutherian mammals

The acrosome of the spermatozoon in eutherian mammals is characterized by a posterior ‘equatorial’ segment which is excluded from the membrane fusion events of the acrosome reaction and apparently remains intact during incorporation of the spermatozoon into the egg. Based on the following considerations: 1. 1. The stability of most of the limiting membrane of the reacted eutherian sperm head, i.e. the inner membrane of the acrosome and post-acrosomal plasmalemma, both of which are underlain immediately by -S-S- crosslinked perinuclear material; 2. 2. The pentalaminar organization and inert character of the equatorial segment membrane; 3. 3. The fact that this appears to be the initial site of sperm/egg fusion we suggest that the equatorial segment of the acrosome functions to preserve a discrete region of ‘labile’ plasmalemma overlying it through which fusion with the egg is achieved. The need for this feature and possible reasons for its restriction to the spermatozoa of eutherian mammals are discussed briefly.