Membranes of Mammalian Sperm and Capacitation

Capacitation prepares the mammalian sperm for interaction with the ovum. It occurs in the female genital tract and involves a change in the properties of the membrane over the head. The head membranes are morphologically specialized in different regions. The portion overlying the acrosome is loose and not linked to a cytoplasmic substratum (Fig. 1). It is differentiated from the tight membrane apposed to the post acrosomal cap (PAC) (1). Following capacitation, the periacrosomal segment fuses with the outer membrane of the acrosome allowing escape of the acrosomal contents, the “acrosome reaction”. The fertilizing sperm contacts the vitellus at the PAC. Thus the outer membrane is separated into a portion concerned with the loss of the acrosome and another with ovum contact. The outer membrane of the acrosome is distinguished from the inner since it participates in the acrosome reaction. Cytochemical analysis of the cell membrane supports the concept that the stimulus for capacitation is a change in the properties of the plasmalemma induced by fluids of the female tract.