FIG. 1. A guinea pig sperm head before and after the acrosome reaction. During the acrosome reaction, the anterior head plasma membrane (except for the posterior-most equatorial region) is lost after fusion with the outer acrosomal membrane. Acrosome intact sperm bind to the zona pellucida by the anterior head plasma membrane. Acrosome reacted sperm bind via the inner acrosomal membrane. The equatorial and/or posterior head region initiates fusion with the egg plasma membrane. The inner acrosomal membrane does not participate in the membrane fusion but is incorporated into the egg cytoplasm. (Reproduced from the Journal of Cell Biology, 1984;99:163-164, by copyright permission of the Rockefeller University Press.)