Sperm Transport in the Female Tract: Activation Acrosome Reaction
Once capacitation has been completed, activation of spermatozoa can proceed. Activation is a calcium-dependent event which involves extensive changes in the sperm. First, the surface membrane of the front of the sperm fuses at many points with the underlying acrosomal membrane (the acrosomal reaction) creating a vesiculated appearance and exposing the enzymatic contents of the acrosomal vesicle and the inner acrosomal membrane to the exterior. Second, the tail beat changes from regular wave-like flagella beats to "whiplashing beats" that push the sperm forward in vigorous lurches. Third, the surface membrane of the middle and posterior half of the sperm head becomes able to fuse to the surface membrane of the ovum. The molecular basis of activation is not yet established. However, a destabilization of the surface membranes may be critical.