Animation courtesy of David Epel and Chris Patton (Stanford University), from their Sea Urchin Embryology web site.

The acrosome reaction is a change in the sperm that is common to many higher animals. In the sea urchin, contact with egg jelly initiates the acrosome reaction, which is a calcium-mediated process. In response to signals transduced by receptors on the surface of the sperm that bind to components of egg jelly (red), actin polymerizes from a pool of globular actin (pink) to form the acrosomal process. The acrosomal vesicle (green) fuses with the plasma membrane, releasing enzymes from the tip of the sperm that aid digestion of egg jelly. At the same time, bindin (blue) is deposited on the surface of the acrosome-reacted sperm.