Membrane events of fertilization in the sea urchin.

Four important events of fertilization in the sea urchin are: 1) the acrosome reaction of the sperm, 2) sperm-egg fusion, 3) the cortical reaction of the egg, and 4) the formation of the fertilization coat. The acrosome reaction is triggered by contact of the sperm with the jelly coat, a complex extracellular matrix surrounding the egg. This causes rapid fluxes of ions, fusion of the acrosome membrane with the plasma membrane, and extension of the acrosomal filament. The acrosome membrane inserted into thesperm plasma membrane covers the acrosomal filament and contacts the plasma membrane of the egg to initiate sperm-egg fusion. One consequence of sperm-egg fusion is insertion of the sperm plasma membrane into the egg plasma membrane, producing a mosaic patch. The sperm components inserted persist in development and can be identified by quantitative methods after gastrulation. Another consequence of sperm-egg fusion is the cortical reaction in which thousands of vesicles fuse with the egg surface, thereby adding their membranes to the egg plasma membrane and releasing their contents upon the egg surface. This results in an approximate doubling of the amount of membrane on the egg surface in a few seconds and produces a mosaic topography. The excess surface membrane is accommodated by elongation of egg microvilli. The cortical reaction causes a detachment of the egg glycocalyx or vitelline membrane, and this layer is elevated from the egg surface. Contents of the cortical granules combine with and alter the vitelline membrane by a hardening reaction to produce the fertilization coat. Hardening involves a peroxidase-mediated tyrosine crosslinking, requiring a burst of oxygen consumption by the egg to generate hydrogen peroxide and resulting in chemiluminescence. These events are followed by activation of metabolic processes in the egg and changes which protect the egg against polyspermy.