Prevention of Polyspermy

Prevention of Polyspermy
At the point of sperm contact with the egg vitelline envelope a fertilization cone appears into which the sperm head is later drawn (see Figure 8-4). This event is followed immediately by important changes in the egg surface that block the entrance of additional sperm, which, in marine eggs especially, may quickly surround the egg in swarming numbers (Figure 8-5). The entrance of more than one sperm, called polyspermy, must be prevented because the union of more than two haploid nuclei would be ruinous for normal development. In a sea urchin egg, contact of the first sperm with the egg membrane is instantly followed by an electrical potential change in the egg membrane that prevents additional sperm from fusing with the membrane. This event, called the fast block, is followed immediately by the cortical reaction, in which thousands of enzyme-rich cortical granules, located just beneath the egg membrane, fuse with the membrane and release their contents into the space between the egg membrane and the overlying vitelline envelope (see Figure 8-4). The cortical reaction creates an osmotic gradient, causing water to rush into this space, elevating the envelope and lifting away all sperm bound to it, except the one sperm that has successfully fused with the egg membrane. One of the cortical granule enzymes causes the vitelline envelope to harden, and it is now called a fertilization membrane. The block to polyspermy is complete. The timing sequence of these early events is summarized in Figure 8-6. Mammals have a similar security system that is erected within seconds after the first sperm fuses with the egg membrane.