Of the 300,000,000 human sperm ejaculated during coitus, only about 200 reach the site of fertilization in the oviduct. Once it finds an egg, the sperm must first migrate through the layer of follicle cells and then bind to and cross the egg coatthe zona pellucida. Finally, the sperm must bind to and fuse with the egg plasma membrane.
Although many sperm can bind to an egg, normally only one fuses with the egg plasma membrane and injects its nucleus and other organelles into the egg cytoplasm.
Mechanisms which operate to ensure that only one sperm fertilizes the egg are
- A rapid depolarization of the egg plasma membrane, which is caused by the fusion of the first sperm, prevents further sperm from fusing
- The egg cortical reaction, which changes the structure of the zona pellucida after fertilisation. The altered zona becomes hardened, so that sperm no longer bind to it.