According to the National Center for Biotechnology Information, only one sperm normally fuses with an egg because if more than one sperm fuses, through a process called polyspermy, development of the zygote usually stops. In most cases, the egg cell blocks polyspermy.
During the fertilization of an egg, a sperm cell binds and fuses to an egg cell and injects its nucleus and other organelles into the egg's cytoplasm, according to the National Center for Biotechnology Information. When polyspermy occurs, multipolar or extra miotic spindles form, which stop the development of the fertilized egg by causing faulty segregation of chromosomes during cell division. To prevent polyspermy from occurring, the egg plasma membrane rapidly depolarizes after the first sperm cell fuses with the egg cell. This prevents further sperm from fusing, therefore acting as a primary block to polyspermy.
A cortical reaction within the egg cell acts as a secondary block to polyspermy, according to the National Center for Biotechnology Information. The sperm cell fuses with an egg plasma membrane and causes an increase in cytosolic calcium within the egg. The calcium activates the egg to begin development, which initiates a cortical reaction that releases various enzymes that change the structure of the outside layer of the egg. The layer, called the zona pellucida, becomes hardened, which prevents another sperm from binding to it.