The union of the haploid sperm and the haploid egg in fertilization creates a single diploid cell, called a zygote, which will develop into an embryo. Fertilization does more, however, than just restore the full genetic complement of the animal. The processes associated with fertilization help the egg and sperm get together, prevent the union of the sperm and egg of different species, and guarantee that only one sperm will enter and activate the egg metabolically. In the accompanying animation, we examine fertilization using an invertebrate animal—a sea urchin—as an example.
The successful fusion of one sperm with one egg requires a sequence of cellular reactions in both of these haploid cells. During fertilization in a sea urchin, the sperm and egg undergo reactions that allow a sperm to recognize and fuse with the egg, followed by other reactions that prevent additional sperm from entering the egg. When more than one sperm cell fuse with one egg, this phenomenon is referred to as polyspermy.
The sperm performs its cellular reactions first, beginning with the release of acrosomal enzymes onto the egg's jelly layer. These enzymes digest through the jelly and allow the sperm's growing acrosomal process access to the egg. The sperm and egg recognize each other through a lock-and-key, species-specific binding between proteins on the surface of the acrosomal process and receptors on the egg. This recognition is especially important for organisms like the sea urchin, in which fertilization is external and the eggs and sperm are likely to encounter gametes from other species.
The species-specific binding allows the sperm to fuse with the egg. Immediately after this fusion, the egg undergoes two reactions that prevent additional sperm from gaining entry. The first, called the fast block to polyspermy, is a quick and short-lived response in which the egg's plasma membrane changes its electric polarity. The change in polarity inhibits other sperm from fusing.
In the mean time, the egg intiates a second, long-lived response, called the slow block to polyspermy. Associated with the slow block to polyspermy is a release of calcium ions into the egg cytoplasm from intracellular stores. The rise in cytoplasmic calcium activates the egg, thus initiating the first processes of development. The rise in calcium triggers cortical granules within the egg to release their contents and chemically alter the egg's outer layers. The vitelline envelope hardens to form the "fertilization membrane," which in turn rises away from the egg's plasma membrane. All of these responses make it more difficult for additional sperm to fertilize the egg.
Textbook Reference: Concept 37.1 Sexual Reproduction Depends on Gamete Formation and Fertilization