25.9–25.11: Sex can lead to fertilization, but it can also spread sexually transmitted diseases.

Sperm fertilizing an egg.

It’s not enough for sperm cells to be in the same general location as an egg—there are some specific events that must take place for fertilization to occur. The process of fertilization in vertebrates involves three separate steps: activation, penetration, and fusion of the nuclei (FIGURE 25-19). And before a sperm can take that first step and then penetrate an egg, it must find it. Recently, researchers discovered that sperm have receptors, like chemical sensors, that cause the sperm cells to swim toward a chemical attractant released by the egg. This attractant, in essence, says, “I’m over here!”

Figure 25.19: Fertilization is the fusion of one sperm with an egg.

After ovulation, the egg is still surrounded by some smaller cells, called granulosa cells. These cells stand between the sperm and the egg, but they’re not the only barrier. Between the granulosa cells and the egg’s membrane, there is a glycoprotein layer called the zona pellucida (pronounced puh-loo-sih-duh). As sperm approach the egg, release of calcium by cells surrounding the egg seems to increase the sperms’ swimming speed and tail movements, in a process known as sperm activation. A second phase of sperm activation occurs as a sperm cell pushes its way through the granulosa cells, and the acrosome (at the head of the sperm) dissolves and releases digestive enzymes that help the sperm penetrate the zona pellucida.

Once a sperm cell gets through the zona pellucida, the oocyte is activated, and the plasma membranes of the sperm and egg fuse. This activation and fusion has a couple of important consequences. First, it changes the egg’s membrane in such a way that it is impossible for any other sperm to also fertilize the egg. After all, it would be a genetic disaster for the fertilized egg to have more than two sets of chromosomes. Second, fusion of the plasma membranes induces the egg to complete its second meiotic division. On doing so, one haploid egg, the ovum, is formed, along with one smaller polar body. The latter disintegrates or is ejected from the egg.

And, finally, the haploid nucleus of the egg fuses with the haploid nucleus of the sperm, creating a diploid cell—the zygote.

In many species, stripping away the zona pellucida makes it possible for the egg to be fertilized by sperm from a different species. This suggests that something in the zona pellucida, perhaps recognition sites, ensures that the egg can be fertilized only by sperm from the same species.