In order to initiate fertilization, a sperm must first fuse with the plasma membrane of the female egg cell (known as the zona pellucida) and then penetrate through the membrane to fertilize it. Generally speaking, many sperm fuse to an egg cell, making it the “easy” part. Actually getting sperm to penetrate through the egg’s external shell can, and often is, the more difficult activity. In order to accomplish this, sperm cells go through a process known as the acrosome reaction, which occurs in the sperm as it approaches the egg.
In simple terms, the acrosome is a cap-like structure over the front half of the sperm’s head. When in the reproductive tract, sperm undergo a process called capacitation, where the acrosomal membrane is destabilized. As the sperm approaches the outer membrane (or zona pellucida) of the egg, the membrane surrounding the acrosome is able to fuse with the plasma membrane of the sperm’s head and expose the contents of the acrosome, which include antigens necessary for binding to the egg’s cell membrane, as well as various enzymes which that help the sperm break through the egg’s tough coating and allows fertilization to begin.
It is critical to note that sperm which are unable to go through the acrosome reaction properly will not be able to fertilize an egg. It should also be noted that certain proteins in the zona pellucida bind to a partner molecule on the sperm. This “lock-and-key” type mechanism of action is species-specific, meaning that the sperm and egg of different species cannot fuse.