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.
In the sea urchin, before a sperm can fertilize an egg, it must first penetrate the egg's outermost barrier, the jelly coat, and attach to the vitelline envelope. Many sperm attach to the vitelline envelope, but only the first one to reach the egg cell membrane achieves fertilization.
The process begins when a sperm touches the jelly coat, triggering the acrosomal reaction.
In the acrosomal reaction, the sperm's cell membrane and acrosomal membrane fuse together, releasing acrosomal enzymes onto the egg. These enzymes are hydrolytic and begin to digest the egg's jelly coat.
A thin extension, called the acrosomal process, grows out from the sperm to meet the vitelline envelope. The acrosomal process contains actin filaments that lengthen through polymerization.
The acrosomal process is covered by proteins, called bindin, that attach to receptors on the egg. The receptors on the eggs of one species recognize only the bindin proteins on sperm of the same species. That is, the interaction is species specific.
The binding of the sperm to the egg completes the acrosomal reaction and allows the sperm's cell membrane to fuse with that of the egg.
After fusion, sodium ions flow into the cell. This sudden influx changes the polarity of the egg's membrane, so that the interior is positively charged relative to the exterior. The positive charge prevents other sperm cells from fusing. This change in polarity is called the fast block to polyspermy.
Activated bindin receptors now initiate the slow block to polyspermy, which involves converting the vitelline envelope to a physical barrier that sperm cannot penetrate. The activated receptors stimulate the egg to release a wave of calcium ions from intracellular stores. The calcium causes cortical granules from the egg to fuse with the cell membrane.
The contents of the cortical granules include enzymes that break the bonds between the vitelline envelope and the cell membrane. Other proteins released from the cortical granules attract water into the space. As a result, the vitelline envelope rises to form a fertilization envelope. Cortical granule enzymes also degrade sperm-binding molecules on the surface of the fertilization envelope and cause it to harden. These many actions prevent additional sperm from contacting the egg cell membrane.
The sperm's nucleus enters the egg's cytoplasm, where it will fuse with the egg's nucleus to create the diploid nucleus of the zygote.
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 cell 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 envelope," which in turn rises away from the egg's cell membrane. All of these responses make it more difficult for additional sperm to fertilize the egg.