The union of the haploid sperm and the haploid egg (fertilization) creates a single diploid cell, a zygote, which will develop into an embryo. Fertilization does more 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. Fertilization involves a complex series of events:

SPECIFICITY IN SPERM–EGG INTERACTIONS Specific recognition molecules mediate interactions between sperm and eggs. These molecules ensure that the activities of sperm are directed toward eggs and not other cells, and they help prevent eggs from being fertilized by sperm from the wrong species. The latter function is particularly important in aquatic species that release eggs and sperm into the surrounding water where the eggs can readily be exposed to sperm of other species. The sea urchin is a good example of such a species, and sea urchin fertilization has been well studied.

Sea urchin eggs release chemical attractants that increase the motility of sperm and cause them to swim toward the egg. These chemical attractants are species-specific. For example, eggs of one species of sea urchin release a specific peptide consisting of 14 amino acids. This peptide binds to receptors present on sperm of the same species. The sperm respond by increasing their mitochondrial respiration and motility. Before exposure to the peptide, the sperm swim in tight little circles, but after binding to the peptide, they swim energetically up the concentration gradient of the peptide until they reach the egg that is releasing it.

When sperm reach an egg, they must get through two protective layers before they can fuse with the egg cell membrane. The eggs of sea urchins are covered with a jelly coat that surrounds a proteinaceous vitelline envelope (Figure 42.5A). The success of a sperm’s assault on these protective layers depends on a membrane-enclosed structure at the front of the sperm head called an acrosome.

The acrosome contains enzymes and other proteins. When a sperm makes contact with an egg of its own species, substances in the jelly coat trigger an acrosomal reaction, which begins with the breakdown of the cell membrane covering the sperm head and the underlying acrosomal membrane (Figure 42.5B). The acrosomal enzymes are released and digest a hole through the jelly coat.

As a result of the polymerization of actin triggered by the acrosomal reaction, an acrosomal process extends out of the head of the sperm. The acrosomal process is coated with species-specific recognition molecules called bindin, and there are bindin receptors on the vitelline envelope of the egg. The interaction of bindin with their receptors enables the sperm to contact the egg cell membrane. That contact results in fusion of the sperm and egg cell membranes and the formation of a fertilization cone that engulfs the sperm head, bringing it into the egg cytoplasm. The sperm mitochondria, which largely constitute the midpiece of the sperm, are also drawn into the egg cytoplasm, but they degrade and disappear; this means that the mitochondria and *mitochondrial genes of the new urchin are derived only from the egg.

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In animals that practice internal fertilization, mating behaviors help guarantee species specificity, but egg–sperm recognition mechanisms still exist. The mammalian egg is surrounded by a thick layer called the cumulus consisting of a loose assemblage of maternal cells in a gelatinous matrix (Figure 42.6). Beneath the cumulus is a glycoprotein envelope called the zona pellucida, which is functionally similar to the vitelline envelope of sea urchin eggs. Mammalian sperm become activated when they are deposited in the female reproductive tract, and they are then capable of an acrosomal reaction should they encounter an egg. An activated sperm can penetrate the cumulus and interact with the zona pellucida.

Question

If two sperm fertilized an egg, there would be two sets of male chromosomes in the egg, which would disrupt the subsequent cell divisions.

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Unlike the jelly coat of sea urchin eggs, the cumulus of mammalian eggs does not trigger the acrosomal reaction. When sperm make contact with the zona pellucida, a species-specific glycoprotein binds to recognition molecules on the head of the sperm. This binding triggers the acrosomal reaction, releasing acrosomal enzymes that digest a path through the zona pellucida. When the sperm head reaches the egg cell membrane, other proteins facilitate its adhesion to and fusion with the egg cell membrane.

BLOCKS TO POLYSPERMY The fusion of the sperm and egg cell membranes and the entry of the sperm into the egg initiate a programmed sequence of events. The first responses to sperm entry are blocks to polyspermy: mechanisms that prevent more than one sperm from entering the egg. Survival of the embryo is unlikely if more than one sperm enters the egg. The sperm contributes a haploid set of chromosomes, so an extra set would disrupt the mitotic division of the egg and subsequent cells. In addition, in most mammals the sperm contributes a centriole to the fertilized egg, and the *centriole forms the centrosome which is critical for organizing the mitotic spindle. Having two centrosomes would disrupt mitosis of the fertilized egg.

Blocks to polyspermy have been studied extensively in sea urchin eggs, which can be fertilized in a dish of seawater. Within seconds after the sperm membrane contacts the egg membrane, an influx of sodium ions changes the electric charge difference across the egg cell membrane. This fast block to polyspermy prevents the fusion of any other sperm with the egg cell membrane, but it is transient. The change in membrane electric charge lasts only about a minute, but that is enough time to allow a slower block to sperm entry to develop.

The slow block to polyspermy involves converting the vitelline envelope to a physical barrier that sperm cannot penetrate. Before fertilization, the vitelline envelope is bonded to the egg cell membrane. Just under the cell membrane are vesicles called cortical granules (see Figure 42.5) which contain enzymes and other proteins.

The sea urchin egg, like all animal cells, sequesters calcium in its endoplasmic reticulum. Sperm entry into the egg stimulates the release of calcium ions from the endoplasmic reticulum and into the egg cytosol. This increase in cytosolic calcium causes the egg’s cortical granules to fuse with the cell membrane and release their contents. Cortical granule enzymes break the bonds between the vitelline envelope and the cell membrane, and other proteins released from the cortical granules attract water into the space between them. 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, thus preventing additional sperm from contacting the egg cell membrane.

In mammalian eggs, sperm entry does not cause a rapid change in membrane potential, but it does trigger a release of calcium from the endoplasmic reticulum. As in the sea urchin egg, increased calcium causes the cortical granules to fuse egg with the egg cell membrane. A fertilization envelope does not form around the mammalian egg, but the cortical granule enzymes destroy the sperm-binding molecules in the zona pellucida. The rise in cytosolic calcium also signals the egg to complete meiosis. The stage is set for the first cell division.