Nffi Fertilization is the union of sperm and egg

■f The union of the haploid sperm and the haploid egg 1 f 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 events and processes associated with fertilization help eggs and sperm get together, prevent the union of sperm and eggs of different species, guarantee that only one sperm will enter an egg, and activate the egg metabolically. Fertilization involves a complex series of events:

► The sperm and the egg recognize each other.

► The sperm is activated so that it is capable of gaining access to the plasma membrane of the egg.

► The plasma membranes of the sperm and the egg fuse.

► The egg blocks entry by additional sperm.

► The egg is metabolically activated and stimulated to start development.

► The egg and sperm nuclei fuse to create the diploid nucleus of the zygote.

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. The sea urchin is such a species, and its mechanisms of fertilization have been well studied.

The eggs of sea urchins and various other marine invertebrates 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 peptide consisting of 14 amino acids. As this peptide diffuses from the egg, it binds to receptors on the sperm the same species. The sperm respond by increasing their mitochondrial respiration and their motility. Before exposure to the peptide, the sperm swim in tight little circles, but after binding the peptide, they swim energetically up the concentration gradient of the peptide until they reach the egg that is releasing it. The peptide released by eggs of one species of sea urchin does not bind to receptors on sperm of other species.

When sperm reach an egg, they must get through two protective layers before they can fuse with the egg plasma membrane. The eggs of sea urchins are covered with a jelly coat, which surrounds a proteinaceous vitelline envelope. The sperm's assault on these protective layers depends on a membrane-enclosed structure called an acrosome containing enzymes and other proteins. The acrosome is located at the front of the sperm head, where it forms a cap over the nucleus (Figure 43.4).

When the 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 plasma membrane covering the sperm head and the underlying acroso-mal membrane. The acrosomal enzymes are released, and they digest a hole through the jelly coat. Next, a structure called an acrosomal process extends out of the head of the sperm. The acrosomal process forms from globular actin proteins behind the acrosome, which polymerize when the acro-somal membrane breaks down.

The acrosomal process extends through the remainder of the jelly coat to make contact with the vitelline envelope. The acrosomal process is coated with a membrane-bound pro

Jelly coat Vitelline envelope Egg plasma membrane

Sperm cell Actin

Sperm cell Actin

Acrosomal Process Sperm

The acrosomal membrane breaks down, releasing enzymes that digest a path through the protective jelly coat of the egg.

An acrosomal process then forms and makes contact with the vitelline envelope...

...which has species-specific receptors for the protein bindin.

Fertilization cone

The acrosomal process membrane and egg membrane fuse.

Egg cytoplasm

43.4 The Acrosomal Reaction The acrosomal reaction allows a sea urchin sperm to recognize an egg of the same species and pass through its protective layers.

The acrosomal membrane breaks down, releasing enzymes that digest a path through the protective jelly coat of the egg.

An acrosomal process then forms and makes contact with the vitelline envelope...

...which has species-specific receptors for the protein bindin.

Fertilization cone

The acrosomal process membrane and egg membrane fuse.

Egg cytoplasm

43.4 The Acrosomal Reaction The acrosomal reaction allows a sea urchin sperm to recognize an egg of the same species and pass through its protective layers.

tein called bindin. Different species have different kinds of bindin molecules. The plasma membrane of the egg has species-specific bindin receptors that extend through the vitelline envelope. The reaction of acrosomal bindin with these receptors stimulates the egg plasma membrane to form a fertilization cone that engulfs the sperm head, bringing it into the egg cytoplasm.

In animals that practice internal fertilization, mating behaviors help guarantee species specificity, but egg-sperm recognition mechanisms still exist. The mammalian egg, for example, is surrounded by a thick layer called the cumulus, which consists of a loose assemblage of maternal cells in a gelatinous matrix (Figure 43.5). Beneath the cumulus is a glycoprotein envelope called the zona pellucida, which is functionally similar to the vitelline envelope of sea urchin eggs. When mammalian sperm are deposited in the female reproductive tract, they become metabolically activated and are made capable of an acrosomal reaction if they should meet an egg. An activated sperm can penetrate the cumulus and interact with the zona pellucida.

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 in the zona 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. When the sperm head reaches the egg plasma membrane, other proteins cause the adhesion of sperm to egg plasma membrane and facilitate fusion of sperm and egg.

The importance of the zona pellucida and its sperm-binding molecules as a species-specific recognition mechanism was revealed in experiments on mammalian eggs and sperm in culture dishes. When the zona was stripped from human eggs and they were exposed to hamster sperm, fertilization took place, resulting in a hamster-human hybrid zygote. The hybrid zygote did not survive its first cell division because of chromosomal incompatibilities, but the experiment demonstrated that the recognition mechanism in mammalian species resides in the zona.

blocks to polyspermy and egg activation. The fusion of the sperm and egg plasma 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—that is, mechanisms that prevent more than one sperm from entering the egg. If more than one sperm enters the egg, the resulting embryo is unlikely to survive.

Blocks to polyspermy have been studied extensively in sea urchin eggs, which can be fertilized in a dish of seawater. Within seconds after a sperm enters a sea urchin egg, there is an influx of sodium ions, which changes the electric charge difference across the egg's plasma membrane. This fast block to polyspermy prevents the fusion of other sperm with the egg plasma membrane.

Membranes Mammalian Egg

Plasma membrane

43.5 A Mammalian Egg Is Surrounded by Barriers to Sperm This human egg is protected by the cumulus and zona pellucida, both of which sperm (shown here in blue) must penetrate to fertilize the egg. Only one sperm will penetrate the zona pellucida.

Plasma membrane

43.5 A Mammalian Egg Is Surrounded by Barriers to Sperm This human egg is protected by the cumulus and zona pellucida, both of which sperm (shown here in blue) must penetrate to fertilize the egg. Only one sperm will penetrate the zona pellucida.

Jelly coat Vitelline envelope Egg plasma membrane

Egg cytoplasm Protein bond

Jelly coat Vitelline envelope Egg plasma membrane

Slow Block Polyspermy

Egg cytoplasm Protein bond

43.6 The Slow Block to Polyspermy Enzymes from the sea urchin egg's cortical granules trigger the slow block to polyspermy.

Enzymes and other substances harden the vitelline envelope to form a fertilization envelope, releasing sperm bound to it.

43.6 The Slow Block to Polyspermy Enzymes from the sea urchin egg's cortical granules trigger the slow block to polyspermy.

form a fertilization envelope. Cortical granule enzymes also degrade sperm-binding molecules on the surface of the fertilization envelope and cause it to harden. The fertilization envelope prevents additional sperm from contacting the egg.

The slow block to polyspermy in the sea urchin is mediated by the phos-phatidyl inositol-bisphosphate (PIP2) second messenger system (see Figure 15.11). Activation of the bindin receptors activates phospholipase C, which cleaves PIP2 in the egg plasma membrane, releasing inositol triphosphate (IP3) into the egg cell cytoplasm. IP3 diffuses to the en-doplasmic reticulum, where it opens calcium channels.

In mammals, sperm entry does not seem to cause a rapid change in membrane potential, but it triggers the PIP2 second messenger system, resulting in several events. Calcium is released from the endoplasmic reticulum, and as in the sea urchin, the increased calcium causes the cortical granules to fuse with the egg plasma membrane. A fertilization envelope does not form around the mammalian egg, but the cortical granule enzymes destroy the molecules in the zona pellucida that bind sperm. The rise in cy-toplasmic calcium also activates the egg's metabolism and signals it to complete meiosis. The pH of the egg's cytoplasm increases, its oxygen consumption rises, protein synthesis increases, and DNA synthesis is initiated. The stage is set for the first cell division.

Enzymes and other substances harden the vitelline envelope to form a fertilization envelope, releasing sperm bound to it.

The slow block to polyspermy takes about a minute and results from the release of calcium (Figure 43.6). Before fertilization, the vitelline envelope is bonded to the egg plasma membrane. Just under the plasma membrane are vesicles called cortical granules, which contain enzymes and other proteins. The sea urchin egg, like all animal cells, contains calcium ions that are sequestered in the endoplasmic reticulum.

Sperm binding to the sea urchin egg stimulates the release of calcium from the egg's endoplasmic reticulum. The increase in cytoplasmic calcium causes the egg's cortical granules to fuse with the plasma membrane and release their contents. The cortical granule enzymes break the bonds between the vitelline envelope and the plasma membrane, and other proteins released from the cortical granules attract water into the space between them. As a result, the vitelline envelope rises to

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Responses

  • katie
    What is the process called when the acrosome membranes of the sperm break down?
    8 years ago
  • aston
    Why is it important for plants to have mechanism that prevents more than one sperm from entering egg?
    8 years ago
  • Ezio
    Where on sea urchin's body do they release sperm and eggs?
    8 years ago
  • pamela
    How does the egg ensure that only ONE sperm's DNA is allowed to enter the egg calcium sodium?
    7 years ago
  • tanja
    How plasma membrane help fertilization of the egg?
    7 years ago
  • dominik
    What is the membrane that surrounds the human egg to insure only human sperm get in?
    7 years ago
  • roma
    Which process forms an eggan egg?
    7 years ago
  • romola
    How is the union of the egg with sperm called?
    7 years ago
  • sayid
    What is the process by which acrosome membranes of speem break down?
    7 years ago
  • ADONAY
    How does a sperm recognize the egg?
    7 years ago
  • david
    Which enzyme causes hardening of fertilization envelope?
    4 years ago
  • Tuulia Lilja
    What happens when the sperm reaches vitteline envelope?
    4 years ago
  • blanca
    What two layers must sperm get through before fertilization which surrond plasma membrrane?
    4 years ago
  • Bowman
    What f is the union of the sperm and the egg cell?
    2 years ago

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