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Surgery => Tubal Ligation => Fertilization
Fertilization
INTRODUCTION
Fertilization, the process in which gametes-a male's sperm and a female's egg or ovum-fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually-that is, when a male and a female are needed to produce an offspring . This article focuses on animal fertilization. For information on plant fertilization see the articles on Seed, Pollination, and Plant Propagation.
Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous-it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo.
Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed-a sperm cell or an egg cell. Each gamete contains only half the genetic material of the original cell. During sperm and egg fusion in fertilization, the full amount of genetic material is restored: half contributed by the male parent and half contributed by the female. In humans, for example, there are 46 chromosomes (carriers of genetic material) in each human body cell-except in the sperm and egg, which each have 23 chromosomes. As soon as fertilization is complete, the zygote that is formed has a complete set of 46 chromosomes containing genetic information from both parents.
The fertilization process also activates cell division. Without activation from the sperm, an egg typically remains dormant and soon dies. In general, it is fertilization that sets the egg on an irreversible pathway of cell division and embryo development.
THE FERTILIZATION PROCESS
Fertilization is complete when the sperm's nucleus fuses with the egg's nucleus. Researchers have identified several specific steps in this process. The first step is the sperm approaching the egg. In some organisms, sperm just swim randomly toward the egg (or eggs). In others, the eggs secrete a chemical substance that attracts the sperm toward the eggs. For example, in one species of sea urchin (an aquatic animal often used in fertilization research), the sperm swim toward a small protein molecule in the egg's protective outer layer, or surface coat. In humans there is evidence that sperm are attracted to the fluid surrounding the egg.
The second step of fertilization is the attachment of several sperm to the egg's surface coat. All animal eggs have surface coats, which are variously named the vitelline envelope (in abalone and frogs) or the zona pellucida (in mammals). This attachment step may last for just a few seconds or for several minutes.
The third step is a complex process in which the sperm penetrate the egg's surface coat. The head, or front end, of the sperm of almost all animals except fish contains an acrosome, a membrane-enclosed compartment. The acrosome releases proteins that dissolve the surface coat of an egg of the same species.
In mammals, a molecule of the egg's surface coat triggers the sperm's acrosome to explosively release its contents onto the surface coat, where the proteins dissolve a tiny hole. A single sperm is then able to make a slitlike channel in the surface coat, through which it swims to reach the egg's cell membrane. In fish eggs that do not have acrosomes, specialized channels, called micropyles, enable a single sperm to swim down through the egg's surface coat to reach the cell membrane. When more than one sperm enters the egg, the resulting zygote typically develops abnormally.
The next step in fertilization-the fusion of sperm and egg cell membranes-is poorly understood. When the membranes fuse, a single sperm and the egg become one cell. This process takes only seconds, and it is directly observable by researchers. Specific proteins on the surface of the sperm appear to induce this fusion process, but the exact mechanism is not yet known.
After fusion of the cell membranes the sperm is motionless. The egg extends cytoplasmic fingers to surround the sperm and pull it into the egg's cytoplasm. Filaments called microtubules begin to grow from the inner surface of the egg cell's membrane inward toward the cell's center, resembling spokes of a bicycle wheel growing from the rim inward toward the wheel's hub. As the microtubules grow, the sperm and egg nuclei are pushed toward the egg's center. Finally, in a process that is also poorly understood, the egg and sperm nuclear envelopes (outer membranes) fuse, permitting the chromosomes from the egg and sperm to mix within a common space. A zygote is formed, and development of an embryo begins.
TYPES OF FERTILIZATION
Two types of fertilization occur in animals: external and internal. In external fertilization the egg and sperm come together outside of the parents' bodies. Animals such as sea urchins, starfish, clams, mussels, frogs, corals, and many fish reproduce in this way. The gametes are released, or spawned, by the adults into the ocean or a pond. Fertilization takes place in this watery environment, where embryos start to develop.
A disadvantage to external fertilization is that the meeting of egg and sperm is somewhat left to chance. Swift water currents, water temperature changes, predators, and a variety of other interruptions can prevent fertilization from occurring. A number of adaptations help ensure that offspring will successfully be produced. The most important adaptation is the production of literally millions of sperm and eggs-if even a tiny fraction of these gametes survive to become zygotes, many offspring will still result.
Males and females also use behavioral clues, chemical signals, or other stimuli to coordinate spawning so that sperm and eggs appear in the water at the same time and in the same place. In animals that use external fertilization, there is no parental care for the developing embryos. Instead, the eggs of these animals contain a food supply in the form of a yolk that nourishes the embryos until they hatch and are able to feed on their own.
Internal fertilization takes place inside the female's body. The male typically has a penis or other structure that delivers sperm into the female's reproductive tract. All mammals, reptiles, and birds as well as some invertebrates, including snails, worms, and insects, use internal fertilization. Internal fertilization does not necessarily require that the developing embryo remains inside the female's body. In honey bees, for example, the queen bee deposits the fertilized eggs into special compartments in the honeycomb. These compartments are supplied with food resources for the young bees to use as they develop.
Various adaptations have evolved in the reproductive process of internal-fertilizing organisms. Because the sperm and egg are always protected inside the male's and female's bodies-and are deliberately placed into close contact during mating-relatively few sperm and eggs are produced. Many animals in this group provide extensive parental care of their young. In most mammals, including humans, two specialized structures in the female's body further help to protect and nourish the developing embryo. One is the uterus, which is the cushioned chamber where the embryo matures before birth; the other is the placenta, which is a blood-rich organ that supplies nutrients to the embryo and also removes its wastes.
RESEARCH ISSUES
Although reproduction is well studied in many kinds of organisms, fertilization is one of the least understood of all fundamental biological processes. Our knowledge of this fascinating topic has been vastly improved by many recent discoveries. For example, researchers have discovered how to clone the genes that direct the fertilization process.
Yet many important questions still remain. Scientists are actively trying to determine issues such as how sperm and egg cells recognize that they are from the same species; what molecules sperm use to attach to egg coats; and how signals on the sperm's surface are relayed inside to trigger the acrosome reaction. With continued study, answers to these questions will one day be known.
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