Fertilization

Fertilization is the fusion of a female's egg cell (oocyte) and a male's sperm cell (spermatozoa) to form the first cell of a new and unique being. While on the surface this sounds like a simple process, there are many factors that make this possible.

Gametes

Gametes are unique from all other cells. Typically, each cell in the human body contains twenty-three pairs of chromosomes (for a total of forty-six). Mature egg and sperm cells contain only one copy of each chromosome (for a total of twenty-three). At fertilization the fusion of the two gametes will create a cell with the appropriate twenty-three pairs of chromosomes (forty-six individual chromosomes) necessary for human development. In this way, one chromosome of each pair will originate from each parent, making the new individual unique from any other person that came before. The specialized process by which the genetic material is shuffled and the chromosome number is cut in half, from forty-six to twenty-three chromosomes, is called meiosis .

Ovulation and Ejaculation

Sperm and egg cells are not only different from other cells, but are different from each other. A female is born with all the eggs she will ever have. At birth, the chromosomes of these eggs have only completed the beginning of meiosis (meiosis I) and will remain dormant (inactive) until the onset of menstrual cycles in puberty. Specific hormones produced during the menstrual cycle around day thirteen or fourteen trigger the continuation of meiosis in one egg each month. Meiosis is suspended for the second time in the middle of meiosis II around three hours prior to ovulation, and does not resume unless fertilization occurs.

During ovulation this egg, enclosed in two layers of protective material, is released from the ovary. The outer layer, the cumulus oophorus, is comprised of cells called cumulus cells, and the inner layer, the zona pellucida, is comprised of a jellylike coating made of protein and sugar. Once released from the ovary, the egg is swept into the fallopian tube. It is receptive to fertilization for only about one day. The sperm must reach the egg during this time, usually in the fallopian tubes, or fertilization will be impossible.

During sexual intercourse, millions of sperm are deposited into the vagina. They travel through the cervix and uterus to the fallopian tubes. Sperm can live within a woman's body for up to three days. Each sperm contain three distinct parts, the head, mid-piece, and tail. Each of these parts has a distinct purpose. The head of the sperm is composed of the nucleus (containing the chromosomes), an acrosome cap (containing enzymes crucial in fertilization), and an outer membrane. The mid-piece contains energy-producing mitochondria, and the tail is the mechanism for movement. Despite the ingenious design, fewer than 1 percent of the sperm released in an ejaculation ever make it to the egg. Factors inhibiting the success of sperm include abnormal formation and premature death from exposure to acidic vaginal secretions. Sperm can also be blocked by excess mucus covering the cervix, or they may travel to the fallopian tube that does not contain the egg. Fortunately, only one sperm is required to fertilize the egg.

In order to fertilize an egg, sperm must undergo the poorly understood process of capacitation. Capacitation involves changes to the acrosome, triggered by the cervical mucus, to prepare it to release the enzymes necessary to break through the zona pellucida. Upon reaching the surface of the zona pellucida, the sperm releases enzymes to break through. Once through the zona pellucida, the head of the sperm fuses to the egg's membrane, the tail of the sperm stops moving, and the egg engulfs the contents of the sperm.

It is crucial that only one sperm enters the egg. If an extra sperm passes through, a lethal condition known as polyspermy (many sperm) will occur. On the rare occasion this occurs, the fetus will be miscarried as a result of the extra set of chromosomes. To prevent this in most instances, a substance is released from the egg that changes the zona pellucida once it has been penetrated, blocking entry of any other sperm. Sperm penetration triggers the completion of the second meiotic division in the egg. With this division, the chromosomes of the sperm and egg come together in their own nucleus. The cell now officially becomes a zygote , the first cell of a new individual.

Variations

In some instances, an ovary releases more than one egg at one time, or both ovaries release an egg simultaneously. Each egg has the potential to be fertilized, resulting in multiple pregnancies. Since each conception originates from a separate egg and sperm, individuals created in this way are as different as those conceived as separate births. These are referred to as fraternal twins. In rare instances, cells from the same embryo separate to create distinct embryos that are genetically identical and are referred to as monozygotic, or identical, twins.

The exact details of fertilization vary from animal to animal. Fertilization does not always take place inside an animal. For example, sea urchins, spiny animals attached to rocks on the ocean floor, release their eggs and sperm directly into the water. Large numbers of each (millions of eggs and billions of sperm) are necessary to ensure that enough eggs will be fertilized to maintain the population. Many other ocean creatures also release egg and sperm cells into the water. However, the eggs are fertilized only by sperm of the same species because of unique proteins on the surface of the egg. As in humans, fertilization immediately triggers a change in the surface of the egg, protecting it from penetration by other sperm, even sperm of the same species.

Reproductive technology has introduced further variations in how eggs may become fertilized, permitting the process to occur outside the fallopian tubes. One of the most common is in vitro fertilization, in which eggs and artificially capacitated sperm are combined in a glass dish and the dividing embryos are later transplanted into the uterus.

see also Meiosis; Reproductive Technology; Twins.

Susan E. Estabrooks

Bibliography

Primakoff, Paul, and Diana G. Myles. "Penetration, Adhesion, and Fusion in Mammalian Sperm-Egg Interaction." Science (Jun. 21, 2002): 2183-2185.

Tobin, Allan, and Jennie Dusheck. Asking about Life, 2nd ed. Orlando, FL: Harcourt,2001.

Wasserman, Paul. "The Biology and Chemistry of Fertilization." Science 235 no. 4788 (1987): 553-560.

Fertilization

Life can persist in one of two ways. First, living things can spurn death in the hope of living forever. This is usually not possible, because death by predation or other unintentional causes is virtually inevitable. Second, living things can make copies of themselves, or reproduce, as a means of "hedging their bets" against death. Then, all that is required for life to persist is for at least one of the copies, or offspring, to remain alive at any one time. Thus, all living things reproduce.

How does reproduction occur? Some organisms reproduce asexually. The single-celled prokaryotes undergo binary fission, meaning literally "to split in half." Binary fission occurs after the cell has doubled the amount of its cellular constituents. The single circular chromosome is replicated from a single initiation point, the replication fork moving bidirectionally around the chromosome. Finally, inward growth of the plasma membrane separates the large cell into two equal halves. Some multicellular organisms reproduce asexually through budding. For example, in hydra, a relative of the jellyfish, a mass of mitotically dividing cells grows on the parent's side and eventually detaches as a small copy of the parent. However, most multicellular organisms reproduce sexually. Sexual reproduction takes place after the production of special reproductive cells termed gametes . Gametes are typically labeled as being of one sex or the other, male or female, with the female gamete being the larger of the two types. Fertilization is the union of a male and a female gamete to form a zygote , the developing offspring. There are many ways in which fertilization can occur in nature.

The sexual organs of animals are called gonads. Gametes are made in the gonads—the female gonads produce eggs, the male gonads produce sperm. Human males produce millions of sperm every day. Females produce one mature egg in each menstrual cycle. When a sperm and an egg fuse during fertilization, a diploid zygote is formed. This zygote divides mitotically until it is an adult organism. Animals are unique in that they have a special type of cells called germ cells , in the gonads. The sole function of these cells is to undergo meiosis to form the precursors of eggs and sperm, oocytes and spermatocytes, which differentiate into the mature gametes necessary for reproduction. One can think of animal bodies, which are made of somatic cells derived from the original germ cells, as the machines that germ cells use to ensure their successful passage to the next generation.

Animals may release gametes into the external environment to be fertilized, or the male may deposit gametes into the female, allowing fertilization to take place inside the female reproductive tract. External fertilization occurs only in aquatic or moist habitats where gametes will not dry out. Sessile animals such as corals often release millions of gametes into the water at one time ensuring that at least some will be fertilized. In such cases, fertilization does not require that members of the opposite sex be near each other, although it is necessary that both males and females release their mature gametes at the same times. They do this by responding to species-specific environmental cues such as light cycles or temperature.

Some animals, including fish and amphibians, use external fertilization but do so only with a particular mate. When a female is receptive to a particular male, she will lay her clutch of eggs in the water and the male will distribute his sperm over them. In this form of external fertilization, unlike internal fertilization in which the female can store and use sperm from many males, the male is assured of paternity and is therefore much more likely to take care of the offspring.

Internal fertilization requires that the male introduce his sperm directly into the female, so there is a much greater probability that any particular gamete will be fertilized. Furthermore, animals are no longer dependent on water for fertilization and may become completely terrestrial. Animals with internal fertilization, especially females who incur most of the cost of reproduction, are selected to be extremely choosy with whom they mate. If they do not pick healthy mates they may spend precious energy and time raising an offspring that cannot compete with the offspring of choosier parents. There are usually species-specific behaviors, courtship displays, and other physical cues that allow females to pick healthy mates of their own species, a phenomenon known as sexual selection.

At the cellular level, a mammalian sperm must undergo several steps before it can fertilize the egg. The first step is termed the acrosomal reaction, in which enzymes from the sperm cap, or acrosome, are released. These enzymes serve to break down the barrier of follicle cells that surround the egg, as well as the zona pellucida, a glycoprotein envelope that encases the egg. The sperm can tunnel through the zona pellucida only if the acrosomal enzymes recognize species-specific molecules of the female's egg. Eventually the sperm gains access to the egg itself, and the sperm and egg plasma membranes fuse. At this stage the egg becomes activated and initiates a rapid sequence of events.

First, the activated egg blocks entry to other sperm, as polyspermy (the fertilization of an egg by more than one sperm) is generally lethal to the developing embryo. When the first sperm fuses with the egg plasma membrane, the egg begins to increase its concentration of positively charged sodium ions from the surrounding environment of the female oviduct. The change in the electric potential across the plasma membrane (the excess positive charge inside the egg) prevents further sperm/egg fusions. Next, development processes begin; the egg increases oxygen consumption and begins protein synthesis. Eventually the nuclei of the sperm and egg fuse to form the diploid nucleus of the new zygote.

see also Reproduction, Asexual and Sexual.

Todd A. Schlenke

Bibliography

Austin, C. R., and R. V. Short, eds. The Evolution of Reproduction. Cambridge, U.K.: Cambridge University Press, 1976.

Dale, Brian, ed. Mechanism of Fertilization: Plants to Humans. New York: Springer-Verlag, 1990.

Gwatkin, Ralph B. L. Fertilization Mechanisms in Man and Mammals. New York: Plenum Press, 1977.

Longo, Frank J. Fertilization, 2nd ed. New York: Chapman and Hall, 1997.

Fertilization

Fertilization is the process by which the nucleus of a sperm (a male reproductive cell) fuses (combines) with the nucleus of an egg (a female reproductive cell; also called an ovum). Fertilization occurs somewhat differently in plants and animals. In flowering plants, two sperm cells are involved in the process of fertilization. The first sperm cell combines with an egg cell, while the second sperm cell combines with two nuclei present in the ovule (the structure that eventually becomes the seed). The structure formed in the second fertilization eventually forms a storage site for nutrients needed by the fertilized egg cell.

A fertilized egg cell is known as a zygote. Once formed, the zygote undergoes continuous cell division that eventually produces a new multicellular organism.

Fertilization in humans

In humans, fertilization occurs in the fallopian tubes of the female reproductive tract. It takes place within hours following sexual intercourse. Approximately 300 million sperm are released into a female's vagina during intercourse. However, only one of these sperm can actually fertilize the single female egg cell (also called an ovum). The successful sperm cell must enter the uterus and swim up the fallopian tube to meet the egg cell. There it passes through the thick coating surrounding the egg. This coating is known as the zona pellucida.

The head of the sperm cell contains enzymes (certain types of chemicals) that break through the zona pellucida and make it possible for the sperm to penetrate into the egg. Once the head of the sperm is inside the egg, the tail of the sperm falls off. The outside of the egg then thickens to prevent another sperm from entering.

In humans, a number of variables affect whether or not fertilization occurs following intercourse. One factor is a woman's ovulatory cycle. The ovulatory cycle is the series of events that bring about the ripening of an egg and its release from the ovaries. Human eggs can be fertilized for only a few days after ovulation, which usually occurs only once every 28 days.

Fertilization in other species

Nearly all forms of terrestrial (land) animals use some form of internal fertilization similar to that in humans. External fertilization, however, is more common among aquatic animals. It is simple enough for aquatic animals simply to dump their sperm and eggs into the water and let currents mix the two kinds of cells with each other.

Reproduction of the sea urchin is a typical example of external fertilization among aquatic animals. A male sea urchin releases several billion sperm into the water. These sperm then swim towards eggs released in the same area. Fertilization occurs within seconds when sperm come into contact and fuse with eggs.

External fertilization in animals

Although it does not occur naturally in animals very often, external fertilization is also a possibility. In the case of humans, for example, some form of external fertilization may be necessary when a male and female wish to have a child but one or the other is biologically incapable of contributing to the normal process of internal fertilization.

An example is the process known as in vitro fertilization. The expression in vitro means "in glass," that is, in a glass test tube or petri dish. The term is used in contrast to in vivo fertilization, where in vivo means "in a living organism."

During in vitro fertilization, eggs are removed surgically from a female's reproductive tract. Those eggs then can be fertilized by sperm that has been taken from a male and then stored in a test tube or petri dish. After the fertilized eggs have divided twice, they are reintroduced into the female's body. If all goes well, the embryo and fetus develop, eventually resulting in a normal birth.

In vitro fertilization has been performed successfully on a variety of domestic animals since the 1950s. In 1978, the first human birth following in vitro fertilization occurred in England. Since that time, the procedure has become a routine treatment for infertile couples who wish to have children.

[See also Reproduction; Reproductive system ]

fertilization (syngamy) The union of male and female gametes (reproductive cells) during the process of sexual reproduction to form a zygote. It involves the fusion of the gametic nuclei (karyogamy) and cytoplasm (plasmogamy). As each gamete contains only half the correct number of chromosomes, fertilization and zygote formation results in a cell with the full complement of chromosomes, half of which are derived from each of the parents. In animals the process involves fusion of the nuclei of a spermatozoan and an ovum. In most aquatic animals (e.g. fish) this takes place in the surrounding water, into which the gametes are shed. Among most terrestrial animals (e.g. insects, many mammals) fertilization occurs in the body of the female, into which the sperms are introduced. In flowering plants, after pollination, the grain pollen produces a pollen tube, which grows down into the female reproductive organ (carpel) to enable a male gamete nucleus to fuse with the egg nucleus (see double fertilization).

In self-fertilization the male and female gametes are derived from the same individual. Among plants, self-fertilization (also called autogamy) is common in many cultivated species, e.g. wheat and oats. However, self-fertilization is a form of inbreeding and does not allow for the mixing of genetic material; if it occurs over a number of generations it will result in offspring being less vigorous and productive than those resulting from cross-fertilization. In cross-fertilization (also called allogamy in plants) the gametes are derived from different individuals. In plants the pollen comes either from another flower of the same plant or from a different plant (see also incompatibility).

fertilization in biology, process in the reproduction of both plants and animals, involving the union of two unlike sex cells (gametes), the sperm and the ovum , followed by the joining of their nuclei. In the flowers of higher plants, the process occurs after pollination has enabled the sperm to contact the egg cell in the plant's ovary. In lower plants and in animals the sperm is actively motile and swims to the egg through an external aqueous medium or through a fluid environment within the reproductive tract of the female. The fundamental principle of fertilization is the same in all organisms. The first sperm to establish successful contact is absorbed by the ovum and the two nuclei unite, thus combining the hereditary material of both parents (see genetics ). In higher forms, the sperm contact initiates cell division in the fertilized egg (zygote), and the subsequent embryo develops into a new individual. Cross-fertilization indicates fusion of a sperm of one hermaphroditic plant or animal with an ovum of another, as distinguished from self-fertilization, in which ovum and sperm of the same individual are fused.

fertilization
1. The union of two gametes to produce a zygote, which occurs during sexual reproduction. Fertilization involves the fusion of two haploid nuclei containing genetic material from two distinct individuals (cross-fertilization) or from one individual (self-fertilization). The resulting zygote then develops into a new individual. Most aquatic animals, e.g. echinoderms, achieve fertilization externally, gametes uniting outside the body of the parents. Some other animals, particularly terrestrial species, have internal fertilization, with the union of gametes inside the female. Some lower plants (e.g. mosses) release their male gametes externally, which then swim like spermatozoa to the female gamete. Most higher plants have the male gamete released internally from the pollen grain directly to the female gamete and double fertilization occurs.

2. The application of plant nutrients (i.e. fertilizers) to land in order to promote the growth of desired plants.

fertilization Union of a male and female gamete: an ovum and a sperm (spermatozoon). If coitus has occurred within a few days of discharge of a ovum from an ovary (normally around the middle of the menstrual cycle or about two weeks from the start of the last period), one among the sperm which have survived the hazards of the journey to the Fallopian tube meets the ovum there, and enters it. This event instantly triggers changes in cellular chemistry and physics in ways which, among other things, prevent access of any rival sperm. A new unique individual is formatted from the conjunction of genetic material, and cell division begins.

Stuart Judge

fertilization Key process in sexual reproduction during which the nuclei of female and male gametes (sex cells) fuse to form a zygote. The zygote contains the genetic material (chromosomes) from both parents (see heredity). In animals, the female sex cell is called the ovum and the male cell sperm. After fertilization, the zygote divides to form an embryo. Fertilization of the female ovum by the male sperm can be external (as in most fish, amphibians and aquatic invertebrates) or internal (as in reptiles, birds, mammals and insects). In plants, the male gamete is found in pollen, and for most higher plants, pollination occurs before fertilization.

fertilization The union of two gametes to produce a zygote, which occurs during sexual reproduction. Fertilization involves the fusion of two haploid nuclei (containing genetic material from two distinct individuals (cross-fertilization) or from one individual (self-fertilization). The resulting zygote then develops into a new individual. Some lower plants, e.g. mosses, release their male gametes externally, which then swim like spermatozoa to the female gamete. Most higher plants have the male gamete released internally from the pollen grain directly to the female gamete and in many double fertilization occurs.

fertilization The union of two gametes to produce a zygote, which occurs during sexual reproduction. Fertilization involves the fusion of two haploid nuclei (see HAPLOID NUMBER), containing genetic material from two distinct individuals (cross-fertilization) or from one individual (self-fertilization). The resulting zygote then develops into a new individual. Most aquatic animals, e.g. echinoderms, achieve fertilization externally, gametes uniting outside the body of the parents. Some other animals, particularly terrestrial species, have internal fertilization, with the union of gametes inside the female.

fer·til·i·za·tion / ˌfərtl-iˈzāshən/ • n. Biol. the action or process of fertilizing an egg, female animal, or plant, involving the fusion of male and female gametes to form a zygote. ∎  the action or process of applying a fertilizer to soil or land.

fertilization (fer-ti-ly-zay-shŏn) n. the fusion of a spermatozoon and an ovum to form a zygote.