fertilization


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fertilization,

in biology, process in the reproductionreproduction,
capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
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 of both plants and animals, involving the union of two unlike sex cells (gametes), the spermsperm
or spermatozoon
, in biology, the male gamete (sex cell), corresponding to the female ovum in organisms that reproduce sexually. In higher animals the sperm is produced in the testis of the male; it is much smaller than the ovum and consists primarily of a head,
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 and the ovumovum
, in biology, specialized plant or animal sex cell, also called the egg, or egg cell. It is the female sex cell, or female gamete; the male gamete is the sperm. The study of the ovum is included in the science of embryology.
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, followed by the joining of their nuclei. In the flowers of higher plants, the process occurs after pollinationpollination,
transfer of pollen from the male reproductive organ (stamen or staminate cone) to the female reproductive organ (pistil or pistillate cone) of the same or of another flower or cone.
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 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 geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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). In higher forms, the sperm contact initiates cell division in the fertilized egg (zygote), and the subsequent embryoembryo
, name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilization until it has become structurally complete and able to survive as a separate organism.
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 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

 

(also called syngamy), the union of male and female sex cells, or gametes, resulting in the formation of a zygote capable of developing into a new organism. Fertilization is the basis of sexual reproduction and ensures the transmission of heritable characteristics from parents to offspring.

In plants. Fertilization occurs in most plants. It is usually preceded by the formation of gametangia, which are the reproductive organs in which gametes develop. These processes are frequently referred to under the general name “sexual reproductive process.” Sexually reproducing plants also include meiosis in their developmental cycle; that is, they exhibit a succession of nuclear phases. No typical sexual reproductive process has been discovered in bacteria, blue-green algae, or some fungi. Several forms of sexual reproduction occur in lower plants. The process can take place in some green algae without the formation of gametes as a result of the union of two unicellular organisms. Sexual reproduction of this sort is called hologamy. The union of flagellated gametes identical in shape and size is called isogamy and is characteristic of many algae.

Unicellular algae, for example, some members of the genus Chlamydomonas, are apparently themselves converted into gametangia, with the resultant formation of gametes. In multicellular algae, for example, Ulothrix and Ulva, certain morphologically indistinguishable cells develop into gametangia. Morphologically distinct gametangia arise in other species of multicellular algae, for example, in Ectocarpus. Many isogamous algae are heterothallic: only physiologically different gametes, designated “plus” and “minus” (+, —), unite. Conjugation is characteristic of algae of the class Conjugatea, for example, Spirogyra: the protoplast of one cell fuses temporarily with that of another cell (the two cells can either belong to the same or different individuals).

Heterogamy is the union of flagellated gametes of different sizes, the larger being female, and the smaller male. Heterogamy occurs, for example, in some members of the genus Chlamydomonas. Oögamy is a form of sexual reproduction that involves the union of a large, nonflagellated female gamete (egg) and a small male gamete. Generally, the male gamete is equipped with flagella, in which case it is called a spermatozoid. Less commonly, the male gamete is nonflagellated, in which case it is called a spermatium. The female gametangia of most oögamous lower plants are called oögonia, and the male gametangia antheridia. Oögamy is characteristic of many green, brown, and diatomaceous algae, as well as of all red algae and some lower fungi. Fertilization takes place in the water in all hologamous, isogamous, and heterogamous plants and in many oögamous plants. In some oögamous plants, for example, Volvox and Vaucheria, it takes place in the female gametangia—oögonia—to which the spermatozoa’s that emerge into the water swim actively. Apparently, chemotaxis plays a role in this process. The spermatia of red algae are passively carried to the oögonia by the water’s current.

The gametes are not differentiated in plants with gametangiogamy. For example, in fungi of the genus Mucor the multinuclear gametangia developing at the ends of the mycelia fuse. (These mycelia are morphologically different in heterothallism.) In so doing, the nuclei also fuse in pairs. This type of gametangiogamy is called zygogamy. In most ascomycetous fungi, the multinuclear protoplast of the antheridium flows into the basal cell of the female gametangium—the ascogonium—which contains a protoplast with numerous nuclei. The nuclei only come together in pairs, forming dikaryons. This first stage of the reproductive process is called plasmogamy. Hyphae are structures that grow out of the ascogonium; the nuclei of the dikaryons divide synchronously within the hyphae. Asci are cells that contain one dikaryon each; these emerge from the tips of the hyphae. The second stage of the reproductive process—karyogamy, or the union of nuclei—occurs within the asci.

Somatogamy is a process characteristic of basidiomycetous fungi. These organisms do not form gametes or gametangia, and plasmogamy occurs when two uninuclear cells, the positive and negative primary mycelia, unite. The resulting binuclear cell gives rise to a secondary mycelium, which consists of cells containing dikaryons. Basidia are formed in this mycelium; karyogamy also occurs within the basidia. Fungi developed gametangiogamy and somatogamy over the course of evolution as adaptations to existence outside of water.

All the higher plants are oogamous, but they differ in the mode of fertilization. The typical gametangia of higher plants—antheridia (male) and archegonia (female)—are multicellular. The cells of the external layer of a gametangium are sterile. One egg is formed in each archegonium, while many spermatozoids are usually formed in a single antheridium. Fertilization in bryophytes and pteridophytes requires water so that the spermatozoids emerging from the antheridia can swim to the archegonia. When it is ready for fertilization, the open top of an archegonium secretes slime, which attracts the spermatozoids. Moving in the slime, the spermatozoids reach the egg, which is then united with only one of the mate gametes.

In pteridophytes, fertilization occurs at or in the prothallium, which is the gametophyte. In seed plants, the prothallium at which fertilization occurs is a structural part of the sporophyte. The prothallia are monoecious in homosporous ferns and dioecius in heterosporous ferns and all seed plants. Seed plants do not have antheridia; spermatozoids in plants of the class Cycadopsida and in the genus Ginkgo or nonflagellated sperm in the form of pollen grains in all other seed plants are formed in the male prothallia. Some gymnosperms (Gnetum, Welwitschia) and all angiosperms lack archegonia, and the eggs are located in the female prothallia.

Seed plants can be fertilized only as a result of pollination—the transfer of pollen grains from the microsporangia to the pollen chambers of the ovules (in gymnosperms) or to the stigmata of the pistils (in angiosperms). In plants of the class Cyca-dopsida and in the genus Ginkgo, the spermatozoids emerge into the archegonial chamber of the ovule and move in the fluid that is produced by the plant until they reach the archegonia. In seed plants with sperm, the sperm move to the eggs through the pollen tubes.

Angiosperms exhibit double fertilization: one sperm unites with an egg, and a second sperm with the central cell of the embryonic sac, or female prothallium. Fertilization independent of the presence of free water is one of the most important adaptations of seed plants to survival on land.

REFERENCES

Maier, K. I. Razmnozhenie rastenii. Moscow, 1937.
Navashin, S. G. Izbr. trudy, vol. 1. Moscow-Leningrad, 1951.
Takhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
Sladkov, A. N. “Polovoi protsess i zhiznennye tsikly u rastenii.” Biologi-cheskie nauki, 1969, nos. 3–4.

A. N. SLADKOV

In animals and man. In animals and man, fertilization involves the union, or syngamy, of two gametes of different sexes—sperm and eggs. The significance of fertilization is twofold: (1) the contact of a sperm cell with an egg disinhibits the egg and triggers its development; (2) karyogamy—the union of haploid nuclei of sperm and egg—leads to the development of a diploid synkaryon in which the paternal and maternal hereditary material is combined. Genetic variety arises as a result of the combination of this material. The various forms of the resultant offspring serve as material for natural selection and the evolution of the species.

The indispensable prerequisite for fertilization is the halving of the number of chromosomes; this occurs during meiotic, or maturation, divisions. These divisions in male gametes always occur before the formation of sperm, while the correlation between fertilization and the meiotic divisions of the egg varies among animals. Sperm can penetrate into the egg before the start of meiosis; this is the case in sponges, in certain worms, in mollusks, and in dogs, foxes, and horses. Penetration of the egg occurs during the metaphase stage of the first meiotic division in mollusks, insects, ascidians, and certain worms. In the lancelet and in many vertebrates, the egg is penetrated during metaphase of the second division, while in coelenterates and sea urchins, after the completion of meiosis.

Syngamy usually occurs as a result of the swimming movements of the male gametes after the gametes are deposited into the water or introduced into the genital tract of the female. The meeting of the gametes is facilitated by gamones, which are produced by the egg to accelerate and prolong the movement of the sperm. Other substances released by the egg promote the aggregation of sperm around the egg. The appearance of such aggregates in certain fishes and in hydrozoan polyps of the genus Campanularia is sometimes considered to be the result of chemotaxis of the kind observed during the fertilization of mosses and ferns. However, the existence of directed movements in animal sperm has not been demonstrated. Animal sperm do not show any specific direction in their movements and come into contact with an egg as a result of accidental collision. The aggregates are probably a result of a mechanism that entraps those sperm that accidentally approach the egg.

The mature egg is surrounded by the egg envelope. In some animals, the envelopes are equipped with micropyles, openings that allow the sperm to enter. Most animals, however, do not have micropyles, and the sperm must penetrate the envelope in order to reach the surface of the oöplasm. The penetration is accomplished by means of a special organoid of the sperm, the acrosome. After the tip of the head of the sperm touches the egg envelope, the acrosome reacts by opening and releasing the contents of the acrosomal vesicle. The enzymes contained in the vesicle dissolve the egg envelope. At the site where the acrosome opens, the acrosomal membrane unites with the membrane of the sperm. The membrane at the base of the acrosome curves outward to form one or more processes, which are filled with subacrosomal matter arranged between the acrosome and the nucleus; the processes lengthen and are transformed into acrosomal filaments, or tubules. These filaments vary in length from 1 to 90 μ, depending on the thickness of the envelope and membrane that the sperm has to penetrate. The acrosomal filament passes through the dissolved portion of the egg envelope and comes into contact and unites with the plasma membrane of the egg. In animals whose sperm penetrate the egg through a micropyle—insects, cephalopods, fishes of the family Acipenseridae, and teleosts—the acrosome loses its importance in the fertilization process and is sometimes reduced or disappears completely (as in certain stone flies and teleosts).

In mammals, the ovulated egg is surrounded not only by an envelope but also by the several layers of follicular cells that form the corona radiata. In horses, cows, and sheep, the follicular cells scatter soon after ovulation, and the sperm easily reach the surface of the egg envelope. In most mammals, the cells of the corona radiata survive for several hours after ovulation; in order to penetrate through this barrier, the sperm release the enzyme hyaluronidase, which dissolves the substance that binds the follicular cells together. Hyaluronidase, like the enzyme that dissolves the egg envelope, is present in the acrosome. Immediately after ejaculation, sperm are incapable of releasing these enzymes. The ability to do so is imparted by the contents of the female genital tract, which cause the sperm to undergo certain physiological changes in a process called capacitation.

As soon as the plasma membranes of the gametes fuse at the point of contact between the acrosomal filament and the surface of the oöplasm, the egg and the sperm become a single cell called the zygote. The first signs of egg activation quickly appear—the cortical reaction and the concentration of the oöplasm at the point of contact with the acrosomal filament of the sperm, which results in the formation of the acrosomal tubule. The oöplasm flows into the acrosomal tubule and surrounds the nucleus, centriole, and mitochondria of the sperm and sometimes the sperm’s flagellum, drawing these structures deep into the egg. The plasma membrane of the sperm remains on the surface and becomes fixed within the membrane of the egg. As a result, the surface membrane of the zygote has a mosaic structure.

Penetrating the oöplasm, the head of the sperm cell turns 180°, and a sperm-aster forms near the tip. The head steadily swells and is transformed into a bladder-like male pronucleus, which appears to be led by the moving sperm-aster. The male pronucleus approaches the female nucleus while the sperm-aster divides in half. The two halves participate in the formation of the spindle for the first cleavage division. In coelenterates, flat-worms, and sea urchins, the pronuclei fuse into a single zygote nucleus. In some nematodes and in mollusks, crustaceans, fish, and amphibians, the pronuclei remain in close contact for a long time but do not fuse, and the union of paternal and maternal nuclear material does not occur until metaphase of the first cleavage division. The metabolic rate increases in the egg during fertilization. Other changes include an increase in the permeability of the plasma membrane and the induction of protein synthesis.

Monospermy is the entry of a single sperm into the egg. In animals that exhibit external semination, monospermy is ensured by secretion of the contents of the cortical bodies; this process prevents the sperm from penetrating into the oöplasm. Among animals in which the sperm is introduced into the genital tract of the female, examples of monospermy and polyspermy—the entry of several sperm—can be found. However, as in monospermy, only one spermatic nucleus unites with the female pronucleus in polyspermy.

REFERENCES

Rothschild, N. M. Oplodotvorenie. Moscow, 1958. (Translated from English.)
Dorfman, V. A. Fiziko-khimicheskie osnovy oplodotvoreniia. Moscow, 1963.
Ginzburg, A. S. Oplodotvorenie u ryb i problema polispermii. Moscow, 1968.
Austin, C. R. Fertilization. Englewood Cliffs, N. J., 1965.
Fertilization, vols. 1–2. Edited by C. B. Metz and A. Monroy. New York-London, 1967–69.
Reproduction in Mammals. Edited by C. R. Austin and R. V. Short. Book 1: Germ Cells and Fertilization. London, 1972.

A. S. GINZBURG

fertilization

[′fərd·əl·ə′zā·shən]
(physiology)
The physicochemical processes involved in the union of the male and female gametes to form the zygote.
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