ovum(redirected from primordial ovum)
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ovum(ō`vəm), 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 spermsperm
, 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,
..... Click the link for more information. . The study of the ovum is included in the science of embryology. The development of a new individual from an unfertilized ovum is called parthenogenesisparthenogenesis
[Gr.,=virgin birth], in biology, a form of reproduction in which the ovum develops into a new individual without fertilization. Natural parthenogenesis has been observed in many lower animals (it is characteristic of the rotifers), especially insects, e.g.
..... Click the link for more information. .
In higher animals the ovum differs from the sperm in that it is larger and is nonmotile, a smooth sphere or oval lacking the flagellum of the sperm. Like that of the sperm, its nucleus contains the chromosomes, which bear the hereditary material of the parent. A gamete, ovum or sperm, contains half the number of chromosomes found in the body cells of the parent, i.e., the gamete is haploid. In animals, ova contain stored food called the yolk, the amount of which varies in different species, depending on the length of time required for the embryo to become self-sufficient in obtaining nourishment outside the egg.
The term ovum is usually restricted to the single female sex cell, but the term egg, in its common use to indicate a bird's egg, refers to a more complex structure, only part of which is produced in the ovary of the bird. The ovum of such an egg is a cell swollen with yolk material. The rest of the egg—e.g., the jelly mass surrounding amphibian eggs, and the shell, membranes, and egg white, or albumen, of bird eggs—is not cellular and is secreted around the ovum as it passes down the oviduct.
Ova are produced in the ovaryovary,
ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone, which control the development of the sexual organs and the secondary sexual characteristics.
..... Click the link for more information. of the female; they are formed from reproductive cells (called primordial germ cells) in a process called oogenesis. In this maturation process a germ cell builds up its food supply and then undergoes a series of cell divisions (called meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
..... Click the link for more information. ), by which the number of chromosomes in the mature ovum is reduced by half. In oogenesis in animals only one of the four cells formed by meiotic division is functional. In this ovum all the yolk from the original cell is collected; the three other, yolkless, cells are called polar bodies and never develop further. Maturation also occurs in the formation of sperm (spermatogenesis), but in spermatogenesis, in contrast to oogenesis, all four of the cells formed by meiotic division are functional.
Fertilization and Maturation
The union of mature sperm and ovum, each bearing half the normal number of chromosomes, results in a single cell (the zygote) with a full number of chromosomes. The zygote undergoes a series of cell divisions (see mitosismitosis
, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
..... Click the link for more information. ) producing a multicellular 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.
..... Click the link for more information. and finally a mature individual. In all sexually reproducing animals the production and maturation of the ovum, its fertilizationfertilization,
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.
..... Click the link for more information. , and its early embryonic development are essentially identical.
In plants that reproduce sexually the pattern is similar to that of animals. In the mosses and ferns, however, the egg cells are formed in special organs called archegonia and are fertilized by sperm that are commonly flagellated and motile like those of animals. In flowering plants the ovary is situated in the base of the pistilpistil
, one of the four basic parts of a flower, the central structure around which are arranged the stamens, the petals, and the sepals. The pistil is usually called the female reproductive organ of a flowering plant, although the actual reproductive structures are microscopic.
..... Click the link for more information. of the flower. After fertilization by a sperm contained in a pollen grain, the zygote develops into the embryo, contained in the seed.
The egg or female sex cell. Strictly speaking, the term refers to this cell when it is ready for fertilization, but it is often applied to earlier or later stages. Confusion is avoided by using qualifying adjectives such as immature, ripe, mature, fertilized, or developing ova. The mature ova are generally spheroidal and large. The number of ova produced at one time varies in different animals, from millions in many marine animals that spawn into the surrounding sea water to about a dozen or less in mammals in which adaptations for internal nourishment of the developing embryo and care of the young are highly developed.
In the ovary the immature ovum is associated with follicle cells through which it receives material for growth. In mammals, as the egg matures, these cells arrange themselves into a structure known as the Graafian, or vesicular, follicle, consisting of a large fluid-filled cavity into which the ovum, surrounded by several layers of cells, projects from the layer of follicle cells that constitutes the inner wall (see illustration). The fluid contains estrogenic female sex hormone secreted by cells in an intermediate layer of the follicular wall.
Yolk, or deutoplasm, is essentially a food reserve in the form of small spherules, present to a greater or lesser extent in all eggs. It accounts largely for the differences in size of eggs. Eggs are classified according to the distribution of yolk. In the isolecithal type there is a nearly uniform distribution through the cytoplasm, as in most small eggs. The yolk in telolecithal eggs is increasingly concentrated toward one pole, as in the large eggs of fish, amphibians, reptiles, and birds. Centrolecithal, or centrally located, yolk occurs in eggs of insects and cephalopod mollusks. See Gametogenesis, Oogenesis