From Big Medical Encyclopedia

The OVUM — a female sex cell of animals and plants, edges as a result of fertilization or, more rare, by a parthenogenesis gives rise to a new organism. Surrounded with covers I. call also egg.

Predecessors I. primary sex cells (see Gametes) forming, as a rule, at the initial stages of an embryogenesis serve. At mammals they appear in an ectoderm at early stages of development of a germ (see), and then pass into an entoderm of a vitellicle. After laying of gonads these cells migrate in them.

At the majority of animals I. has rounded shape. Sizes Ya. vary over a wide range and depend on type of development of an organism. At the animals passing in development a stage of a free larva, I. rather small, in them not enough yolk collects, to-ry it is evenly disseminated through cytoplasm (gomoletsitalny, oligole-tsitalny, izoletsitalny I.). From such I. the larva capable to eat independently develops. At animals with direct development the sizes Ya. increase due to accumulation of a yolk in them. So, at fishes I. reach 10 — 70 mm and more (large sharks), at birds — 30 — 100 mm and more. In large I. the yolk can occupy the central part of cytoplasm (tsentroletsitalny I.) or to concentrate near a vegetative pole (telolecithal I.). In the course of evolution with emergence of placental mammals, including the person, the sizes Ya. again decrease (to 50 — 180 microns). Diameter I. the person is averaged by 88 — 91 microns. Seasonal polymorphism I occurs at a number of animals of the same look. E.g., nek-ry Crustacea postpone small I in the summer., capable to parthenogenetic development (see the Parthenogenesis), and in the fall — large, development to-rykh begins only after fertilization.

I. it is surrounded with covers, among to-rykh distinguish primary, secondary and tertiary. Carry the plasma membrane which is available in all zooblasts and plants to primary covers. Secondary covers are formed as a result of secretory activity of follicular cells (e.g., zona radiata of oviparous vertebrata or z. pellucida of mammals), and tertiary — as a result of secretion of cells of oviducts or skin glands. Under a plasma membrane there is a cortical layer. In ova of many animals in it the cortical granules containing glikozoamino-glycanes are located. Diameter of cortical granules I. the person is made by 0,3 — 0,5 microns. At activation I. contents of granules are allocated in space between a plasma membrane and z. pellucida (so-called cortical reaction) and cause agglutination spermiyev, blocking a polyspermia (see &Oplootvoreniye).

Kernel I. has a haploid set of chromosomes (see). In cytoplasm Ya. organoids — mitochondrions (see), Golgi's complex (see Golgi a complex), ribosomes (see), and also lipidic inclusions, granules of a glycogen and vitelline granules contain. In it is available a reserve of various proteins, including histones, a tuba-linov, actin, RNA polymerases and DNA polymerases. Besides, in cytoplasm Ya. mammals, including the person, multivezi-kulyarny bodies contain, to-rye carry to secondary lysosomes (see).

Created I. has polarity — a certain arrangement of structures and organoids along the main axis, trailer points a cut correspond to animalny and vegetative poles. In addition, cytoplasm Ya. it is characterized by regional distinctions (an ooplazmatichesky segregation), thanking the Crimea primary differentiation of cells in development is carried out: affiliated kernels I., getting during crushing to different sites of cytoplasm, under the influence of the specific factors operating on a genome give rise to different cellular lines, in particular the line of primary sex cells.

The follicular type of an oogenesis is characteristic of the majority of animals (see), at Krom somatic cells of a mesodermal origin form I around. epithelial layer. For the first time the follicle of mammals was described in 17 century by the Dutch scientist de R. de Graaf, to-ry the domestic scientist took it for Ya. Lish in 1827 To. M. The rem was found at mammals and the person in a wall of a mature follicle true by me. The mature follicle (gra-af a bubble) has complex structure: the multilayer follicular epithelium is surrounded connective tissue teky, containing blood vessels. Release I. (see the Ovulation) comes from follicular covers at a stage of metaphase of the second meiotic division. Women during each menstrual cycle (see) in a bast layer of an ovary have several growing follicles, however one ovulates, as a rule, I. On ovulating I. a part of the follicular cells forming around it a so-called radiate crown (corona radiata) remains. After escaping of a follicle I. it is taken fimbrias of a uterine tube and begins to move ahead on a pipe towards a uterus. In a uterine tube at a meeting I. to a spermatozoon there is a fertilization (see). If it did not come, I. perishes in 5 — 6 days after an ovulation.

Necessary condition of development normal I. preservation in not changed type of its genetic material concluded in a kernel is capable to give rise to a new organism. However in the course of a gametogenesis disturbances in structure of chromosomes (aberation chromosomes) or change of their number — an aneuploidy are possible (see of an ametopatiya).

Frustration of hormonal regulation in an organism or the damaging action of environmental factors can be the reasons of these disturbances. The majority of the germs developing from abnormal I., perishes at different stages of pregnancy (see the Misbirth).

Bibliography: Ayzenshtadtt. B. Tsitologiya of an oogenesis, M., 1984;

Volkova O. V. Functional morphology of female reproductive system, page 43, M., 1983; D y and A. P. N and B and r and N about in V. S. Oogenez of mammals, in book: Sovr.

probl. an oogenesis, under the editorship of M. S. Mickiewicz, page 200, M., 1977; Nikitina. And., etc., Follicular oocytes of the person, Ontogenesis, t. 13, No. 2, page 123, 1982; Ham A. and To about r m and to D. Gistologiya, the lane with English, t. 5, M., 1983; The vertebrate ovary, comparative biology and evolution, ed. by R. E. Jones, N. Y., 1978. T. B. Ayzenshtadt.