From Big Medical Encyclopedia

CRUSHING OF EGG — the initial stages of development of a germ which are directly following fertilization of egg and representing a consecutive number of its mitotic divisions on special type of cells called by blastomeres. Blastomeres only share, but do not grow, with each division decreasing in sizes.

At the most various representatives of fauna communication between D.'s processes I is noted. and later stages of an ontogeny. It can be expressed in so-called organoobrazuyushchy sites of plasma (see. Organogenesis ), or in communication between the planes D. I. and the plane of symmetry, or in early identification, at the first stages of D. I., separate blastomeres or their groups from which certain laying of an organism develops. D.'s process I. comes to an end with education blastulas (see) after what process begins gastrulations (see).

Fig. 1. The diagrammatic representation of crushing of egg (and and — radial; in — e — spiral): and — an eight-cellular stage; — a sixteen-cellular stage; in — transition from four to an eight-cellular stage by the dextrorotatory division caused by change of an inclination of an axis of nuclear spindles in relation to the equator of egg to the right; — an eight-cellular stage; d — transition from eight-cellular to a sixteen-cellular stage by the left-handed division caused by change of an inclination of an axis of nuclear spindles in relation to the equator of egg to the left; e — a sixteen-cellular stage. In positions in and d the provision of nuclear spindles of mitotic division is specified; in positions of and e shooters specified the cells formed as a result of division; 1 — a spindle of mitotic division; 2 — a centriole.

D.'s character I. considerably differs at representatives of different groups of animals. Distinguish radial, spiral (fig. 1), bilateral D. I., and also full (fissio in toto) and incomplete (fissio ex parte). At a total cleavage all cell shares, at incomplete — its only one part. In turn, the total cleavage can be equal (fissio equalis) and unequal (fissio inequalis), and incomplete — superficial (fissio superficialis) and diskoidalny (fissio discoidalis). Allocate, besides, two-symmetric and anarchical crushing. Depending on time and an order of cell fission crushing can be synchronous and asynchronous. At synchronous D. I. all blastomeres share at the same time therefore stages 2, 4, 8, 16, 32 etc. of cells appear. At asynchronous D. I. blastomeres share not at the same time, and alternately (e.g., from two cells only one shares, it turns out three) in this connection there are stages 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 cells etc.

The radial cleavage (fissio radialis) inherent a nek-eye to the lowest vertebrata, an erinaceous, sponges, is in crude form characterized by isocytic and simultaneous (synchronous) divisions. Nuclear spindles during the first two divisions are located perpendicularly to morfol, axes of egg, in the third division they are located parallel to it, and further there is an alternation of the perpendicular and parallel directions of axes of spindles of mitoses to morfol, axes of egg.

Spiral crushing (fissio spiralis) observed in eggs of annlides, nemertin of mollusks, is characterized by alternation of the right-and left-handed divisions caused by alternate change of an inclination of an axis of nuclear spindles in relation to the equator of egg to the right or to the left.

The bilateral cleavage (fissio bilateralis) is observed at astsidiya, roundworms, etc. The symmetric arrangement of blastomeres in relation to the plane passing through the plane of symmetry of a germ is characteristic of this cleavage pattern.

The two-symmetric cleavage pattern of egg occurs only in one group of animals — at dogstails and is characterized by two planes of symmetry which come to light already on early (four-cellular) reduction stages.

The anarchical cleavage pattern of egg was for the first time described by I. I. Mechnikov at a hydrojellyfish of Oceania armata. During it the small group of blastomeres of various size forms, in a cut their any natural arrangement is absent. By movement of cells the stage of a blastula forms.

The superficial cleavage (fissio superficialis) occurs at arthropods (cancroid, arachnoid, insects, etc.). It is characterized by the fact that the kernel of an oosperm is exposed to consecutive divisions while the body of an ovum does not share. Only after in the central part of egg there is a large number of kernels, they begin to move together with the sites of cytoplasm surrounding them to a surface of egg then the surface layer of cytoplasm of the last breaks into separate cells.

The discoidal cleavage (fissio discoidalis) occurs at animals with large eggs (scorpions, bony fishes, legless amphibians, oviparous mammals) and with huge eggs (akulovy fishes, cephalopod mollusks, reptiles, birds). It is characterized by the fact that the disk of cytoplasm which is on an animalny pole of egg is divided into cells only small (in proportion to the sizes of egg) and containing a kernel. During evolution the discoidal cleavage arose independently at three types of animals: at mollusks, arthropod and vertebral. Among vertebrata it appeared at akulovy fishes, at some amphibians and at reptiles, from the last this type is inherited by birds and oviparous mammals.

The person has a crushing of egg full, asynchronous. As well as in other cases of a total cleavage, it at first leads to formation of the dense complex of blastomeres reminding a fruit of a mulberry tree (Morus) in a form and therefore called by a morula. Early determination is characteristic of mammals and the person (identification at the first stages of D. I. separate blastomeres or their groups from which certain laying of an organism develops); already at early stages laying of the germ (a germinal node, or embrioblast) from provisional (temporary) body of food — a trophoblast stands apart. At late stages of D. I. are determined morfol, axes and the planes of symmetry. D.'s stages I. the person are for the first time described by Hertig, Rock, Adams and Malligen (A. T. Hertig, J. Rock, E. Adams, W. Mulligan, 1954).

From the very beginning of crushing there are two sorts of blastomeres: large (dark) and smaller (light).

In the first three days crushing happens in a uterine tube, and during this time the germ reaches a stage of 8 blastomeres (on average one division a day). Then during the next three days crushing of a germ in a uterus continues. At the same time it lies in a cavity of the uterus in «stand-at-ease» (not implanted) and eats secretory material of uterine glands.

Fig. 2. Oosperm of the person: 1 — a bicellular stage; 2 — a stage of 58 blastomeres (light sites — the beginning of formation of a puzyrkovidny germ — a blastocyste).

By 4 — 4,5 days after fertilization a puzyrkovidny germ — the blastocyste reaches a 58-cellular stage (fig. 2), in it a bookmark is outside well-marked trophoblast (see), and in the center there is an internal cellular weight (embrioblast). The period of stay in a uterus is characterized by big activity of division. By 5,5 days the germ reaches a stage of a hollow blastocyste. Such blastocyste, e.g., at a stage of 107 cells has the sizes of 153 X 115 microns in a live state is already deprived of egg covers and can be considered as a preimplantation stage. Embrioblast, being flattened, takes the form of a disk — a germinal guard; it corresponds actually to a blastula (see).

See also Germ , Fabric determination .

Bibliography: Bodemer Ch. Modern embryology, the lane with English, M., 1971, bibliogr.; Ivanov a-K and z and with O. M. Comparative embryology of backboneless animals, Novosibirsk, 1975; Knorre A. G. A short sketch of embryology of the person with elements of comparative, experimental and pathological embryology, L., 1967, bibliogr.; Sh m and d G. A. Embriologiya's t of animals, p.1, M., 19 51 * Analysis of development, ed. by B. H. Wil-lier a. o., p. 213, Philadelphia — L., 1955 bibliogr.; S i e w i n g R. Lehrbuch der vergleichenden Entwicklungsgeschichte der T i ere, Hamburg — B., 1969, bibliogr.; W olpert L. The mechanics and mechanism cleavage, Int. Rev. Cytol., v. 10 p. 163, 1960, bibliogr.

G. A. Schmidt