EMBRYONIC DEVELOPMENT

From Big Medical Encyclopedia

EMBRYONIC DEVELOPMENT

(Greek embryon a uterine fruit, a germ) — a stage of an ontogeny of an organism from the moment of fertilization before completion of basic processes of an organogenesis. At a number of animals this period proceeds before release from egg covers. As for mammals, embryologists to them E. rubles usually carry the period of all pre-natal development and divide it on germinal and fetal (fetalis) the periods. At various mammals of border between these periods fall on different time of development and are defined unequally. Sometimes instead of the concept «embryonal development» use the term «embryogenesis». In obstetrics under E. rubles mean a stage till the fetal period, duration to-rogo at the person corresponds to 8 weeks. During this period there are basic processes of an organogenesis. AA. the river at the person in the first 3 days after fertilization occurs in a uterine tube, then in a uterus.

On the basis morfol. criteria in E. rubles allocate several periods: the period of a one-celled germ, or zygote (see), the period of crushing of egg (see), the period of a gastrulation (see), the period of isolation of the main rudiments of bodies and fabrics, the period of an organogenesis (see) and a histogenesis (see). Bases of allocation of the periods E. rubles are put by K. M. Ber. The period of the zygote representing the one-celled germ formed as a result of merge of sex cells of parent organisms (see Fertilisation), at the person and many mammals lasts apprx. 1 days. In cytoplasm of a zygote at the same time there are active physical. - chemical processes, movement of organellas and inclusions, the plane of a bilateralism is defined.

The period of crushing of egg proceeds from division of a zygote into 2 cells (blastomere) before formation of a single-layer multicellular germ — a blastula (see). At the person it begins in 1 days after fertilization and 6 days last. At this time the germ moves ahead on a uterine tube and moves to a uterus. Crushing of a germ in a uterine tube happens to

speed of one division a day. Rate of crushing sharply increases in a cavity of the uterus; at the same time increase in number of cells is followed by the progressing reduction of their sizes. The person and viviparous mammals have a crushing of all material of a zygote (total cleavage), in the course to-rogo larger and dark blastomeres — embrioblast and small, light blastomeres, the acquiring cells of an embrioblast — a trophoblast stand apart (see). In the split-up germ the amount of liquid appears and increases, it takes a form of a bubble (blastocyst). Cells of an embrioblast concentrate on one pole of a blast of a cyst, cells of a trophoblast make its walls. By the end of the 6th days, by the time of implantation, the germ represents the organism consisting of several honeycombs cells, the vast majority to-rykh makes a trophoblast. The trophoblast is early differentiated in specialized epithelial fabric and is a source of formation of an epithelial cover of fibers of chorion; the extraembryonic mesoderm created from the cells moved from an embrioblast (see) forms their connective tissue basis.

Embrioblast it is flattened, taking the form of a disk and forming a germinal guard.

The period of a gastrulation includes transformation of a single-layer germ into three-layered. At the highest vertebrata and the person embrioblast by a delamination (splitting) at first turns into the two-layer education consisting of an outside germinal leaf — the epiblast containing elements of an ectoderm (see) both mesoderm, and internal germinal leaf — a hypoblast, or an entoderm (see). Formation of a two-layer germ happens on the 2nd week of development of a germ (the 1st phase of a gastrulation). At vertebrata on

the 3rd week of development from an epiblast the third germinal leaf — a mesoderm (the 2nd phase of a gastrulation) forms. A result of a gastrulation is formation of an axial complex of rudiments: a medullary plate, vpos

the ledstviya which is becoming isolated in a neurotubule, chords and mesoderms, edge from 4th week it is actively dismembered on somites (see). In development between embryonal bookmarks there are contacts and interaction that causes

determination of their cellular material.

An ectoderm, a mesoderm, an entoderm — sources of development of all fabrics in the course of ontogenesis (see) — represent the unspecialized cells with basphilic cytoplasm, large kernels deprived of specialized organellas, with high mitotic activity, active growth, having ability to purposeful movements. As a part of germinal leaves there are diverse rudiments of bodies and fabrics, further development to-rykh proceeds with different intensity and comes to an end in different terms, even without being limited to the period of pre-natal development. In a little differentiated cellular material of embryonal rudiments there is a reproduction of cells, their specialization (differentiation), growth, space movements of separate cells and cellular masses, their close interaction, change of biochemical structure. At the beginning of development there are distinctions in sizes and a form of cells of different rudiments, then gradually there are qualitative changes of structures, features of a metabolism. In cells of different rudiments unequal organellas and specific inclusions form, extracellular derivatives are formed (e.g., intercellular substance). Differentiations result diverse rudiments, specialized fabrics and bodies and, respectively, their functional distinctions. In parallel with process of a differentiation process of integration develops and amplifies (association of parts of a germ in one harmoniously developing whole), degree a cut increases in process of development of a germ. The vozaimodeystviye of parts of a germ is the cornerstone of integration, a cut with continuation of development becomes more and more perfect. At first integration is expressed in interaction of cells, in the subsequent the integrating function is performed by nervous and endocrine systems. At the same time at each stage of development these or those components of process of a histogenesis (reproduction, growth, migration of cells, intercellular and interwoven interactions — correlations, dying off of cells) can have preferential value.

Age of a germ in the period of E. the river is estimated in hours in the beginning, then in days and weeks. From the moment of the beginning of segmentation of a mesoderm (20 — the 21st days of development) the age of a germ is determined by quantity of somites, during isolation of a germ from provisional bodies by measurement of length of his body from a darkness to a tailbone, and with development of extremities — from a darkness to heels (see the Germ, the Fruit).

In the period of E. river in connection with high metabolic rate and high sensitivity of a germ to various disturbing factors (to pharmaceuticals, ionizing radiation, bacterial toxins, etc.) there can be disturbances in development (dizembriogenez), leading to developing of diseases, malformations and even to death of an embryo. Extent of display of various diseases (hereditary and not hereditary) is also closely connected with conditions, in to-rykh there is E. river. Diseases of mother, the use by it during pregnancy of a number of medicines, unfavorable conditions of the environment in this period can have serious effects for a germ and be shown both in the post-natal period, and at an adult organism; under favorable conditions pathology can not be shown. In the course of E. rubles are available the temporary intervals matching the most responsible morphogenetic processes when the germ is especially sensitive to the damaging influences — so-called critical periods (see. Antenatal period). It is the period of implantation corresponding to the end of the 1st and the beginning of the 2nd week after conception and the period of formation of a placenta corresponding 3 — the 7th to weeks of development. Damage of germs, especially at this time, can cause lag it in development, the lowered body resistance, abortion. At damage of separate rudiments local anomalies of development of bodies appear (a crevice of a lip, lack of an extremity, etc.). Owing to asynchrony of a differentiation for different bodies there are, inherent only to them temporary periods when they are the most sensitive to the damaging agents. The earlier the influence of adverse factors leading to deviations from normal development is noted the in the bigger number of bodies and fabrics they can be shown (see. Antenatal pathology, Hereditary diseases, Malformations, Heart diseases inborn, Embryopathies, Enzymopathies).

Bibliography: See bibliogr. to St. Germ, Embryology. O. V. Volkova.

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