ONTOGENESIS

From Big Medical Encyclopedia

ONTOGENESIS (ontogenesis; Greek on, ontos real + genesis origin, an origin) — the process of an ontogeny of an organism representing set of the natural, interconnected, characterized by a certain temporary sequence morphological, physiological and metabolic transformations in an organism from the moment of its isolation as identity to death or the termination of existence in former quality (e.g., as a result of division of a unicell).

The term «ontogenesis» is entered in 1866 it. zoologist Gekkel (E. Haeckel) in connection with the formulation of the basic biogenetic law (see), according to Krom ontogenesis is the short and compressed repetition of phylogeny — process of historical development of separate types, classes, groups, families, childbirth, types of live organisms. According to modern views of O. is result not only phylogenesis (see), but also its basis, and phylogenesis appears as a historical number of ontogeneses.

The lake can be divided on embryonal (germinal, prenatal) and postembryonal (after and germinal, post-natal) the periods. The first covers a time term from fertilization till the birth (or before escaping of egg covers), the second — from the birth to death.

Duration of every period of O. is defined by features of organisms of this look, and also action of ecological factors. The amount of nutrients, a cut can use this species of animals for providing the initial stages of development, depends on duration of the embryonal period.

On the basis of set of properties of an organism at rather early stages of an ontogeny providing its adaptive relations with the environment allocate free larval, secondary larval and not larval types of development.

Free larval type of development — development with metamorphosis (see), or indirect development — - is characterized by existence in the life cycle of a special inserted stage — larval, on an extent a cut the organism differs from a puberal individual and has necessary devices for an independent way of life (see. Larvae ). Transition from larval to a puberal form is called metamorphosis. The larval type of development is characteristic of the animals laying eggs with rather trace amount of nutritious material.

At not larval type of development called also direct germs a long time develop under cover of egg covers, and the young organism leaves them, being capable to independent getting of food. At nek-ry types the juvenile after hatching receives food from parents (e.g., oviparous mammals). At not larval type of development the organism by the time of its exit from egg covers corresponds according to the main plan of a structure, a set of bodies and systems to a puberal form, differing from it only in the smaller sizes of a body, an underdevelopment (hl. obr. in the quantitative plan) nek-ry systems, including reproductive, etc.

This type of development is characteristic of the animals laying the eggs with a significant amount of a yolk concluded in special covers (a shell, a cocoon, etc.).

The lake of placental mammals (including and the person) has a number of features. Early stages O. proceed as pre-natal development, at Krom the oospore (germ) develops in a maternal organism and nutrients in the course of all embryogenesis come to a germ in the beginning directly from a uterus, and then through a placenta.

This type of development phylogenetic the youngest and in the best degree provides survival of an individual.

The pre-natal period of development of the person can be divided into actually embryonal and fetal periods. The first of them takes the first 7 weeks of development. From 8th week the fetal period which is characterized by growth, a further differentiation of bodies, formation of their functions and coming to an end with the birth begins.

The lake of the person begins with fertilisations (see). Fertilization at the person internal, is made in internal generative organs of the woman. Most often fertilization happens in a gleam of an ampoule of an oviduct (uterine tube). The period of a one-celled germ lasts at the person about days, beginning from the moment of insemination then the period of crushing (begins see. Crushing of egg ). Process of crushing is characterized by progressive reduction of the sizes of cells (blastomeres), increase of total quantity of DNA. In the course of crushing the compact complex of cells in a form reminding a fruit of a mulberry tree (morus) and called by a morula is formed (see. Germ ). At the same time blastomeres are located on a surface of a morula in one layer, and is central the lying cells form internal cellular weight. As a result already by the end of the first week of development the germ of the person represents the education divided into two parts: a surface layer of cells — trophoblast (see) and internal cellular weight — embrioblast. At this stage of development the germ of the person is called a blastocyste.

In the course of crushing there is a constant reorganization of cells of a germ of the person. The most important in this process is formation of ribosomes (with - m) that, apparently, is connected with early approach of specific proteinaceous sintez. The last is considered as feature of the period of crushing of mammals.

To transformations in a body of a germ at the end of the period of crushing (6 — the 7th day of development) there is an immersion of a germ in a mucous membrane of a uterus — implantation.

The following stage of prenatal O. substantially defining all further process is gastrulation (see), proceeding during the period from the 7th to the 19th day of pre-natal development.

Dynamics of process of a gastrulation can be divided into two phases (a phase of education two-layer and a phase of formation of a three-layered germ). At the same time each of layers of a germ (so-called germinal leaves) contains the cellular material which is determined in the direction of education absolutely certain though still wide, a circle of fabric derivatives (see. Germinal leaves ). So, the entoderm gives rise to an epithelium of enterodermalny type, an ectoderm — to fabrics of epidermal type and elements of a nervous system. Thus, as a part of each germinal leaf the determined embryonal rudiments stand apart diversely, the specific differentiation to-rykh is resulted by various fabric derivatives. In the course of a gastrulation there is also an emergence of an axial complex of rudiments.

In parallel there is a transformation of a trophoblast, formation of primary, and then secondary fibers of chorion and as a result formation of haemo chorial placentae (see). Early and powerful development of a trophoblast and extraembryonic mesoderm in mammals is caused by an insignificant reserve of nutrients in their ova that causes the necessity of transition to food from a maternal organism.

On the 4th week of development the germ of the person undergoes a number of the consecutive changes leading to isolation of a body of a germ from extraembryonic parts, to formation of a neurotubule, segmentation of a mesoderm and its differentiation on somites, nephrotomes and splanchnotomes, to emergence of germinal connecting fabric, or a mesenchyma. The mesenchyma will give afterwards rise to all types of connecting fabric and smooth muscular tissue.

During the subsequent period of prenatal O. there are processes organogenesis (see), having specific character for each body and leading finally to formation of the structures capable to provide viability of a fruit or the child at each stage of its development.

In O. of the person and animals there are so-called critical periods (by P. G. Svetlov's definition, 1978) which are characterized by special sensitivity of an organism to various influences from the environment. Such periods in development of the person are the period of implantation, placentation, the periods of a differentiation of this or that body, a neonatality, puberty. In these time terms manifestation of a dizontogenez — different emergence patol is the most probable. shifts in fetation.

In process of studying of patterns of O. numerous theories of an ontogeny appeared. Part of them, though exerted the defined impact on a research of patterns of O. in due time, kept only historical value. These are theories of W. Roux, A. Weiss-mana, R. Goldschmidt, H. Driesch. Other theories (from among keeping the value) are connected with an explanation of processes of an embryogenesis, the mechanisms and factors providing determination and differentiation of parts of a germ, and also development of a germ as whole.

According to G. Shpemann's theory (this theory was supported and made for its further development D.II. Filatov, i I much. P. Ivanov, G. V. Lopashov, Uoddipgton (S. of Wad-dington), J. Needham, Zh. Brashe, etc.), determination of development of parts of a germ and the arranged O.'s course in general are connected with consecutive inclusion of the so-called organizational centers, or organizers. Function of these organizers is performed by the structures arising in the course of an embryogenesis and rendering the inducing action on susceptible (competent) cellular material, inducing it to change in a certain direction. Data of numerous experiments opened a number of contradictions of the theory of the «organizational centers» connected by hl. obr. with underestimation of the fact of integration of the developing live system into a whole.

Awareness of the principle of integrity of an organism at any stage of its O., coordinate change of various rudiments induced to searches of factors of integration that led to emergence of the concept of «the morphogenetic field». Representation is their cornerstone that in the developing structure the separate forces or numbers of forces (the concept of Uoddington) causing orderliness and nonrandom result of formoobrazovatelny processes which are correctly distributed in space work. The similar concepts differing in many details were created by A. G. Gurvich, N. K. Koltsov and other researchers. The question of the material nature of the field is the least developed in them. In particular, N. K. Koltsov specified that potentials on poles of morphogenetic zero can have the electric, gravitational, mechanical, chemical nature. According to the theory physiological, or axial, gradients, offered by Chayld (S. M. of Child), intensity of oxidation-reduction metabolic processes is not identical in different parts of a body of a germ. Assume that sites with the highest activity exert impact on development of the neighboring sites, perhaps, coordinate formoobrazovatelny processes and correspond to zones of primary organizers (e.g., a dorsal lip a blastopore, to a genzenovsky small knot of primary strip). Also integrating role is attributed to these gradients.

Achievements of molecular biology and genetics furnished the clue to understanding physical. - chemical and tsitol, bases of mechanisms of a cellular differentiation and intercellular interactions. Lakes of an individual began to represent as implementation process of hereditary information in specific conditions of the environment. In process on the basis of the specified information the phenotype inherent to organisms of this look is reproduced. It is supposed that the amount of information at the initial moments of O. is less, than in the organism which reached adulthood. Additional information, according to Uoddington, arises because the specific basis of O. is made by system of natural and consecutive interactions of products of activity of genes — proteins, and also structures of the developing organism among themselves and the organism as whole with environmental factors. These representations, correctly reflecting the most essential party of processes of O., emphasize its system character, create premises for fruitful criticism of concepts preformizm (see) and epigenesis (see).

The problem O. of animals in the after and germinal period found reflection in theories of growth and maturing of an organism from the moment of the birth before puberty. In particular, M. M. Zavadovsky offered the theory indicating importance of interaction of bodies in post-natal development and a role of hemadens in this process. Also the theories of aging underlining a role of genetic factors or the rates of aging of environmental factors indicating a preferential role in definition are offered (see. Gerontology ).

O.'s theory connecting together on the basis of representations of thermodynamics processes of an oogenesis, embryonic and post-natal development, aging, regeneration, malignant growth was offered by A. I. Zotin. According to this theory of O. of the live organism which is an open circuit is considered in general as process of approach of energy level of an organism to more probable final stationary state, the corresponding transition live in lifeless and characterized by the smallest indicators of dispersion of energy. This process begins with the moment of fertilization and is possible thanks to the process of an oogenesis preceding it when future organism in the form of the ovum formed by a maternal organism due to its life activity gets energetically less probable, but such state, a cut does possible development of a new individual. It is possible to tell that during an oogenesis the ovum gains ability to provide O.'s process in general.

According to the considered representations, the initial stages of processes of regeneration, healing of wounds, malignant growth are provided with internal resources of an organism, transition of the last to energetically less probable state, but differing in the raised plastic opportunities.

Post-natal O.'s division into the age periods reflects change of morfofunktsionalny indicators of an organism in time after its birth. The existing options of such division are aimed at providing the solution of specific objectives of age physiology and medicine. Division of a piece of O. from the moment of the birth before achievement of a condition of puberty is most in details developed. Allocate the periods of a neonatality, chest age, the childhood and school age, pubertal and mature age (see. Age ), and also the periods of presenile and senile changes (see. Old age, aging ).

See also Histogenesis , Sistemogenez , Embryonic development .


Bibliography: Zavarzin A. A. Synthesis of DNA and kinetics of cell populations in ontogenesis of mammals, D., 1967, bibliogr.; Zotin A. I. Thermodynamic approach to problems of development, growth and aging, M., 1974; Knorre A. G. Embryonal histogenesis, JI., 1971; Koltsov N. K. The organization of a cell, M. — JI., 1936; Petten B. M. Embryology of the person, the lane with English, M., 1959; Svetlov P. G. Physiology (mechanics) of development, t. 1—2, JI., 1978; T about to and B. P N. General embryology, M., 1977; Uoddingtonk. A morphogenesis and genetics, the lane with English, M., 1964; it, the Main biological concepts, in book: On the way to teort. biol., the lane with English, under the editorship of B. of JI. Astaurova, t. 1, page 11, M., 1970; Shmalgauzen I. I. Problem of Darwinism, JI., 1969; Schmidt G. A. Types of an embryogenesis and their adaptive value, M., 1968; Yablokov A. V. and Yusufov A. G. Theory of evolution, M., 1976.


V. N. Yarygin, S. B. Tarabrin.

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