From Big Medical Encyclopedia

GERMINAL LEAVES (synonym germinal layers) — the layers of cells of a germ of all metazoans (Metazoa) and the person which are standing apart in the course of a gastrulation from whom at the subsequent development there are fabrics having a certain specificity.

Fig. 1. The diagrammatic representation of formation of germinal leaves at early stages of embryonic development of the person: and — a single-layer germ (discoblastula) at the age of 5,5 days; the germ has an appearance of a hollow bubble — a blastocyste which wall is formed by one layer of cells of the trophoblast (1) participating later in formation of chorion; actually the germ is presented diskovidno by the flattened layer of cells — the germinal guard (2) which is not stratified on germinal leaves yet; — a two-layer germ of the person at the age of 7 — 8 days (the first phase of a gastrulation): 1 — an outside germinal leaf (primary ectoderm); 2 — an internal germinal leaf (primary entoderm); in — a fragment of a three-layered germ of the person at the age of 17 — 18 days (the second phase of a gastrulation): 1 — an outside germinal leaf, or an ectoderm; 2 — elements of an average germinal leaf, or a mesoderm; 3 — an internal germinal leaf, or an entoderm; 4 — primary strip.

Germs of all metazoans at early stages of development pass a stage of isolation of two (at sponges and coelenterates) or at first two, and then three (at all higher types — Bilateralia) cellular layers (fig. 1): outside, or ectoderms (see), internal, or entoderm (see), and average, or mesoderms (see). Cells 3. l. are usually densely close, forming layer, and are quite often located epiteliopodobno. However, unlike epiteliyev — tightly specialized fabrics, 3. l. consist of the low-differentiated cells with wide potentialities of development. Interposition and form of cells 3. l. in the course of an embryogenesis can change (e.g., an epithelial-like mesoderm, being loosened, can form a mesenchyma).

K. F. Wolf (1759) for the first time noted that the germinal disk of egg at a nasizhivaniye is stratified on leaves, and showed (1769) that the internal leaf gives rise to intestines. X. I. Pander (1817) tracked participation of each of three leaves in creation of bodies of a chicken germ in more detail. K. M. Ber (1828, 1837) showed a role 3. l. in formation of bodies at germs of all vertebrata. In works of these biologists emergence 3. l. as homogeneous material, from to-rogo diverse bodies and fabrics novoobrazutsya, served as the actual justification of the theory epigenesis (see) — doctrines about germinal development as the process which is carried out by consecutive new growths. R. Remak (1855) connected this doctrine with the put-forward T. Sh bathing (1838, 1839) the cellular theory. Having for the first time studied 3. l. vertebrata it is microscopic, R. Remak showed features of a structure and interposition of cells of each of them and tracked their further transformations at development of various fabrics.

A. O. Kowalewski (1865, 1871) showed universal prevalence and a homology 3. l. in development of all metazoans, having given these important embriol, confirmation of the idea of Ch. Darvin on unity of an origin of fauna. Gekkel (E. Haeckel, from 1866 to 1874, the theory of a gastrea) and I. I. Swordsmen (1886, the theory of a fagotsitella) for the first time regarded education 3. l. in an embryogenesis of modern animals as reconciliation (reproduction) of formation of primary bodies at the most ancient ancestors of Metazoa also connected the theory 3. h.p. problem of an origin of metazoans. According to I. I. Mechnikov, the inner layer of a body of the first Metazoa resulted from immigration of the separate cells making hollow colony from a surface inside for implementation of function of digestion.

Theory 3. l. — the widest of generalizations of embryology explaining universal prevalence, similarity of gistogenetichesky potentialities 3. l. and their homology at representatives of all Metazoa groups unity of an origin (relationship) of types of fauna and reconciliation in ontogenesis of those primary cellular layers of which the body of the general ancestral form of all Metazoa consisted.

Division of functions between cells and their space division into two layers at early stages of evolutionary development was unstable. Later on this basis there were two main layers of a body: outside — the kinoblast specialized on motive function (blinking of cilia), rekapituliruyemy at germs of modern animals an ectoderm, and internal — fagotsitoblast, (phagocytosis) specialized on digestion and protection, rekapituliruyemy two layers — an entoderm and a mesoderm. During further evolution 3. l. lost value of primary bodies of a mature animal, but kept the value in a chain of morphogenetic processes at development of a germ.

Fig. 2. The scheme of formation of fabric derivatives from three germinal leaves at a germ of the highest vertebrata (cross section; names of the formed fabric derivatives are put in brackets after the name of the corresponding germinal leaf or a rudiment): 1 — a skin ectoderm (epidermis); 2 — a ganglious plate (sensitive and vegetative neurons, a peripheral neuroglia, chromatophores); 3 — a neurotubule (neurons, a neuroglia); 4 — a dermatome (a connective tissue basis of skin); 5 — a myotome (skeletal muscular tissue); 6 — a sclerotome (cartilaginous and bone fabrics); 7 — a nephrotome (a renal epithelium); 8 — a parietal layer of a splanchnotome (mesothelium); 9 — an extraembryonic ectoderm (an epithelium of amnion); 10 — a visceral layer of a splanchnotome (a mesothelium, tissue of a cardiac muscle); 11 — an intestinal entoderm (an intestinal epithelium); 12 — a mesenchyma (connecting fabric, blood, smooth muscle fabric); 13 — an endothelium of an aorta; 14 — a chord; 15 — whole; 16 — a vitelline entoderm (an epithelium of a vitellicle).

Works K. M. Bera, R. Remak (for vertebrata), A. O. Kowalewski, I. I. Mechnikov (for invertebrates) and many others it was established that at the most various animals of change 3. l. during normal embryonic development are generally identical (fig. 2) though in process of increase in level of the organization of animals during evolution fabric derivatives 3. l. become more various.

O. Gertvig, De G. R. De Beer, Van-Beneden (E. van Beneden) and some other embryologists in 19 and especially in 20 century put purely topographical contents in the concept «germinal leaves», denying their gistogenetichesky specificity. Such tendencies — a direct consequence of the wrong, subjective interpretation of achievements of the experimental embryology which showed great regulatory opportunities for a germ and its parts, including 3. l., and a consequence of underestimation of the historical principle in biology, in particular phylogenetic conditionality of properties 3. l., including their gistogenetichesky potentialities.

Given to experimental histology, and also (1946) genetic system of fabrics developed by N. G. Hlopin showed that the origin of each of types of fabrics is connected at vertebrata and the person with any one of 3. l. or even sites of a leaf. So, an epithelium of skin type develop only from a skin ectoderm and a prechordal plate, an epithelium of intestinal type — only from an entoderm, an epithelium of renal and coelomic type — only from a mesoderm (nephrotomes and splanchnotomes) etc. Each of the called types of epithelial fabrics differs from others resistant morfol., funkts, and cytochemical features is normal also characteristic behavior in experimental conditions and pathology: the special type finds kambialnost (i.e. distribution and recovery potentialities of the small differentiated reserve cells), regenerations, the tumoral growth, growth in culture etc. The same treats deep differences of muscular tissue of a mesodermal and ectodermal (neyralny) origin. Than the organization of an animal is higher, specificity (determinancy), fixedness of properties of fabric derivatives of each leaf, as a rule, is more sharply expressed to those and the more so these properties are with firmness shown under any experimental conditions.

This conclusion has a direct bearing most on a number of important problems of theoretical and applied medicine. At the person, as well as at other highest vertebrata, already during embryonic development fabric derivatives various 3. l. (including also cambial cells of these fabrics) get so strong determination that under all studied conditions of fabric defined (e.g., entodermalny) origins lose ability to arise (or to be recovered after damage, i.e. to regenerate) at the expense of tissues of other (ekto-or mesodermal) origins. Fabric specificity should be considered also at transplantations (see). Even during the receiving cellular clones in culture from the only parent cell and in the conditions of tripsinizirovanny cultures of a cell of any certain fabric (vertebrata and the person) are capable to give rise to elements only the same, but not other fabric.

Fabric specificity of derivatives various 3. l. leaves a mark and on properties of the tumors developing from these fabrics. The essential differences of cancer tumors of an epidermal and enterodermalny origin, difference of cancer from sarcoma etc. shown, in particular, by N. G. Hlopin (1947), A are connected with it. D. Timofeevsky (1947), Kaudri (E. Cowdry, 1958). Early the arising gistogenetichesky specificity 3. l. (and embryonal rudiments, derivative of them) it is possible to explain also an origin of teratomas, gamarty and some other dizembriogenetichesky tumors (see. Dizontogenetichesky tumors ). Without such specificity the cellular material which got to an unusual environment could assimilate to this environment, and there would be no paradoxical combinations of fabric and organ structures taking place in such cases.

In the course of evolution change of space relationship 3 is possible. l. and gistogenetichesky potentialities of their certain sites. So, at vertebrata the prechordal plate which is formally a part internal 3. l., all possesses others, than other internal 3. l., and besides wide potentialities: at the expense of it the epithelial vystilka (skin, but not intestinal type) of a front gut and respiratory tracts, the mesoderm of the first two segments going for formation of oculomotor muscles, etc. are formed. In phylogenesis of vertebrata the entodermalny epithelium of the front end of the intestinal channel is forced out ectodermal so the border between them in a digestive tube moved in the caudal direction. It is confirmed also experimental gistol. analysis. Pearlworts, and on a nek-eye to data and at insects, had a full replacement in comparison with other invertebrates of an entodermalny vystilka of all intestinal tube vystilky an ectodermal origin. These and similar facts find an explanation in the theory of a metorizis, i.e. phylogenetic shift of borders between 3. l. or separate rudiments and respectively between their fabric derivatives (see. Metorizis ). This theory — important addition to the classical theory 3. l.

Mammals and the person in a neck of uterus where the epithelium relating to renal coelomic fabric type and a vulval epithelium (skin type) border, have regular reversible shifts of border caused by cyclic changes of hormonal balance between both diverse epiteliya. Change of character of an epithelial vystilka in this area, and also emergence in a wall of a uterus of cancer tumors of skin type are quite often mistakenly treated as the phenomena of a metaplasia, actually it is result of shift of borders between fabric derivatives of two 3. l. — ectoderms and mesoderms (metoristichesky modulation).

Thus, theory 3. l. it is necessary to understand taking into account evolutionary dynamics. See also Germ , Embryonic development .

Bibliography: Ivanov A. V. Origin of metazoans, M., 1968; Ivanov a-Kazas O. M. Comparative embryology of backboneless animals, M., 1977; Knorre A. G. Embryonal histogenesis, L., 1971, bibliogr.; Kowalewski y A.O. Chosen works, L., 1951; Hlopin N. G. All-biological and experimental fundamentals of histology, L., 1946, bibliogr.; Siewing R. Lehrbuch der vergielchen-den Entwicklungsgeschichte der Tiere, Hamburg — B., 1969.

A. G. Knorre.