ECTODERM

From Big Medical Encyclopedia

ECTODERM [ectoderma (LNE); Greek ektos out of, outside + derma skin] — the outside germinal leaf of a body of a germ which is formed in the course of a gastrulation.

At germs of all metazoans primary E., or the epiblast, arises in the first phase gastrulations (see). At the highest vertebrata, including at the person, actually E., or secondary E., there is that part of an epiblast, edges remains outside after isolation of a mesoderm and release of cellular material, originative to a chord in the second phase of a gastrulation.

Secondary E. already during a gastrulation it is subdivided into three parts: the neuroectoderm (or a neyralny rudiment) presented by a medullary plate skin E. and in not germinal E.

A medullary plate, caving in, forms a nervous fillet. Edges of a fillet (neural folds) are closed, creating a neurotubule, edges is a rudiment back and a brain. At short circuit of a neurotubule the so-called nervous crest is formed, to-ry on each side from a neurotubule creates two ganglious plates. Fabric derivatives of a nervous crest are neurons of sensitive and vegetative (sympathetic) nodes, all peripheral neuroglia, chromatophores (pigment cells), chromaffin tissue of adrenal glands. From cells of a nervous crest there is also a part mesenchymas (see) — an ektomezenkhima. Also small isolated islands of a neyralny rudiment which are localized in skin E are a part of a neuroectoderm. heads (so-called placode). Rudiments of sensitive nodes craniocereberal concern to them (cranial, T.) nerves, including statoacoustic, or «acoustical», a placode — a rudiment of a webby labyrinth of an inner ear and the related nodes of an eighth cranial nerve, and also a lenticular placode — site E., the forming rudiment of a crystalline lens.

Skin E. represents a rudiment of an epithelium of skin (epidermis) and its derivatives — grease and sweat glands, hair, nails, and at mammals also of claws, hoofs, horns, at birds — a beak, feathers. In the field of oral and proctal bays of a germ E. it vpyachivatsya towards the front and back ends of an average gut. These embolies break in a cavity of an average gut, forming respectively a front and back gut. Vystilka of a front gut arising from material E. and a prechordal plate, gives rise to development of an epithelium of an oral cavity, a throat, trachea, bronchial tubes, thymic, thyroid, epithelial bodies, a front share of a hypophysis, a throat, a gullet. From an ectodermal vystilka of a back gut the epithelium of a bladder, an ureter, a part of a rectum develops.

Extraembryonic E., being continuation skin E., is a part of temporary germinal bodies, including embryonic membranes, and forms an outside epithelium of a serous cover, an epithelium of amnion at germs of the highest vertebrata (see. Germ , Fetal membranes ). At mammals and the person a part extraembryonic E. already at the beginning of crushing of an oosperm stands apart in a look trophoblast (see).


Bibliography: Klishov A. A. Histogenesis and angenesis, L., 1984; Knorre A. G. Embryonal histogenesis, L., 1971; Stanek I. Embryology of the person, the lane with slovatsk., Bratislava, 1977.

A. G. Knorre, A. A. Klipov.

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