Germ Layers


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Germ layers

The primitive cell layers, or first tissues, which appear early in the development of animals and from which the embryo body and its auxiliary membranes, when present, are constructed. These are more or less distinct anatomically, but do not necessarily have sharp boundaries of demarcation. Germ layers are almost universal among animal embryos and appear to establish discontinuities of architectural importance without complete loss of continuity. Three kinds of germ layers are recognizable: (1) the ectoderm or outer skin, (2) the endoderm or inner skin, and (3) the mesoderm or middle skin. The layers have been named in accordance with their positions in the spherical type of gastrula such as that of the sea urchin or amphibian. The terms epiblast, mesoblast, and hypoblast are sometimes used as synonyms for ectoderm, mesoderm, and endoderm, respectively. The three primary germ layers are present as a basic structural plan in all Metazoa with the exception of the coelenterates and the Porifera, in which a distinct mesodermal layer is absent. See Gastrulation

Germ Layers

 

layers of the body of the embryo of multi-cellular animals and man, which are formed in the process of gastrulation.

The majority of organisms have three germ layers: the exterior, or ectoderm; the interior, or endoderm; and the middle, or mesoderm. Exceptions are the sponges and coelenterates, in which only two germ layers are formed, the exterior and the interior. Derivatives of the ectoderm perform integumental, sensory, and motor functions: in the course of embryonic development they give rise to the nervous system; the skin integument and the skin glands that are formed from it; the hair, plumage, scales, nails, and so forth; the epithelium of the anterior and posterior sections of the digestive system; the connective-tissue foundation of the skin; the pigment cells; and the visceral skeleton. The endoderm forms the lining of the intestinal cavity and provides nutrition for the embryo; from it originate the mucous membrane of the digestive system, the digestive glands, and the organs of respiration. The mesoderm effects communication between the parts of the embryo and performs supportive and trophic functions; from it are formed the organs of excretion, the sex organs, the circulatory system, and the serous membranes that line the secondary body cavity (coelom) and clothe the internal organs and muscles; in vertebrates the skeleton is also formed from the mesoderm. Analogous germ layers in different groups of organisms may have, in addition to common traits, substantial differences in their manner of formation and their structures, because of the adaptation of the embryos to different conditions of development.

The science of germ layers—one of the most important generalizations in embryology—has a long history and is connected with fundamental biological teachings and discoveries, such as epigenesis, cell theory, and Darwinism. Among those responsible for its development were K. F. Wolff, C. H. Pander, K. E. Baer, R. Remak, E. Haeckel, O. Hertwig, A. O. Kovalevskii, and I. I. Mechnikov. The latest data of experimental embryology, including the results of in vivo staining of various sections of the wall of the blastula and tracing of their migration during gastrulation and neurulation, have made it possible to determine, as early as the blastula stage, the positions of groups of cells from which the various germ layers and their derivatives will be formed and to construct a map of the future rudiments of organs and their systems. Through experiments in transplanting and removing the material of various germ layers in the blastula stage and during the period of gastrulation, the properties of the matter of various germ layers and their capacity for differentiation have been elaborated: at the beginning, the sections of the various germ layers can still be substituted for one another when transplanted, but by the end of gastrulation they lose that capacity.

T. A. DETLAF

References in periodicals archive ?
A) In utero exposure to BaP induced a significant dose-dependent increase in mutant frequencies in somatic tissues originating from distinct primary germ layers, including the bone marrow (mesoderm), brain (ectoderm), and liver (endoderm).
We also showed that these cell lines had AP activity and expressed pluripotent markers such as Oct4, Sox2, and Nanog, The cell lines also formed EBs, revealed normal karyotypes, and were capable of differentiating into the three germ layers.
Mature teratoma is benign and contains the mature tissues of at least two of the three germ layers (1-3).
From histological point of view, the teratomas within the neuraxis are similar to the extragonadal ones, and consist of a mixture of cells derived from all three germ layers.
Endoderm The innermost layer of cells in an embryo at the stage when the three germ layers (ectoderm, mesoderm, and endoderm) are established.
Perhaps iPS cell researchers will discover that skin cells can be driven back even further in development to a totipotent state--that is, to a single zygote-like cell capable of generating not only all three germ layers but also all the supporting extra-embryonic tissues.
A clonal line of human embryonal carcinoma cells was derived that yields tissues from all three primary germ layers in 1989.
In 2006, at the age of 15, Lupita was diagnosed with an ovarian teratoma (a congenital tumor containing one or more of three embryonic germ layers, occurring mainly in young women).
Embryonic stem cells are able to differentiate into the cells of the three primary germ layers including germ cells.
After examining the anatomical, experimental, and genetic approaches to studying the initiation and construction of organisms, this textbook describes early embryonic development, the tissues formed by the vertebrate ectoderm, formation of the mesodermal and endodermal germ layers, and sex determination.
Teratomas are tumors that consist of tissues arising from at least 2 embryonic germ layers.