MESODERM (grech, mesos average + derma skin) — an average germinal leaf, or layer; set of the cells of an embryo which are located in primary perigastrium (blastotsela) between outside and internal germinal leaves (fig. 1).
The m appears in phylogenesis at the lowest worms and corresponds, according to I. I. Mechnikov, a peripheral fagotsitoblast of coelenterates. Ways of a bookmark and further development of M. are various at different groups of animals. At the highest vertebral M. develops during the second phase of a gastrulation since future mesodermal material remains after isolation of the lower (internal) germinal leaf (in the first phase of a gastrulation) as a part of a periblast of a blastodisc. As a result of reproduction of cells and their movement mesodermal material forms in a periblast of a blastodisc at birds and mammals the longitudinal thickening called by primary strip (edges it is gradually extended in the direction from caudal to head department). A front, most reinforced part of primary strip, carries the name primary, or genzenovsky, a small knot.
At vertebrata, including and the person, M., expanding is farther and farther on both sides from the place of the emergence, it is subdivided into sites, originative to different bodies and fabrics. The dorsal part M. lying on both sides of a chord and a medullary plate (after a neurotubule), is metamericly dismembered, or segmented, on somites (see), or back segments (fig. 2). Segmentation begins on the head end and gradually extends to tail; the number of couples of segments of a germ increases with age (see. Metamerism ). Somites remain during nek-ry time not segmented ventral departments of M., splanchnotomes, or side plates connected with, by means of the segmented narrowed intermediate sites — segment legs, or nephrotomes.
Then somites are differentiated on the medioventralny site — a sclerotome, the dorsolateral site — a dermatome, or a skin plate, and the myotome, or a muscular plate located between them. The relative sizes of a sclerotome in phylogenesis strongly increase; at the highest vertebrata it is connected with more considerable development of tissues of skeleton.
In splanchnotomes germs of vertebrata have a slit-like cavity — splankhnotsel, whole, or the secondary perigastrium dividing them into two leaves: outside — a somatopleure, or the parietal layer adjacent to an ectoderm, and internal — a splanchnopleura, or the visceral layer adjacent to an entoderm. Mesodermal segments and chord form together with a nervous rudiment — a medullary plate (in later terms of development — a neurotubule) so-called back, or axial, the complex of rudiments which is idiosyncrasy of all type of chordate animals. Two front segments stand apart after the third and besides in the kaudokranialny sequence (a larvaceous mesoderm). They are presented only by somites, oculomotor muscles at their expense develop. Larvaceous, or larval, segments (I and II) are inherited from the remote ancestors of the vertebrata who had a stage of a three-segment larva and the undergoing metamorphoses. Other (post-larvaceous) M.'s segmentation gradually extends to tail department.
Sclerotomes and dermatomas are loosened, forming setevidny connections of cells of a star-shaped form and separate free cells. Their set call entomezenkhimy, or mesenchyma (see). The cells which are moved from parietal and visceral leaves of splanchnotomes also join a mesenchyma. The parts of splanchnotomes keeping later allocations from their structure of a mesenchyma a close structure designate as an embryonal coelomic epithelium. Myotomes are the main source of development of skeletal (cross-striped) muscles.
The mesenchyma formed of different departments of M. gives rise to all kinds of connecting fabric, cartilaginous and bone educations, the hemopoietic bodies, blood, a lymph and smooth muscles of vessels and interiors.
From nephrotomes the epithelium of renal tubules develops. A large number of various derivatives is formed of a splanchnotome after eviction of cells of a mesenchyma from it: mesothelial cover of serous covers, cardiac muscle. the person has bark of adrenal glands, epithelial parts of seed plants and ovaries, a single-layer epithelium of a uterus and oviducts (at the person of uterine tubes).
Peripheral parts of splanchnotomes expand out of limits of a germinal guard and are a part of embryonic membranes. These parts of an average germinal leaf are called extraembryonic
M. U of the person and subhuman primates in connection with early and extraembryonic M.'s emergence is shifted by snowballing of provisional (temporary) bodies on very early stages of uterine life and strongly changed in comparison with other mammals. Extraembryonic M. is put at the person before germinal and irrespective of it. It fills a cavity of a blastocyste and spreads its wall consisting from trophoblast (see), and also walls of amnion, a vitellicle and an allantois, forming their connective tissue basis (see. Germ ). The germinal M. developing from primary strip is connected with extraembryonic the peripheral parts. Therefore after their reunion (at the person by the end of the 4th week of an embryogenesis) extraembryonic M. is represented direct peripheral continuation of splanchnotomes. From extraembryonic M. the first centers of a hemopoiesis, a peculiar connecting fabric of a number of provisional germinal bodies, napr develop, vartonova jelly (see) umbilical cord and vystilka of an extraembryonic tselom. Disturbances of differentiation of M. during pre-natal development conduct to various to malformations (see).
See also Germinal leaves .
Bibliography: Ivanova-Kazas O. M. Comparative embryology of backboneless animals, t. 1, page 54, Novosibirsk — M., 1975; To N about r r e A. G. Short sketch of embryology of the person, L., 1967; about N e, the Embryonal histogenesis, page 340, L., 1971: S i e w i n g R. Lehrbuch der ver-gleichenden Entwicklungsgeschichte der Tie-re, Hamburg, 1969.
H. G. Hlopin.