The mesoderm

The mesoderm (synonym mesoblast) is the average embryonic leaf, consisting of cells found in the primary body cavity between the ectoderm (see) and entodermal (see). From mesoderm formed embryonic beginnings, serving as the source of the development of muscles, epithelium serous cavities organs of the urogenital system.
Cm. also the Germ.

The mesoderm (from the Greek. mesos - average and derma - skin layer; synonym: average embryonic leaf, mesoblast)is one of the three embryonic leaves multicellular animals and humans in the early stages of development.
Topographically, the mesoderm occupies an intermediate position between the outer embryonic leaf - ectoderm (see) and internal - entodermal (see). The germ of sponges and most coelenterates the mesoderm is not formed; these animals are for life remain dvuhmetrovye. At representatives of the higher types of animals, as a rule, the mesoderm appears in the process of development of the embryo (see) later ecto - and endoderm, with arises from different animals at the expense of one of these leaflets or by both (respectively distinguish ecto - and encomendero). In vertebrates, the mesoderm is formed as an independent (third) layer of the embryo already in the second phase of gastrulation (Fig. 1).

the cut of the embryo vertebral
Fig. 1. A cross-section of the embryo spinal at the end of the second phase of gastrulation (three embryonic leaf and axial complex germs): 1 - the ectoderm (I - skin ectoderm, 2 - nervous album); II - the mesoderm (3 - mesoderm, 4 - hardly serious); III - endoderm.

In a series of vertebrates there is a gradual change in the way education mesoderm. For example, in fish and amphibians it occurs in the boundary between anti - and ectoderm region, educated side lips primary mouth (blastopore). In birds, mammals and humans cellular material of the future mesoderm first collected in the form of primary strips in the composition of external embryonic leaf (a person - on the 15th day of fetal development), and then plunges into the space between the outer and inner leaflets and lies on both sides of the embryo spinal chords (chord), entering with him and the embryo of the nervous system in the axial composition of complex germs. Next to the germ of a chord part of M (axial) are part of the body of the embryo and take part in the formation of its permanent bodies. Peripheral same areas grow between marginal parts of ecto - and endoderm and are part of the temporary bodies of germ - yolk SAC, amnion and chorion.
The mesoderm the body of vertebrate embryo and man falls into dorsal areas of the spinal segments (comity), intermediate - segment legs (nephrotomy) ventral and lateral plates (splanchnotomy). Comity and nephrotomy are gradually in the direction from front to back (the man first pair of Samitov occurs on 20-21 days prenatal development, the last, 43 or 44-I, para - by the end of the 5-th week). Splanchnotomy remain non-segmented, but are split on the parietal (parietal and visceral (vnutrenney) sheets between which occurs secondary cavity of the body (General). Comity are divided into dorsolateral areas (dermatomes), Medio-ventral (sclerotomy) and intermediate between them (myotomy). The dermatomes and sclerotomy, getting more loose arrangement of the cells that form mesenchyme (see). Many cells in the mesenchyme moved from splanchnotomy. Scheme of organogenesis in embryo, the higher the spinal shown in Fig. 2. Thus, in particular, from mitomo develops arbitrary striated muscle skeletal muscles. Nephrotomy give rise to kidney epithelium, of the fallopian tubes and uterus. Splanchnotomy turn into a single-layer flat epithelium lining the whole, mesotheli (see). Are formed also by the adrenal cortex, follicular epithelium of the gonads and muscular tissue of the heart.

scheme of organogenesis in embryo, the higher the vertebral
Fig. 2. Scheme of organogenesis in embryo, the higher the vertebral (name tissue derived put in brackets after the name of the corresponding Bud): 1 - skin entoderm (epidermis); 2 - ganglion plate (sensitive and sympathetic neurons, peripheral neuroglia, chromatophores from); 3 - neural tube (neurons, neuroglia); 4 - chord; 5 - dermatome (connective tissue framework of the skin); 6 - myotome (skeletal muscle tissue); 7 - sclerotome (cartilage and bone tissue); 8 - nematom (kidney epithelium); 9 - parietal a piece of splanchnotomy (mesotheli); 10 - visceral piece of splanchnotomy (mesotheli, the tissue of the heart muscle); 11 - intestinal entoderm (intestinal epithelium); 12 - mesenchyme (connective tissue, blood, smooth muscle tissue); 13 - nazaralieva the ectoderm (epithelial amnion);- 14 - aortic endothelium; 15 - the yolk entoderm (epithelium yolk SAC); 16 - General.

Cm. also Embryonic leaves.