Mesoderm

MEODERM (from Greek mesos-middle and derma-skin), the middle germ layer (see Germ layers), represents in the classical understanding the collection of cells forming the wall of the secondary body cavity, or coelom, in embryos. From the composition of M. at early stages of development, the rudiment of the so-called support-trophic system is isolated, i.e., all connective tissues, including blood and lymph. This rudiment, called mesenchyme (see), is isolated in the form of isolated cells or separate cellular complexes. In some cases, it cannot be sharply demarcated from the rest of M., which for the most part has the character of continuous cellular sheets. - The formation of M. in vertebrates was first traced by Kolliker (Kolliker, 1879) in the chick embryo and mammals. The precise formulation of the concept itself applicable to all classes of vertebrates and many invertebrates was given by the Hertwigs (O. and R. Hertwig, 1881) on the basis of the comparative-embryological method. M. in vertebrates is formed in most cases as a bilaterally symmetrical rudiment along the edges of the blastopore (see) or from the corresponding area according to the blastopore theory (Hertwig), i.e., the lateral groove of selachians and bony fishes, the primary plate of reptiles, the primitive streak of birds and mammals. This so-called peristomal mesoderm then grows out to the sides and forward along the sides of the rudiment of the notochord, which forms in front of the blastopore and is included at some stages of development in the wall of the primary gut (head diverticulum in amniotes); this last area of M. represents the so-called parachordal, or gastral M. [see separate table {pp. 743-744}, fig. 1]. Only in the lancelet, as well as in some invertebrates, M. is isolated after the closure of the blastopore in the form of segmented paired outgrowths from the wall of the primary gut. In all other cases, the rudiment of M. in vertebrates initially has the character of a continuous paired cellular plate, wedged between the ecto- and endoderm of the embryo. The body cavity arises later by the splitting of the cellular plates of M. into two layers. - The rudiment of M., growing farther in both directions from its place of origin, divides into several areas having different prospective significance, i.e., forming different products of differentiation in the future. The dorsal part of M., lying on both sides of the notochord and neural tube, undergoes metameric segmentation (becomes segmented) into the so-called dorsal segments, or myotomes. Segmentation begins from the head end of the embryo and gradually extends to the tail end, and the number of pairs of segments gradually increases with the age of the embryo. Myotomes remain for some time in connection with the unsegmented ventral parts of M., the splanchnotome, and via segmented intermediate narrowed areas, the so-called segmental legs, or nephrotomes. In the myotomes, a special medio-ventral area, the sclerotome, should also be distinguished, which is then isolated from the proper M. and is one of the sources of mesenchyme formation (see). Only in the lancelet do unsegmented splanchnotomes form by the secondary fusion of the ventral parts of the primary segments. The secondary body cavity, or coelom, reaches its greatest development in the splanchnotomes, in which it is called the splanchnocoel. The part of the splanchnotome adjacent to the ectoderm, which has split due to the formation of the coelom into two layers, is called the parietal (wall), somatic, fibrous layer, or somatopleura. The layer bordering the endoderm is called the visceral (internal), intestinal fibrous layer, or splanchopleura. It should be noted that the greater part of the wall of the myotome, facing the ectoderm and designated as the dermal plate, as well as the aforementioned sclerotome, represents a second source of mesenchyme formation and soon becomes isolated from the proper M., the remainder of which is called the muscular plate. Within the parietal and visceral layers of the splanchnotomes, diffuse mesenchymal rudiments are also embedded, gradually becoming isolated from their composition. All these relationships are most clearly observed in the embryos of selachians, while in other vertebrates they often turn out to be considerably obscured and can be deciphered only by comparison with these. In vertebrates possessing special auxiliary organs during embryonic life - the yolk sac and embryonic membranes (see Embryo) - the ventral parts of the splanchnotomes enter into the composition of these latter and are called extra-embryonic mesoderm. This latter, in its structure and in part in its further fate, cannot be sharply demarcated from mesenchyme (see). In humans, extra-embryonic M. (or mesenchyme) is formed at very early stages of development independently of its embryonic part and apparently does not show bilateral symmetry. The part of the body cavity, bounded by extra-embryonic mesoderm, is designated as the extra-embryonic coelom. Thanks to the latest advances in experimental embryology, it must be admitted (H. Spemann, O. Mangold, W. Vogt, L. Graper and others), that the cellular material from which M. should form already at the blastula stage (see) or at the very beginning of the gastrulation process (see Gastrula) has a strict localization. This cellular material received the name of presumptive mesoderm. In the lancelet, according to Conklin (E. Conklin), even in the fertilized egg cell, it is possible to distinguish a special area of protoplasm, from which M. is formed in the future. With further differentiation, from different areas of the embryonic M. in vertebrates, very different tissues develop. Myotomes give rise to voluntary somatic musculature, nephrotomes, or segmental legs - the epithelium of excretory organs and parts of the excretory ducts of the male, and in some lower vertebrates possibly also of the female reproductive apparatus. From the splanchnotomes, the internal lining (see Mesothelium) of the serous cavities, the so-called interrenal organs (see Adrenal glands), cardiac muscle, and in higher vertebrates presumably part of the Müllerian duct (see) and its derivatives are formed. Therefore, one cannot but recognize that M. is in essence a collective concept, containing within it very heterogeneous rudiments. This gives rise to the fact that many biologists do not consider it possible to look upon M. as a single germ layer.