Ontogeny of any organism is characterized by the formation of germ layers. In such primitive types of animals as coelenterates and sponges, the embryo consists of only two layers: endoderm and ectoderm. Over time, more progressive forms of organisms have a third leaf - the mesoderm.
What is mesoderm?
Ontogeny is a consistent development of the embryo, which is accompanied by a number of changes in the morphology and anatomy of the future young organism. The mesoderm is the germ layer that plays an important role in the formation of many organs and tissues. Such primitive multicellular animals as hydras, jellyfish, corals or sponges are not in vain called two-layered animals, because in the process of ontogenesis they formed only two germ layers.
Mesoderm formation
The process of laying the middle germ layer in different taxonomic groups is different. There are three most well-known ways in which the mesoderm is formed: they are teloblastic, enterocoelic, andectodermal.
1. The teloblastic path of mesoderm development is characteristic of many protostomes and is based on the formation of blastomeres. Some of them specialize in laying the middle germ layer, which eventually takes the form of two longitudinal parallel ribbons. These ribbons give rise to the mesoderm.
2. The enterocoel method is fundamentally different in that mesoderm progenitor cells form an invagination (invagination) together with the endoderm. This invagination in the future forms the primary intestine. The boundary between the two sheets remains indistinguishable for a long time, and only after a long period of time does the mesoderm, as an independent layer, detach from the endoderm. This way of development is typical for animals such as the lancelet or the starfish.
3. An ectodermal way of mesoderm development is possessed by such types of animals as reptiles, birds and mammals (including humans). The bottom line is that after invagination, only the endoderm is formed. If we imagine a picture of the embryo in a section, then after gastrulation (the formation of an invagination), free space will appear between the ento- and ectoderm. Cells of ectodermal origin “bud off” there, which give rise to a new germ layer.
Mesoderm morphology
Mesoderm plays a big role in the formation of the embryo. This is a good evolutionary sign in biology, because the difference in the morphology of the middle germ layer in different groups of animals is used in taxonomy.
Ifconsider two longitudinal ribbons that are formed during the teloblast mode of development, then the mesoderm will be represented by metamerically repeating areas. The dorsal side of each such tape is divided into somites, the lateral side into nephrotomes, and the ventral side into splanchnotomes.
What role does the mesoderm play? Human organs derived from mesoderm
Each germ layer is a kind of precursor to organ systems and tissues of the future organism. The topology of the generatrix sheets to a greater extent determines their further fate. Since the mesoderm is the middle germinal layer, it is involved in the formation of tissues and organs that are between the human integument and the innermost layers of the body. What structures are of mesodermal origin?
- The formation of connective tissue occurs just from the cells of the mesoderm. This tissue is the boundary between the outer and inner layers of almost any animal organ.
- The musculoskeletal system, which consists of the skeleton and muscle system, also has a mesodermal origin. Here we mean not only skeletal muscles, but also the muscular wall of blood vessels, the heart and other internal organs and structures. The human skeleton is represented mainly by bone tissue, and to a lesser extent by cartilage tissue. When it comes to chordates, at the embryonic stage of which the notochord is formed, one should not confuse the origin of this axial structure with the spine. If the latter is of true mesodermal origin, then the notochord issplitting of the intestine, which means that its origin is associated with the endoderm.
- The reproductive and excretory systems are also formed from the mesoderm. In most chordates, they are interconnected, which means they are formed from the same germ layer.
- The circulatory system also has a mesodermal origin. Both the heart and blood vessels are formed by cells of the middle germ layer.
Conclusion
Mesoderm is a complex structure of the embryo, which eventually gives rise to many vital organs and tissues. In different taxonomic groups of animals, the formation and development of the middle leaf is different, and this is one of the evolutionary signs. The presence of mesoderm indicates the three-layered nature of the animal, which is a significant sign of the advancement of the group.
