Ontogenesis of any organism is characterized by the formation of germ layers. In such primitive types of animals as intestinal and sponges, the embryo consists of only two layers: endoderm and ectoderm. Over time, in more progressive forms of organisms, a third leaf appears - the mesoderm.
What is mesoderm?
Ontogenesis 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. Mesoderm is an embryonic leaf 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 double-layer animals, because in the process of ontogenesis they formed only two germ layers.
Mesoderm formation
The process of laying the middle germinal leaf is different for different taxonomic groups. There are three best-known methods in which mesoderm is formed: teloblastic, enterocele, and ectoderm.
1. The teloblastic pathway of mesoderm development is characteristic of many primary animals, and is based on the formation of blastomeres. Some of them specialize in laying the middle germinal leaf, which ultimately takes the form of two longitudinal parallel ribbons. These ribbons give rise to the mesoderm.
2. The enterocele method is fundamentally different in that the mesoderm progenitor cells form intussusception (invagination) together with the endoderm. This intussusception in the future forms the primary gut. The boundary between the two leaves remains indistinguishable for a long time, and only after a long period of time the mesoderm as an independent layer is disconnected from the endoderm. This developmental pattern is characteristic of animals such as the lancelet or starfish.
3. The 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 intussusception, only the endoderm is formed. If you imagine a picture of the embryo in the context, then after gastrulation (the formation of an indentation), free space will appear between the ento and ectoderm. There cells of ectodermal origin “bud”, which give rise to a new germinal layer.
Mesoderm morphology
A large role in the formation of the embryo is played by the mesoderm. This in biology is a good evolutionary trait, because the difference in the morphology of the middle germinal leaf in different groups of animals is used in taxonomy.
If we consider two longitudinal ribbons that are formed during the teloblastic method of development, then the mesoderm will be represented by metamerically repeating sections. The dorsal side of each such tape is divided into somites, the lateral into nephrotomes, and the ventral into splanchnotomas.
What role does mesoderm play? Human organs formed from the mesoderm
Each germ layer is a kind of predecessor of the organ systems and tissues of the future organism. The topology of the forming leaves to a greater extent determines their future fate. Since the mesoderm is the middle germinal layer, it participates 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 organ of the animal.
- The musculoskeletal system, which consists of the skeleton and the muscle system, also has a mesodermal origin. This refers not only to skeletal muscles, but also the muscular wall of blood vessels, heart and other internal organs and structures. The human skeleton is represented mainly by bone tissue, and to a lesser extent by cartilage. When it comes to chordate animals, at the embryonic stage of which chord formation occurs, do not confuse the origin of this axial structure with the spine. If the latter has a truly mesodermal origin, then the chord is a detachment of the intestine, which means that its origin is associated with endoderm.
- The reproductive and excretory systems are also formed from the mesoderm. In most chordal animals, they are interconnected, which means that they are formed from the same germinal leaf.
- The circulatory system also has a mesodermal origin. Both the heart and blood vessels are formed by cells of the middle germinal leaf.
Conclusion
The mesoderm is the embryo’s complex structure , which ultimately 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 characters. The presence of a mesoderm indicates a three-layer animal, which is a significant sign of the advancement of the group.