He Mesoderm Is one of the three embryonic cell layers that arise during the gastrulation process, around the third week of gestation. It is present in all vertebrates, including humans.
It is defined as a blastodermal lamina that is located between the ectoderm and endoderm layers. Before the Gastrulation , The embryo only has two layers: the hypoblast and the epiblast.
While, during gastrulation, the epithelial cells of the epiblast layer become mesenchymal cells that can migrate to other areas. These cells are invaginated to give rise to the three layers or embryonic layers.
The mesoderm is the last layer that originates, and is formed by a process of mitosis that occurs in the ectoderm. The animals that present this layer are called"triblásticos"and enter the group"bilateria".
This structure differs in three areas on each side of the notochord: the axial mesoderm, the paraxial and the lateral. Each of these parts will give rise to different structures of the organism.
From this layer, skeletal muscles, connective tissue, cartilage, components of the circulatory and lymphatic system, epithelium of certain endocrine glands and part of the genitourinary system are derived.
It creates muscles and connective tissues throughout the body, except in the part of the head, where many structures come from the ectoderm.
On the other hand, it has the capacity to induce the growth of other structures such as the neural plate, which is the precursor of nervous system .
All these embryonic processes are directed by refined genetic mechanisms that, if altered, can lead to severe malformations, genetic syndromes and even death.
The term mesoderm comes from the Greek"μέσος". It is divided into"mesos", which means middle or intermediate and"dermos", which means"skin". This layer can also be called a mesoblast.
Development of mesoderm and its derivatives
The mesoderm mainly results in muscles, bones and blood vessels. During the early stages of embryonic development, cells form two kinds of tissue:
Epithelia: cells are connected through strong junctions by building sheets. The mesoderm forms numerous epithelia.
Mesenchyme: the cells are distributed leaving wide spaces between them, constituting a filling tissue. The mesenchyme is the connective tissue, and much of it comes from the mesoderm. A small part arises from the ectoderm.
The derivatives of this structure are best explained by dividing it into different areas: axial, paraxial and lateral mesoderm. Since each of them gives rise to different structures.
Axial mesoderm
This corresponds to a fundamental structure in the development called notochord. This is shaped like a cord, and is located in the midline of the dorsal part of the embryo. It is the axis of reference that will determine that both sides of the body develop symmetrically.
The notochord begins to form at 18 days of gestation, through cellular movements that occurred during the gastrulation period. It begins with a superficial crack that is folded, and is invaginated until forming an elongated cylinder.
This structure is fundamental for determining the position of the nervous system and subsequent neural differentiation. The notochord has the important function of displaying inductive signals that regulate the development of the embryo.
Thus, this structure sends inductive signals to the ectoderm (the layer that is just above the mesoderm) so that some of its cells differentiate into precursor nerve cells. These will constitute the central nervous system.
In some living beings, such as Chordate , The axial mesoderm remains throughout the life as axial support of the body. However, in most vertebrates it ossifies in the interior of the vertebrae. Even so, some remains persist in the nucleus pulposus of the Invertebral discs .
Paraxial mesoderm
It is the thickest and broadest part of the mesoderm. About the third week, it is divided into segments (called somitámeros) that appear in cephalic to caudal order.
In the cephalic area, the segments are related to the neuronal plate, forming neurons. These will give rise to a large part of the cephalic mesenchyme.
Whereas, in the occipital area, the segments are organized in Somites . They are transient structures fundamental to the first segmental distribution of the early embryonic phase.
As we develop, most of this segmentation disappears. However, it remains partly in the spine and spinal nerves.
The somites are arranged on both sides of the neural tube. On the fifth week, there are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8-10 coccygeal somites. These will form the axial skeleton. Each pair of somites will evolve into three groups of cells:
- Sclerotoma: is formed by cells that have migrated from the somites to the ventral part of the notochord. This will become the spine, ribs, skull bones and cartilage.
- Dermotoma: arises by the cells of the dorsal part of the somites. It gives rise to the mesenchyme of the connective tissue, that is, to the dermis of the skin. In birds, the dermotome is the one that produces the appearance of the feathers.
- Myotoma: gives rise to skeletal muscles. Its precursor cells are the myoblasts, which migrate into the ventral region of the somites.
The shorter and deeper muscles usually arise from individual myotomes. While superficial and large, they derive from the fusion of several myotomes. The process of muscle formation in the mesoderm is known as myogenesis.
Lateral mesoderm
It is the outermost part of the mesoderm. At about 17 days of gestation, the lateral mesoderm is divided into two sheets: the mesothelium is photonopleural, which is next to the endoderm; And the somatopleural mesoderm, which is located adjacent to the ectoderm.
For example, the walls of the intestinal tube come from the reciprocal mesoderm. While from the somatopleural mesoderm arise the serous membranes that surround the peritoneal, pleural and pericardial cavities.
From the lateral mesoderm arise cells that will constitute the cardiovascular and blood system, the lining of the body cavities and the formation of membranes extraembryonic. The latter have the mission of bringing nutrients to the embryo.
In particular, it gives rise to the heart, blood vessels, blood cells like red and white blood cells, etc.
Other classifications include the"intermediate mesoderm", a structure that connects the paraxial mesoderm with the lateral one. Its development and differentiation gives rise to genitourinary structures such as the kidneys, gonads and associated ducts. They also originate part of the adrenal glands.
References
- Derivatives of the mesoderm. (S.f.). Retrieved on April 29, 201, from University of Córdoba: uco.es.
- Mesoderm. (S.f.). Retrieved on April 29, 2017, from Embryology: embryology.med.unsw.edu.au.
- Mesoderm. (S.f.). Retrieved on April 29, 2017, from Wikipedia: en.wikipedia.org.
- Mesoderm. (S.f.). Retrieved on April 29, 2017, Dictionary of Medical Terms, Royal National Academy of Medicine: dtme.ranm.es.