MESODERM: DEVELOPMENT, PARTS AND DERIVATIVES - BIOLOGY - 2025

The 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 blastodermic lamina that is located between the ectoderm and endoderm layers. Before gastrulation, the embryo has only two layers: the hypoblast and the epiblast.

Whereas, during gastrulation, the epithelial cells of the epiblast layer become mesenchymal cells that can migrate to other areas. These cells invaginate to give rise to the three embryonic laminae or layers.

The mesoderm is the last layer that originates, and it is formed by a process of mitosis that occurs in the ectoderm. Animals that present this layer are called "triblastics" and fall within the group "bilateria".

This structure differs into three areas on each side of the notochord: the axial, paraxial, and lateral mesoderm. Each of these parts will give rise to different structures of the body.

Skeletal muscles, connective tissue, cartilage, components of the circulatory and lymphatic system, the epithelium of certain endocrine glands, and part of the genitourinary system are derived from this layer.

It creates muscles and connective tissues for the entire body, except for the part of the head, where many structures come from the ectoderm. On the other hand, it has the ability to induce the growth of other structures such as the neural plate, which is the precursor of the nervous system.

All of these embryonic processes are driven 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 medium or intermediate, and "dermos", which means "skin". This layer can also be called a mesoblast.

Development of the mesoderm and its derivatives

The mesoderm gives rise mainly to muscles, bones, and blood vessels. During the early stages of embryonic development, cells form two classes of tissues:

Epithelia: cells connect through strong junctions building sheets. The mesoderm forms numerous epithelia.

Mesenchyme: cells are distributed leaving wide spaces between them, constituting a filling tissue. Mesenchyme is connective tissue, and much of it comes from the mesoderm. A small part arises from the ectoderm.

The derivatives of this structure are better explained 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 development called the notochord. This is cord-shaped, 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 occur during the gastrulation period. It begins with a superficial crevice that folds over and invaginates until it forms an elongated cylinder.

This structure is essential to determine 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 just above the mesoderm) so that some of its cells differentiate into nerve precursor cells. These are going to make up the central nervous system.

In some living beings, such as chordates, the axial mesoderm remains throughout life as axial support of the body. However, in most vertebrates it ossifies within the vertebrae. Even so, some remains persist in the nucleus pulposus of the invertebral discs.

Paraxial mesoderm

It is the thickest and widest part of the mesoderm. On the third week, it is divided into segments (called somitamers) that appear in order from cephalad to caudal.

In the cephalic area, the segments relate to the neuronal plate, forming neuromeres. These will give rise to a large part of the cephalic mesenchyme.

While, in the occipital area, the segments are organized into somites. They are fundamental transitory structures for the first segmental distribution of the early embryonic phase.

As we develop, most of this segmentation disappears. However, it is partly maintained in the spinal column and spinal nerves.

Somites are arranged on both sides of the neural tube. Around the fifth week, 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8-10 coccygeal somites were observed. These will form the axial skeleton. Each pair of somites will evolve originating three groups of cells:

- Sclerotome: it is made up of cells that have migrated from the somites to the ventral part of the notochord. This is going to become the spine, ribs, skull bones and cartilage.

- Dermotome: arises from the cells of the most dorsal part of the somites. It gives rise to the mesenchyme of the connective tissue, that is, the dermis of the skin. In birds, the dermotome is the one that produces the appearance of the feathers.

- Myotome: gives rise to skeletal muscles. Its precursor cells are myoblasts, which migrate towards the ventral region of the somites.

The shorter and deeper muscles generally 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 approximately 17 days of gestation, the lateral mesoderm divides into two sheets: the splacnopleural mesoderm, which is next to the endoderm; and the somatopleural mesoderm, which is located adjacent to the ectoderm.

For example, from the splacnopleural mesoderm come the walls of the intestinal tube. While the somatopleural mesoderm arises the serous membranes that surround the peritoneal, pleural and pericardial cavities.

Cells arise from the lateral mesoderm that will constitute the cardiovascular and blood system, the lining of the body cavities and the formation of extra-embryonic membranes. The latter have the mission of bringing nutrients to the embryo.

Specifically, it gives rise to the heart, blood vessels, blood cells such as red and white blood cells, etc.

Other classifications include the "intermediate mesoderm," a structure that connects the paraxial to the lateral mesoderm. Its development and differentiation give rise to genitourinary structures such as the kidneys, gonads and associated ducts. They also originate from part of the adrenal glands.

References

1. Derivatives of the mesoderm. (sf). Retrieved on April 29, 201, from the University of Córdoba: uco.es.
2. Mesoderm. (sf). Retrieved on April 29, 2017, from Embriology: embryology.med.unsw.edu.au.
3. Mesoderm. (sf). Retrieved on April 29, 2017, from Wikipedia: en.wikipedia.org.
4. Mesoderm. (sf). Retrieved on April 29, 2017, from Dictionary of Medical Terms, Royal National Academy of Medicine: dtme.ranm.es.