Axial structures and dorsal mesoderm
The segregation of primordia begins simultaneously with the formation of the middle germ layer itself. One of the first elements of the axial organs is the notochord (notochord), which is laid down by the end of the third week of development.
Flanking the notochord is the dorsal mesoderm, which undergoes segmentation—division into dense segments called somites. This process does not occur all at once, but follows a strict craniocaudally oriented gradient (the wave of segmentation spreads from front to back). The chronology of segmentation is as follows:
- Day 22: 7 pairs of somites form.
- Day 30: the number increases to 30 pairs.
- Day 35: segmentation is fully completed with the formation of 44 pairs.
In the fourth week (30-pair stage), each somite differentiates into three specialized zones:
- Dermatomes (outer part) — via an intermediate mesenchymal stage, they give rise to the connective tissue framework of the skin (dermis).
- Myotomes (middle part) — serve as the sole source for the development of all skeletal muscle tissue.
- Sclerotomes (inner part) — cells actively migrate, surrounding the notochord for the subsequent formation of the spine. In addition, bones and cartilage develop from their mesenchyme.
Intermediate mesoderm (Nephrogonotomes)
Lateral to the somites lies the nephrogenic cord, or intermediate mesoderm. This region is critically important as it gives rise to the epithelial lining of the excretory organs (kidneys, urinary tract) and gonads.
The structural organization of the intermediate mesoderm is heterogeneous. In the anterior and middle regions, the primordium is divided into nephrotomes (segmental stalks). In the posterior region, segmentation is absent, and the tissue forms a continuous nephrogenic cord.
The development of the kidneys from this source goes through three sequential stages:
- Pronephros: forms in the third week from the anterior 8–10 segmental stalks.
- Mesonephros: develops in weeks 4–5 from the subsequent 25 segmental stalks.
- Metanephros: the definitive organ, which forms by the end of the second month from the unsegmented nephrogenic cord and structures of the mesonephric duct.
Lateral plate (Splanchnotome)
The most lateral part of the mesoderm — the splanchnotome — is completely devoid of segmentation. During embryonic body folding, this region shifts and ultimately assumes a ventral position.
A characteristic feature of the lateral plate is its longitudinal splitting into two independent layers:
- Visceral layer (splanchnopleure): adjoins the internal organs and forms the myoepicardial plate. It is from this layer that the muscular (myocardium) and outer (epicardium) coats of the heart subsequently develop.
- Parietal layer (somatopleure): the outer layer, which serves as the source of mesothelium — the epithelial lining of all serous cavities.
Between these layers forms the secondary body cavity — the coelom. During the second month of embryogenesis, the coelomic space is divided into definitive serous cavities: pericardial, pleural, and peritoneal.
Mesenchyme and internal environment tissues
A special place in embryonic development is occupied by mesenchyme — embryonic tissue consisting of mobile, stellate cells that fill the spaces between the formed germ layers.
The bulk of mesenchymal cells emigrate from the mesodermal primordia—sclerotomes and dermatomes. However, it also partially forms through the migration of cells from the endoderm and ectoderm. The direction of further differentiation depends directly on the source:
- Mesenchyme of sclerotomal origin is used to build cartilage and bone tissue.
- Mesenchyme of dermatomal origin forms the connective tissue framework of the skin dermis.
In general, mesenchyme possesses tremendous potential and gives rise to a wide range of derivatives:
- All types of connective tissue.
- Blood and hematopoietic tissue.
- Blood vessels.
- Smooth muscle tissue of internal organs.
- Microglia (glial macrophages of the central nervous system).
Briefly: answers to topical questions
What structures develop from ectomesenchyme (neuromesenchyme)?
Ectomesenchyme (neuromesenchyme), which represents migrating neural crest cells, gives rise to the tissues of the facial and head region. Replacing the mesoderm in the head region, this embryonic source gives rise to the following structures:
- Supporting tissues — connective, cartilage, and bone tissues of the facial region;
- Dental tissues — connective tissue of the dental pulp, dentinoblasts, and cementoblasts;
- Eye tunics — sclera and vascular tunic of the eye.
From which embryonic source does the adrenal cortex develop?
The adrenal cortex is of mesodermal origin and develops from the layers of the splanchnotome. The primary embryonic source is the coelomic epithelium in the region of the mesentery root. During embryogenesis, it proliferates and forms the interrenal body, in which two layers differentiate:
- Fetal cortex — the inner zone composed of large acidophilic cells that degenerate after birth;
- Definitive cortex is the outer zone of small basophilic cells that transforms into the final cortex of an adult.
The fate of somite parts is easy to remember by their names: "Dermatome" — dermis (skin), "Myotome" — myocytes (skeletal muscles), "Sclerotome" — sclerosis/density (spine, bones, cartilage).
Class and exam questions
What is formed from the visceral layer of the splanchnotome?
The myoepicardial plate is formed from the visceral layer (splanchnopleura), which gives rise to the myocardium and epicardium of the heart.
Where does mesenchyme come from in the embryo?
The main sources of mesenchyme are cells migrating from the sclerotomes and dermatomes of the mesoderm. Endoderm and ectoderm also partially participate in its formation.
How does the secondary kidney develop?
The secondary kidney (metanephros) is formed by the end of the second month of development from the posterior unsegmented region of the intermediate mesoderm (nephrogenic cord) and the mesonephric duct.
What else to study on this topic
- Mechanisms of the craniocaudal gradient of somite segmentation
- Development of the myoepicardial plate and heart primordium
- Stages of formation of the secondary body cavity (coelom) and its division
- The role of the mesonephric duct in the development of the urogenital system
- Origin and functions of microglia in the central nervous system
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