Axial Structures and Dorsal Mesoderm
The process of primordia segregation begins simultaneously with the formation of the middle germ layer itself. One of the first elements of the axial organs is the notochord, which is laid down by the end of the third week of development.
Flanking the notochord is the dorsal mesoderm, which undergoes segmentation into dense blocks called somites. This process does not occur all at once, but follows a strict craniocaudally directed gradient (the wave of segmentation spreads from head to tail). 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 complete with the formation of 44 pairs.
During the fourth week (the 30-pair stage), each somite differentiates into three specialized zones:
- Dermatome (outer part) — through an intermediate mesenchymal stage, it gives rise to the connective tissue framework of the skin (dermis).
- Myotome (middle part) — serves as the exclusive source for the development of all skeletal muscle tissue.
- Sclerotome (inner part) — cells actively migrate, surrounding the notochord to subsequently form the vertebral column. Additionally, bones and cartilages develop from their mesenchyme.
Intermediate Mesoderm (Nephrogonotomes)
Lateral to the somites lies the nephrogenic cord, or intermediate mesoderm. This region is critical 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 caudal region, segmentation is absent, and the tissue forms a continuous nephrogenic cord.
Kidney development from this source occurs in 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 mesonephric duct structures.
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 (splanchnopleuric layer): adheres to internal organs and forms the myoepicardial mantle. It is from this layer that the muscular (myocardium) and outer (epicardium) layers of the heart subsequently develop.
- Parietal layer (somatopleuric layer): the outer layer, which serves as the source of mesothelium—the epithelial lining of all serous cavities.
- Coelom: The secondary body cavity forms between these layers. During the second month of embryogenesis, the coelomic space is partitioned into definitive serous cavities: pericardial, pleural, and peritoneal.
Mesenchyme and Internal Environment Tissues
A special place in embryonic development is occupied by mesenchyme—embryonic connective tissue consisting of mobile, stellate cells that fill the free spaces between established germ layers.
The bulk of mesenchymal cells delaminates from the mesodermal primordia (sclerotomes and dermatomes). However, it is also partially formed by cell migration from the endoderm and ectoderm. The direction of further differentiation directly depends on the source:
- Mesenchyme of sclerotomal origin is utilized to build cartilaginous and bone tissues.
- Mesenchyme of dermatomal origin forms the connective tissue framework of the skin dermis.
Overall, mesenchyme possesses immense potential and gives rise to a wide spectrum 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).