Mesoderm Differentiation
The development of the middle germ layer proceeds in several directions, forming key axial structures.
- Notochord (Chorda) — a dense, unpaired cord of cells located strictly along the central axis. Cells for the notochord migrate from the epiblast through the primitive pit. The notochord defines the body axis around which the vertebral column will later form.
- Somites (Somiti) — dorsal paraxial mesoderm. These are compact, paired cell clusters located on either side of the notochord. Somite segmentation proceeds in a craniocaudal direction (from head to tail). Each somite divides into three functional zones:
- Dermatome (outer part) — the source of the dermis of the skin.
- Myotome (middle part) — the precursor of skeletal musculature.
- Sclerotome (inner part) — the source of bones and cartilage of the axial skeleton.
- Nephrogonotomes (Nephrogonotomes) — intermediate mesoderm (segmental stalks), connecting the somites to the lateral plate. At the cranial end, they are segmented, while in the lower half of the body, they merge into a continuous nephrogenic cord. These are the precursors of the urinary and reproductive systems (specifically, forming tubules).
- Splanchnotome — the lateral, unsegmented part of the mesoderm. It splits into two layers: parietal (adjacent to the ectoderm) and visceral (adjacent to the endoderm). The space between them is the coelomic cavity (future pleural, pericardial, and peritoneal cavities). Both layers give rise to the mesothelium of serous membranes, while the visceral layer specifically develops into the myocardium and epicardium.
Origin and Role of Mesenchyme
Mesenchyme is an embryonic tissue consisting of motile, stellate cells that fill the spaces between organ primordia. Mesenchymal cells migrate predominantly from the somites (dermatomes and sclerotomes) and splanchnotomes.
Tissues of the internal environment and smooth muscle form from this universal precursor. Derivatives of the mesenchyme include all types of connective tissue (including bone and cartilage), hematopoietic tissue, blood cells, and blood vessels (at the stage of established axial primordia, primary blood cells are already visible within the vessel lumens).
Ectoderm Differentiation and Neurulation
The outer germ layer gives rise to two lineages: neuroectoderm and surface ectoderm.
Neurulation (formation of the nervous system) begins under the inducing influence of the notochord. The embryo stage during this period is called the neurula. The process includes several steps:
- Thickening of the ectoderm — formation of the neural plate.
- Invagination of the plate, forming the neural groove and neural folds at the margins.
- Fusion of the folds into the neural tube, which sinks beneath the ectoderm. The central nervous system (brain and spinal cord) develops from it.
- Formation of the neural crest (ganglionic plate) from the material of the neural folds. It gives rise to the peripheral nervous system, the adrenal medulla, and melanocytes.
The remaining surface ectoderm serves as the source for the epidermis and its derivatives (hair, nails, glands), as well as the epithelium of the initial (oral cavity) and terminal (anal canal) sections of the digestive tract.
Endoderm Transformations
Initially, the intestinal endoderm is a thin, single-layered sheet of squamous cells. During notogenesis, this sheet, together with the tightly adjoining visceral layer of the splanchnotome, rolls up to form a closed primitive gut tube.
This axial precursor subsequently gives rise to the epithelial lining of the stomach and intestines, as well as the parenchyma of the major digestive glands—the liver and pancreas.