Notogenesis

Mesodermal differentiation

The development of the middle germ layer proceeds in several directions, forming the most important axial structures.

1. Notochord (Chorda) — a dense unpaired cord of cells located strictly along the central axis. Cells for it migrate from the epiblast through the primitive pit. The notochord defines the body axis around which the vertebral column will later form.
2. Somites (Somiti) — dorsal paraxial mesoderm. These are compact paired cell clusters on either side of the notochord. Somite segmentation proceeds in a craniocaudal direction (from head to tail). Each somite divides into three functional zones:
3. Dermatome (outer part) — source of the dermis of the skin.
4. Myotome (middle part) — precursor of skeletal musculature.
5. Sclerotome (inner part) — source of development for the bones and cartilage of the axial skeleton.
6. 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, tubules form within them).
7. Splanchnotome — 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 abdominal cavities). Both layers give rise to the mesothelium of the serous membranes, while the myocardium and epicardium develop specifically from the visceral layer.

Origin and role of mesenchyme

Mesenchyme — an embryonic tissue consisting of mobile, branching cells that fill the spaces between organ primordia. Mesenchymal cells migrate predominantly from the somites (dermatomes and sclerotomes) as well as from the splanchnotomes.

This universal precursor gives rise to internal environment tissues and smooth musculature. Derivatives of mesenchyme include all types of connective tissue (including bone and cartilage), hematopoietic tissue, blood cells, and blood vessels (at the stage of formed axial primordia, primary blood cells are already visible within the vessel lumens).

Ectodermal 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 embryonic stage during this period is called the neurula. The process includes several steps:

1. Thickening of the ectoderm — formation of the neural plate.
2. Invagination of the plate with the formation of the neural groove and neural folds at the edges.
3. Fusion of the edges into the neural tube, which sinks beneath the ectoderm. The central nervous system (brain and spinal cord) will develop from it.
4. Formation of the neural crest (ganglion plate) from the material of the folds. It gives rise to the peripheral nervous system, adrenal medulla, and melanocytes.

The remaining surface 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) parts of the digestive tract.

Endodermal transformations

Initially, the gut endoderm is a thin, single-layered sheet of flat cells. During notogenesis, this sheet, together with the tightly adjacent visceral layer of the splanchnotome, rolls up to form a closed primary gut tube.

This axial precursor subsequently forms the epithelial lining of the stomach and intestines, as well as the parenchyma of the major digestive glands — the liver and pancreas.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of embryonic induction (role of the notochord)
• Histogenesis of internal environment tissues from mesenchyme
• Formation and division of the coelomic cavity
• Development of surface ectoderm derivatives
• Spatial reorganization of the embryo (body folds)

Other topics in "Gastrulation, formation of axial organ rudiments, and extraembryonic membranes"

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