Histogenesis and organogenesis

Neurulation and Axial Complex

The initial stage of organogenesis is called neurulation, and the embryo at this stage is called a neurula. During this period, the axial organ complex characteristic of all chordates is formed, which proves their complete homology and evolutionary continuity.

The complex includes three main structures:

1. Notochord (chorda) — develops from the dorsal endoderm.
2. Intestinal tube — forms from the endoderm and lies directly beneath the notochord.
3. Neural tube — forms from the ectoderm on the dorsal side of the embryo. This process is induced by the notochord and mesoderm. First, a neural groove appears, the edges of which fuse into a tube with an internal cavity called the neurocoel. In craniates, this tube differentiates into the brain and spinal cord.

Mesodermal differentiation

The mesoderm is located on either side of the notochord, between the surface ectoderm and endoderm. During development, the unified mesodermal sheet dynamically divides into three primordia:

• Dorsal part (somites). These are paired segmented structures that develop in a craniocaudal direction. Somites differentiate into:
• Sclerotomes (ventromedial part): transform into secondary mesenchyme, migrate to the notochord, and form the vertebrae, ribs, and scapulae.
• Myotomes (inner dorsolateral part): give rise to striated skeletal muscles.
• Dermatomes (outer dorsolateral part): form the inner layer of the skin (dermis).
• Intermediate mesoderm (nephrotome and gonotome). These are segmented stalks connecting the somites to the lateral plate. They give rise to the excretory and reproductive organs.
• Ventral part (lateral plate). A thin unsegmented layer of cells splitting into two leaves, between which a secondary body cavity arises — the coelom (lined with mesothelium). The inner visceral leaf lies adjacent to the entoderm and gives rise to the myocardium, lung parenchyma, and mesentery. The outer parietal leaf lies adjacent to the ectoderm and forms the outer layers of the serous membranes (peritoneum, pleura, pericardium).

Germ layer derivatives

Although the classification of organs by germ layers is conventional (many organs have a mixed origin), each layer gives rise to specific tissues.

Ectoderm:

• Neural tube (brain and spinal cord).
• Skin epidermis and its derivatives (nails, feathers, hair, mammary glands).
• Organs of vision, hearing, and smell.
• Tooth enamel, oral cavity epithelium, anterior and posterior sections of the gut.
• Posterior pituitary, pineal gland.

Mesoderm:

• Dermis.
• Skeleton, muscles, connective tissue.
• Circulatory, lymphatic, excretory, and reproductive systems.
• Serous membranes (coelom, peritoneum, etc.).

Endoderm:

• Epithelium of the midgut and stomach, tongue, oral and pharyngeal cavities.
• Digestive glands (liver, pancreas).
• Epithelium of the lungs and respiratory tract.
• Anterior and intermediate lobes of the pituitary gland, thyroid and parathyroid glands, thymus.

Class and exam questions

What else to study on this topic

• Induction mechanisms: how the notochord and mesoderm influence neural tube development.
• Migration of sclerotome cells and the formation of secondary mesenchyme.
• Differentiation of brain and spinal cord regions in craniates.
• Formation and topography of the mesothelium in serous cavities.

Other topics in the section «Periodization of Ontogenesis»

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