Development of nervous tissue

Stages of neurulation

The process of neural system formation is called neurulation. It proceeds in several consecutive stages, at each of which the shape of the embryonic material changes significantly:

1. Neural plate. A thickening of the ectoderm area occurs in the median dorsal part of the embryo's back.
2. Neural groove. The median part of the plate bends inward (invaginates). Elevations called neural folds form along the edges of this depression.
3. Neural tube. The edges of the groove gradually approach and fuse together. A completely closed tube is formed, whose wall is represented by neuroepithelial cells.
4. Neural crests (ganglionic plates). The material of the neural folds that was not incorporated into the tube transforms into loose cell clusters. These structures are located in the space between the separated neural tube and the surface ectoderm.

Derivatives of embryonic primordia

Various regions of the future nervous system differentiate from strictly defined embryonic structures.

The neural tube gives rise to the entire Central Nervous System (CNS), meaning the brain and spinal cord. In addition, a paired optic vesicle-like evagination of the forebrain gives rise to crucial structures of the eye: the retina, secretory epithelium, and iris muscles that regulate pupil size.

From the neural crests, cells actively migrate in three different directions, differentiating into:

• Ganglia: cells migrate deep into the mesoderm, where they give rise to the peripheral nervous system, specifically sensory spinal ganglia and autonomic nervous system ganglia.
• Pigment cells: a portion of the population remains directly beneath the ectoderm and transforms into skin melanocytes.
• Neuroendocrinocytes: cells form the adrenal medulla and become the hormone-producing elements of the diffuse endocrine system.

From neural placodes (specific ectodermal thickenings located on the sides of the embryonic head), certain cranial ganglia are formed.

Cellular mechanisms of neurogenesis

Blastic, i.e., immature, dividing cells—neuroblasts and glioblasts—appear in the wall of the developing primordia. A key role in their formation is played by glial progenitor cells (GPCs), also known as radial glia.

Radial glia serve as a common "progenitor" for neurons and glial elements. GPC cell bodies are located in the so-called ventricular zone (in the region of the developing third ventricle), and their long processes radially penetrate the entire thickness of the future brain wall, reaching its outer surface.

These cells divide asymmetrically: one daughter cell retains the properties of a stem progenitor cell, while the second differentiates into a migratory neuroblast. Subsequently, the neuroblast uses the long process of the radial glia as a sort of rail along which it migrates into the developing cortex. By the end of the prenatal period, GPCs lose their radial shape and finally differentiate into astrocytes.

Neuron selection and apoptosis

In embryogenesis, nervous tissue is formed with a giant excess: about $10^{12}$ neurons are produced. Each cell has three potential fates: it can become a functioning neuron (establishing synaptic contacts), remain in reserve, or die.

The scale of programmed cell death (apoptosis) is colossal—from 40% to 85% of all initial cells are eliminated. The main causes of such rigorous selection are genetic defects (DNA or chromosomal damage) and the absence of functional load. If a neuron fails to successfully establish connections with its target cells or sensory organs, it undergoes elimination. Non-functioning neurons are destroyed by the organism much more frequently than actively working ones.

Postnatal neurogenesis

In adult humans, the process of new neuron formation is severely limited and persists only in strictly defined brain zones: beneath the third ventricle and in the region near the hippocampus (this zone is closely associated with memory and emotion mechanisms).

In other parts of the CNS, new neurons are not formed, and cell death is uncompensated, resulting in a steady decline in the total number of neurons. A noticeable loss of the cell population begins at age 50–60, and by 90 years the total loss can range from 10% to 40% of the initial number of neurocytes.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of the notochord's inductive influence on the ectoderm
• Retinal histogenesis and formation of the iris muscular apparatus
• Molecular factors of neural crest cell migration
• Genetic control of asymmetric division in glial progenitor cells
• The role of apoptosis in the formation of correct synaptic networks

Other topics in the section "Nervous tissue: neurocytes, gliocytes, nerve fibers"

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