Stages of Neurulation
The process of nervous system formation is called neurulation. It proceeds in several sequential stages, with the shape of the embryonic material changing significantly at each step:
- Neural plate. A thickening of the ectoderm occurs in the median dorsal region of the embryo.
- Neural groove. The median part of the plate invaginates. Elevations known as neural folds form along the edges of this depression.
- Neural tube. The edges of the groove gradually approach and fuse. A fully closed tube is formed, whose wall consists of neuroepithelial cells.
- Neural crests (ganglionic plates). Material from the neural folds that is not incorporated into the tube transforms into loose cellular clusters. These structures locate in the space between the separated neural tube and the surface ectoderm.
Derivatives of Embryonic Primordia
Different parts of the future nervous system differentiate from strictly designated embryonic structures.
The neural tube forms the entire Central Nervous System (CNS), namely the brain and spinal cord. In addition, paired cup-like evaginations of the forebrain give rise to crucial eye structures: the retina, secretory epithelium, and iris muscles that regulate pupil diameter.
The neural crest cells actively migrate in three distinct directions, differentiating into:
- Ganglia: cells migrate deep into the mesoderm, giving rise to the peripheral nervous system, specifically sensory spinal ganglia and autonomic ganglia.
- Pigment cells: a population population remains directly beneath the ectoderm and transforms into skin melanocytes.
- Neuroendocrine cells: cells form the adrenal medulla and hormone-producing elements of the diffuse neuroendocrine system.
Neural placodes (specific ectodermal thickenings located laterally on the embryonic head) form certain cranial nerve ganglia.
Cellular Mechanisms of Neurogenesis
Blastic, immature, dividing cells—neuroblasts and glioblasts—appear within the walls of the forming primordia. Glial precursor cells (GPCs), also known as radial glia, play a key role in their formation.
Radial glia serve as the common "progenitor" for neurons and glial elements. GPC cell bodies are located in the ventricular zone (near the forming third ventricle), while their long processes radially traverse the entire thickness of the future brain wall, reaching its outer surface.
Division of these cells is asymmetrical: one daughter cell retains stem/progenitor properties, while the second differentiates into a migratory neuroblast. Subsequently, the neuroblast uses the long radial glial process as a guide rail to migrate into the developing cortex. By the end of the prenatal period, GPCs lose their radial shape and finally differentiate into astrocytes.
Neuronal Selection and Apoptosis
During embryogenesis, nervous tissue is produced in massive excess: approximately $10^{12}$ neurons are generated. Each cell faces three potential fates: it may become a functioning neuron (establishing synaptic contacts), remain in reserve, or perish.
The scale of programmed cell death (apoptosis) is colossal—eliminating 40% to 85% of all initial cells. The main drivers of this strict selection are genetic errors (DNA or chromosomal damage) and a lack of functional load. If a neuron fails to successfully establish connections with its target cells or sensory organs, it is eliminated. Non-functioning neurons are destroyed by the organism much more frequently than actively working ones.
Postnatal Neurogenesis
In adult humans, the formation of new neurons is severely restricted and persists only in strictly defined brain zones: beneath the third ventricle and in the hippocampal region (closely linked to memory and emotion mechanisms).
In other regions of the CNS, new neurons do not form, and cell loss is not compensated, leading to a steady decline in the total number of neurons. Noticeable cell population loss begins around age 50–60, and by age 90, total loss can range from 10% to 40% of the initial number of neurocytes.