Neural Development and Brain Growth: Beyond Mitosis

Contrary to popular belief, neurons, the primary cells of the brain, do not divide through the process of mitosis like most other cell types. Instead, they develop and grow through a variety of mechanisms that allow for the ongoing growth and adaptation of the brain.

Understanding Neurogenesis and Neural Growth

Neurogenesis, the process by which new neurons are generated, is a key aspect of neural development. Unlike somatic cells that undergo regular mitosis, neural stem cells in certain regions of the brain, such as the hippocampus, can produce new neurons throughout life. This process enables the brain to form new neurons in response to learning experiences and environmental factors, contributing to the brain's ongoing development.

Structural Changes and Synapse Formation

Neurons can grow and form new connections, synapses, with other neurons. This growth occurs as neurons extend their dendrites, the branch-like structures that receive signals, and axons, the long projections that send signals. The formation of new synapses is critical for learning and memory. Neurons exhibit plasticity, meaning they can change in response to experience and activity, including the strengthening or weakening of existing synapses and the formation of new synaptic connections throughout life.

Myelination and Signal Transmission

While neurons themselves do not divide, the process of myelination, the formation of a protective myelin sheath around axons, significantly enhances the speed and efficiency of signal transmission. Myelination is carried out by glial cells, specifically oligodendrocytes in the central nervous system. Even though neurons do not increase in number after a certain age, they constantly produce new branches or neurites, especially dendrites, and form new synapses. This contributes more to brain volume than the nucleated cell bodies or neuron count.

The Role of Glial Cells and Growth Factors

The white matter of the brain consists largely of myelin, a crucial component that is relatively scarce at birth. Myelin is produced after birth and significantly contributes to brain growth. In addition, glial cells, which include astrocytes, remain capable of mitosis and play a significant role in brain development. Astrocytes and other glial cells support neurons in various ways, including promoting neuron survival, growth, and differentiation.

Brain Growth Beyond Mitosis

Brain growth is a multifaceted process involving both the production of new neurons and the structural changes that occur in existing neurons. New neurons are produced from neural stem cells during childhood, and only half of the brain cells are neurons—mainly supporting cells called glial cells. Astrocytes, a type of glial cell, constitute over 90% of the tissue in some brain regions and remain capable of mitosis. Myelin, formed by oligodendrocytes, is a critical component of the brain, with much of it being produced after birth.

Figure 1. Myelinated axon of the brain. Myelin consists of spiral, compressed layers of oligodendrocyte membrane, significantly increasing the volume of the myelin sheath compared to the underlying axons.

Figure 2. Neurons of the cerebral cortex visualized by fluorescence microscopy. The volume of the cell bodies is far less than the volume of all their branches (neurites), especially dendrites, which can grow and retract constantly throughout life.

Figure 3. Nervous tissue of the medulla oblongata. Notice that the tiny glial cells greatly outnumber the neurons, but most of the tissue volume is composed of the fibrous-looking neurites of the nerve cells.

Long-Term Developments in Brain Size

Research has shown that the brain grows and changes throughout life. Gray matter volume increases until late childhood, white matter volume increases until young adulthood, and the cerebral cortex grows thicker until late infancy or early childhood. After age 40, there is a rapid increase in the volume of the ventricles, the spaces within the brain that contain cerebrospinal fluid.

For further insights into the complex processes of brain development and growth, please refer to the comprehensive articles available on the subject.

Conclusion

Neurons and the brain do not rely solely on mitosis for growth and development. Instead, they employ a range of mechanisms including neurogenesis, structural changes, and the influence of growth factors to ensure continuous adaptation and expansion.