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The brain is a sensitive organ. Unlike other organs, any damage to brain cells is considered permanent and irreversible - or so it was thought. Recent research has indicated that the brain may have some capacity to regenerate and repair damaged cells. With the possibilities that stem cell research may one day offer, hope may be on the horizon for individuals suffering such crippling diseases as Huntington's, Parkinson's, and Alzheimer's disease. The ability for brain cells to regenerate themselves is known as neurogenesis.
Through the process of mitosis, new cells are formed from existing brain cells. These new stem cells are born without a function. Stimulation from their physical environment causes these new cells to differentiate, or specialize, into neuronal cells. The differentiated cells migrate to different locations of the brain by means of a chemical signal. Once they move away from their origin, these cells are either adapt and develop into mature neuronal cells, or they do not adapt and die. The ability of these cells to adapt to their new environment is known as plasticity.
At their final migration sites, the neuronal cells mature in the presence of chemical hormones known as neurotrophic growth factors and acquire their lifelong functions. The new neurons become integrated into the existing synaptic circuitry. This "regenerative" development from stem cell to mature neuronal cell is the basis of neurogenesis.
The concept of brain cell repair and regeneration in adult humans is not a new phenomenon, and certainly not exclusive to humans. First discovered in the 1960s by researchers Altman and then by Kaplan and Hinds, brain cells were observed to regenerate as axons in the brain and in the spinal cord. This revolutionary concept was later found to occur only in particular regions of the brain. In 1998, Eriksson demonstrated the repair capacity of brain cells in the hippocampus of humans, where learning and memory are affected.
Present day research has found that that neuronal stem cells proliferate and migrate to their final destinations in the subventricular zone (SVZ), which is located in the lateral ventricles of the brain, and the dentate gyrus (DG) in the hippocampal formation. Here, they develop into cells that will aid in the brain's reception and processing of olfactory information. The regenerative capabilities have been observed in mice, and other vertebrates and invertebrates.
Many external and environmental factors affect the capacity of neuronal cell birth. Neurogenesis is affected by physical activity. Increasing physical activity increases the neuron's ability for self-repair, and therefore enhances mental sharpness. Increasing levels of stress causes the body to secrete corticosteroid hormones which acts to inhibit neurogenesis by reducing the growth factor production, which is vital to new cell growth. Increasing levels of testosterone, serotonin, and glutamate, on the other hand, have been known to lead to increased neuronal cell proliferation.
Neurogenesis introduces a host of possibilities for people suffering from degenerative brain diseases. Much debate has ensued in recent years as to the use of embryonic stem cells in creating new therapies for those suffering from currently incurable genetic diseases. However, as neurogenesis has demonstrated, stem cell research brings promising results to medical applications. A brain injury today means destruction and despair; in the future, it can mean regeneration and repair.
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