Since the end of the 20th century, it has been known that the brain is capable of forming new cells even in adulthood through neurogenesis. Accordingly, neurogenesis is the formation of new neurons from progenitor and stem cells, which occurs both during embryogenesis and in the adult nervous system.
What is neurogenesis?
Neurogenesis is the formation of new neurons from progenitor and stem cells, which occurs both during embryogenesis and in the adult nervous system. Neurogenesis during the embryonic period includes the development as a whole, starting with the secretion of neural crest cells, the formation of the first vesicles in the brain and ending with the differentiation of the central nervous system together with the functional supply of organs, cells and tissues in the organism. Neurogenesis begins with the formation of the neural tube, which detaches from the ectoderm in the early stage of embryogenesis, which in turn is called neurulation. This lays the foundation for the development of the spinal cord, brain and nervous system. The first embryonic precursor structures of the brain, so-called cerebral vesicles, which later form the forebrain, midbrain and rhomboid brain, are formed at the upwardly oriented portions of the neural tube. Here, the anlagen of the spinal cord and the central nervous system are built up, likewise the sense organs such as eyes, ears and nose. There is also still a reservoir of stem cells in the brain of an adult human. For a long time, researchers assumed that the formation of new nerve cells in the central nervous system no longer occurs. This has proven to be wrong. Stem cells proliferate, nerve cells are formed even in old age, and the process is dependent on physical and mental activity. Adult neurogenesis is regulated in this way.
Function and task
There are two brain regions, more specifically the hippocampus and the subventricular zone, where neurogenesis occurs even in the adult brain. The hippocampus enables memory and learning processes in humans. If the area is disturbed, neurodegenerative diseases occur. These include depression and anxiety, but can also lead to Parkinson’s disease. Adult neurogenesis takes place in the subgranular zone. This lies between the granule cell layer and the hilus. Cell bodies of neural stem cells have apical cell processes of neurons, also called dendrites, which serve to receive stimuli, which at this point lead into the molecular cell layer and which have a high density of voltage-independent potassium and sodium channels. These do not respond to stimuli and excitation there, but have the ability to build new cells by mitotic division, which are called progenitor cells. Division proceeds at different levels. The rate of division is affected when, for example, stress factors come into play and trigger an inhibitory effect. Physical activity, in turn, increases the rate of division and is even the most effective way to promote neurogenesis in the brain. Exercise increases blood flow to the heart and brain, increases levels of the growth factor BDNF and the nerve growth factor GDNF, and activates neurogenesis. Released endorphins lower the level of the stress hormone cortisol. Especially in old age, the aging process is delayed here and cognitive performance is improved. Equally helpful are meditative exercises. The processes in the brain are calmed down in this way, mental clarity is gained and the focus is on the moment of the here and now. The density of gray matter of certain regions in the brain, including that of the hippocampus, is thus increased, chemical messengers in the brain are balanced, including those responsible for adult neurogenesis. Integrative functions are activated, leading to long-term neural changes in the brain and increasing melatonin production. Equally important is a healthy diet. The brain is made up of about sixty percent fat. Accordingly, a healthy high-fat diet is necessary and provides important omega-3 fatty acids and docosahexaenoic acid, which the brain needs to form new brain cells. In addition, adequate sleep, sun exposure and well-being are important to maintain neurogenesis and bring about healthy brain function. Lack of sleep, for example, slows down the process in the hippocampus and even upsets the entire hormone balance.Cell proliferation is inhibited. Sun exposure, in turn, supplies the body with vitamin D, increasing serotonin levels. Ultimately, the exact function of adult neurogenesis has not yet been sufficiently researched. What is known is that newly formed granule cells have an influence on long-term memory. Also, environmental and living conditions probably have an influence on neurogenesis.
Diseases and disorders
Diseases of the central nervous system, including post-traumatic stress disorder are thought to be cured with the stimulation of neurogenesis. Thus, the finding that the adult brain also forms new neurons from stem cells has been helpful in promoting improved therapy for people suffering from Alzheimer’s or Parkinson’s disease, for example. In the brain of a person with Parkinson’s disease, the proliferation (tissue proliferation) of neural progenitor cells is severely inhibited. In such cases, doctors mainly examine the olfactory bulb in the brain. There, the nerve cells die very early, which is why a disturbance of the olfactory perception occurs. Dopamine is then supposed to alleviate the motor symptoms, although the death of the nerve cells cannot be prevented. Stimulation of neurogenesis in the brain should compensate for the cell loss. This could be done, for example, in the premotor phase of Parkinson’s disease, which is particularly noticeable in sleep and olfactory disturbances, as well as initial cognitive and psychiatric symptoms. At this stage, attempts can be made to stimulate neurogenesis and in this way somewhat limit the degenerative process.
