Are neurons regenerating?

Are neurons regenerating? The answer to this question is not simple and for years scientific studies have supported the claim that neurogenesis or neuronal regeneration occurs from birth and throughout our lives.

However, more recent research goes in the opposite direction and suggests that neurogenesis does not occur in the adult brain, or not the way it was believed.

In this article, we explain what neurogenesis is and give you the keys to understanding the current controversy regarding whether or not neurons regenerate in adulthood.

    Neurogenesis: what does it consist of?

    Neurogenesis refers to the process by which new neurons are generated in the brain. This phenomenon is crucial during embryonic development, but apparently also continues in some areas of the brain after birth and for the rest of our lives.

    The mature brain has a multitude of specialized areas and neurons that differ in structure and connections. The hippocampus, for example, which is an area of ​​the brain that plays an important role in memory and spatial navigation, has at least 27 different types of neurons.

    This incredible neuronal diversity in the brain is the product of neurogenesis during embryonic development. During pregnancy, and thanks to stem cells, cell differentiation occurs, the process by which these neurons undergo genetic changes and acquire the morphology and functions of a specific cell type, at certain times and regions of the brain.

    Stem cells can divide indefinitely to generate more stem cells or to differentiate, giving rise to more specialized cells, such as progenitor cells. These can be differentiated into several specific types of neurons (neurogenesis); or, they can differentiate into glial progenitor cells, which give rise to glial cells such as astrocytes, oligodendrocytes, and microglia (gliogenesis).

    Are neurons regenerating?

    Neuroplasticity refers to the nervous system’s adaptive capacity to change throughout life based on learning gained through behavior and experiences. The brain can create new or strengthen existing connections between neurons and different neural circuits. This process of improving communication between neurons is called synaptic plasticity.

    On the other hand, the brain is also capable, at least in certain regions, of producing progenitor cells which produce neurogenesis. Until recently, neuroscientists believed that adult neurogenesis did not occur; that is, the birth of neurons was assumed to be limited to the period of time comprising embryonic development and the first years of childhood, and after this period of rapid growth, the nervous system was unable to regenerate.

    This belief stems from the fact that unlike most cells in our body, mature neurons do not undergo cell division, a process by which a cell (the parent cell) divides into two or more new cells (daughter cells. about 20 years ago, when it was first reported that neurons were regenerating in the adult human brain.

    Since, Numerous studies have determined that new neurons are born throughout life in specific neurogenic areas of the brain, As the sub-granular area of ​​the dentate gyrus of the hippocampus and the subventricular area (the ejido located under the lateral ventricles), not from the division of mature cells, but from the differentiation of neural stem cells.

    Neural stem cells

    Stem cells are undifferentiated biological cells that can generate different types of specialized cells through cell differentiation. Some can become any type of differentiated cell in our body: these are the so-called totipotent stem cells; and others can become almost any cell: pluripotent stem cells.

    Other types of stem cells already have a certain degree of specialization and can only be transformed into specific and closely related cells (multipotent stem cells), such as different types of cells in a tissue.

    There are also stem cells that have already committed to being a specific type of cell (unipotent stem cells)., But which retain the ability to self-renew through cell division. This capacity for self-renewal is another distinctive characteristic of stem cells.

    In short, neural stem cells are self-renewing multipotential stem cells of the nervous system and are capable of generating both new neurons and glial cells (non-neuronal brain cells that serve as support and protection for neurons).

    Neurogenesis in the adult brain: the controversy

    Most research on neurogenesis in adults has focused on one region of the brain: the dentate gyrus of the hippocampus. Neurogenesis has been observed in this area of ​​the brain in all mammalian species studied to date.

    In the adult human brain, this process of neuronal regeneration seems to occur in the hippocampus, An area particularly important for learning and memory, emotions, mood, anxiety or stress response.

    Another area where evidence of adult neurogenesis has been found in humans is the striatum, a region of the brain known for its role in motor coordination, but also in other processes such as the regulation of reward, dislike, motivation or pleasure.

    The striated body has been identified as a key structure in higher cognitive functions, particularly in cognitive flexibility, the ability to adapt behavioral goals in response to changing environmental demands.

    However, the controversy is served, as recent research has shown that the formation of new neurons in the structures of the hippocampus decays in childhood and is very rare or nonexistent in the adult brain.

    The study, published in 2018 in the journal Nature, concluded that the recruitment of young neurons into the hippocampus declines rapidly during the first years of life, and that neurogenesis in the dentate gyrus of this brain structure does not persist or is extremely rare in adult humans.

    The explanation for the latter could lie in the fact that, although markers have been frequently found associated with new neurons, these markers can also be found in neurons that are born during development and have remained in cells for years..

    However, the reverse explanation has also been put on the table by neuroscientists in favor of adult neurogenesis, and it has been argued that just because no new neurons are observed does not mean that they are not there, but simply that we are not in a position to detect them.

    In addition, this study also suggests that plasticity in the adult hippocampus does not require the continuous generation of new neurons; according to the authors, it is possible that the brain has a “reservoir” of neurons that never fully mature, or that they do so slowly and can make changes, so there is no need to integrate new neurons. A hypothesis that remains to be tested.

    In both cases, today there is no clear consensus in the scientific community on whether or not neurons regenerate in the adult brain. The evidence is conflicting, and more recent research seems to call into question decades of research on neurogenesis in adults.

    So the only certainty we have at the moment is that there is still a long way to go.

    Bibliographical references:

    • Kempermann, G. (2016). Adult neurogenesis: an evolutionary perspective. Cold Spring Harbor Perspectives in Biology, 8 (2).

    • Kozorovitskiy, Y., and Gould, E. (2008). Adult neurogenesis in the hippocampus. Handbook of Cognitive Developmental Neuroscience, 51-62.

    • Song, J., Zhong, C., Bonaguidi, MA, Sun, GJ, Hsu, D., Gu, Y., … and Deisseroth, K. (2012). Mechanism of neural circuits that regulates the destination decision of resting neuronal stem cells in adults. Nature, 489 (7414), 150.

    • Sorrells, SF, Paredes, MF, Cebrian-Silla, A., Sandoval, K., Qi, D., Kelley, KW, … and Chang, EF (2018). Human hippocampal neurogenesis drops sharply in children to levels undetectable in adults. Nature, 555 (7696), 377.

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