Abstract
Brain plasticity is important for translational purposes since most neurological disorders and brain aging problems remain substantially incurable. In the mammalian nervous system, neurons are mostly not renewed throughout life and cannot be replaced. In humans, the increasing life expectancy explains the increase in brain health problems, also producing heavy social and economic burden. An exception to the “static” brain is represented by stem cell niches leading to the production of new neurons. Such adult neurogenesis is dramatically reduced from fish to mammals, and in large-brained mammals with respect to rodents. Some examples of neurogenesis occurring outside the neurogenic niches have been reported, yet these new neurons actually do not integrate in the mature nervous tissue. Non-newly generated, “immature” neurons (nng-INs) are also present: Prenatally generated cells continuing to express molecules of immaturity (mostly shared with the newly born neurons). Of interest, nng-INs seem to show an inverse phylogenetic trend across mammals, being abundant in higher-order brain regions not served by neurogenesis and providing structural plasticity in rather stable areas. Both newly generated and nng-INs represent a potential reservoir of young cells (a “brain reserve”) that might be exploited for preventing the damage of aging and/or delay the onset/reduce the impact of neurological disorders.
Highlights
The aging of the brain, especially in the light of a progressive increase of life expectancy, will impact the majority of people during their lifetime, putting at stake their later life and that of their relatives
Some hopes have been recognized in structural plasticity: The possibility for a “generally static” brain to undergo structural changes throughout life that may go beyond the modifications of synaptic contacts between pre-existing neuronal elements
The real roles and functions of adult neurogenesis are far from being elucidated, and it appears clear that the new neurons can mainly serve physiological functions within the neural circuits, rather than being useful for repair
Summary
The aging of the brain, especially in the light of a progressive increase of life expectancy, will impact the majority of people during their lifetime, putting at stake their later life and that of their relatives This cannot be seen only as a health problem for patients but as a more general, worrisome, social, and economic burden. And adding further complexity, non-newly generated, immature neurons sharing the same molecular markers of the newly born cells are present in the mature brain. While all these aspects of plasticity were revealed, some remarkable differences started to emerge among mammalian species, indicating that, speaking of brain structural changes, mice and humans can be different due to evolutionary choices linked to different ecological niches and behavioral needs. Particular attention will be paid to the comparative aspect: The challenge of translating the results of neurobiological research to humans needs a thorough comprehension of the cellular and molecular mechanisms of brain structural plasticity, and further knowledge of their evolutionary aspects and constraints
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