Constitutive and induced neurogenesis in the adult mammalian brain: Manipulation of endogenous precursors toward CNS repair

Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons. During the past 3 decades, research exploring potential neuronal replacement therapies has focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain. Recent advances in our understanding of related events of neural development and plasticity, including the role of radial glia in developmental neurogenesis and the ability of endogenous precursors present in the adult brain to be induced to produce neurons and partially repopulate brain regions affected by neurodegenerative processes, have led to fundamental changes in the views about how the brain develops as well as to approaches by which endogenous precursors might be recruited to repair the adult brain. Recruitment of new neurons can be induced in a region-specific, layer-specific and neuronal-type-specific manner, and, in some cases, newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow the development of neuronal replacement therapies for neurodegenerative disease and other CNS injuries that do not require transplantation of exogenous cells.