Neurogenesis in the damaged mammalian brain

Once considered to be a structurally rigid organ, the adult mammalian brain has recently been the subject of a series of discoveries of constant remodeling at multiple levels, including synapses, dendrites, axons, and neuronal soma under physiological conditions (for example, see a recent review by Abraham, 2008). Continuous production of new neurons (neurogenesis) in the mature brain is among recent additions to such structural plasticity. Moreover, recent studies have revealed its previously unrecognized capacity for self-repair, i.e., supply of new neurons and glia after damage. Studies of injured brains have also revealed that endogenous neural stem/progenitor cells serve as sensitive responders to various injury signals and actively participate in tissue repair in many ways. This review summarizes the current understanding of this injury-induced neurogenesis/gliogenesis in the adult mammalian brain and critically evaluates its significance in the context of brain repair. Emphasis is on the comparison between persistent and injury-induced neurogenesis and regulators and outcomes of neuronal and glial production in damaged/diseased brains. Several important issues, in particular, those that remain controversial, as well as the recently emerging idea that consider stem/progenitor cells as injury sensors and responders are also highlighted. Finally, prospects of future research aiming at utilizing endogenous repair capacity for therapy for various neurological disorders are discussed.