Young at heart: Insights into hippocampal neurogenesis in the aged brain.

While the existence and importance of adult hippocampal neurogenesis in young adult rodents has been well-established, such qualities in aged animals and humans have remained poorly understood. Most evidence in humans has come from hippocampal volumetric changes that provide no direct proof of new neurons in adulthood. Here, we review the basic neurobiological evidence for adult hippocampal neurogenesis in the aged brain of experimental animals with short and long lifespans, and humans. The rate of cell cycling and addition of new hippocampal neurons to the existing hippocampal circuit undoubtedly decreases with age. Yet, neural stem/progenitor cells that persist into senescence may activate and produce a substantial number of functional new neurons that exhibit enhanced survival and integration given the right set of conditions. There thus exists remarkable potential for newly-generated neurons in the senescent hippocampus to make important circuit- and behavioral-level contributions, which may serve as a target for future therapeutics.