Neurogenesis, cellular plasticity and cognition: the impact of stem cells in the adult and aging brain--a mini-review.

The hippocampus is a structure equipped with a high degree of flexibility and adaptation. In contrast to most structures of the adult central nervous system, the hippocampus can rely on a form of plasticity known as neurogenesis. The continuous provision of new neurons derived from resident adult neural stem cells appears to facilitate the execution of hippocampal-dependent tasks since reduction or blockage of neurogenesis is associated with cognitive impairments. Importantly, however, although hippocampal neurogenesis is maintained all throughout life, its levels decrease steadily along with aging. Notwithstanding some evidence that in age-matched animals neurogenesis levels and learning performance are tightly associated, these two parameters do not appear to be directly coupled when comparing individuals of various age groups. Additional components, and in particular experience, appear to play a fundamental roles in hippocampal functions. In this review, we speculate on the impact of neurogenesis level modulation on cognitive performances, putting in perspective recent studies made in the aging human population and in rodent models of aging.