In the human and mouse brain, continued neurogenesis has been documented throughout adulthood. These new neurons derive from neural stem cells (NSCs) residing in the subventricular zone and in the subgranular zone of the hippocampal dentate gyrus13. Progeny of stem cells located in the subventricular zone travel through the rostral migratory stream into the olfactory bulb to differentiate into interneurons, whereas progeny from the subgranular zone of the dentate gyrus migrate to the granular layer, where they integrate with its resident cells13. Although a few studies have demonstrated that the human brain experiences a decline in neurogenesis with ageing, much of our understanding of age-associated changes in neurogenesis comes from rodent studies. Old mice (>2 years of age) experience a twofold reduction in the number of NSCs relative to young mice, a loss that mirrors the respective numbers of neurospheres derived from young and old mouse brains14,15. Accompanying this age-associated decline is an overall drop in the number of newly generated neurons, a drop that has apparent functional consequences. For example, in the ageing mouse brain, the decline in neurogenesis
