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 is associated with an impairment of olfactory discrimination due to the presence of fewer NSCs in the subventricular zone and, thus, a diminished ability to replenish the olfactory epithelium continually16.
