Research Perspective Volume 4, Issue 3 pp 176—186

Figure 1. Potential mechanisms responsible for the age-related decline in neurogenesis. (A-E) Schematic representation of adult neurogenesis in the young (A) or senescent (B-E) hippocampus. Proliferating and quiescent neural stem and progenitors cells (NSCs; pink) are indicated by the presence or lack of mitotic spindles, respectively. Proliferating NSCs divide to generate (arrows) additional NSCs or immature neurons (blue). During maturation, many neurons undergo apoptosis (crosses). A reduced number of neurons in the senescent hippocampus (B-E) can be explained by an increase in quiescence i.e. a lower proportion of proliferating NSCs (B), a change in NSCs fate i.e. an increase in differentiative at the expense of proliferative and/or gliogenic at the expense of neurogenic division (C), depletion of NSCs by cell death (D), or a higher proportion of newborn neurons undergoing apoptosis (E). Continuous or dashed arrows (A-D) indicate a constant or decreases proportion of proliferating NSCs and neurons generated from the total pool of NSCs, respectively. Note that NSCs in E are not colored because neuronal death could potentially occur concomitantly to any of the previous conditions (A-D).