9 Neurogenesis in the Adult Hippocampus

As noted previously in this volume, adult neurogenesis is a process, not an event. Adult neurogenesis comprises a series of sequential developmental events that are all necessary for the generation of new neurons under the conditions of the adult brain. In the original publications on adult neurogenesis, the precursor cell population from which neurogenesis originates was identified only by the detection of proliferative activity and the absence of mature neuronal markers (Altman and Das 1965; Kaplan and Hinds 1977; Cameron et al. 1993; Kuhn et al. 1996). The new neurons, in contrast, were identified by the presence of mature neuronal markers in cells that had been birthmarked with the thymidineoder BrdU (bromodeoxyuridine) method (see Chapters 2 and 3) a couple of weeks earlier. The expression of the polysialilated neural cell adhesion molecule (PSA-NCAM) with neurogenesis has been noted early but could not be clearly linked to either proliferation or mature stage (Seki and Arai 1993a,b). PSA-NCAM expression was the first indication of the developmental events that take place, filling the gaps between the start and endpoint of development. Today, we have quite detailed knowledge about the course of neuronal development in the adult hippocampus, and although many detailed questions remain open, a clear overall picture has emerged (Kempermann et al. 2004; Abrous et al. 2005; Ming and Song 2005; Lledo et al. 2006). Although we coarsely talk about neurogenesis in the hippocampus, it should be noted that neurogenesis occurs only in the dentate gyrus (DG), not other regions; in an...