The granule cell population of the dentate gyrus is produced in three distinct phases occurring during gestation, the early postnatal period and in adulthood. During the embryonic period, new neurons arise from the ventricular zone and migrate across the hippocampal rudiment to populate the incipient dentate gyrus (Schlessinger et al., 1975; Altman & Bayer, 1990). Progenitor cells also migrate into this region and continue to produce new neurons well into the postnatal period – these new granule cells help to form the granule cell layer (Schlessinger et al., 1975; Altman & Bayer, 1990). In young adulthood, progenitor cells are located on the border of the granule cell layer and hilus, a region called the subgranular zone (sgz). These cells divide and produce new granule cells throughout adult life. Although the rate of adult neurogenesis slows considerably with advancing age (Seki & Arai, 1995; Kuhn et al., 1996; Simon et al., 2005; Leuner et al., 2007), some new granule cell production is evident even in the dentate gyrus of the very old.
