Numerous studies have suggested that the rate of neurogenesis in both SVZ and DG declines with age, raising the possibility that reduced neurogenesis may account, to some degree, for the impaired learning and memory and cognitive deterioration in the elderly (Tropepe et al., 1997; Seki and Arai, 1995; Kuhn et al., 1996; Kempermann et al., 1998; Kempermann et al., 2002). Examination of neurogenesis in brain tissue of AD patients revealed increased expression of immature neuronal marker proteins (Jin et al., 2004a). However, these observations have been challenged recently (Boekhoorn et al., 2006). Other reports suggest that in the aged and AD brain, there is a significant decline in extent of proliferation of progenitor cells and their numbers [For review see (Brinton and Wang, 2006)]. A recent study suggests that levels of stem cell factor (SCF), a hematopoietic growth factor that supports neurogenesis in the brain, are reduced in the plasma and cerebrospinal fluid of individuals affected with AD (Laske et al., 2008).
