The issue of whether neurogenesis occurs in the adult mammalian DRG has been debated, suggesting that late migrating neural crest cells may enter DRG and then remain in an undifferentiated state, thus explaining a potential source of NSCs (6,13). Studies on age-related neurogenesis in adult DRG provide some evidence for the existence of latent NSCs. Several studies found an age-related increase in the number of DRG neurons (10,32). It has also been proposed that the number of DRG neurons decrease with age (1,13). Yet another possibility is that there is no change in the number of DRG neurons with increasing age (22,31). In DRG from post-natal and adult rats, a significant difference was found in the total number of neurons between neonatal and adult ages, yet bromodeoxyuridine (BrdU) labeling, an indicator of proliferation, was not evident (9). These studies argue that a quiescent NSCs population in DRG may continue neurogenesis through adulthood with declines in neuron number occurring in old age. Recently, we and others have demonstrated that a protracted maturation process persists in trigeminal and dorsal root ganglia and
