It is now established that adult neurogenesis, the generation of functional neuronal cells from adult neural progenitors, occurs continuously in two distinct regions of the mammalian brain. In the subgranular zone (SGZ) of the dentate gyrus in the hippocampus adult neural stem cells differentiate into dentate granule cells. Following a lengthy period of maturation, these cells integrate into the pre-existing neural circuitry and are believed to functionally contribute to the processes of learning and memory. In the subventricular zone (SVZ) of the lateral ventricle, adult neural stem cells form neuroblasts and a small number glia. The neuroblasts travel through the rostral migratory stream to the olfactory bulb where they differentiate into granule cells and periglomerular cells that function in olfaction.
Adult neurogenesis is regulated by both intrinsic and extrinsic factors. Epigenetic mechanisms represent the major intrinsic influence on adult neurogenesis. Epigenetics refers to changes in gene expression that are independent of alterations to the genomic DNA sequence. This may be achieved by DNA methylation, histone modification, chromatin remodeling, and/or transcriptional feedback loops. In addition, adult neurogenesis is affected by extrinsic factors including a broad range of physiological, pharmacological, and environmental stimuli. Several extracellular growth factors, hormones, and neurotransmitters have been shown to promote adult neurogenesis. The most studied of these factors are BDNF, FGF, and VEGF.