Microenvironmental elements supporting adult hippocampal neurogenesis

Neurogenesis continues into adulthood in two distinct regions, the subventricular zone of the forebrain and the subgranular zone of the dentate gyrus. Transplantation experiments have suggested that the two neurogenic regions have a special microenvironment to support the proliferation and differentiation of stem or progenitor cells. As the candidates of the microenvironment, three elements have so far been proposed: (i) astrocytes; (ii) polysialyl neural cell adhesion molecule (PSA-NCAM)-expressing immature neurons; and (iii) blood vessels. In the early developmental process of neurogenesis, newly born cells make clusters within the neurogenic regions and the clusters are found to interact structurally with astrocytes, polysialic acid-expressing immature cells, endothelium and extravascular basal laminae of blood vessels. Furthermore, recent reports have shown that astrocytes support the proliferation and differentiation of stem cells in vitro. These results suggest that these microenvironmental elements contribute to the cell proliferation and differentiation of stem or progenitor cells. However, it remains to be determined how the microenvironmental elements support adult neurogenesis functionally and coordinate with each other.