For the long run: maintaining germinal niches in the adult brain.

The adult mammalian brain retains neural stem cells that continually generate new neurons within two restricted regions: the subventricular zone (SVZ) of the lateral ventricle and the dentate gyrus subgranular zone (SGZ) of the hippocampus. Though these cellular populations are spatially isolated and subserve different brain systems, common themes begin to define adult neurogenic niches: (1) astrocytes serve as both stem cell and niche cell, (2) a basal lamina and concomitant vasculogenesis may be essential components of the niche, and (3) "embryonic" molecular morphogens and signals persist in these niches and play critical roles for adult neurogenesis. The adult neurogenic niches can be viewed as "displaced" neuroepithelium, pockets of cells and local signals that preserve enough embryonic character to maintain neurogenesis for life.