Neural stem cells and the regulation of neurogenesis in the adult hippocampus.

Neurogenesis continues in the hippocampal dentate gyrus of adult rodents and primates including humans. Neurons are born in the underlying subgranular layer (SGL) and move into the granule cell layer (GCL) to become mature granule neurons. Recent work indicates that the primary precursors for these new neurons correspond to radial astrocytes whose cell body is in the SGL and their processes traverse the GCL. These astrocytes divide to give rise to intermediate precursors, D cells that likely become mature granule neurons. Here we propose that the anatomy of radial astrocytes may allow for signals within the GCL to regulate neurogenesis in the SGL. Levels of neuronal activity within the granule cell layer may regulate the proliferation rates of radial astrocytes and determine the number of new neurons produced in the dentate gyrus.