The timing of neuronal development in adult hippocampal neurogenesis.

The granule cell layer (GCL) of the adult dentate gyrus (DG) is a heterogeneous structure formed by neurons of different ages because a significant proportion of neurons continues to be generated throughout life. The subgranular zone of the DG contains neural progenitor cells (NPCs) that divide, differentiate, and migrate to produce functional dentate granule cells (DGCs) that become incorporated into the existing hippocampal circuitry. New available tools to identify adult-born neurons in live and fixed brain sections have allowed the transition from NPC to functional neuron to be characterized in great detail. Maturation of the neuronal phenotype includes changes in membrane excitability and morphology as well as the establishment of appropriate connectivity within the existing circuits, a process that lasts several weeks. The events leading to neuronal maturation share many of the features of the developing brain, and electrical activity is emerging as a key modulator of neuronal development in the adult DG. The underlying mechanisms are now beginning to be understood.