Neurogenesis in the adult mammalian brain

The production of neurons in the mammalian brain is typically restricted to a discrete developmental period ending, for the most part, prior to parturition. However, in certain regions of the brain, including the dentate gyrus, new neurons continue to be produced well into adulthood. In the adult brain, new cells arise from progenitors located within the hilus and subgranular zone of the dentate gyrus, and then migrate into the dentate granule cell layer. Morphological, biochemical, and ultrastructural evidence indicate that many of these new cells become granule neurons, the principal projection neuron of the dentate gyrus. Anatomic studies have demonstrated that adult-generated granule neurons contribute axonal projections to area CA3 of Ammon's horn, while electron microscopy studies have revealed that these cells possess dendritic processes that extend into the dentate molecular layer to form synapses. Collectively, these data indicate that adult-generated granule neurons become functionally incorporated into the pre-existing neural circuitry of the dentate gyrus. The production and survival of adult-generated granule neurons are significantly influenced by experiential and neuroendocrine factors, suggesting that adult neurogenesis represents a substrate by which the environment may affect the structure and function of the adult brain. Although the precise function of adult-generated granule neurons is unknown, the formation of entirely novel neural circuits, and the regulation of this process by neuroendocrine and experiential factors, is likely to represent an important mode of neural plasticity. Moreover, the persistence of neural progenitors within the adult brain provides hope that an understanding of the process of adult neurogenesis may ultimately be of therapeutic relevance.