Postnatal and adult neurogenesis in the development of human disease.

The mammalian brain contains a population of neurons that are continuously generated from late embryogenesis through adulthood-after the generation of almost all other neuronal types. This brain region-the hippocampal dentate gyrus-is in a sense, therefore, persistently immature. Postnatal and adult neurogenesis is likely an essential feature of the dentate, which is critical for learning and memory. Protracted neurogenesis after birth would allow the new cells to develop in conjunction with external events-but it may come with a price: while neurogenesis in utero occurs in a protected environment, children and adults are exposed to any number of hazards, such as toxins and infectious agents. Mature neurons might be resistant to such exposures, but new neurons may be vulnerable. Consistent with this prediction, in adult rodents seizures disrupt the integration of newly generated granule cells, whereas mature granule cells are comparatively unaffected. Significantly, abnormally interconnected cells may contribute to epileptogenesis and/or associated cognitive and memory deficits. Finally, studies increasingly indicate that new granule cells are extremely sensitive to a host of endogenous and exogenous factors, raising the possibility that disrupted granule cell integration may be a common feature of many neurological diseases.