Aging and neuronal replacement.

Neural stem cells contribute to neurogenesis in both the embryonic and adult brain. However, while adult neural stem cells produce new neurons that populate the olfactory bulb and the granule cell layer of the hippocampus, they do not normally participate in reparative neurogenesis following injury or disease affecting regions distant from the subventricular zone or the dentate gyrus. Here we review differences between neural stem cells found in the embryo and the adult, and describe factors that enhance neuronal output from these cells in vivo. Additionally, we review evidence that neural stem cells can be transplanted into injured regions of the adult brain to enhance compensatory neurogenesis from endogenous precursors. Pre-differentiation of neural stem cells into immature neurons prior to transplantation can also aid in functional recovery following injury or disease.