Astrocytic nature of adult neural stem cells in vivo

The identification of adult brain regions harboring neural stem cells and of their continual generation of new neurons throughout life challenges traditional views of the adult brain's germinal potential and suggests new alternatives for brain repair. Surprisingly, cells with characteristics of mature astrocytes have been shown to function as the primary precursors of new neurons in vivo. This observation goes against the generalized assumption that the rare cells with undifferentiated appearance are neural stem cells. It suggests that neural stem cells in vivo have important structural functions in addition to their role as primary progenitors. Surprisingly, neurospheres - a commonly used model of neural stem cells in culture - are not necessarily derived from the in vivo primary precursors but from transit amplifying cells. We suggest that neural stem cells, when activated to become transit amplifying cells, upregulate the expression of growth factor receptors on their surface, allowing their isolation as neurospheres.