Glia as neural progenitor cells.

Recent studies have substantially expanded our conception of the roles for glia in function and maintenance of the adult nervous system. Of these reports, several have re-examined the lineage relationships among neural stem cells, their early radial glial derivatives and their mitotically competent neurogenic daughters. These studies have highlighted the role of radial cells in development, and of their glial progeny postnatally, as both progenitors and regulators of neuronal production and phenotype. In the adult mammalian brain, radial cell populations are scant, but their glial derivatives participate in a gliovascular network that organizes not only the structural and functional architecture of the brain but also its generative niches for resident progenitors - glial as well as neuronal. As in other organs, these progenitors can reside as transit-amplifying pools, by which lineage-biased progenitors expand to replenish discrete mature phenotypes. This review will consider the types of transit-amplifying progenitor cells persistent in the adult mammalian CNS, and the extent to which these derive from glial phenotypes. It will also discuss the interactions of progenitor cells with their brethren that could specify their phenotype and fate, while defining the permissive niches for cell genesis in the adult CNS.