Neural progenitor cells of the adult brain.

Recent studies have substantially expanded our conception of the types of progenitor cells that continue to reside in the adult nervous system, and their respective roles in the normal maintenance of the brain and spinal cord. In the adult, neural stem cells persist within the forebrain ventricular zone, and give rise to a variety of more restricted progenitor phenotypes. The major progenitor pools of the adult human brain, each of which has now been isolated to purity, include ventricular zone neuronal progenitor cells, hippocampal neuronal progenitors and parenchymal glial progenitor cells. Each of these phenotypes exists within a local environmental niche, which tightly regulates both the mitotic activity and derivatives of its resident progenitors. Within these niches, both neuronal and glial progenitor cells may reside as transit amplifying pools, by which lineage-biased progenitors expand to replenish discrete mature phenotypes. The largest such pool appears to be that of the parenchymal glial progenitor cell. These cells are restricted to glial phenotype by their local white matter environment, but upon removal from that setting, can expand to generate neurons as well as glia. Thus, at least some populations of adult glial progenitors retain both multilineage capacity and mitotic competence, and as such may be able to regenerate the tissue-specified stem cells from which they derived. This presentation will focus on the interactions of the adult human white matter with its resident glial progenitor cells, emphasizing those pathways that regulate the maintenance of these cells in their undifferentiated state, in particular those that may be shared with ventricular zone neural stem cells.