Glial progenitors in the CNS and possible lineage relationships among them.

Glial cells are derived from stem cells that mature through specific stages of development to generate fully differentiated astrocytes and oligodendrocytes. Several types of intermediate precursors have been described and in some cases lineage relationships identified although this remains a subject of controversy. We review recent findings and discuss some possibilities. Motoneuron-oligodendrocyte precursors (MNOPs), white matter progenitor cells (WMPCs), polydendrocytes, glial restricted precursors (GRPs), astrocyte precursor cells (APCs), and oligodendroblasts are likely all derived from earlier appearing stem cells but segregate at different stages in development. Some of these precursors persist in the adult, and it is these glial progenitors rather than stem cells that respond after injury and participate in the repair process. Although which specific glial progenitor responds remains unclear, the availability of new markers will likely resolve this issue. We believe that the development of consensus sets of markers and an improvement in our ability to define stages of glial maturation will lead to a clearer appreciation of the importance of glia in the etiopathology of disease.