considered as a possible treatment for neurological diseases which cause demyelinatior. However, it is difficult to generate a suitable number of quality cells for clinical use. Finding a source for these cells remains impractical as of 2016. Should adult cells be used for transplantation, a brain biopsy would be required for each patient, adding to the risk of immune rejection. Embryonically derived stem cells have been demonstrated to carry out remyelination under laboratory conditions, but some religious groups are opposed to their use. Adult central nervous system stem cells have also been shown to generate myelinating oligodendrocytes, but are not readily accessible. Even if a viable source of OPCs were found, identifying and monitoring the outcome of remyelination remains difficult, though multimodal measures of conduction velocity and emerging magnetic resonance imaging techniques offer improved sensitivity versus other imaging methods. In addition, the interaction between transplanted cells and immune cells and the effect of inflammatory immune cells on remyelination have yet to be fully characterized. If the failure of endogenous remyelination is due to an unfavorable differentiation environment, then this will have to be addressed prior to transplantation. History It had been known since the early 1900s that astrocytes, oligodendrocytes, and microglia make up the major glial cell populations in the mammalian CNS. The presence of another glial cell population had escaped recognition because of the lack of a suitable marker to identify them in tissue sections. The notion that there exists a population of glial progenitor cells in the developing and mature CNS began to be entertained in the late 1980s by several independent groups. In one series of studies on the development and origin of oligodendrocytes in the rodent CNS, a population of immature cells that appeared to be precursors to oligodendrocytes was identified by the expression of the GD3 ganglioside. In a separate series of studies, cells from perinatal rat optic nerves that expressed the A2B5 ganglioside were shown to differentiate into oligodendrocytes in culture. Subsequently, A2B5+ cells from other CNS regions and from adult CNS were also shown to generate oligodendrocytes. Based on the observation that these cells require PDGF for their proliferation and expansion, the expression of the alpha receptor for platelet-derived growth factor (Pdgfra) was used to search for the in vivo correlates of the A2B5+ cells, which led to the discovery of a unique population of Pdgfra+ cells in the CNS whose appearance and