Figure 1.

Origins, migration and development of OPCs and Schwann cells. (a) In the rodent spinal cord, OPCs originate from the pMN domain in the ventral ventricular zone at approximately embryonic (E)12.5. This is followed by a second wave of progenitors in more dorsal regions, which then migrate throughout the spinal cord to myelinate white matter tracts [23]. Similarly, in the rodent telencephalon, the first wave of OPCs arise from ventral progenitor cells in the medial ganglionic eminence (MGE) at E12.5. Subsequently, a second wave of OPCs is generated several days later at about E15.5 by dorsal progenitor cells in the lateral ganglion eminence (LGE). After birth, cortex-derived progenitors give rise to the final wave of OPCs. (b) During the formation of the neural tube, neural crest cells arise from the tips of the neural folds. Neural cells initially form and accumulate at the dorsal surface of the tube, but soon migrate along with different pathways to differentiate into SC precursor cells, as well as progenitors of melanocytes, autonomic neurons, dorsal root sensory glia, chromaffin cells and other peripheral glia). SC precursor cells then transition into immature SCs upon neuregulin-1 (NRG1), fibroblast growth factor 2 (FGF2) and Notch signalling. While all immature SCs are thought to possess myelinating potential, only immature SCs in close association with large-diameter axons differentiate into myelinating SCs. By contrast, SCs that associate with smaller-diameter axons form bundles of non-myelinating Remak cells [32].