Fig. 6. Summary of SHH-dependent and - independent induction of oligodendrocytes. SHH-dependent: generation of oligodendrocytes in vivo is confined to the pMN (and V3) region, which earlier in development produces motoneurones and which is characterised by the expression of the class II gene Nkx6.1 prior to induction of Olig2. Subsequent co-expression of NKX2.2 in the pMN OLIG2-expressing domain is associated with the generation of migratory OPCs. SHH-independent: neural stem cells derived from the developing spinal cord that are Nkx6.1, Olig2 and Nkx2.2 negative can generate Olig2- and Nkx2.2-positive progeny following FGF2 treatment in the absence of SHH signalling, and without induction of the V3 or pMN associated genes Nkx2.9 and Nkx6.1. This differentiation is MAP kinase dependent and can be opposed by BMP4.