Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneuron s. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog, We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial expl ants, isolated at various development stages, are unable to generate oligod endrocytes in culture until E5 but become able to do so in an autonomous wa y from E5.5. This indicates that the induction of oligodendrocyte precursor s is a late event that occurs between E5 and E5.5, precisely at the time wh en the ventral neuroepithelium stops producing somatic motoneurons. Analysi s of the spatial restriction of oligodendrocyte progenitors, evidenced by t heir expression of O4 or PDGFR alpha, indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not i n the adjacent domain characterized by Pax6 expression from which somatic m otoneurons emerge. We then confirm that Shh is necessary between E5 and E5. 5 to specify oligodendrocyte precursors but is no longer required beyond th is stage to maintain ongoing oligodendrocyte production. Furthermore, Shh i s sufficient to induce oligodendrocyte formation from ventral neuroepitheli al explants dissected at E5, Newly induced oligodendrocytes expressed Nkx2. 2 but not Pax6, correlating with the in vivo observation. Altogether, our r esults show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.