Sonic hedgehog signaling is required during the appearance of spinal cord oligodendrocyte precursors

Spinal cord oligodendrocyte precursors arise in the ventral ventricular zon e as a result of local signals. Ectopic oligodendrocyte precursors can be i nduced by sonic hedgehog (Shh) in explants of chick dorsal spinal cord over an extended developmental period. The role of Shh during normal oligodendr ocyte development is, however, unclear. Here we demonstrate that Shh is loc alized to the ventral spinal cord immediately prior to, and during the appe arance of oligodendrocyte precursors. Continued expression of Shh is requir ed for the appearance of spinal cord oligodendrocyte precursors as neutrali zation of Shh signaling both in vivo and in vitro during a defined developm ental period blocked their emergence. The inhibition of oligodendrocyte pre cursor emergence in the absence of Shh signaling was not the result of inhi biting precursor cell proliferation, and the neutralization of Shh signalin g after the emergence of oligodendrocyte precursors had no effect on the ap pearance of additional cells or their subsequent differentiation. Similar c oncentrations of Shh induce motor neurons and oligodendrocytes in dorsal sp inal cord explants. However, in explants from early embryos the motor neuro n lineage is preferentially expanded while in explants from older embryos t he oligodendrocyte lineage is preferentially expanded.