The role of Sonic hedgehog in neural tube patterning

In the developing neural tube of vertebrate embryos, many types of neuronal and nonneuronal cells differentiate in response to the secreted signalling molecule, Shh. Shh shows a spatially restricted pattern of expression in c ells located at the ventral midline, yet governs the differentiation of div erse cell types throughout the ventral half of the neural tube. Here, we de scribe how the distinct fate assumed by cells in response to Shh is depende nt upon their position with respect to both the dorso-ventral and anterior- posterior axes of the neural tube and describe the ways in which a single f actor, Shh, is able to pattern the developing nervous system. We first disc uss the evidence that Shh does impose ventral identity on cells in the neur al tube, then focus on the role of a graded Shh signal in patterning the ne ural tube and finally discuss the interaction of Shh with other factors tha t affect its signalling outcome.