During vertebrate nervous system development, a gradient of sonic hedgehog(Shh) ligand secreted by the notochord specifies ventral cell identities in the adjacent neural tube. To visualize Shh signal distribution during this process, Chamberlain and colleagues have engineered mice in which the Shh locus produces bioactive, fluorescently labelled Shh (see ). Using these mice, the researchers show that Shh ligand produced by the notochord forms a dynamic gradient that spreads through the neural target field as the ventral pattern emerges, and that Shh associates with the apically localized basal body - an organelle that sits at the base of cilia - of neural progenitors during this process. Other experiments indicate that the profile of the Shh gradient depends on negative feedback from the neural target cells and on Shh lipidation and that Shh might move from the notochord into the neural target field along microtubules. Together, these results provide important new insights into Shh signal distribution and transduction during neural tube patterning.
