The cloning of the founding member of the Hedgehog (HH) family of secreted proteins two decades ago inaugurated a field that has diversified to encompass embryonic development, stem cell biology and tissue homeostasis. Interest in HH signalling increased when the pathway was implicated in several cancers and congenital syndromes. The mechanism of HH signalling is complex and remains incompletely understood. Nevertheless, studies have revealed novel biological insights into this system, including the function of HH lipidation in the secretion and transport of this ligand and details of the signal transduction pathway, which involves Patched 1, Smoothened and GLI proteins (Cubitus interruptus in Drosophila melanogaster), as well as, in vertebrates, primary cilia.

Distinct classes of neurons are generated at defined positions in the ventral neural tube in response to a gradient of Sonic Hedgehog (Shh) activity. A set of homeodomain transcription factors expressed by neural progenitors act as intermediaries in Shh-dependent neural patterning. These homeodomain factors fall into two classes: class I proteins are repressed by Shh and class II proteins require Shh signaling for their expression. The profile of class I and class II protein expression defines five progenitor domains, each of which generates a distinct class of postmitotic neurons. Cross-repressive interactions between class I and class II proteins appear to refine and maintain these progenitor domains. The combinatorial expression of three of these proteins--Nkx6.1, Nkx2.2, and Irx3--specifies the identity of three classes of neurons generated in the ventral third of the neural tube.

Distinct classes of motor neurons and ventral interneurons are generated by the graded signaling activity of Sonic hedgehog (Shh). Shh controls neuronal fate by establishing different progenitor cell populations in the ventral neural tube that are defined by the expression of Pax6 and Nkx2.2. Pax6 establishes distinct ventral progenitor cell populations and controls the identity of motor neurons and ventral interneurons, mediating graded Shh signaling in the ventral spinal cord and hindbrain.