John Saunders' ZPA, Sonic hedgehog and digit identity - How does it really all work?

Among John Saunders' many seminal contributions to developmental biology, his discovery of the limb 'zone of polarizing activity' (ZPA) is arguably one of the most memorable and ground-breaking. This discovery introduced the limb as a premier model for understanding developmental patterning and promoted the concept of patterning by a morphogen gradient. In the 50 years since the discovery of the ZPA, Sonic hedgehog (Shh) has been identified as the ZPA factor and the basic components of the signaling pathway and many aspects of its regulation have been elucidated. Although much has also been learned about how it regulates growth, the mechanism by which Shh patterns the limb, how it acts to instruct digit 'identity', nevertheless remains an enigma. This review focuses on what has been learned about Shh function in the limb and the outstanding puzzles that remain to be solved.