Integration of growth and specification in chick wing digit-patterning

In the classical model of chick wing digit- patterning(1), the polarizing region - a group of cells at the posterior margin of the early bud - produces a morphogen gradient, now known to be based on Sonic hedgehog (Shh)(2,3), that progressively specifies anteroposterior positional identities in the posterior digit- forming region(4). Here we add an integral growth component to this model by showing that Shh- dependent proliferation of prospective digit progenitor cells is essential for specifying the complete pattern of digits across the anteroposterior axis. Inhibiting Shh signalling in early wing buds reduced anteroposterior expansion, and posterior digits were lost because all prospective digit precursors formed anterior structures. Inhibiting proliferation also irreversibly reduced anteroposterior expansion, but instead anterior digits were lost because all prospective digit precursors formed posterior structures. When proliferation recovered in such wings, Shh transcription was maintained for longer than normal, suggesting that duration of Shh expression is controlled by a mechanism that measures proliferation. Rescue experiments confirmed that Shh- dependent proliferation controls digit number during a discrete time- window in which Shh- dependent specification normally occurs. Our findings that Shh signalling has dual functions that can be temporally uncoupled have implications for understanding congenital and evolutionary digit reductions.