Uncoupling the role of sonic hedgehog in limb development: growth and specification

Sonic hedgehog (Shh) signaling is essential for the anterior-posterior (A-P) patterning of the embryonic limb bud. An additional role for Shh has now been identified in the mouse and chick, in which Shh promotes the proliferation of limb mesenchymal cells, which increases the number of digits that form. Surprisingly, these data are not consistent with any of the previous models in the mouse that described roles for Shh and the zone of polarizing activity in the patterning of the A-P axis. Therefore, new light is shed on the mechanisms of Shh action, which both challenges our understanding of limb development and gives insights into the evolutionary changes that result in digit reductions.