How do we get a perfect complement of digits?

A crucial issue in limb development is how a correct set of precisely shaped digits forms in the digital plate. This process relies on patterning across the anterior–posterior axis of the limb bud, which is under the control of Sonic hedgehog emanating from the zone of polarizing activity. Recently, Sonic hedgehog function in the limb bud has been shown to have a dual character controlling both growth and patterning of the digital field. This finding has prompted the proposal of new models of how these two functions are achieved, and this will be discussed in this review.