Several signaling centers play crucial roles in limb development. The apical ectodermal ridge (AER), located at the distal tip of the limb bud, acts as a key signaling center and is essential for the outgrowth and patterning of the limb. The zone of polarizing activity (ZPA), located in the posterior part of the limb bud, is responsible for the anterior-posterior axis patterning through the secretion of Sonic hedgehog (Shh).
