What are the signaling centers controlling limb bud formation? Apical Ectodermal Ridge The Apical Ectodermal Ridge (AER) controls identity along the proximal/distal axis. The AER is a ridge of thickened ectoderm that forms along the entire edge of the limb bud at the dorsal/ventral boundary. This thickened ridge is analogous to ectodermal placodes in that [...]