Apical ectodermal ridge

The Apical Ectodermal Ridge (AER) arises from the Paraxial Mesoderm and is a major component in Limb Development. It signals to the Progress Zone and the Zone of Polarizing Activity (ZPA) via the Fibroblast Growth Factor (FGF-8) throughout the development of the limbs.

If for any reason the AER is not present during development of an embryo it will result in reduced cell proliferation in the progress zone, limb truncation, and loss of distal structures.