The layer of transient surfaceectodermal cells at the apex of the embryonic limb bud (see figure below). Considered to exert an inductive influence on the condensation of underlying mesenchyme, and thereby to stimulate elongation of the limb along the proximal-distal axis and maintain signals required for patterning the limb. The majority of cell division (mitosis) occurs just deep to AER, and this region is known as the progress zone.
