What happens when an extra AER is grafted onto a developing limb bud?
AER serves as distal outgrowth engine generating proximal-distal extension. Its FGF signals maintain progress zone proliferation, keep Meis genes repressed distally, and progressively specify more distal fates through Hox temporal activation and RA antagonism. Transplanting an additional AER onto dorsal or ventral surface of limb bud creates a second active signaling center, bifurcating underlying mesenchyme into an ectopic outgrowth with its own progress zone and autopod. Result is limb bifurcation and supernumerary skeletal elements branching from graft site. Removal instead causes truncatio
Ref: Saunders 1948, Gilbert Chapter 20: Extra AER graft induces supernumerary limb structures.
