Practice question

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 truncation, early removal yields only humerus formation.