Practice question

Extra AER grafted onto dorsal or ventral surface of limb bud creates ectopic signaling center secreting FGF8, FGF4 independent of endogenous ridge. Local subjacent mesenchyme remains proliferative and initiates secondary outgrowth axis with its own proximal-distal polarity. Depending on graft position, supernumerary limb elements including duplicated stylopod, zeugopod or autopod form branching from primary limb. This experiment demonstrated AER sufficiency for outgrowth induction and reprogramming of surrounding mesenchyme. Niswander and colleagues confirmed FGF bead implantation similarly induces duplicated structures mimicking AER transplantation.