Practice question

Classic AER substitution proved diffusible FGFs mediate ridge function. Saunders removed chick AER causing distal arrest, then Niswander implanted heparin beads soaked in FGF8, FGF4 or FGF2 into distal mesenchyme beneath wound ectoderm. Progress zone proliferation resumed, FGF10 and Gremlin expression recovered, and a largely normal skeleton with stylopod zeugopod autopod formed. This demonstrated chemical sufficiency of FGF signaling for outgrowth and maintenance of FGF10-FGF8 feedback loop. ZPA grafts instead duplicate digits anterior-posteriorly, TBX5 deletion blocks forelimb initiation, neither addressing AER replacement.