Practice question

Apical ectodermal ridge acts as distal organizer secreting FGF8, FGF4, FGF9 that maintain underlying progress zone mesenchyme in proliferative, undifferentiated state. These FGFs sustain FGF10 in lateral plate mesoderm, establishing reciprocal positive feedback essential for proximal-distal elongation and preventing precocious differentiation. Early surgical removal abolishes loop, induces distal apoptosis, terminates outgrowth rapidly. Saunders demonstrated timing-dependent truncation: early excision stops development completely, later removal deletes only distal phalanges. Hence early AER loss halts limb extension and distal structures never form in chick embryos.