Practice question

Early limb bud induction depends on fibroblast growth factor 10 originating in lateral plate mesoderm, functioning as upstream inducer even though technically growth factor rather than transcription factor. FGF10 activates beta-catenin and Wnt3a in overlying ectoderm triggering formation of functional apical ridge producing FGF8 that feeds back maintaining FGF10. Hoxd13 acts later distally, Islet1 modulates hindlimb outgrowth, Pitx1 specifies hindlimb identity. FGF10-null phenotype shows complete limb agenesis proving indispensable inductive function, supporting its classification as essential early initiator despite nomenclature nuance in embryonic signaling categories.