Practice question

Forelimb versus hindlimb identity is encoded by T-box transcription factors expressed in lateral plate mesoderm before any morphological bud. Tbx5 is restricted to presumptive forelimb field, where it upregulates Fgf10 and Wnt2b and launches forelimb-specific enhancer program leading to arm or wing formation. Tbx4 together with Pitx1 and Islet1 marks hindlimb field and specifies leg development. Genetic ablation of Tbx5 abolishes forelimb initiation entirely, while ectopic Tbx5 expression in flank can induce supernumerary wing-like buds, establishing its master selector status for forelimb.