Practice question

Limb bud initiation begins with lateral plate mesoderm signaling to overlying ectoderm through paracrine FGFs. Under control of TBX5 in forelimb and combined TBX4 Pitx1 and Islet1 in hindlimb, mesenchyme expresses and secretes FGF10 inducing receptor FGFR2b. FGF10 acts via FGFR2b in ectoderm to induce AER establishment, directional proliferation, and bud evagination with cytoskeletal rearrangement. It also promotes mesenchymal survival through MAPK. Without FGF10, buds fail to form or degenerate rapidly. It does not directly maintain SHH, inhibit BMP, or confer DV polarity; its core evolutionary conserved role is initiating bud formation.