Practice question

Limb bud initiation and sustained outgrowth depend on mesenchymal FGF10 induced by TBX5 in forelimb and TBX4-Pitx1-Islet1 in hindlimb. FGF10 activates Wnt3a and beta-catenin in overlying ectoderm, inducing AER formation expressing FGF8. Reciprocal FGF10-FGF8 loop then drives proliferation of progress zone and maintains distal identity. Fgf10 null mice exhibit complete limbless phenotype with no AER induction. Replacement of FGF10 with implanted FGF bead rescues outgrowth, proving necessity and sufficiency. Hence FGF10 constitutes essential initiating and maintenance signal for tetrapod limb outgrowth, elongation and patterning competence.