Practice question

Proof that mesodermal FGF10 initiates limb budding came from gain-of-function implantation experiments. Implanting FGF10-soaked heparin beads beneath flank ectoderm of chick embryos outside normal limb fields induced ectopic Wnt3a, beta-catenin activation and AER formation expressing FGF8, followed by complete extra limb containing stylopod zeugopod autopod and digits with muscle and cartilage. Conversely, AER removal causes truncation, TBX5 knockout prevents forelimb, AER graft from another species still supports outgrowth due to permissive nature. Thus FGF10 bead experiment proved mesodermal signals sufficient to induce full limb formation program de novo.