Practice question

Instructive capacity for limb formation resides in lateral plate mesoderm, demonstrated by classical transplantation in chick embryos. Excision of forelimb-destined lateral plate mesoderm and grafting to ectopic flank induced complete supernumerary limb, including an apical ridge newly induced from host flank ectoderm. Ectoderm grafts alone were non-inductive, Hox gene knockouts altered axial position without testing induction, and ridge removal truncates rather than initiates. Molecular mechanism centers on FGF10 secreted by lateral plate mesoderm activating Wnt3a-FGF8 signaling cascade in overlying ectoderm, initiating budding and pattern.