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(A) Illustration of the main current hypotheses explaining limb bud elongation. (1) Directed division of mesenchymal cells. (2) Intercalation of mesenchymal cells. (3) Formation and resolution of rosettes in the ectoderm a form of ectodermal molding. (B) Tail bud elongation: Cell motility in the tail bud is controlled by an FGF gradient. Highly motile cells at the tail end are sparser compared to less motile cells at lower FGF concentrations.
