Limb bud development begins with the activation of a group of mesenchymal cells in the lateral mesoderm. 1. (Hox) genes regulate patterning in vertebrate limb development. The limb buds form deep to a thick (AER), At the apex of each limb bud the ectoderm thickens to form an 3. band of 2. (FGFS) in the AER is a multilayerd epithelial structure. Expression of endogenous 4. ectoderm involved in the interaction with mesenchyme induction process. fibroblast growth factor apical ectodermal ridge