Limb bud. Apical ectodermal ridge extends from anterior to posterior along dorsal/ventral boundary of growing limb bud. Proximal to apical ectodermal ridge is progress zone (an area of proliferating mesodermal cells). Within posterior mesoderm is zone of polarizing activity, which is important signaling center. These centers are interconnected, so limb patterning and growth partly depend on coordinated function.