Development of the limbs
Limb buds and apical ectodermal ridge

From S11, the ectoderm thickens on the lateral body wall to form the ectodermal ring. This extends down from the pharyngeal arches to near the caudal eminence. Somatopleural mesoderm proliferates under the ectodermal ring. By S12, the upper limb buds arise at the level of somites 7-12. By S13, the lower limb buds arise at the level of somites 25-29. In these regions the ectoderm forms a longitudinal ridge of high columnar epithelium, the apical ectodermal ridge (AER). The AER is present until the embryo is aged six weeks.