Limb development

Limb development in tetrapods — animals with four limbs — is an area of active research in developmental biology. Limb formation begins in the limb field, as mesenchymal cells from the lateral plate mesoderm proliferate to the point that they cause the ectoderm above to bulge out, which is known as the limb bud. Fibroblast growth factor (FGF) induces formation of an organizer, called the apical ectodermal ridge (AER), which guides further development and controls cell death. Apoptosis — programmed cell death — is necessary to eliminate webbing between digits.