Pattern of Limb Development
The first visible sign of limb development is the appearance of ridge or thickening on each lateral side of the embryo of amniotes. The ridge called Wolffian Ridge extends antero-posteriorly between the brachial and pelvic regions; but only its anterior and posterior ends persist from which the forelimb and hindlimb develop respectively. In the amphibians there is no such ridge and separate pairs of ridges arise in the respective limb regions. The ridge is formed from proliferation of cells of the somatic (parietal) layer of lateral plate mesoderm. The mesodermal cells of the ridge or disc lose their epithelial connections and become mesenchymal migrate as separate cells and form a loose mass of mesenchyme beneath the embryonic ectoderm opposite the ridge or thickening.
Their continued proliferation elevates the covering ectoderm and a limb bud is formed. The early limb bud has a core of mesoderm and a covering of ectoderm; the ectoderm is 2-cell layered. The outer, peridermal layer is of flattened cells and the inner layer has cuboidal cells, separated from the mesoderm by a basement membrane. The bud is at fist a small mound but soon grows in size due to proliferation of mesodermal cells; as this happens the cells of the inner layer of ectodermal covering along its distal, edge become tall columnar and form a pseudostratified layer. In the amniotes (lizard, birds, mammals) this thickening of the apical ectoderm appears as a prominent ridge running antero-posteriorly along the free distal margin of the bud. It is called the Apical Ectodermal Ridge (AER). It has also been described in xenopus and at least one frog (Rana tigrina) and may be present in other amphibians also, though not so prominently. In chick limb development AEU has been found to play a very important role.