How do arms and legs appear and develop in human embryos?
Limb buds first appear during the end of the 4th week of development (the upper limb buds appear around day 26-27 and the lower limb buds appear around day 27-28). They are simply elevations of mesenchyme covered by ectoderm that grow out of the body wall.
At the apex of each limb bud, there is a thickening of ectoderm called the apical ectodermal ridge (AER). This interacts with the mesenchyme below it in order to initiate growth in the proximal-distal direction (i.e. growth in the length of the limb). Key proteins involved in the initiation of growth are fibroblast growth factors (FGFs) expressed in the AER.
Another region involved in the early growth of the limb is the ...