Cells from the lateral plate mesoderm and the myotome migrate to the limb field and proliferate to create the limb bud. The lateral plate cells produce the cartilaginous and skeletal portions of the limb while the myotome cells produce the muscle components. The lateral plate mesodermal cells secrete a fibroblast growth factor (FGF7 and FGF10, presumably) to induce the overlying ectoderm to form an important organizing structure called the apical ectodermal ridge (AER).The AER reciprocatively secretes FGF8 and FGF4 which maintains the FGF10 signal and induces proliferation in the mesoderm. The position of FGF10 expression is regulated by Wnt8c in the hindlimb and Wnt2b in the forelimb. The forelimb and the hindlimb are specified by their position along the anterior/posterior axis and possibly by two T-box containing transcription factors: Tbx5 and Tbx4, respectively.
^Tonegawa A, Funayama N, Ueno N, Takahashi Y (1997). "Mesodermal subdivision along the mediolateral axis in chicken controlled by different concentrations of BMP-4". Development124 (10): 1975–84. PMID9169844.
^ abLarsen, William J. (1998). Essentials of human embryology. Edinburgh: Churchill Livingstone. ISBN0-443-07514-X.