Embryonic origin and structure of the forelimb and hindlimb.
Top, Limb buds are the early signs for the developing limbs in the embryo.
Middle, Stylopod (purple), zeugopod (blue) and autopod (olive) fates are specified throughout mesenchyme proximo-distal growth. Stylopod – the proximal segment, is the first to be determined. The cells of progress zone (PZ, dashed) – the most distal mesenchyme of the limb bud, continue to proliferate. They become committed to zeugopod fate upon the FGF signaling from the Apical Ectodermal Ridge (AER, green). Following further outgrowth, the newly-formed PZ cells underneath the AER acquire the Autopod specification, while previously committed regions maintain their characteristics.
Bottom, Human upper arm: stylopod region corresponds to the Humerus bone, zeugopod to the Ulna and Radius bones, autopod to the Carpal, metacarpal and phalangs bones.