Fig. 1. (A) Representation of the prospective limb territories in a stage 15 chick embryo. The forelimbs and hindlimbs are derived from discrete regions of the lateral plate mesoderm (LPM). Tbx5 (purple) is expressed in the prospective forelimb, whereas Tbx4 (green) is expressed in the prospective hindlimb. At these stages, Pitx1 (orange) is expressed in a caudal domain that overlaps with Tbx4. The somites (brown) are numbered, and serve as a reference for the axial position of the forelimb and hindlimb fields. (B) Representations of the three primary axes of the developing limb and the equivalent structures of the mature human limb that ultimately form. (Top) The apical ectodermal ridge (AER; shown in red) runs along the distal extent of the limb and is crucial for patterning the proximodistal axis. Cells of the zone of polarizing activity (ZPA; shown in yellow) in the posterior of the limb are important in patterning the anteroposterior axis of the limb. (Bottom) The AER bisects the dorsal and ventral ectoderm.