The zone consists of mesodermal cells and is situated underneath the apical ectodermal ridge of the vertebrate limb bud (see figure below). It acts to provide positional information that enables cells to develop into a limb. The transmission of this information appears to depend on the time the cells spend in the zone during outgrowth of the bud (proximal-distal axis of the limb): the shorter the time, the more it receives signals from the ridge (with timing perhaps determined by transcription factors). Alternatively, some research suggests the cells are prespecified.
