NCIt definition : Interdependent positional signaling by the apical ectodermal ridge, the posterior
mesenchyme zone of polarizing activity, and the non ridge ectoderm coordinate limb
bud outgrowth and patterning, Limb Development, by migrated lateral plate mesoderm
and somite cells. Proximal-distal differentiating mesenchymal cells condense to form
the limb skeletal primordia.;