NCIt definition : Human Skeletal Development involves growth and differentiation of axial, limb, and
cranial elements of the skeletal system. Axial vertebrae and ribs derive from cartilage
precursors of the paired segmented paraxial mesodermal somite sclerotomes. 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 by migrated lateral plate mesoderm and somite cells. Proximal-distal differentiating
mesenchymal cells condense to form the limb skeletal primordia. The skull forms by
direct ossification of, or cartilage condensation from neural crest-derived, ectomesenchyme.;