The limb's skeletal elements are prefigured by tight aggregates of mesenchymal cells called precartilage condensations. Mesenchymal condensation is mediated by extracellular matrix and cell adhesion molecules. Cartilage differentiates from the condensations, giving rise to the skeletal primordia. In most tetrapod limb skeletons (though not in some amphibians), the cartilage skeleton is replaced by bone later in development.
Read more about this topic: Limb Development