Bones and Cartilages Development:
At the end of the fourth week, the sclerotome cells form a mesenchyme, which has bone forming capacity. In the beginning, bones appear as condensation of mesenchymal cells which form bone models. Then, mesenchymal models of most limb bones are differentiated cartilage bone models. At the end, cartilage bone models become ossified by endochondral bone formation. Most flat bones develop from pre-existing membranous sheaths, in mesenchyme; this type of osteogenesis is called intramembranous bone formation.
Histogenesis of Cartilage
During the fifth week, the development of cartilage starts from mesenchyme. The mesenchyme condenses to form chondrification centres. First, the mesenchymal cells differentiate into precondrocytes and later into chondroblast which secrete collagenous fibrils and extracellular matrix. After that, collagenous and/or elastic fibres are deposited in the intracellular matrix. There are three types of cartilage: hyaline cartilage, fibrocartilage and elastic cartilage.
Histogenesis of Bone:
Primarily, bone develops into mesenchyme and cartilage.Bone consists of cells and bone matrix. Studies of the cellular events during embryonic bone formation show that osteogenesis and chondrogenesis are programmed early in development.