Long bones are formed by endochondral ossification, which is characterized by the presence of a cartilaginous model in which chondrocytes differentiate and mineralized cartilage is replaced with bone through remodeling. For this process, osteoblasts differentiate directly from mesenchymal cells to form the bone matrix.

Intramembranous ossification is the process by which flat bones are formed. In the normal adult skeleton, remodeling is coupled such that the level of resorption is equal to the level of formation and bone density remains constant. Bone is remodeled throughout life through a coordinated sequence of events which involve the sequential actions of osteoclasts and osteoblasts, replacing old bone with new bone.

This process is performed by the macrophage-related osteoclast. Resorption of the bone matrix is required for adaptation to growth, repair and mineral mobilization. Most components of the bone matrix are synthesized and secreted by osteoblasts.

Hydroxyapatite crystals bind to both types of proteins.

The organic matrix of bone is formed mostly of collagen, but also non-collagenous proteins. Recently, a fourth function has been attributed to the skeleton: an endocrine organ. Bones have three major functions: to serve as mechanical support, sites of muscle insertion and as a reserve of calcium and phosphate for the organism.

Flat bone vs long bone