BONE METABOLISM
Background
Bone is a supportive connective tissue. It has multiple functions such as supporting the body weight, protection of the internal organs, and storage of the minerals. It is also a primary site of hemopoiesis, the formation of blood cells, in the adult. Hemopoiesis occurs in the red marrow of the long bones, as well as in the flat bones such as the sternum, the ribs, the ileum, and the vertebral bodies. The cartilage-covered ends of the bone form articulate joints to allow multiplication of the force of the muscles attached to them. As any connective tissue, bone tissue consists of cells and extracellular matrix.
There are three types of cells in mature bone tissue: osteoblasts, osteocytes, and osteoclasts. Osteoblasts and osteocytes are involved in the deposition of bone matrix. These cells produce collagen type I, which is then calcified. These cells also produce non-collagenous proteins, as well as regulatory factors. Osteoblasts are located at the surfaces of bone tissue, while osteoclasts are located within the calcified matrix of bones. Osteoclasts are involved in the resorption of bone tissue. The extracellular matrix can be subdivided into organic matrix composed of collagen fibers and ground substance, and inorganic matrix composed mainly of a complex of calcium and phosphate in the form of hydroxyapatite [Ca-10-(PO-4)-6-(OH)-2].
