Bone is a dynamic tissue that is continually being built, broken down, and rebuilt in a process called bone remodeling. Bone tissue is broken down and reserved by multinucleated cells known as osteoclasts. These cells are derived from monocytes which originate within bone marrow.
Osteoclasts play an important role in liberating minerals and other molecules stored within the bone matrix. Bone tissue serves as a repository for vital minerals including calcium phosphate and various biologically active molecules, such as growth factors. The release of calcium from the bone can play a role in maintaining its homeostasis within the body. The cells responsible for building new bone tissue are known as osteoblasts.
Osteoblasts are thought to be derived from cells found to be associated with blood vessels. Once active, they start to produce the organic component of bone, osteoid, which is predominantly made of collagen. Minerals start to crystallize around the collagen scaffold to form hydroxyapatite, the major inorganic constituent of bone, which contains calcium phosphate. Bone mineral density, or BMD, can be used to estimate the strength of bone and to assess the risk of fracture.
As osteoblasts form new bone tissue, many become embedded within the matrix and differentiate into osteocytes. The structure, composition and cellular processes that occur within bone allow it to simultaneously serve as a calcium reservoir, while providing structural support for the vital organs and for locomotion.