Bone matrix is the intercellular substance of the bone that forms most of the mass of the bone. It is a non-living, mineralized extracellular substance that provides the structural framework and mechanical support for bones. The bone matrix is composed of organic and inorganic components. The organic portion of the bone matrix consists of collagen fibers (type I), proteoglycans, and glycoproteins (e.g. osteonectin and osteocalcin), which give bone tissues their flexibility and tensile strength. The inorganic portion makes up about half of the weight of the bone and mostly consists of calcium hydroxyapatite, but there are also significant amounts of bicarbonate, citrate, magnesium, potassium, and sodium ions. The bone matrix is produced by osteoblasts, which are bone cells that store calcium in bone by making bone matrix. Osteocytes are the terminally differentiated immobilized cells in the bone matrix that form contacts with each other and with bone lining cells, which aid bone growth and repair. The main function of the bone matrix is to provide mechanical support for the bones and to exert an essential role in bone homeostasis.