Biological Role of Synthetic Octacalcium Phosphate in Bone Formation and Mineralization

Bone formation, including matrix mineralization by calcium phosphate crystals, is essentially organized under the control of osteoblasts. Great efforts have been conducted to establish the linkage between bio-crystals and synthetic crystals to determine the physicochemical properties of these crystals. Octacalcium phosphate (OCP) has been proposed to be a precursor of hydroxyapatite (HA), the prototype in bone and tooth apatite crystals, and suggested to have a role in apatite crystal development. Studies using synthetic OCP provided evidence that this mineral phase stimulates bone formation more than synthetic HA if implanted in bone defects during conversion from OCP to HA. Furthermore, the enhancement of bone formation was usually accompanied by osteoclastic resorption of OCP. In vitro studies showed that OCP is capable of stimulating mouse bone marrow stromal cells to differentiate into osteoblastic cells and osteoclast formation in co-culture by enhancing the expression of the receptor activator of NF-κB ligand (RANKL) in osteoblastic cells. Although the precise mechanism of the precursor precipitation and subsequent apatite formation has not been fully elucidated in normal bone mineralization, it is likely that the OCP precursor plays a role in the stimulation of bone-forming cells through interaction with the surrounding tissue environment if this mineral phase is involved in bone formation.