Mechanism of mineral formation in bone.

The mechanism of mineral formation in bone is seen best where active new bone formation is occurring, e.g., in newly forming subperiosteal bone of the embryo, in the growing bone of young animals, and in healing rickets where the calcification process in osteoid is reactivated. A large body of ultrastructural evidence, using conventional and anhydrous methods for tissue preparation, has shown convincingly that extracellular matrix vesicles are present at or near the mineralization front in all of the above, and that these vesicles are the initial site of apatite mineral deposition. Thus bone resembles growth plate cartilage, predentin, and turkey tendon in having calcification initiated by matrix vesicles. Once the calcification cascade is begun, matrix vesicles are no longer needed to support mineralization and are consumed by the advancing mineralization front in which performed crystals serve as nuclei for the formation of new crystals. The rate of crystal proliferation is promoted by the availability of Ca2+, PO4(3-), and the presence of collagen, and retarded by naturally occurring inhibitors of mineralization such as proteoglycans and several noncollagenous calcium-binding proteins of bone including bone-Gla protein (osteocalcin), phosphoproteins, osteonectin, and alpha-2HS-glycoproteins. New electron microscopic immunocytochemical findings in our laboratory suggest that the origin of alkaline phosphatase-positive bone matrix vesicles is polarized to the mineral-facing side of osteoblasts and may be concentrated near the intercellular junctions of human embryonic osteoblasts.