Calcium deposits of atherosclerotic plaque consist of hydroxyapatite and may appear identical to fully formed lamellar bone, including trabeculae, lacunae, and areas resembling marrow. Possible mechanisms for bone formation in artery walls are developmental retention of pluripotent cells or osteoblastic immigration coupled with loss of molecular regulatory control that unmasks an embryonic osteogenic program. In situ hybridization of calcified human atherosclerotic lesions shows expression of bone morphogenetic protein type 2, a potent osteogenic differentiation factor. Medial cells of bovine aorta cultured (Dulbecco's modified Eagle's medium plus 15% fetal calf serum) for > 2 weeks form nodules similar to those formed by cultured osteoblasts, including the elaboration of hydroxyapatite.