Hardening of the arteries, or atherosclerosis, is the primary cause of heart disease. It's caused by calcium accumulation in the blood vessels, which leads to obstruction of blood flow and heart complications. Although many risk factors for atherosclerosis have been identified, the cause isn't known and there's currently no way to reverse it once it sets in. In a new study, researchers have characterized the cells responsible for driving this calcium build-up in vessel walls.