Via EurekAlert!: "For the first time, we are showing evidence that vascular diseases are actually a kind of stem cell disease. ... It is generally accepted that the buildup of artery-blocking plaque stems from the body's immune response to vessel damage caused by low-density lipoproteins ... Such damage attracts legions of white blood cells and can spur the formation of fibrous scar tissue ... The scar tissue, known as neointima, has certain characteristics of smooth muscle, the dominant type of tissue in the blood vessel wall. Because mature smooth muscle cells no longer multiply and grow, it was theorized that in the course of the inflammatory response, they revert, or de-differentiate, into an earlier state where they can proliferate ... However, no experiments published have directly demonstrated this de-differentiation process ... researchers turned to transgenic mice with a gene that caused their mature smooth muscle cells to glow green under a microscope. In analyzing the cells from cross sections of the blood vessels, they found that more than 90 percent of the cells in the blood vessels were mature smooth muscle cells. They then isolated and cultured the cells taken from the middle layer of the mouse blood vessels. ... Notably, none of the new, proliferating cells glowed green, which meant that their lineage could not be traced back to the mature smooth muscle cells originally isolated from the blood vessels. ... We did further tests and detected proteins and transcriptional factors that are only found in stem cells. No one knew that these cells existed in the blood vessel walls because no one looked for them before. ... In the later stages of vascular disease, the soft vessels become hardened and more brittle. Previously, there was controversy about how soft tissue would become hard. The ability of stem cells to form bone or cartilage could explain this calcification of the blood vessels. ... Other tests in the study showed that the multipotent stem cells were dormant under normal physiological conditions. When the blood vessel walls were damaged, the stem cells rather than the mature smooth muscle cells became activated and started to multiply." Though if you want to consider root causes, look at mechanisms like accumulated damage to mitochondria that leads to a greater level of oxidized low-density lipoproteins in the blood.