The layer of cells that lines the heart, blood vessels, lymph vessels, and certain tissue-lined cavities throughout the body. Researchers are especially interested in the vascular (blood vessel) endothelium and its role in the development of such medical conditions as heart disease and diabetic retinopathy (eye disease). Researchers who study atherosclerosis (narrowing of the arteries) and heart disease are keenly interested in the function—and dysfunction—of the vascular endothelium. Endothelial dysfunction, a condition in which the blood vessels are unable to dilate fully when needed, is thought to be one of the earliest signs of atherosclerosis. Research shows that endothelial dysfunction tends to develop before the blood vessel walls start to thicken and before atherosclerotic plaques begin to appear on them. Researchers think that it may cause atherosclerosis by promoting stiffening of the blood vessels, the adhesion of platelets and other substances to the inner walls of blood vessels, inflammation, and blood clotting. Endothelial dysfunction may be an important mechanism by which Type 2 diabetes raises the risk of heart disease. Recent research has shown that insulin resistance (a condition in which the body’s cells do not respond to insulin properly) may be a major contributor to endothelial dysfunction. Other risk factors for endothelial dysfunction may include poor high blood glucose levels, blood lipid (cholesterol and triglyceride) abnormalities, and high blood pressure. Researchers believe that taking steps to control these risk factors, including dietary modification, exercise, weight loss, and drug therapy to control diabetes, lipid levels, and blood pressure, could help improve endothelial dysfunction—and lower the risk of atherosclerosis and heart disease. Researchers are also studying the vascular endothelium in the context of angiogenesis, or the growth of new blood vessels. Cells within the endothelium tend to promote angiogenesis, with the help of growth factors such as vascular endothelial growth factor (VEGF). Angiogenesis can adversely affect diabetic retinopathy by promoting the development of abnormal new blood vessels in the retina, which tend to leak and cause scarring. Researchers hope someday to stop or delay the progression of diabetic retinopathy by using drugs that block the action of VEGF.