To appreciate the role of the arterial smooth muscle cell in atherosclerosis, it is necessary to know something about the structure of normal arteries. Normal arteries are composed of three morphologically distinct layers (Fig. I). The intima, or inner layer of the artery, consists in the newborn and in children of a narrow region bounded on one side by a single layer of endothelial cells that line the lumen of the vessel and on the other side by the internal elastic lamina, a perforated sheet of elastic tissue. Between these two boundaries are occasional smooth muscle cells surrounded by various extracellular components of connective tissue matrix. With increasing age. smooth muscle cells and extracellular matrix components accumulate slowly within the intima, increasing its thickness. The media, or middle layer of the artery, consists of large numbers of smooth muscle cells, each surrounded by small amounts of collagen and varying numbers of small elastic fibers and other connective tissue matrix components. There are no fibroblasts lOJ,. Lm