The intima in muscular arteries generally refers to the region between the endothelial cell surface and the internal elastic lamina. However, in elastic-type arteries, especially at branching points and curvatures, the internal elastic lamina is often indistinct, so the intima is typically considered to extend from the endothelial cells to the innermost elastic fibers. The endothelial cells are arranged on a specialized connective tissue known as the basement membrane. The general understanding is that the subendothelial layer sandwiched between the basement membrane and the internal elastic lamina is very thin, morphologically almost unrecognizable, and contains only a small amount of extracellular matrix with few cellular components. This understanding applies to most mammals, including often-used experimental animals such as mice, rats, and rabbits. However, in human coronary arteries and the aorta, a fibrocellular thickened intima is known to exist and increase with age (Stary 1989; Nakashima et al. 2002; Schwartz 1999). In other words, humans possess both a thin intima consisting almost entirely of endothelial cells, as well as a thickened intima with a clear expanse. Since a thickened intima is almost constantly observed in arteries prone to atherosclerosis, it is believed that most atherosclerosis develops on the soil (substrate) of a pre-existing thickened intima.