Coronary artery. Coloured transmission electron micrograph (TEM) of a cross-section through the wall of a coronary artery. The inside (lumen) of the artery is at upper right. The wall of the artery is made up of an endothelial lining (green), smooth muscle (red/blue/green) and elastic tissue (yellow/green). Coronary arteries supply the thick muscular wall of the heart with blood. Magnification: 3000x when printed 10 centimetres wide.