Artery. Coloured transmission electron micrograph (TEM) of a transverse section through an artery. Red blood cells (erythrocytes, red) and white blood cells (leucocytes, pale purple) can be seen in the central canal (lumen). The lumen is lined with a layer called the tunica intima, which is made up of epithelial cells (pale pink) and elastic fibres (dark pink). This layer lies underneath a layer of smooth muscle and elastic fibres (yellow), called the tunica media. The outermost layer of the artery is known as the tunica externa (not shown) which is mainly composed of collagen. Magnification: x1500 when printed 10cm wide.