Blood vessel endothelium. Coloured scanning electron micrograph (SEM) of endothelial cells lining the inside of a blood vessel. The entire vascular system is lined internally by a single layer of endothelial cells. The cells are flat and thin and their nuclei bulge outwards as seen here. Together with a membrane, the endothelial cells make up the tunica intima, the inner layer of a blood vessel. Magnification: x1000 at 6x7cm size.