Light micrograph of an arteriole wall. The arteriole runs from middle left to top right. Seen here is the tunica media, the largest layer of the arteriole wall. It consists of smooth muscle cells that control the diameter of the arteriole. Along the bottom are fat cells (white). Arterioles and venules are the finest blood vessels that can still be seen with the naked eye. Magnification: x400 when printed at 15cm wide.