Aortic smooth muscle cells, fluorescence light micrograph. Smooth muscle is a type of muscle found in the walls of blood vessels (such as veins and arteries), and the intestines and stomach. These cells are from the wall of the aorta, the body's main artery. The cells have been stained for alpha-actin (green), a form of the protein actin found in muscle cells. The cell nuclei have been stained blue. This sample is from human tissue.