Smooth muscle tissue. Transmission electron micrograph (TEM) of a longitudinal section through smooth muscle cells. Smooth muscle cells are spindle-shaped and often closely packed together, particularly when forming a wall or sheets of muscle. They have a central nucleus (large ovals), and the cytoplasm is filled with myofilaments of the contractile proteins actin and myosin. These filaments are not arranged into orderly sarcomeres such as those found in cardiac and skeletal muscle cells. Magnification: x21,000, when printed 10 centimetres wide.