07/25/2002 - The Image of the Week was submitted by Faye Siranni in Dr. David Walker's laboratory. It is a Transmission Electron Microscope ( TEM ) image from part of a heart muscle cell (cardiomyocyte) showing the structures that cause muscles to contract during a heartbeat or exercise. The dark bands that run vertically are called Z lines and the faint vertical lines in between are called M lines. When contracted, the Z lines come closer together. This magnification is 50,000 X.