Transmission electron microscope image of a thin longitudinal section cut through an area of human skeletal muscle tissue. Image of muscle sarcomeres shows distinct banding pattern: the darker bands are called A bands(the A band includes a lighter central zone, called the H band), and the lighter bands are called I bands. Each I band is bisected by a dark transverse line called the Z-line). Paired mitochondria are on either side of the electron opaque Z-line. The Z-Line marks the longitudinal extent of a sarcomere unit.