Light microscopy of skeletal muscle fibres in longitudinal section. The repeating striations of dark and light bands are myosin and actin filament proteins assembled into long myofibrils within each muscle fibre. A sarcomere is the region between successive light bands. Fibres are the product of fusions of many hundreds or thousands of earlier precursor muscle cells called myoblasts. Their nuclei become displaced to the edges of the fused cells and these form a multinucleated muscle fibre. Magnification x600 when printed at 10cm.