Figure 428 from Chapter 16 (Cytoplasmic matrix and cytoskeleton) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Papillary muscle of cat heart shows one sarcomere length of two adjacent myofibrils at high magnification. The darker A bands are composed of a parallel array of myosin filaments, about 10-11 nm thick and about 1.5 µm long. The lighter I bands of actin filaments are about 6 nm thick extending from the dense central Z line. The bracket labeled X indicates the region of overlap of the two sets of interdigitating filaments. The bracket labeled Y indicates the central region of the A band, composed of only myosin filaments. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.