The structural arrangement of striated muscle sarcomeres. (a) Schematic diagram of a vertebrate striated muscle sarcomere with the thick myosin filaments (TF) forming the A-band and overlapping actin filaments (A), which extend to the Z-line (Z). Myosin filaments are cross-linked at the M-band (M), on each side of which are the bare regions (B), where the myosin filament backbone appears triangular (see also Fig. 4 a). The I-band extends from the A-band edges to the Z-line. (b and c) The appearance in electron micrographs of cross sections through the bare regions in panel a, for the simple lattice (b) and the superlattice (c). (d–f) Illustrations of the no-three-alike rules, and a superlattice unit cell where the no-three-alike rules have been applied (f). Adapted from Luther et al. (1996), with permission.