(A) Electron micrograph of a longitudinal section through zebrafish myotomal muscle showing the typical sarcomere striations of vertebrate striated muscle. The sarcomere (B), which extends between Z-bands (Z) and is ∼2.2 μm long, consists of the centrally placed A-bands and the less densely packed I-bands, which extend between successive A-bands. The A-band is formed by an array of myosin filaments carrying myosin head projections and cross-linked halfway along their length at the M-band (M). Each side of the M-band are the bare regions where the myosin filament backbones appear triangular. (C) Electron micrograph of zebrafish myotomal muscle in cross section showing myosin filament profiles near to the M-band (M) and in the adjacent bare regions (BR). The triangular profiles in one bare region all point in the same direction indicating the presence of a simple lattice arrangement.