Figure 1.

Sarcomere organization in vertebrate and nematode muscle. Sarcomeres in vertebrate striated muscle cells are linked longitudinally in cytoplasmic myofibrils, while sarcomeres in nematode BWM cells are packed in a continuous layer beneath the cell surface that faces the outer body wall. Sarcomeres of both muscles appear similar in longitudinal y/z-sections: Z-lines at sarcomere ends anchor thin filaments; M-lines at sarcomere centers anchor thick filaments; A-bands containing thick filaments are centered on the M-lines; and I-bands lacking thick filaments are centered on the Z-lines. However, the different sarcomere arrangements result in very different appearances in x/y-and x/z-sections. Vertebrate sarcomeres in neighboring myofibrils are in register, resulting in cross striations perpendicular to the thin and thick filaments, visible in y/z - and x/z-sections. In x/y-cross sections of vertebrate striated muscle cells, all sarcomeres are cut at the same position. Thus, the x/y-section at the end of the modeled muscle cuts through I-bands in all sectioned sarcomeres, while the x/y-section near the middle of the muscle cuts A-bands in all sectioned sarcomeres. Conversely, nematode sarcomeres are offset from their neighbors by approximately 1 µm along the z-axis. As a result, striations appear at an oblique angle from thin and thick filaments in x/z-views. In x/y-cross sections of nematode muscle, different sarcomere positions are revealed in neighboring sarcomeres. Thus, the Z-line structure (dense body) for one sarcomere in an x/y-section is bordered by the I-bands of neighboring sarcomeres, which in turn are adjacent to the A-bands of their neighbors.