Transition in the thin-filament arrangement in rat skeletal muscle

The transition in thin-filament arrangement from tetragonal near the Z-band to trigonal in the A-band was investigated by computer-assisted analysis of thin-filament arrangement in serially cross-sectioned rat muscle. Extensor digitorum longus (EDL; fast) muscle from adult rats and adult, 9-day and 3-day neonatal soleus (slow) muscle were serially cross-sectioned from the H-zone of one sarcomere to the H-zone of an adjacent sarcomere. Thin-filament arrangement was analysed throughout the I-band, and particularly at three levels of the sarcomere: in the I-band, one section (0.06 microns) from the Z-band; six sections (0.36 microns) from the Z-band; in the A-band, two sections from the A-I junction (0.72 microns from the Z-band). Data for radical distributions and annular distributions were obtained by computer. In all muscles studies, thin-filament arrangement exhibited four-neighbour ordering throughout the I-band. Thin-filament arrangement exhibited three-neighbour ordering only in the A-band. The transition in thin-filament arrangement from four-neighbour to three-neighbour ordering occurred within 0.12 microns of the A-I junction in muscles fixed at rest length. In adult soleus that had been stretched 20% so that the A-I junction moved away from the N2-line, the transition in thin-filament arrangement occurred in the I-band within a region 0.4-0.5 microns from the Z-band. This region corresponds to the N2-line region of the I-band. Thus, the transition from four-neighbour to three-neighbour ordering occurs in the I-band independent of the thin filament-thick filament interaction. We conclude that some inherent feature of the I-band or thin filament-thin filament interaction imposes a four-neighbour ordering on thin filaments from the Z-band to the N2-line.