A- and I-segments, which are the naturally occurring assemblies of the myosin-containing and actin-containing filaments, respectively, have been released by mechanical disruption of frog skeletal myofibrils in a relaxing medium. In the electron microscope, negatively stained A-segments (length ~1.6 μm) show periodic structure in much more detail than is observed in isolated filaments or sectioned fibrils. The structure is bipolar. The central M-band (width ~495 Å) is flanked on either side by a heavily stained "bare" zone (width ~395 Å), following which is a series of ten bands of equal width (420 Å ± 15 Å) showing a polarised substructure; the end of the segment has a less regular structure. Bands 1, 2 and 3 differ from bands 4 to 10, which all look alike. Optical diffraction patterns from the region of the ten bands resemble, in certain respects, the patterns from micrographs of light meromyosin paracrystals. The arrangement of the heavy meromyosin projections is not apparent; probably they have been damaged by the stain. No evidence was found of a 442 Å periodicity. It is concluded that the band pattern observed represents mainly the bipolar structure of the light meromyosin filament backbone. © 1971.
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