Myofilament diameters: an ultrastructural re-evaluation.

In situ, ultrastructural measurements of diameters of contractile filaments in skeletal and heart muscle differ considerably from those previously reported. Past measurements have been made in thin, transverse epoxy sections that were non-specifically stained with heavy metal salts to overcome background scattering of epoxy polymer. In our images from transverse de-embedded sections, the hexagonal lattice has some considerable differences from that seen in epoxy sections. Muscle samples from rat atrium and frog sartorius were fixed, dehydrated, embedded in polyethylene glycol, and sectioned. Sections were de-embedded in graded polyethylene glycol/ethanol, mounted on coated grids, critical point dried, and viewed in the electron microscope without staining. The backbone diameters of thick filaments were measured in the M band region and have an average value, after correction for shrinkage, of 25 nm. Thin filament diameters range from 6.5-9.5 nm. In regions of overlap of thin and thick filaments, the thick filament profiles varied from circular to asymmetric; diameters range up to 36 nm and yield eccentricity ratios varying from 1.5 to 1.0 (circular profiles). Portions of thick filaments touch or partially envelope neighboring thin filaments. The relative contributions of cytoskeletal components to these images of overlap regions remains to be determined, but the backbone diameters in glycerinated frog sartorius are not significantly different from control samples. The present results are consistent with those reported for rotary shadowed thick filaments; from recent experiments in muscles whose myofilament lattice is osmotically compressed; and with estimates of A band mass. This lattice geometry yields relatively low surface-to-surface distances between filaments. Steric considerations and their implications for cross bridge theory are discussed.