The influence of fixation upon the fine structure of the z-disk of rat striated muscle

The fine structure of the Z-disk in rat striated muscle has been found to be dependent upon the manner of its fixation. Material primarily fixed in osmium tetroxide shows a square or ‘woven’ lattice in transverse sections, with a spacing of around 22 ran, and with its axes off-set by 45° from the axes of alignment of the files of I-filaments prior to their entry into the Z-disk. In longitudinal sections the disk may have a zig-zag appearance with individual I-filaments of one sarcomere apparently linked obliquely to 2 or more I-filaments of the next. Transverse sections of Z-disks in muscle primarily fixed in glutaraldehyde show a square-lattice pattern with a spacing of about 11 nm, its axes coinciding with the axes of alignment of the adjacent I-filaments. In longitudinal sections oblique linkages of the ends of the I-filaments are not seen, and they appear either to interdigitate, or to be longitudinally continuous with those of the next sarcomere. This second fine structure is interpreted as being the consequence of the superimposition of 2 basic square lattices, each of 22-nm period, 50 % out of register along each axis, and each formed from the lateral linkage of the ends of the interdigitating I-filamente with the ends of adjacent filaments from their own sarcomere. The relationship of this postulated structure to the lattice patterns previously described, the nature of the cross-linking material, and some possible functional implications are briefly discussed.