Filamentous and matrix components of skeletal muscle Z‐disks

Abstract

The fine structural appearance of Z-disk lattices in vertebrate skeletal "fast" muscle varies depending upon whether osmium or glutaraldehyde has been employed as the primary fixative. Prior investigators have attributed the differences to change in the extent of actin filament overlap within the Z-disk and/or to rearrangement of Z-disk filaments.Adult frog and young newt "fast" muscle has been studied under various degrees of stretch, with several different aldehyde and osmium fixation procedures, and after plastic section digestion techniques utilizing Pronase or pepsin. Serial cross sections of Z-disks were correlated with oriented cross and longitudinal sections. Fixation with collidine-buffered osmium and Verona1 acetate-buffered glutaraldehyde seems to provide the greatest and most distincitly contrasting differences. A consistently arranged phase, the filamentous lattice, can be discerned after either fixation. However, a second phase, termed "Z-disk matrix," appears variable, perhaps due to extraction during primary osmium fixation procedures. Glutaraldehyde-fixed frog muscle Z-disks display a copious matrix, one which is seldom totally depleted by osmium fixation. In young newt muscle Z-disks, little matrix is present after glutaraldehyde fixation and none of it remains after primary osmium. In Z-disks fixed by either method, matrix that is retained appears to be deposited in lattice-like patterns. It is suggested that these matrix patterns, or their loss, are the basis for the varying images of Z-disks observed under different fixation conditions, and that the filamentous lattice js relatively stable. The Z-disk is more rapidly obliterated by Pronase or pepsin digestion than is any other muscle component, including actin (which appears notably unreactive). The rapid digestion effect is limited to the region postulated to include the matrix phase. Models for the structural interrelationship of filamentous and matrix phases are discussed and compared to prior Z-disk models.

Localization of [alpha]-actinin and tropomyosin in muscle and nonmuscle systems

“…This observation, therefore, supports Kelly and Cahill's proposal (Kelly and Caliill, 1972) that alteration of the matrix material by fixation causes formation of the large and small lattices. It is interesting to note that distinguishing between the two phases of material in the Z-disk (Kelly and Cahill, 1972) and attributing the large and small lattices to one of the two phases is teleologically similar to the explanation involving two kinds of Z-filaments that MacDonald and Engel (1971) proposed to account for the large parallel square lattices and the large angled lattice in Z-disks.…”

“…If osmium-fixed newt muscle, which con tains no matrix component, was examined in an identical manner, the parallel rows of actin dots seen as the 1-filaments approached the Z-disk were trans formed directly into the large angled lattice and then back again into parallel rows of actin dots as the serial sections passed through the Zdisk; no small or large parallel lattices were ever seen. Kelly and Cahill (1972) also point out that although the small and large parallel lattices had been called filamentous by previous investigators, the areas of den sity that form the sides of these lattices that the previous investigators termed filaments were very pleomorphic. The strands of density varied in diameter from 2 to 6 nm, and their presence varied greatly from place to place within a lattice and from specimen to specimen, even when glutaraldehyde fixation was used.…”

“…Matrix that is re tained after fixation by either glutaraldehyde or osmium appears to be deposited in lattice-like patterns in Z disks. Kelly and Cahill (1972), therefore, suggest that these lattice-like patterns formed by deposition of matrix material or the absence of these patterns, are the basis for the varying images of Z-disks observed under different fixation conditions, and that the underlying filamentous lattice is relatively stable to fixa tion. This interpretation was corroborated by serial cross-sections that followed individual filaments or sets of filaments as they coursed through the Z-disk in newt or frog skeletal muscle.…”

“…Although MacDonald and Engel (1971) favor the first explan ation because it requires existence of only one set of filaments, recent evidence on two types of material found in Z-disks (Kelly and Cahill, 1972) suggests that their second explanation may actually be the more nearly correct one. Kelly and Cahill (1972) have recently called attention to the fact that Z-disks in most skeletal muscles consist of two primary phases: 1) a filamentous lattice phase that seems relatively unaltered by either glutaraldehyde or osmium fixation; and 2) an amorphous matrix phase that surrounds the Z-filaments and seems highly variable in content and form depending on the muscle examined and on the type of fixation used. Glutar aldehyde-fixed frog muscle Z-disks have a copious matrix phase and this phase is seldom totally depleted by osmium fixation.…”

“…The strands of density varied in diameter from 2 to 6 nm, and their presence varied greatly from place to place within a lattice and from specimen to specimen, even when glutaraldehyde fixation was used. Consequently, Kelly and Cahill (1972) suggest that the large and small parallel lattices are formed by denaturation of the matrix material by glutaraldehyde fixation and deposition of this de natured material as strands between adjacent actin filaments. Because osmium tetroxide extracts the matrix material, the large and small lattices are not present in osmium-fixed muscle.…”