The Ultrastructure of the M Line in Skeletal Muscle

Abstract

By electron microscopy, the ultrastructure of the M line was investigated in fibers from frog nonglycerinated semitendinosus muscles at body length and at different degrees of shortening and stretch. The M line appeared as a line of high electron opacity in the middle of the A band. Its framework consists of: (i) three (four or five) arrays of transverse M bridges, 200 A apart, connecting each A filament with its six neighbors; (ii) M filaments, parallel to the A filaments, passing through the M line and linking each set of M bridges together. In the shortened fiber the M line remained distinct. At high degrees of stretch, the M line became fainter or indiscernible. This appearance reflects a misalignment of the M components caused by a staggering of the A filaments. The M line reappeared after release of fibers stretched 70-80% above equilibrium length. On the basis of the structural analysis, the possible function of the M line is compared with that of the Z line, and a model is suggested for the M line.

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

“…Knappeis and Carlsen (1962) observed that I-filaments in longitudinal sections terminate as rod-like projections on either side of the Z-disk with an I-filament on one side of the Z-disk lying between two I-filaments on the opposite side (see Figure 1). Thus, I-filaments are not continuous through the Z-disk.…”

“…A, whereas the ostensible diameter of the Z-filaments is 50 -7 A. Con sequently, one of the principal criticisms of the Knappeis and Carlsen (1962) model has now been mitigated, and it seems quite likely that the thin fila ment may indeed split into two or perhaps more smaller filaments as it enters the Z-disk. Reedy (1964) described che scruccure in transverse sections of Z-disk from rat striated muscle as having a "basketweave" appearance.…”

“…Most of this inquiry has involved Z disks of "fast" skeletal muscles (fast-twitch glycolytic fibers; Peter £t al., 1972), and a number of models have been proposed to explain Z-disk images obtained by high resolution electron microscopy techniques applied to vertebrate skeletal muscle (Knappeis and Carlsen, 1962;Huxley, 1963;Franzini-Armstrong and Porter, 1964;Reedy, 1964;Kelly, 1967Kelly, , 1969. It is important to note that al though creation of each of these Z-disk models was based on similar images of osmium-fixed material, it was necessary in every instance to incorporate individual assumptions that could not be confirmed with the available techniques.…”

“…One of the first ultrastructural descriptions of Z disks was given by Knappeis and Carlsen (1962), and this early description is still widely accepted as the best general model of Z-disk architecture. Knappeis and Carlsen (1962) observed that I-filaments in longitudinal sections terminate as rod-like projections on either side of the Z-disk with an I-filament on one side of the Z-disk lying between two I-filaments on the opposite side (see Figure 1).…”

“…Diagrammatic representation of the Z-disk model proposed by Knappeis and Carlsen (1962) (A and C show two identical longitudinal sections through the Z-disk. I-filaments (all white or all black bars) on opposite sides of the Z-disk are displaced with respect to each other so that I-filaments on one side of the Z-disk lie between two I-filaments on the opposite side of the Z-disk.…”