Structure and function of the skeletal muscle extracellular matrix

Abstract

The skeletal muscle extracellular matrix (ECM) plays an important role in muscle fiber force transmission, maintenance, and repair. In both injured and diseased states, ECM adapts dramatically, a property that has clinical manifestations and alters muscle function. Here we review the structure, composition, and mechanical properties of skeletal muscle ECM; describe the cells that contribute to the maintenance of the ECM; and, finally, overview changes that occur with pathology. New scanning electron micrographs of ECM structure are also presented with hypotheses about ECM structure–function relationships. Detailed structure–function relationships of the ECM have yet to be defined and, as a result, we propose areas for future study.

Keywords: Skeletal muscle; biomechanics; collagen; extracellular matrix; fibrosis.

Figure 1

Schematic diagram of the gross organization of muscle tissue and muscle ECM-tendon organization. (A) Muscle ECM can be categorized as epimysium (surrounding the muscle), perimysium (surrounding muscle fascicles), and endomysium (surrounding muscle fibers). (B) Cross-section of muscle tissue indicating that the perimysium may be continuous with the tendon, whereas endomysium is contained within muscle fascicles.

Figure 2

Scanning EM of the collagen us endomysial network around muscle fibers observed after digesting fibers with NaOH. (A) Low power overview of theendomysium reveals an array of “tubes” into which muscle fibers insert (arrows) as well as a thickened area surrounding the fibers that is presumably perimysium (arrowhead). (B) Higher power view of endomysial network revealing the fine structure of the endomysial surfaces (arrow) as well as some undigested muscle fiber (arrowhead). This picture suggests that muscle fibers are embedded in a complex connective tissue matrix and are intimately associated with ECM. Figure from reference used with permission.

Figure 3

Mouse extensor digitorum longus muscles formalin-fixed at resting length, dehydrated in graded ethanol, lyophilized, and observed by SEM(same preparatory method used for Figs 4, 5, 6, and 7). (A) Patch of ECM pulled away from muscle fibers during sample preparation. Each white rectangle is enlarged in subsequent figure components. (B) A regular, longitudinal organization of ECM structures is observable on the fiber surface and its periodicity is noted with lines. (C) Central region of the patch of ECM pulled away from muscle fiber surface showing the wavy collagen fiber organization and collagen fibril network. (D) Apparent connection between the ECM patch pulled away and the muscle fiber surface. The patch appears to be continuous with the ECM on the fiber surface.

Figure 4

Scanning electron micrograph of 7 adjacent muscle fibers. Note that the surface topology varies among the fibers. In 5 cases, surface striations are visible, some of which clearly show A-band (curved brackets) and Z-band periodicity (dots). In 3 fibers, stout longitudinal “cables” extend a distance of at least 100 μm. In one case, a discrete connection is seen on the surface of a fiber that presumably represents the “perimysial plate” (circled) that connects adjacent fibers. Micrograph was obtained from a mouse EDL muscle stretched to a sarcomere length of 3.3 μm.

Figure 5

Scanning electron micrograph of mouse EDL muscle stretched about 30% beyond resting length showing longitudinally aligned perimysial collagen cables. (A) Two muscle fibers separated by stretched collagen cables. The collagen cables are distinct from the muscle fiber surface. (B) A collagen cable becomes frayed as it traverses across collagen cables.

Figure 6

Montage of scanning electron micrographs from shortened mouse EDL muscle. Large collagen cables arranged in a slack configuration are visible on the surface of the muscle fiber and integrated with the fiber surface. Coils in the collagen cables may indicate that the cables have a strain relief function (arrow). These large cables are believed to be perimysial in nature while the mesh in the background of the montage (arrow heads) is believed to be endomysial. It is clear that these two levels of ECM are intimately associated.

Figure 7

Epimysial layer of a mouse EDL muscle viewed in cross-section. A connective tissue sheath is clearly seen surrounding muscle fibers. (A) Survey view of the muscle where a region can be observed with sarcomere periodicity through the connective tissue layer. An individual muscle fiber is outlined (dashed line) (B) Closer view of the epimysium reveals longitudinal periodicities (lines) with approximately 1 μm of spacing.

Figure 8

Mechanical contribution of the ECM to muscle bundle modulus. (A) Individual fibers display linear stress-strain behavior, whereas muscle bundles composed of these fibers and ECM display a nonlinear relationship. (B) Fiber groups do not contain ECM (see text), and the modulus value is comparable to that of single fibers. Fiber bundles do contain ECM and have a significantly higher modulus, suggesting that the addition of ECM is responsible for the increased modulus (data replotted from reference79). Asterisk indicates p<0.05 compared to fibers, dagger indicates p<0.05 compared to fiber groups.

Figure 9

Cross-sectional view of normal and neurotoxin-injected rat tibia is anterior muscle stained with hematoxylin and eosin. (A) Normal muscle showing endomysium connective tissue (sample outlined in solid line) separating muscle fibers and perimysium (sample outlined in dashed line) separating bundles of muscle fibers (artery, a; vein, v). (B) Increased connective tissue around atrophied fibers in neurotoxin-injected muscle (nerve, n). Unaffected regions show normal fiber morphology (image courtesy of Drs. Sam Ward and Viviane Minamoto).