Alignment, cross linking, and beyond: a collagen architect's guide to the skeletal muscle extracellular matrix

Abstract

The muscle extracellular matrix (ECM) forms a complex network of collagens, proteoglycans, and other proteins that produce a favorable environment for muscle regeneration, protect the sarcolemma from contraction-induced damage, and provide a pathway for the lateral transmission of contractile force. In each of these functions, the structure and organization of the muscle ECM play an important role. Many aspects of collagen architecture, including collagen alignment, cross linking, and packing density affect the regenerative capacity, passive mechanical properties, and contractile force transmission pathways of skeletal muscle. The balance between fortifying the muscle ECM and maintaining ECM turnover and compliance is highly dependent on the integrated organization, or architecture, of the muscle matrix, especially related to collagen. While muscle ECM remodeling patterns in response to exercise and disease are similar, in that collagen synthesis can increase in both cases, one outcome leads to a stronger muscle and the other leads to fibrosis. In this review, we provide a comprehensive analysis of the architectural features of each layer of muscle ECM: epimysium, perimysium, and endomysium. Further, we detail the importance of muscle ECM architecture to biomechanical function in the context of exercise or fibrosis, including disease, injury, and aging. We describe how collagen architecture is linked to active and passive muscle biomechanics and which architectural features are acutely dynamic and adapt over time. Future studies should investigate the significance of collagen architecture in muscle stiffness, ECM turnover, and lateral force transmission in the context of health and fibrosis.

Keywords: collagen architecture; fibrosis; muscle matrix; muscle mechanics; perimysium.

Graphical abstract

Figure 1.

Schematic of collagen architectural features within the skeletal muscle extracellular matrix (ECM). The muscle ECM contains a significant amount of fibrillar collagen, which has a complex architecture that dictates many functional properties of muscle. Composition of collagens relates to their total content and distribution among types of collagens. Cross linking relates to the degree of collagen cross links, both enzymatic and nonenzymatic, that exist between collagen molecules and fibrils. Density relates to how tightly collagen fibrils and fibers are packed together within the matrix. Orientation of collagen fibers and fibrils relates to their directional alignment and their crimp. These properties of collagen architecture relate to passive mechanics, collagen turnover, and lateral force transmission. AGE, advanced glycation end product. Figure made using BioRender.com.

Figure 2.

Collagen alignment within the epimysium and perimysium in the D2 wild-type and D2.mdx diaphragms. Images taken using second harmonic generation microscopy show collagen fiber organization in decellularized D2 wild-type (WT) and D2.mdx whole diaphragms. The top layer of extracellular matrix (ECM), the epimysium, is oriented approximately 40° to the myofibers in the WT and 80° to the myofibers in the D2.mdx diaphragm. However, the perimysium is closely aligned along the myofiber angle in both WT and D2.mdx diaphragms. These images highlight the difference in diaphragm collagen architecture between ECM layers and genotypes. The rose plots show the distribution of collagen orientations within the representative images. Scale bars = 25 μm.

Figure 3.

Healthy and fibrotic collagen architecture across the layers of skeletal muscle extracellular matrix (ECM). Fibrotic skeletal muscle has visibly altered collagen architecture compared to healthy muscle. In general, fibrosis is accompanied by increases in collagen content, cross links, and fiber alignment. The healthy epimysium contains collagen fibers that run at a steeper angle to the myofibers than in the perimysium, and in fibrosis, the collagen may run transversely to the myofiber angle. In the perimysium, collagen fibers run in a direction similar to the myofibers, and in fibrosis, the collagen fibers become more aligned and straighter (less crimped). The endomysial collagen matrix is composed of the fibrous layer, which is mostly fibrillar collagen, and the reticular layer, which is largely collagen IV with some collagen VI that acts to link the fibrous and reticular layers. Fibrillar collagen within the endomysium forms a sheath that runs circumferentially around each myofiber. In fibrosis, there is an increase in collagens in both the reticular and fibrous layers and an increase in cross links in the fibrous layer. deH-DHLNL, dehydro-dihydroxylysinonorleucine; deH-HLNL, dehydro-hydroxylysinonorleucine; HLP, hydroxylysyl pyridinoline; LP, lysyl pyridinoline. Figure assembled using BioRender.com.