Chapter 6 - Ultrastructure and Biomechanics of Skeletal Muscle ECM: Implications in Tissue Regeneration

Abstract

Volumetric muscle loss (VML) not only causes impaired function or a nonfunctional limb, but may also lead to limb amputation. Current treatments of VML are not able to address the functional deficiency of the damaged muscles due to a lack of effective regeneration. Tissue engineering and regenerative approaches, such as stem cell therapy and the use of scaffolds to promote skeletal muscle regeneration, prove to be a promising strategy for skeletal muscle regeneration and VML treatment. The knowledge of the biophysical properties of the skeletal muscle extracellular matrix (ECM) is thus important and benefits tissue engineering and regeneration efforts from two aspects: (1) it helps better design scaffolds that mimic the skeletal muscle ECM, and (2) skeletal muscle ECM is itself a very promising scaffold/biomaterial for muscular regeneration. In this chapter, we have reviewed the hierarchical organization, ultrastructure, and biomechanics of the skeletal muscle ECM, covering the epimysial ECM, perimysial ECM, endomysial ECM, and the basement membrane. We further reported a comparative biomechanical study between porcine native skeletal muscle and skeletal muscle ECM obtained via 1% sodium dodecyl sulfate (SDS) decellularization. We found that the skeletal muscle has a nonlinear, anisotropic behavior, with the longitudinal direction being stiffer than the transverse direction. However, the skeletal muscle ECM shows an overall softening trend and a switch of anisotropic directions, i.e., the longitudinal direction turns softer and more extensible than the transverse direction. Via histological observation, we showed that the switching of anisotropy in the skeletal muscle ECM most likely results from the removal of muscle fibers, the intrinsic collagen fiber arrangement of ECM lacunae, and the possible collagen fiber network disruption due to decellularization. Uniaxial mechanical testing shows that both the skeletal muscle and skeletal muscle ECM have a nonlinear stress–strain behavior, while the decellularized skeletal muscle ECM is more extensible, with a smaller initial tensile modulus and a larger maximum tensile modulus. After reviewing our data and the previous studies, we emphasize that the compositional, structural, and mechanical properties of the skeletal muscle ECM are highly dependent on the decellularization methods being chosen. Lastly, we reviewed the most recent research and applications of skeletal muscle ECM in muscular regeneration, particularly the use of skeletal muscle ECM scaffolds as grafting materials for tissue engineering and regeneration, as well as the fabrication of skeletal muscle ECM hydrogel for injection therapy.