Elucidation of extracellular matrix mechanics from muscle fibers and fiber bundles

Abstract

The importance of the extracellular matrix (ECM) in muscle is widely recognized, since ECM plays a central role in proper muscle development (Buck and Horwitz, 1987), tissue structural support (Purslow, 2002), and transmission of mechanical signals between fibers and tendon (Huijing, 1999). Since substrate biomechanical properties have been shown to be critical in the biology of tissue development and remodeling (Engler et al., 2006, Gilbert et al., 2010), it is likely that mechanics are critical for ECM to perform its function. Unfortunately, there are almost no data available regarding skeletal muscle ECM viscoelastic properties. This is primarily due to the impossibility of isolating and testing muscle ECM. Therefore, this note presents a new method to quantify viscoelastic ECM modulus by combining tests of single muscle fibers and fiber bundles. Our results demonstrate that ECM is a highly nonlinearly elastic material, while muscle fibers are linearly elastic.

Introduction

Extracellular matrix (ECM) is essential for the development, maintenance and regeneration of skeletal muscle (Buck and Horwitz, 1987, Purslow, 2002). ECM is involved in the macrostructure of muscle, arranging fibers into bundles, bundles into fascicles and integrating muscle structure with aponeurosis and tendon. Additionally, ECM is thought to play a vital role in mechanotransduction and transmitting force laterally from the fiber to the tendon and vice versa (Fomovsky et al., 2010, Huijing, 1999, Purslow and Trotter, 1994, Street, 1983). The mechanical strength and elasticity of the ECM are critical to its functional performance—it must be strong enough to sustain the loads of contraction yet elastic enough to prevent tearing under externally applied strains (Purslow, 2002). These properties change both with age and disease, where chronic alterations to the ECM appear to impair muscle function (Lieber et al., 2003, Zhou and Lu, 2010). Unfortunately, there are almost no data available regarding skeletal muscle ECM viscoelastic properties. This is primarily due to the impossibility of isolating and testing muscle ECM.

Attempts to remove muscle cells chemically from the ECM to test its properties directly have all met with some degradation or compromised mechanical properties (Borschel et al., 2004, Qing and Qin, 2009). Additionally, the geometry of the ECM structure is modified when part of its composite structure (the fiber) is removed, which likely affects the orientation of collagen fibers and thus the modulus of elasticity. This report describes a new technique for indirectly determining the mechanical properties of the ECM without digestion, by combining tests from single muscle fibers and fiber bundles and using the analytical approach of composite theory.