Extracellular matrix integrity affects the mechanical behaviour of in-situ chondrocytes under compression

Abstract

Cartilage lesions change the microenvironment of cells and may accelerate cartilage degradation through catabolic responses from chondrocytes. In this study, we investigated the effects of structural integrity of the extracellular matrix (ECM) on chondrocytes by comparing the mechanics of cells surrounded by an intact ECM with cells close to a cartilage lesion using experimental and numerical methods. Experimentally, 15% nominal compression was applied to bovine cartilage tissues using a light-transmissible compression system. Target cells in the intact ECM and near lesions were imaged by dual-photon microscopy. Changes in cell morphology (N_cell=32 for both ECM conditions) were quantified. A two-scale (tissue level and cell level) Finite Element (FE) model was also developed. A 15% nominal compression was applied to a non-linear, biphasic tissue model with the corresponding cell level models studied at different radial locations from the centre of the sample in the transient phase and at steady state. We studied the Green-Lagrange strains in the tissue and cells. Experimental and theoretical results indicated that cells near lesions deform less axially than chondrocytes in the intact ECM at steady state. However, cells near lesions experienced large tensile strains in the principal height direction, which are likely associated with non-uniform tissue radial bulging. Previous experiments showed that tensile strains of high magnitude cause an up-regulation of digestive enzyme gene expressions. Therefore, we propose that cartilage degradation near tissue lesions may be due to the large tensile strains in the principal height direction applied to cells, thus leading to an up-regulation of catabolic factors.

Introduction

Every day movements of synovial joints occur nearly without friction on a thin layer of articular cartilage covering the ends of bones. Articular cartilage consists of four major components: water (65–80%), collagen fibres (10–20%), proteoglycans (4–7%) and chondrocytes (1–10%) (Mow et al., 1984). The collagen fibres form a structural network that provides tensile strength and structural integrity to the tissue, while proteoglycans (PGs) give the tissue compressive strength through their water-retaining properties (Poole, 1997). Collagen and PGs form the extracellular matrix (ECM) that provides protection for the chondrocytes from excessive deformation during joint loading. Chondrocytes, in turn, maintain the integrity of the ECM by load-dependent release of matrix molecules (Bachrach et al., 1995, Honda et al., 2000, Poole, 1997, Sauerland et al., 2003).

Damage to articular cartilage often leads to joint degeneration and osteoarthritis (OA) (Link et al., 2003), a debilitating disease that causes joint pain and stiffness, and affects the quality of life of many people, especially the elderly (Wieland et al., 2005). Focal cartilage lesions may be found in otherwise healthy people with no symptoms of OA (Cicuttini et al., 2005). Once a lesion is formed, damage seems to spread from the lesion to the rest of the cartilage (Squires et al., 2003) resulting in cartilage erosion (Wang et al., 2006). There is also evidence for the presence of digestive enzymes in the vicinity of tissue lesions (Shlopov et al., 1997), suggesting a change in local chondrocyte behaviour, and an active release of proteolytic enzymes from cells near cartilage lesions (Wieland et al., 2005).

The mechanical microenvironment of chondrocytes is known to influence their biosynthetic activity (Grodzinsky et al., 2000, Guilak et al., 1997, Stockwell, 1987). Depending on the type (e.g., compression, tension) and nature (e.g., magnitude, frequency) of tissue loading, the biosynthetic response of chondrocytes is anabolic or catabolic (Bachrach et al., 1995, Griffin and Guilak, 2005, Honda et al., 2000, Sauerland et al., 2003). An intact ECM has also been shown to be crucial in the proper regulation of cell volumes (Turunen et al., 2011). Since collagen fibres and proteoglycans are compromised at the site of tissue lesions, chondrocytes adjacent to lesions are thought to experience an altered mechanical milieu resulting in a catabolic response. However, the details as to which mechanical factors drive this detrimental change in chondrocyte behaviour remain unclear.

Therefore, the purpose of this study was to investigate the effects of cartilage tissue lesions on the mechanical behaviour of in-situ chondrocytes using experimental and numerical approaches. Differences in cell mechanics between intact and lesioned ECM conditions may provide insight into the mechanical factors that induce catabolic responses in chondrocytes near tissue lesions. Since the ECM near lesions is damaged, we hypothesised that cells near lesions experience excessive deformations during joint loading, thus producing different mechanical signals than cells surrounded by an intact matrix.