Biomechanics of meniscus cells: regional variation and comparison to articular chondrocytes and ligament cells

Abstract

Central to understanding mechanotransduction in the knee meniscus is the characterization of meniscus cell mechanics. In addition to biochemical and geometric differences, the inner and outer regions of the meniscus contain cells that are distinct in morphology and phenotype. This study investigated the regional variation in meniscus cell mechanics in comparison with articular chondrocytes and ligament cells. It was found that the meniscus contains two biomechanically distinct cell populations, with outer meniscus cells being stiffer (1.59 ± 0.19 kPa) than inner meniscus cells (1.07 ± 0.14 kPa). Additionally, it was found that both outer and inner meniscus cell stiffnesses were similar to ligament cells (1.32 ± 0.20 kPa), and articular chondrocytes showed the highest stiffness overall (2.51 ± 0.20 kPa). Comparison of compressibility characteristics of the cells showed similarities between articular chondrocytes and inner meniscus cells, as well as between outer meniscus cells and ligament cells. These results show that cellular biomechanics vary regionally in the knee meniscus and that meniscus cells are biomechanically similar to ligament cells. The mechanical properties of musculoskeletal cells determined in this study may be useful for the development of mathematical models or the design of experiments studying mechanotransduction in a variety of soft tissues.

Fig. 1. Schematic of cytocompression experiments

(A) Cells were seeded onto a glass slide and compressed at a rate of 4 µm/second by a tungsten probe. The height (h) and width (w) of the cell, as well as the probe deflection (δ) was measured at various stages of each compression event and used to calculate cellular biomechanical properties. (B) Representative frames of an inner meniscus cell seeded onto a glass slide initially (i), at equilibrium compression (ii), and during the recovery stage (iii), of a cytocompression event

Fig. 2. Stress versus strain correlations for different cell types

Linear regression analysis was performed on stress-strain data for chondrocytes, inner and outer meniscus cells, and ligament cells. Significant correlations were observed for all cell types (p < 0.001). For each graph, the equation obtained from linear regression is shown with 95% confidence bounds for the slope in parentheses below. Chondrocytes showed the highest cell stiffness (slope) than all other cell types, while ligament cell stiffness was not statistically different from either meniscus cell type. Inner meniscus cells, however, were statistically less stiff than outer meniscus cells

Fig. 3. Correlation of residual vs. applied strain of compressed cells

Linear regression analysis was performed on residual strain versus applied strain data for chondrocytes, inner and outer meniscus cells, and ligament cells. Significant correlations were observed for all cell types (p < 0.001). The equation obtained from linear regression is shown in each graph with 95% confidence bounds for the slope in parentheses below. Positive correlations were observed for all cell types, but chondrocytes displayed a statistically lower slope than outer meniscus cells

Fig. 4. Apparent compressibility versus applied strain correlations

Linear regression analysis was performed on compressibility versus applied strain data for chondrocytes, inner and outer meniscus cells, and ligament cells. Significant correlations were observed for all cell types (p < 0.05). The equation obtained from linear regression is shown in each graph with 95% confidence bounds for the slope in parentheses below. Positive correlations were observed for outer meniscus cells and ligament cells, while negative correlations were observed for chondrocytes and inner meniscus cells

Fig. 5. Actin and FAK immunocytochemistry

Articular chondrocytes, inner and outer meniscus cells, and ligament cells were seeded onto glass slides and fluorescently stained for cell nuclei, actin, and focal adhesion kinase (FAK). Cells from all groups stained positively for actin and FAK, and FAK staining was similar across cell types. Actin staining was most defined in articular chondrocytes, and more diffuse in meniscus and ligament cells

Fig. 6. α-tubulin immunocytochemistry

Single articular chondrocytes, inner and outer meniscus cells, and ligament cells were fluorescently stained for microtubules using an anti-α-tubulin antibody. All cells stained positively for microtubules, with no gross differences noted