Probing mechanical heterogeneity in chondrocytes using passive microrheology

Abstract

Characterising chondrocytes mechanics is important for understanding mechanotransduction. The bulk viscoelastic response of chondrocytes have been measured. However, their mechanical properties have not been resolved on a subcellular scale. Microrheolgy is a technique in which the mechanical properties of a material are found by analysing the Mean Square Displacement (MSD) of tracerparticles. Microrheology can resolve mechanical properties with subcellular resolution. However, a limitation of microrheology is that large sequences of images of the particles are required for accurate measurements. These are not always available due to particles moving out of focus and photobleaching. This paper presents a microrheology-based study on mechanical heterogeneity in chondrocytes using short sequences of images. Bovine Articular chondrocytes were seeded into agarose constructs [2]. The mitochondria were fluorescently labeled and imaged every 30 seconds for 15 minutes with a confocal microscope. Digital Image Correlation was used to quantify the motion of the mitochondria and their MSDs were calculated. An average MSD was found for every cell and the variability in mitochondrial motion was obtained by comparing the distribution of measured MSDs to the distribution of MSDs obtained from Monte-Carlo simulations of particles embedded within heterogeneous media. Measured mitochondrial motion was consistent with directed diffusion. The diffusion coefficient of the mitochondria varied by about 50% within single cells. Calculations based on statistical mechanics showed that directed diffusion can only occur if the cytoplasm behaves like a fluid on large time-scales. It is probable that this viscous behavior is connected to the non-equilibrium nature of the cytoskeleton