Mitochondrial Dynamics in Chondrocytes and Their Connection to the Mechanical Properties of the Cytoplasm

[+] Author and Article Information
Ze’ev Bomzon

Faculty of Civil and Environmental Engineering,  Technion-Israel Institute of Technology, Haifa 32000, Israel and Centre for Micro-Photonics,  Swinburne University of Technology, PO Box 218, Hawthorn, 3122, Victoria, Australiazbomzon@swin.edu.au

Martin M. Knight

Medical Engineering Division, Department of Engineering and IRC in Biomedical Materials,  Queen Mary University of London, London, UKm.m.knight@qmul.ac.uk

Dan L. Bader

Medical Engineering Division, Department of Engineering and IRC in Biomedical Materials,  Queen Mary University of London, London, UKd.l.bader@qmul.ac.uk

Eitan Kimmel

Faculty of Civil and Environmental Engineering and Department of Biomedical Engineering,  Technion-Israel-Institute of Technology, Haifa 32000, Israeleitan@bm.technion.ac.il

J Biomech Eng 128(5), 674-679 (Feb 12, 2006) (6 pages) doi:10.1115/1.2246239 History: Received November 06, 2005; Revised February 12, 2006

Background: The motion and redistribution of intracellular organelles is a fundamental process in cells. Organelle motion is a complex phenomenon that depends on a large number of variables including the shape of the organelle, the type of motors with which the organelles are associated, and the mechanical properties of the cytoplasm. This paper presents a study that characterizes the diffusive motion of mitochondria in chondrocytes seeded in agarose constructs and what this implies about the mechanical properties of the cytoplasm. Method of approach: Images showing mitochondrial motion in individual cells at 30s intervals for 15min were captured with a confocal microscope. Digital image correlation was used to quantify the motion of the mitochondria, and the mean square displacement (MSD) was calculated. Statistical tools for testing whether the characteristic motion of mitochondria varied throughout the cell were developed. Calculations based on statistical mechanics were used to establish connections between the measured MSDs and the mechanical nature of the cytoplasm. Results: The average MSD of the mitochondria varied with time according to a power law with the power term greater than 1, indicating that mitochondrial motion can be viewed as a combination of diffusion and directional motion. Statistical analysis revealed that the motion of the mitochondria was not uniform throughout the cell, and that the diffusion coefficient may vary by over 50%, indicating intracellular heterogeneity. High correlations were found between movements of mitochondria when they were less than 2μm apart. The correlation is probably due to viscoelastic properties of the cytoplasm. Theoretical analysis based on statistical mechanics suggests that directed diffusion can only occur in a material that behaves like a fluid on large time scales. Conclusions: The study shows that mitochondria in different regions of the cell experience different characteristic motions. This suggests that the cytoplasm is a heterogeneous viscoelastic material. The study provides new insight into the motion of mitochondria in chondrocytes and its connection with the mechanical properties of the cytoplasm.

Copyright © 2006 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 1

Intracellular motion of mitochondria in a single cell 1, 5, and 10min after the beginning of the experiment. The arrows which depict the direction and magnitude of the motion are superimposed on confocal images of the mitochondria.

Grahic Jump Location
Figure 2

Experimentally measured MSD for mitochondria in a single cell. (a) MSD of mitochondria in ten different ROIs. (b) Average MSD for all mitochondria in the cell.

Grahic Jump Location
Figure 3

Distribution of fij for a typical cell, as well as the F distribution expected for a homogeneous sample (solid line), and the distribution of F values obtained for a simulation of 500 freely diffusing particles when the variance of the diffusion coefficient is 25% and 58%

Grahic Jump Location
Figure 4

Average correlation between mitochondrial motion as function of the distance between the mitochondria



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