Linking Particle Dynamics to Intracellular Micromechanics in Living Cells

Colloidal particles embedded in the cytoplasm of living mammalian cells have been found to display remarkable heterogeneity in their amplitude of motion. However, consensus on the significance and origin of this phenomenon is still lacking. We conducted experiments on Hmec-1 cells loaded with about 100 particles, to reveal the intracellular particle dynamics as a function of both location and time. Central quantity in our analysis is the amplitude (A) of the individual Mean Squared Displacement (iMSD), averaged over a short time. Histograms of A were measured: 1) over all particles present at the same time and 2) for individual particles as a function of time. Both distributions showed significant broadening compared to particles in Newtonian liquid, indicating that the particle dynamics varies with both location and time. However, no systematic dependence of A on intracellular location was found. Both the (strong) spatial and (weak) temporal variations were further analyzed by correlation functions of A. Spatial crosscorrelations were rather weak down to interparticle distances of 1 micron, suggesting that the precise intracellular probe distribution is not crucial for observing a dynamic behavior that is representative for the whole cell. Temporal correlations of A decayed at ~ 10 s, possibly suggesting an intracellular reorganization at this time scale. These findings imply 1) that both individual particle dynamics and the ensemble averaged behavior in a given cell can be measured if there are enough particles per cell, and 2) that the amplitude and power-law exponent of iMSDs can be used to reveal local dynamics. We illustrate this by showing how superand subdiffusive behaviors may be hidden under an apparently diffusive global dynamics. -----------------------------------------------------------------This chapter has been published in Physics Review E 79, 051910 (2009) Mapping of Spatiotemporal Heterogeneous Particle Dynamics in Living Cells 48 3.