Probing Single-Cell Micromechanics In Vivo: The Microrheology of C. elegans Developing Embryos

Abstract

Cells are not directly accessible in vivo and therefore their mechanical properties cannot be measured by methods that require a direct contact between probe and cell. Here, we introduce a novel in vivo assay based on particle tracking microrheology whereby the extent and time-lag dependence of the mean squared displacements of thermally excited nanoparticles embedded within the cytoplasm of developing embryos reflect local viscoelastic properties. As a proof of principle, we probe local viscoelastic properties of the cytoplasm of developing Caenorhabditis elegans embryos. Our results indicate that unlike differentiated cells, the cytoplasm of these embryos does not exhibit measurable elasticity, but is highly viscous. Furthermore, the viscosity of the cytoplasm does not vary along the anterior-posterior axis of the embryo during the first cell division. These results support the hypothesis that the asymmetric positioning of the mitotic spindle stems from an asymmetric distribution of elementary force generators as opposed to asymmetric viscosity of the cytoplasm.

Hydrodynamic property of the cytoplasm is sufficient to mediate cytoplasmic streaming in the Caenorhabiditis elegans embryo

“…These results indicate that a Newtonian fluid is a good approximation and thus, nonNewtonian properties appear to play a minor role in cytoplasmic streaming in C. elegans. This finding is consistent with a previous microrheology study showing that simple diffusion was observed in the cytoplasm of the C. elegans embryo (35). Although nonNewtonian properties may appear when much larger or smaller forces are applied, the cytoplasm of the C. elegans embryo appears to behave as a Newtonian fluid under physiologically relevant amplitudes of applied force.…”

Direct laser manipulation reveals the mechanics of cell contacts in vivo

“…The viscosity associated with the observed damping is, thus, on the order of 1 Pa·s, which is consistent with our previous measurements of cytosol viscosity using beads. This value is also consistent with microrheological measurements made in the cytosol of Caenorhabditis elegans embryo (19).…”

Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

“…Single cells can be manipulated to probe the strength and location of receptor binding 16 and adhesion or to measure traction and adhesion forces 17 . Viscoelastic properties can be measured on short length-scales and in small volumes 18 , such as within cells 19 . Early applications of single-molecule force and displacement measurements included the characterization of conventional motor proteins such as kinesins 20,21 and myosins 22 .…”