Physics in biology: soft cells
Physical Review Letters 87, 148102 (13 September 2001) Living cells have defined shapes, and can robustly oppose being deformed. This structural integrity is primarily a result of the cells’ cytoskeleton — a mesh of protein fibres producing a gel-like architecture. Indeed, the physical properties of cells are usually viewed in such terms, with cellular motion explained by transitions between this gel structure and a disassembled, more fluid-like sol. But now, in Physical Review Letters, Ben Fabry and colleagues report that their measurements made on single cells show that the bulk properties of cells more closely resemble those of a soft glass. Fabry and colleagues coated ferrimagnetic beads with short peptides. This made the beads stick to the surface of living cells, the peptides connecting directly to the cytoskeleton beneath. The beads were magnetized horizontally, then made to roll back and forth on the cell surface under the torque generated by an applied, oscillating magnetic field. By watching the movement of the beads through a CCD camera, a number of physical properties of the cell, such as its elastic modulus, could be estimated. The elastic modulus was found to depend on the frequency of the bead’s oscillation. When the bead oscillated below 10 Hz, the elastic modulus showed a weak power-law dependence on frequency; at higher frequencies there was a much stronger frequency dependence, approaching simple, newtonian-viscosity behaviour. Similar results were obtained for several types of cells, including smooth muscle cells, white blood cells and carcinoma cells, the only difference being the precise value of the power-law exponential. This is not typical gel behaviour, but rather corresponds to the somewhat enigmatic group of soft-glass materials, a group that also includes foams, slurries and colloidal suspensions. In soft-glass materials, the power-law exponential is a manifestation of the energy in the system, specifically the ‘effective noise temperature’. A unique feature of the cytoskeleton is that its components are in constant flux, assembling, disassembling and dissipating energy. It may be possible to somehow incorporate this relentless activity into a description of the cell as a soft glass, perhaps as a component of the effective noise temperature. Such an approach is certainly appealing, as it provides an explanation of how the cell deforms and flows without resorting to a phase transition as in the gel/sol model. But however useful the soft-glass model proves to be, careful studies, such as this by Fabry et al., will be needed to test and define the bulk properties of life’s most basic unit, the single cell.
Scaling the Microrheology of Living Cells
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