The Mechanical Properties of the Cell Surface

ABSTRACT A new instrument for measuring the mechanical properties of the cell surface is described. It consists of a micropipette connected to a moveable reservoir of water. The tip of the pipette is brought up to a cell and a bulge sucked out of the surface by lowering the reservoir. A plot of deformation (i.e. degree of bulging) against negative hydrostatic pressure gives a straight line for the sea-urchin egg at all stages. We have called the slope of this line the ‘stiffness ‘of the cell membrane ; it varies with the stage of the egg, the size of the pipette and the speed at which readings are taken. A comparison of the straight-line plots actually obtained with the curves to be expected from pure surface tension or from a thin-walled elastic sphere, leaves no doubt that the cell membrane is not behaving like either of these systems. It is concluded that the membrane is sufficiently thick to resist deformation by virtue of its own rigidity, resembling therefore a tennis ball rather than a rubber balloon or a fluid drop. An analysis of the problem in terms of a thick membrane is mathematically intractable, and is only possible by means of models. Experiments, using a large-scale pipette to suck bulges out of rubber balls, show that over a considerable range of wall thicknesses, the pressure-deformation curves are in fact linear. It is shown that the slope of the pressure-deformation curve or ‘stiffness’ depends on Young’s modulus and the internal pressure. It is not possible to separate these two variables by direct means. A dimensional analysis shows that it is permissible to scale down from model experiments to arrive at Young’s modulus and the internal pressure of the cell. For any given value of ‘stiffness’ there is a series of solutions, with values for the modulus decreasing from a certain figure, and values for the internal pressure increasing from nil. A limit can, however, be set to this series by other measurements.