Abstract
During mitosis, adherent cells round up, by increasing the tension of the contractile actomyosin cortex while increasing the internal hydrostatic pressure. In the simple scenario of a liquid cell interior, the surface tension is related to the local curvature and the hydrostatic pressure difference by Laplace's law. However, verification of this scenario for cells requires accurate measurements of cell shape. Here, we use wedged micro-cantilevers to uniaxially confine single cells and determine confinement forces while concurrently determining cell shape using confocal microscopy. We fit experimentally measured confined cell shapes to shapes obeying Laplace's law with uniform surface tension and find quantitative agreement. Geometrical parameters derived from fitting the cell shape, and the measured force were used to calculate hydrostatic pressure excess and surface tension of cells. We find that HeLa cells increase their internal hydrostatic pressure excess and surface tension from ≈ 40 Pa and 0.2 mNm−1 during interphase to ≈ 400 Pa and 1.6 mNm−1 during metaphase. The method introduced provides a means to determine internal pressure excess and surface tension of rounded cells accurately and with minimal cellular perturbation, and should be applicable to characterize the mechanical properties of various cellular systems.
Highlights
We performed a parallel plate confinement assay on HeLa cells using a combined confocal microscopy and AFM setup (Fig. 1)
The average distance, d . between measured surface points and the fit surface is smaller than 300 nm for all fits, demonstrating the good agreement between the measured cell shape and the cell shape predicted by the model (Fig. 2b)
The model specifies the rotationally symmetric shape and the mechanics of non-adherent cells confined between parallel plates (Fig. 1 and 2)
Summary
Correspondence and requests for materials should be addressed to J.H. Measuring the force exerted by confined mitotic HeLa cells, Stewart et al inferred that the increasing contractile stress in the cell cortex is balanced by an increasing internal hydrostatic pressure[17]. This conclusion was based on cells modeled as pressurized liquid sacks bounded by a shell in which contractile in-plane tensions are present. The thereby obtained accurate geometrical parameters of cell shape are used to calculate the internal hydrostatic pressure excess and the surface tension of the cell from the confinement force exerted by the micro-cantilever on the cell. We measure pressure excess and surface tensions of cells undergoing mitosis and compare these values with those obtained for non-adherent interphase cells
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