Fourier transform traction microscopy

The distribution of traction fields arising at the interface between each adherent cell and its substrate were evaluated using FTTM. In brief, cells were plated sparsely on gel blocks, and allowed to adhere and stabilize for 24 h. For each adherent cell, images of fluorescent microbeads (0.2 µm in diameter, Molecular Probes, Eugene, OR) embedded near the gel apical surface were taken at different times; the fluorescent image of the same region of the gel after cell detachment with trypsin was used as the reference (traction-free) image. The displacement field between a pair of images was then obtained by identifying the coordinates of the peak of the cross-correlation function²⁶. From the displacement field and known elastic properties of the gel (Young’s modulus of 1300 Pa with a Poisson’ ratio of 0.48) the traction field was computed using both constrained and unconstrained FTT cytometry²⁶. The computed traction field was used to obtain net contractile moment, which is a scalar measure of the cell’s contractile strength. Net contractile moment is expressed in units of pico-Newton meters (pNm).