Traction force microscopy

We measured the mechanical outputs of colonies using TFM, a technique for obtaining the stress field exerted by adherent cells on their environment (Sabass et al., 2008 ). In a TFM experiment, cells adhere to a thick, flexible substrate with embedded fiducial markers. For our experiments, we used PAA gels for their ease of preparation and versatility in stiffness, prepared with 0.11 µm fluorescent microspheres as markers. While cells are attached, they contract inward and deform the gel. By imaging the beads underneath cells while they are attached and subsequent to removal via SDS, we can obtain the displacement of the gel due to cell traction stresses. On the basis of the substrate displacement, we use Fourier transform traction cytometry (Butler et al., 2002 ; Sabass et al., 2008 ) to ascertain both the magnitude and location of traction stresses, as well as derive bulk quantities such as the total mechanical output of a colony. TFM thus provides an effective means for measuring the mechanical characteristics of colonies and tissues, which makes it an attractive technique for experimentally testing predictions made by mechanical models of cells and colonies. As TFM is compatible with other imaging techniques, it also allows us to directly compare morphological changes and mechanical outputs.