Image of the actin cytoskeleton in a COS-7 cell showing the free volumes contained in the “sponge” of the cytoskeleton. Panels b and c are zoomed versions of the box in a. The colors code the distance of the actin from the substrate. These images are made using only labeled actin, and there are many other proteins that make up the cytoskeleton, so the protein density and the free volumes are smaller. Bars, 2 µm (Xu et al., 2012). Reprinted by permission from Macmillan Publishers Ltd: Nature Methods. 9:185–188. 2012. Intermediate filaments bear a lot of stress (Fudge et al., 2008).