A,B) Fibroblast clustering on collagen matrices was carried out in DMEM/FBS with blebbistatin added to the incubations as indicated. At the end of the incubations, samples were fixed and stained for cell nuclei (Hoechst). Data shown are the appearance of cell clusters from a representative experiment (Bar = 200 μm) and the averages ± SD of quantitative clustering measurements (cell cluster = ≥10 nucleus in 100 μm2 ) for triplicate samples. C) Using the same samples as in A,B, contraction of attached collagen matrices was determined as the difference in height between control matrices (no cells) and experimental matrices. Data shown are the averages ± SD for triplicate samples. Blebbistatin inhibited cell clustering at 2.5-5 μM, whereas 10-20 μM blebbistatin was required to block matrix flattening.