This videoclip shows the deformation of collagen matrix (left, bright fields) and migration of fibroblasts on the matrix (right, fluorescence micrographs). This videoclip illustrate that, when a large number of fibroblasts migrate all together (i.e., en masse), cells can collectively deform the matrices. Fibroblasts are seeded on the left half of each image. The matrix deformation is measured by microbeads embedded in the collagen matrix. The cell movement trajectories are tracked by labeling cells with quantum dots. This result is published in Ozcelikkale et al (2017) J Royal Soc Interface.