  Key to collective cell migration is the ability of cells to rearrange their
position with respect to their neighbors. Recent theory and experiments
demonstrated that cellular rearrangements are facilitated by cell shape, with
cells having more elongated shapes and greater perimeters more easily sliding
past their neighbors within the cell layer. Though it is thought that cell
perimeter is controlled primarily by cortical tension and adhesion at each
cell's periphery, experimental testing of this hypothesis has produced
conflicting results. Here we studied collective cell migration in an epithelial
monolayer by measuring forces, cell perimeters, and motion, and found all three
to decrease with either increased cell density or inhibition of cell
contraction. In contrast to previous understanding, the data suggest that cell
shape and rearrangements are controlled not by cortical tension or adhesion at
the cell periphery but rather by the stress fibers that produce tractions at
the cell-substrate interface. This finding is confirmed by an experiment
showing that increasing tractions reverses the effect of density on cell shape
and rearrangements. Our study therefore reduces the focus on the cell periphery
by establishing cell-substrate traction as a major physical factor controlling
cell shape and motion in collective cell migration.
