Figure 1.

(a) Simplified representation of the physical relationship between cell-substrate tractions, T, which have been reported previously¹⁵, and intercellular stresses, σ, which are reported for the first time here. Intercellular stresses arise from the accumulation of unbalanced cell-substrate tractions. At any point within the monolayer (b), the intercellular stresses, defined in laboratory frame (x, y), (c), have shear (σ_xy, and σ_yx) and normal (σ_xx, and σ_yy) components. This frame can be rotated locally to obtain the principal frame (x', y'), (d), where shear stresses vanish and the resulting normal stresses are called principal stresses (σ_max and σ_min). The corresponding axes are called maximum, aligned with x', and minimum, aligned with y', principal orientations.