  The vertex model is widely used to simulate the mechanical properties of
confluent epithelia and other multicellular tissues. This inherently discrete
framework allows a Cauchy stress to be attributed to each cell, and its
symmetric component has been widely reported, at least for planar monolayers.
Here we consider the stress attributed to the neighbourhood of each tricellular
junction, evaluating in particular its leading-order antisymmetric component
and the associated couple stresses, which characterise the degree to which
individual cells experience (and resist) in-plane bending deformations. We
develop discrete potential theory for localised monolayers having disordered
internal structure and use this to derive the analogues of Airy and Mindlin
stress functions. These scalar potentials typically have broad-banded spectra,
highlighting the contributions of small-scale defects and boundary-layers to
global stress patterns. An affine approximation attributes couple stresses to
pressure differences between cells sharing a trijunction, but simulations
indicate an additional role for non-affine deformations.
