  This study investigates the relationship between cell shape and
cell-generated stresses in confluent cell layers. Using simultaneous
measurements of cell shape orientation and cell-generated contractile forces in
MDCK and LP-9 colonies, we report the emergence of correlated, dynamic domains
in which misalignment between the directors defined by cell shape and by
contractile forces reaches up to 90$^o$, effectively creating extensile domains
in a monolayer of contractile cells. To understand this misalignment, we
develop a continuum model that decouples the orientation of cell-generated
active forces from the orientation of the cell shapes. This challenges the
prevailing understanding that cells throughout a tissue create either
contractile or extensile forces, and the validity of the usual active nematic
models of cell motility where active forces are strictly slaved to cell shape
orientation.
