  Biological cells can actively tune their intracellular architecture according
to their overall shape. Here we explore the rheological implication of such
coupling in a minimal model of a dense cellular material where each cell exerts
an active mechanical stress along its axis of elongation. Increasing the active
stress amplitude leads to several transitions. An initially hexagonal crystal
motif is first destabilized into a solid with anisotropic cells. Increasing
activity further, we find a re-entrant transition to a regime with finite
hexatic order and finite shear modulus, in which cells arrange according to a
rhombile pattern with periodically arranged rosette structures. The shear
modulus vanishes again at a third threshold beyond which spontaneous tissue
flows arise. In this last regime, we observe the emergence of cell shape
patterns called topological defects, with flow and stress fields around defects
agreeing with those observed in epithelial tissue experiments. We further
provide a testable prediction of cell-cell rearrangement hotspots near
topological defects. Overall, our work connects seemingly distinct features -
e.g. rosettes and topological defects - observed across various types of
epithelial tissues.
