Mechanical stress combines with planar polarised patterning during metaphase to orient embryonic epithelial cell divisions

The orientation of cell division (OCD) in the plane of epithelia drives tissue morphogenesis and relaxes stresses, with errors leading to pathologies. Stress anisotropy, cell elongation and planar polarisation can all contribute to the OCD, but it is unclear how these interact in vivo . In the planar polarised Drosophila embryonic ectoderm during axis elongation, planar OCD is highly variable. We show that both a temporary reversal of tissue stress anisotropy and local compression from neighbouring dividing cells re-orient mitotic spindles during metaphase, independently of interphase cell elongation. Isotropic cells align their OCD to the anterior-posterior (AP) embryonic axis, mediated by tissue-wide planar polarised Myosin II, while the spindle of elongated cells is sterically constrained to cell long axes. Thus AP-patterning ensures that cell division combines with cell rearrangement to extend the body axis, except when strong local stress anisotropy is dissipated by cells dividing according to their elongation. ### Competing Interest Statement The authors have declared no competing interest.