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

The planar orientation of cell division (OCD) is important for epithelial morphogenesis and homeostasis. We ask how mechanics and antero-posterior (AP) patterning combine to influence the first divisions after gastrulation in the Drosophila embryonic epithelium. We analyse hundreds of cell divisions and show that stress anisotropy, notably from compressive forces, can reorient division directly in metaphase. Stress anisotropy influences the OCD by imposing cell elongation, despite mitotic rounding and over-riding interphase cell elongation. In strongly elongated cells, the mitotic spindle adapts its length to, and hence its orientation is constrained by, the cell long axis. Alongside mechanical cues, there is a tissue-wide bias of the mitotic spindle orientation towards AP-patterned planar polarised Myosin-II. This spindle bias is lost in an AP-patterning mutant. Thus, a patterning-induced mitotic spindle orientation bias over-rides mechanical cues in mildly elongated cells but the spindle is constrained to the high stress axis in strongly elongated cells.