Despite decades of research, the global cues that establish the orientation of planar cell polarity (PCP) within a tissue are still relatively poorly understood. In some contexts, Wnt signals can define the axis of PCP, but other factors must also be involved. A number of studies have implicated mechanical forces in the establishment of PCP, and Tatsuo Michiue and colleagues here set out to investigate how mechanical signalling might affect PCP in the Xenopus neuroectoderm. Having established that tension is required for PCP proteins to be appropriately localised, the authors use tissue explant experiments to show that the orientation of stretch influences the PCP axis. However, Wnt signals are also important in this system, and strong PCP establishment requires the alignment of the Wnt gradient with the orientation of stretch. Further analyses provide evidence that the cells do not read tissue tension directly, but rather that the PCP axis is determined by cell shape – being oriented in the direction of cell elongation. Although the mechanisms by which cell elongation controls PCP orientation have yet to be investigated, this work underscores the importance of mechanical cues, acting in concert with chemical signals, in defining tissue polarity.
