PT - JOURNAL ARTICLE AU - Jordi Comelles AU - SS Soumya AU - S. Anvitha AU - Guillaume Salbreux AU - Frank Jülicher AU - Mandar M. Inamdar AU - Daniel Riveline TI - Epithelial colonies <em>in vitro</em> elongate through collective effects AID - 10.1101/755181 DP - 2020 Jan 01 TA - bioRxiv PG - 755181 4099 - http://biorxiv.org/content/early/2020/04/09/755181.short 4100 - http://biorxiv.org/content/early/2020/04/09/755181.full AB - Epithelial tissues of the developing embryos experience symmetry breaking through elongation and shape transformations by different mechanisms, such as neighbour exchange, cell elongation, and oriented cell division. Although the molecular actors involved in these phenomena are known, the mesoscopic processes leading to the different mechanisms remain elusive. This knowledge gap exists, in part, because autonomous tissue self-organization in vivo is influenced by interactions with external inputs, such as morphogen gradients or neighbouring tissues, and it is difficult to distinguish intrinsic from directed tissue behaviour. Here we use in vitro experiments to observe the spontaneous elongation behaviour of spreading circular epithelial colonies prepared with microfabrication. By using cell biology methods and by quantifying colony deformation kinematics at multiple scales, we report that the global elongation direction of the colony correlates with the direction of local protrusions at the colony boundary and also with the anisotropy in the average cell elongation within colony. Although we see no mean bias in the direction of elongation for a freely spreading colony, the axis of this global symmetry breaking can be imposed by an external time-periodic stretch. Moreover, tissue elongation happens primarily due to cell elongations and orientated neighbour exchange, respectively, in the absence and in the presence of external force. These different behaviours are confirmed by theory using a vertex model for collective cell behaviour. This provides a framework to understand autonomous tissue elongation and its cellular origins.Competing Interest StatementThe authors have declared no competing interest.