Epithelial tissues of the developing embryos elongate by different mechanisms, such as neighbor exchange, cell elongation, and oriented cell division. Since autonomous tissue self-organization is influenced by external cues such as morphogen gradients or neighboring tissues, it is difficult to distinguish intrinsic from directed tissue behavior. The mesoscopic processes leading to the different mechanisms remain elusive. Here, we study the spontaneous elongation behavior of spreading circular epithelial colonies in vitro. By quantifying deformation kinematics at multiple scales, we report that global elongation happens primarily due to cell elongations, and its direction correlates with the anisotropy of the average cell elongation. By imposing an external time-periodic stretch, the axis of this global symmetry breaking can be modified and elongation occurs primarily due to orientated neighbor exchange. These different behaviors are confirmed using a vertex model for collective cell behavior, providing a framework for understanding autonomous tissue elongation and its origins.

Tissue elongation is a central morphogenetic event occurring in all organisms in development 1,2 . During this process, symmetry of cells and tissues is broken by different mechanisms, such as neighbor exchange 3,4 , cell elongation 5,6 and oriented cell division 7 . While the phenomenon is known to involve remodeling of adherens junctions 4 and acto-myosin 4,8 at the molecular level, mesoscopic mechanisms leading to distinct morphogenesis processes are poorly understood. This is partly because inputs from morphogen gradients 9 or from neighboring tissues 10,11 can affect tissue autonomous self-organization in vivo. It is therefore difficult to disentangle cell intrinsic from externally mediated behaviors. Here we use in vitro experiments and numerical simulations to characterize the spontaneous behavior of a growing cell colony in vitro. We show that in vitro tissue elongation arises from anisotropy in the average cell elongation. This anisotropy sets the direction along which boundary cells migrate radially resulting in a non-isotropic elongation that arises primarily through cell elongation. For colonies submitted to a time periodic stretch, the axis of global symmetry breaking can be imposed by external force, and tissue elongation arises through oriented neighbor exchange. Emergence of radially migrating cells and the interplay between cell elongation and cell rearrangements are confirmed by numerical simulations based on a vertex model. Our results suggest that spontaneous shape deformation is related to the mean orientation of the nematic cell elongation field in the absence of any external input. This provides a framework to explain autonomous tissue elongation and how contributions from different mesoscopic mechanisms can be modulated by external forces.