Epithelia remove excess cells through extrusion, and prevent accumulation of unnecessary or pathological cells. The extrusion process can be triggered by apoptotic signaling1, oncogenic transformation2,3, and overcrowding of cells4–6. Despite the important links of cell extrusion to developmental7, homeostatic5 and pathological processes2,8,9 such as cancer metastasis, its underlying mechanism and connections to the intrinsic mechanics of the epithelium are largely unexplored. Here, we show that apoptotic cell extrusion is provoked by singularities in cell alignments9,10 in the form of comet-like topological defects. We find a universal correlation between the extrusion sites and positions of nematic defects in the cell orientation field in different epithelium types. We model the epithelium as an active nematic liquid crystal and compare numerical simulations to strain rate and stress measurements within cell monolayers. The results confirm the active nematic nature of epithelia for the first time, and demonstrate that defect-induced isotropic stresses are the primary precursor of mechanotransductive responses in cells such as YAP (Yes-associated protein) transcription factor activity11, caspase-3 mediated cell death, and extrusions. Importantly, the defect-driven extrusion mechanism depends on intercellular junctions, since the weakening of cell-cell interactions in α-catenin knockdown (α-catKD) monolayer reduces the defect size and increases both the number of defects and extrusion rates, as also predicted by our model. We further demonstrate the ability to control extrusion hotspots by geometrically inducing defects through microcontact-printing of patterned monolayers. Together we propose a novel mechanism for apoptotic cell extrusion: spontaneously formed topological defects in epithelia govern cell fate. This new finding has important implications in predicting extrusion hotspots and dynamics in vivo, with potential applications to tissue regeneration and metastasis suppression. Moreover, we anticipate that the analogy between the epithelium and active nematic liquid crystals will trigger further investigations of the link between cellular processes and the material properties of epithelia.

Actomyosin machinery endows cells with contractility at a single cell level. However, within a monolayer, cells can be contractile or extensile based on the direction of pushing or pulling forces exerted by their neighbours or on the substrate. It has been shown that a monolayer of fibroblasts behaves as a contractile system while epithelial or neural progentior monolayers behave as an extensile system. Through a combination of cell culture experiments and in silico modeling, we reveal the mechanism behind this switch in extensile to contractile as the weakening of intercellular contacts. This switch promotes the buildup of tension at the cell-substrate interface through an increase in actin stress fibers and traction forces. This is accompanied by mechanotransductive changes in vinculin and YAP activation. We further show that contractile and extensile differences in cell activity sort cells in mixtures, uncovering a generic mechanism for pattern formation during cell competition, and morphogenesis.

Singapore# These authors contributed equally to this work. SummaryThe control of tissue growth, which is a key to maintain the protective barrier function of the epithelium, depends on the balance between cell division and cell extrusion rates [1,2]. Cells within confluent epithelial layers undergo cell extrusion, which relies on cell-cell interactions [3] and actomyosin contractility [4,5]. Although it has been reported that cell extrusion is also dependent on cell density [6,7], the contribution of tissue mechanics, which is tightly regulated by cell density [8][9][10][11][12], to cell extrusion is still poorly understood. By measuring the multi-cellular dynamics and traction forces, we show that changes in epithelial packing density lead to the emergence of distinct modes of cell extrusion. In confluent epithelia with low cell density, cell extrusion is mainly driven by the lamellipodia-based crawling mechanism in the neighbor nondying cells in connection with large-scale collective movements. As cell density increases, cell motion is shown to slow down and the role of a supra-cellular actomyosin cable formation and its contraction in the neighboring cells becomes the preponderant mechanism to locally promote cell

Collective epithelial behaviors are essential for the development of lumens in organs. However, conventional assays of planar systems fail to replicate cell cohorts of tubular structures that advance in concerted ways on out-of-plane curved and confined surfaces, such as ductal elongation in vivo. Here, we mimic such coordinated tissue migration by forming lumens of epithelial cell sheets inside microtubes of 1–10 cell lengths in diameter. We show that these cell tubes reproduce the physiological apical–basal polarity, and have actin alignment, cell orientation, tissue organization, and migration modes that depend on the extent of tubular confinement and/or curvature. In contrast to flat constraint, the cell sheets in a highly constricted smaller microtube demonstrate slow motion with periodic relaxation, but fast overall movement in large microtubes. Altogether, our findings provide insights into the emerging migratory modes for epithelial migration and growth under tubular confinement, which are reminiscent of the in vivo scenario.

We introduce the notion of cell division-induced activity and show that the cell division generates extensile forces and drives dynamical patterns in cell assemblies. Extending the hydrodynamic models of lyotropic active nematics we describe turbulent-like velocity fields that are generated by the cell division in a confluent monolayer of cells. We show that the experimentally measured flow field of dividing Madin-Darby Canine Kidney (MDCK) cells is reproduced by our modeling approach. Division-induced activity acts together with intrinsic activity of the cells in extensile and contractile cell assemblies to change the flow and director patterns and the density of topological defects. Finally we model the evolution of the boundary of a cellular colony and compare the fingering instabilities induced by cell division to experimental observations on the expansion of MDCK cell cultures.