ABSTRACT Epithelial cells maintain an essential barrier despite continuously undergoing mitosis and apoptosis. Biological and biophysical mechanisms have evolved to remove dying cells while maintaining that barrier. Cell extrusion is thought to be driven by a multicellular filamentous actin ring formed by neighbouring cells, the contraction of which provides the mechanical force for extrusion, with little or no contribution from the dying cell. Here, we use live confocal imaging, providing time-resolved three-dimensional observations of actomyosin dynamics, to reveal new mechanical roles for dying cells in their own extrusion from monolayers. Based on our observations, the clearance of dying cells can be subdivided into two stages. The first, previously unidentified, stage is driven by the dying cell, which exerts tension on its neighbours through the action of a cortical contractile F-actin and myosin ring at the cell apex. The second stage, consistent with previous studies, is driven by a multicellular F-actin ring in the neighbouring cells that moves from the apical to the basal plane to extrude the dying cell. Crucially, these data reinstate the dying cell as an active physical participant in cell extrusion.

Developing epithelial tissues are characterised by the disordered cell packing caused by ongoing cell proliferation and changes in tissue size. However, cell packing in adult epithelial tissues exhibits a high level of order, and typically, the apical tissue surface resembles a regular hexagonal lattice of planar polygons. One of the central questions in tissue development concerns the mechanisms which induce cells to repack. The change in packing may transform the tissue into a regular pattern of hexagonal cells, as seen during the refinement of Drosophila M. wing and notum tissue, or it can occur as a mechanism which drives tissue shape change, as seen during embryonal axis elongation during Drosophila convergent extension. We study cell repacking in epithelia effected by the forces that act at the interface between adjacent cells. To this end, we develop a mechanical model of epithelial tissue based on the ideas of the cellular Potts model and building on previous vertex models. Analysing expanding and fixed-size tissues, we find that steady state packing geometries depend on the regularity in the timing of cell divisions. We predict that cells in topologically active epithelia leave the tissue in response to mechanical compression and geometric anisotropy. Through a collaboration with biologists Eliana Marinari and Buzz Baum, we find that such mechanically driven cell delamination indeed occurs in the Drosophila notum. We thus identify a novel process of tissue homeostasis, whereby live cells delaminate from developing epithelium in order to limit overcrowding. Analysing the relation between stable packing geometries and the mechanical parameters, we suggest that an increase in the strength of acto-myosin contractility alone could cause tissue to repack into a regular lattice. Modifying the model to describe polarised acto-myosin localisation, we computationally reproduce cell intercalation and actin cable and rosette formation during convergent extension in Drosophila.