Abstract Delamination within stratified epithelia like the skin epidermis describes the detachment and upward motion of cells originating from the basal layer. Despite its fundamental importance for tissue development, homeostatic regeneration and repair, the mechanisms that drive delamination remain a longstanding open question. Upward motion follows cell shape changes, which are inherently driven by physical forces, but their role is elusive. Here, we investigate delamination in stratifying keratinocytes by combining imaging, force measurements and theoretical modeling. We identify a local change in force balance between differentiating cells and their environment as the key step initiating delamination. Within a homogeneous cell layer with apically polarized contractility, differentiation leads to actomyosin remodeling, redistributing cellular force exertion to the basal side. Such mechanical heterogeneity then results in differentiating cells experiencing and inward basal and outward apical forces that manifest in the formation of a +1 force defect and promote shape changes culminating in upward motion. Simultaneously, delaminating cells actively pull on their underlying neighbors, generating convergent tissue flows which close the basal layer below. Together, we propose a general physical description of delamination initiation, which may act across various multilayered epithelia.

Cell competition is a highly conserved mechanism through which cells with lower fitness levels than surrounding cells are actively removed from tissues. Differences in fitness may result from intrinsic tissue heterogeneity or be caused by differentiation, infections, or mutations. The resulting competition dynamics act as a key regulator of various biological processes during development and homeostasis. The underlying mechanical factors often remain unclear. Here, we discuss the biophysical principles of cell competition and elimination via extrusion or delamination. Recent advances have uncovered how fitness is determined by cellular mechanical properties, which can regulate winning or losing, and how cells use forces to outcompete each other. Furthermore, forces can influence the fate and direction of eliminated loser cells, which govern functional tissue development and disease progression.

In April 2026, the first international symposium on cell extrusion (CellExIt#1) was held in St Jean-Cap-Ferrat in the south of France. The meeting gathered investigators who, through different perspectives and disciplines, are dissecting the mechanisms leading to the seamless expulsion of cells from epithelial tissues. In this Meeting Report, we provide a brief overview of cell extrusion as well as a panoramic view of the research on this crucial physiological process that has recently been performed in the different laboratories represented at CellExIt#1, outlining ongoing projects, the diversity of experimental angles and important open questions in the field.

Abstract Cell extrusion is a fundamental process in tissue homeostasis, morphogenesis, and cancer progression, facilitating the removal of cells either alive or through apoptosis. While biochemical signaling pathways are known to regulate extrusion, recent advances have underscored the importance of mechanical forces in this process. Here, using optogenetic control of RhoA activation in epithelial monolayers combined with Bayesian Inversion Stress Microscopy (BISM) and three-dimensional cell-based modeling, we uncover a counterintuitive mechanism whereby elevated tension, instead of stabilizing the monolayer, actively drives extrusion in highly contractile cells. We show that local RhoA activation enhances myosin II–dependent contractility and F-actin reorganization, which promotes cell stiffening, resulting in localized tension buildup. The ensuing tensile stress amplifies vertical mechanical fluctuations, which in turn trigger cell extrusion. Remarkably, these tension-induced extrusions occur both apically and basally. Furthermore, our findings show that RhoA-mediated contractility is not merely an effector of extrusion but also an active promoter of basal extrusion, independently of caspase activation. Our study demonstrates that tensile stress can directly initiate extrusion events and bias their outcome toward apical or basal fates. By identifying tension as a driver rather than a suppressor of extrusion, this work revises current models of epithelial homeostasis and highlights mechanical control as a targetable axis in disease and regeneration.

Intracellular transport of macromolecules is crucial for the proper functioning of most cellular processes. Although intracellular crowding is known to strongly alter macromolecule mobility, how cytoplasmic structures physically modulate diffusion remains largely unexplored. Here, we investigated the mechanisms by which cytoplasmic crowding controls diffusivity using live-cell experiments and porous media modeling approaches. Confocal microscopy combined with fluorescence recovery after photobleaching and fluorescence correlation spectroscopy measurements revealed an anticorrelation between free-green fluorescent protein diffusivity and the heterogeneous cytoplasmic structure abundance in live mammalian cells. This motivated the development of a multiscale model, where the cytoplasm is treated as a hierarchical porous medium with nanometric and micrometric obstacles. Numerically solving the model allowed us to predict the effective cytoplasmic diffusion coefficient for various obstacle volume fractions, and to identify tortuous and porous hydrodynamic hindrances as key diffusion reduction mechanisms. Comparison with our experimental results highlighted the importance of hydrodynamic interactions between diffusing molecules and nanometric obstacles. Importantly, we found that the effective cytoplasmic diffusivity was not dependent on specific intracellular regions but rather on the local intracellular obstacle volume fraction. Finally, the model was extended to predict the diffusivity of larger macromolecules, showing excellent agreement with literature data for several macromolecules and cell lines. This study provides insights into the physical mechanisms impeding intracellular diffusion, demonstrating the potential of porous media modeling approaches to predict transport mechanisms in dynamic or heterogeneous intracellular structures, as in cell motility, blebbing, and apoptosis.

Epithelial tissues maintain organ integrity while continuously remodeling during morphogenesis, repair, and disease. At high cell densities, these tissues often appear mechanically arrested in a disordered, solid-like state, raising the question of how they retain the ability to reorganize. Here, we show that, unlike thermal glasses, dense epithelial tissues do not exhibit caging behavior but instead behave as a complex fluid. Cells display subdiffusive creep together with Fickian yet non-Gaussian dynamics and compressed exponential relaxation, hallmarks of stress-driven fluidity. This fluidity arises from the tissue's structural and mechanical organization rather than from cell division or extrusion, which only transiently enhance local dynamics. Fast-moving cells organize into collective, anisotropic clusters whose spatial heterogeneity correlates with local structural entropy and soft vibrational modes. Together, these findings reveal a hidden fluidity in densely packed epithelia that supports mechanical stability while preserving the capacity for remodeling during development, wound healing, and early tumor invasion.

Collective cell migration is central in development and disease. Vimentin is an intermediate filament protein expressed by epithelial cells at the edge of wounds where collective cell migration is most efficient. Yet, its functional role in this context remains underexplored. Here, we show that vimentin, over-expressed in cells undergoing partial epithelial to mesenchymal transition at the edge of epithelial monolayers, has a multiscale impact on the whole monolayer mechano-dynamics. Vimentin knock-down delays wound closure, reduces cell coordination, while increasing traction forces exerted by cells on the substratum. It also disrupts the directionality of leader cells migration, as well as the cohesion and coordinated motion of cells deep in the monolayer. We further show that vimentin promotes the conversion of polarized cell locomotion into coordinate collective migration by polarizing actin, focal adhesions and traction forces, sustaining leader cell's lamellipodium protrusive activity and directionality, while allowing mechanical coupling of leader with follower cells. Altogether, we show that vimentin is essential for bridging polarized single cell locomotion and coordinated collective migration to allow efficient collective migration. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Cancer-associated fibroblasts are major architects of the tumour stroma, where their aligned, elongated morphology forms a capsule that mechanically restrains tumour expansion. However, it is unclear how this supracellular organization emerges and persists. Here we show that fibroblasts generate a fibronectin matrix that progressively acquires the same nematic order as the cell layer, and that this matrix in turn feeds back to immobilize both cells and topological defects. Using long-term live imaging, traction force microscopy, matrix microfabrication and hydrodynamic modelling, we find that this reciprocal coupling induces an ageing process in which cellular flows and defect motion slow dramatically and ultimately freeze. Despite this arrest, the monolayer remains active, with defects concentrating contractile forces that may represent mechanical weak points. Disrupting fibronectin production fluidizes the capsule, reactivates defect dynamics and compromises its barrier-like function. These findings reveal a self-organizing mechanism by which fibroblasts and their matrix co-evolve to create a mechanically stable, yet active, stromal architecture with direct implications for tumour dissemination.

Cells within biological tissue are constantly subjected to dynamic mechanical forces. Measuring the internal stress of tissues has proven crucial for our understanding of the role of mechanical forces in fundamental biological processes like morphogenesis, collective migration, cell division or cell elimination and death. Previously, we have introduced Bayesian Inversion Stress Microscopy (BISM), which is relying on measuring cell-generated traction forces in vitro and has proven particularly useful to measure absolute stresses in confined cell monolayers. We further demonstrate the applicability and robustness of BISM across various experimental settings with different boundary conditions, ranging from confined tissues of arbitrary shape to monolayers composed of different cell types. Importantly, BISM does not require assumptions on cell rheology. Therefore, it can be applied to complex heterogeneous tissues consisting of different cell types, as long as they can be grown on a flat substrate. Finally, we compare BISM to other common stress measurement techniques using a coherent experimental setup, followed by a discussion on its limitations and further perspectives.

Organs and tissues consist of a precise arrangement of different cell types, all playing a specific role to fulfil the biological function of the tissue. Small changes in cellular phenotypes or behaviour can lead to developmental defects, tissue malfunctions or the emergence of diseases. Therefore, tissue integrity, health and function are maintained through different quality control mechanisms. One highly conserved mechanism is cell competition, through which cells of reduced fitness are eliminated. Cells can employ various strategies to eliminate each other. Those include the exertion of mechanical forces, but its role in determining the competition outcome remains unclear. Here, we report that heterogeneities in force transmission capabilities mediated by cell-cell adhesion differences lead to cell competition. We show that increased force transmission endows collectives of cells with a fitness advantage, as it provides increased resistance to elimination forces. Elimination forces are generated from large stress fluctuations, emerging at the interfaces of competing cell populations. Besides promoting the removal of unfit cells in a wide range of biological conditions where local cell-cell adhesion heterogeneities are observed, this mechanism might be of general importance for the generation and maintenance of tissue boundaries.

Myoblast fusion into multinucleated myotubes is essential for skeletal muscle development and repair, yet how tissue-scale mechanics contributes to this process remains poorly understood. Here, we show that primary myoblasts behave as an evolving active nematic system in which actomyosin-dependent stresses, extracellular-matrix (ECM) remodeling and fusion-driven myotube growth are dynamically coupled. As myoblasts fuse into elongated myotubes, orientational order increases and the nematic field is progressively reshaped. We identify a strong coupling between cellular and ECM nematic organization, whereby cytoskeleton-dependent ECM remodeling stabilizes topological defects and reinforces their associated stress patterns. Fusion events preferentially accumulate near comet-shaped +1/2 defects, which correspond to regions of high compressive stress predicted by our theoretical model. Our findings support a model in which the intrinsic fusion machinery provides fusion competence, while ECM-stabilized nematic stress patterns spatially bias the localization of fusion events. Fusion-driven myotube growth then feeds back on the mechanical landscape, increasing nematogen length and stress magnitude. Together, these results reveal a self-reinforcing biomechanical mechanism that contributes to the organization of myoblast fusion and myotube growth, with potential relevance for developmental and regenerative morphogenesis. Active self-organization of myoblasts, coupled with extracellular-matrix interactions, create mechanical stress patterns that bias cell fusion toward specific locations, providing a mechanistic link between tissue mechanics and muscle development.

Epithelia are intricate tissues whose function is intimately linked to mechanics. While mechanobiology has primarily focused on factors such as cell-generated contractility and mechanical properties of extracellular matrix, a interesting mechanobiological paradigm highlights the role of osmotic and mechanical pressures in shaping epithelial tissues. In our study, we developed an in vitro model of cell-coated microsized hydrogel spheres (MHSs) which allows to decipher the interplay between cellular activities and tissue mechanics. Drastic, isotropic MHS compressions were observed once the epithelia reached confluence. Further studies revealed that the compression was a process independent of cell contractility but rather regulated by active transepithelial fluid flow. Compressive stresses of about 7 kPa are generated by such an active hydraulic mechanism. Tissue homeostasis is then maintained by a fine balance between cell proliferation and extrusion. Our findings demonstrate the critical role of fluid transport in generating mechanical forces within epithelial tissues. Supported by a theoretical mechanohydraulic model, a mechanistic framework for understanding the intricate interplay between cellular processes and tissue mechanics was established. These results challenge traditional views of epithelial tissue mechanics, emphasizing the pivotal influence of osmotic and mechanical pressures in shaping tissues. We anticipate that this study will advance the understanding of epithelial tissue development, the maintenance of homeostasis, and the mechanisms underlying pathological conditions.

Cell competition is a tissue surveillance mechanism for eliminating unwanted cells, being indispensable in development, infection and tumourigenesis. Although studies have established the role of biochemical mechanisms in this process, due to challenges in measuring forces in these systems, how mechanical forces determine the competition outcome remains unclear. Here we report a form of cell competition that is regulated by differences in force transmission capabilities, selecting for cell types with stronger intercellular adhesion. Direct force measurements in ex vivo tissues and different cell lines reveal that there is an increased mechanical activity at the interface between two competing cell types, which can lead to large stress fluctuations resulting in upward forces and cell elimination. We show how a winning cell type endowed with a stronger intercellular adhesion exhibits higher resistance to elimination and benefiting from efficient force transmission to the neighbouring cells. This cell elimination mechanism could have broad implications for keeping the strong force transmission ability for maintaining tissue boundaries and cell invasion pathology.

Tissues eliminate unfit, unwanted or unnecessary cells through cell extrusion, and this can lead to the elimination of both apoptotic and live cells. However, the mechanical signatures that influence the fate of extruding cells remain unknown. Here we show that modified force transmission across adherens junctions inhibits apoptotic cell eliminations. By combining cell experiments with varying levels of E-cadherin junctions and three-dimensional modelling of cell monolayers, we find that these changes not only affect the fate of the extruded cells but also shift extrusion from the apical to the basal side, leading to cell invasion into soft collagen gels. We generalize our findings using xenografts and cysts cultured in matrigel, derived from patients with breast cancer. Our results link intercellular force transmission regulated by cell–cell communication to cell extrusion mechanisms, with potential implications during morphogenesis and invasion of cancer cells. Tissues eliminate unwanted cells through cell extrusion, but the factors determining whether these extuded cells live or die are not fully understood. Now force transmission across adherens junctions is shown to have a role in shaping their fate.

Epithelial tissues line the surfaces of vital organs and are often densely packed in a disordered, mechanically arrested state, referred to as ‘jammed’ or ‘glassy state. While collective migration at low-density regimes has been extensively studied, the microscopic dynamics within such jammed epithelia remains poorly understood. Here, we reveal that contrary to expectations from thermal systems with glassy dynamics, jammed epithelial monolayers do not exhibit cage effects that reflect the temporary spatial trapping of the particles. Instead, cells display sub-diffusive creep and Fickian yet non-Gaussian dynamics, accompanied by compressed exponential relaxation, features that reveal stress-driven fluidity. We show that cell divisions and extrusions transiently do enhance local motion, they are insufficient to fluidize the tissue globally. Fast-moving cells form collective, anisotropic clusters, and these dynamic heterogeneities correlate with local structural entropy and low-frequency vibrational modes. These findings challenge the conventional view that jammed tissues are static and inert structures, uncovering a hidden fluidity that can be expected to play a critical role in morphogenesis, wound healing, and early tumor progression. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, Adv-101019835 Alexander von Humboldt Foundation, Alexander von Humboldt Professorship Agence Nationale de la Recherche, “STRATEPI” DFG-ANR-22-CE92-0048, “VISCOMAG2” ANR-24-CE42-6142 Institut National du Cancer, INCa_16712, INCa_18429 Initiatives d’excellence, Idex ANR-11-IDEX-0005-02 BioMechanOE, TP5 International Human Frontier Science Program Organization, https://ror.org/02ebx7v45, LT0007/2023-C

Epithelial cells can resemble nematic liquid crystals, exhibiting long-range alignment and topological defects. Bera et al.1 show that these defects do not arise randomly but that they are pre-patterned by coordinated cell forces and motions. Some of these defects can move in both directions within the same tissue, driven by distinct distributions of cell-generated traction forces and internal stresses.

A leading paradigm for understanding the large-scale behavior of tissues is via generalizations of liquid crystal physics; much like liquid crystals, tissues combine fluid-like, viscoelastic behaviors with local orientational order, such as nematic symmetry. Whilst aspects of quantitative agreement have been achieved for flat monolayers, the most striking features of tissue morphogenesis-including symmetry breaking, folding and invagination-concern surfaces with complex curved geometries in three dimensions. As yet, however, characterizing such behaviors has been frustrated due to the absence of proper image analysis methods; current state-of-the-art methods almost exclusively rely on two-dimensional intensity projections of multiple image planes, which superimpose data and lose geometric information that can be crucial. Here, we describe an analysis pipeline that properly captures the nematic order and topological defects associated with tissue surfaces of arbitrary geometry, which we demonstrate in the context of in vitro multicellular aggregates, and in vivo zebrafish hearts.

The collective motion of epithelial cells is a fundamental biological process which plays a significant role in embryogenesis, wound healing, and tumor metastasis. While it has been broadly investigated for over a decade both in vivo and in vitro, large-scale coherent flocking phases remain underexplored and have so far been mostly described as fluid. In this work, we report an additional mode of large-scale collective motion for different epithelial cell types in vitro with distinctive features. By tracking individual cells, we show that cells move over long time scales coherently not as a fluid, but as a polar elastic solid with negligible cell rearrangements. Our analysis reveals that this solid flocking phase exhibits signatures of long-range polar order, accompanying with scale-free correlations of the transverse component of velocity fluctuations, anomalously large density fluctuations, and shear waves. Based on a general theory of active polar solids, we argue that these features result from massless orientational Goldstone mode, which, in contrast to polar fluids where they are generic, require the decoupling of global rotations of the polarity and in-plane elastic deformations in polar solids. We theoretically show and consistently observe in experiments that the fluctuations of elastic deformations diverge for large system sizes in such polar active solid phases, leading eventually to rupture and thus potentially loss of tissue integrity at large scales.