Nature Physics

Sheets of confluent cells are often considered as active nematics, with accumulation at [Formula] topological defects and escape from [Formula] defects being widely recognized. However, collective dynamics surrounding integer-charge defects remain poorly understood, despite its biological importance. By using microfabricated patterns, we induce diverse +1 topological defects (aster, spirals, and target) within monolayers of neural progenitor cells. Remarkably, cells are consistently attracted to the core of +1 defects regardless of their type, challenging existing theories and the conventional extensile/contractile dichotomy. We trace back the origin of this accumulation behavior to previously overlooked nonlinear active forces using a combination of experiments and a continuous theory derived from a cell-level model. Our findings demonstrate that +1 topological defects can reveal key features of active nematic systems and offer a new way to characterize and classify cell layers.

Glassy dynamics in active biological cells remain a subject of debate, as cellular activity rarely slows enough for true glassy features to emerge. In this study, we address this paradox of glassy dynamics in epithelial cells by integrating experimental observations with an active vertex model. We demonstrate that while crowding is essential, it is not sufficient for glassy dynamics to emerge. A mechanochemical feedback loop (MCFL), mediated by cell shape changes through the contractile actomyosin network, is required to drive glass transition in dense epithelial tissues, as revealed via a crosstalk between actin-based cell clustering and dynamic heterogeneity in experiments. Incorporating MCFL into the vertex model reveals contrasting results from those previously predicted by theories- we show that the MCFL can counteract cell division-induced fluidisation and enable glassy dynamics to emerge through active cell-to-cell communication. Furthermore, our analysis reveals, for the first time, the existence of novel collective mechanochemical oscillations that arise from the crosstalk of two MCFLs. Together, we demonstrate that an interplay between crowding and active mechanochemical feedback enables the emergence of glass-like traits and collective biochemical oscillations in epithelial tissues with active cell-cell contacts.

The active regulation of tissue material properties via phase transitions is central in morphogenesis. Transitions abruptly occur at critical points in diverse control parameters, including cell density, shape or adhesion. Whether these parameters are interdependent, performing redundant or distinct functions, is unknown. Here we show that co-regulation of multiple control parameters impacts not only tissue deformability, but also cell polarization. We theoretically define a new phase diagram capturing the material states of zebrafish pluripotent tissues and show that they cross simultaneously critical points in cell density, connectivity and adhesion strength. Combining optogenetics, biophysical measurements and quantitative morphometrics, we independently modulate each parameter, identifying adhesion as the main determinant of tissue rheology. Unexpectedly, uncoupling adhesion-driven from density-driven rigidification in amorphous tissues triggers epithelial organization via tricellular junction formation, followed by luminogenesis and apicobasal polarization. Altogether, this work reveals the non-linear dynamics of emergent tissue mechanics as instructive mechanisms of tissue organization.

The spatio-temporal patterning of multicellular tissues is driven by the collective dynamics of cell proliferation and active movement. These processes are mediated by the extracellular matrix environment via a combination of biomolecular and physical cues. Here we show that the passive viscoelastic properties of the matrix that encapsulate a proliferating ball of cells (e.g. a developing organoid) play a critical role in guiding tissue organization in space and time. By varying the viscoelasticity of well-defined model matrices, we show how a spheroidal tissue of breast epithelial cells breaks symmetry and forms finger-like protrusions that invade the matrix. A computational model allows us to recapitulate these observations and leads to a phase diagram that demarcates the regions of morphological stability and instability as a function of matrix viscoelasticity, tissue viscosity, cell motility and cell division rate. Experiments that use biomolecular manipulations to independently vary these parameters confirm our predictions. To further test our theory, we also study the self-organization of an in-vitro intestinal organoid and show that the morphological changes of this system also fits within our paradigm. Altogether, our studies demonstrate the role of stress relaxation mechanisms in determining the dynamics of tissue growth and the symmetry breaking instabilities associated with branching, a fundamental process in morphogenesis and oncogenesis, and suggest ways of controlling tissue form using the extracellular matrix.

Thin shells buckle and wrinkle when compressed. While this behavior is generally detrimental in engineering, it has been widely implicated in epithelial morphogenesis and patterning during development. Yet the rules governing buckling of active viscoelastic shells like epithelia remain unclear. Here we delineate those rules by combining an experimental system that allows us to sculpt epithelial shells and subject them to controlled deflation with a 3D computational model linking cytoskeletal dynamics to tissue mechanics. Experiments and simulations across several orders of magnitude in time and space reveal that buckling emerges for fast deflation relative to the cortexs relaxation time, and is suppressed by high contractility. We show, further, that the tissue develops wrinkle patterns with different degrees of symmetry breaking that depend on its size and viscous confinement. Strikingly, we find that epithelial buckling is a multiscale phenomenon involving long-lived supracellular folds but also short-lived subcellular wrinkles in the actin cortex. Finally, by forming epithelial shells with anisotropic curvature we rationally direct buckling into predictable wrinkle patterns. Our study shows that epithelial tissues can be understood as hierarchical materials with mechanical instabilities that can be harnessed to engineer epithelial morphogenesis.

Epithelial cortinoids, fluid filled shells formed from induced pluripotent stem cells (iPSCs), must accommodate large deformations during growth and morphogenesis. Using inflation-deflation assays and high-resolution imaging, we find that these fluid-filled shells are weakly-pressurized and achieve extreme deformability through reversible soft modes of deformation accommodated by the cytoskeleton. We show that cytoskeletal elements such as actin localized along lateral cell edges undergo tilt and bend instabilities that buffer mechanical load by decoupling apico-basal stretching from lateral extension. These reversible instabilities act as elastic safety valves, permitting large shape changes without loss of epithelial hydraulic and topological integrity. A minimal theoretical and computational model demonstrates how tilt and bend reduce effective resistance to radial thinning and explains the observed pressure-strain softening. Thus, iPSC shells exploit reversible cytoskeletal instabilities as mechanical buffers, enabling robust tolerance of large deformations in developing epithelia.

Evolution of multicellularity from early unicellular ancestors is arguably one of the most important transitions since the origin of life1,2. Multicellularity is often associated with higher nutrient uptake3, better defense against predation, cell specialization and better division of labor4. While many single-celled organisms exhibit both solitary and colonial existence3,5,6, the organizing principles governing the transition and the benefits endowed are less clear. Using the suspension-feeding unicellular protist Stentor coeruleus, we show that hydrodynamic coupling between proximal neighbors results in faster feeding flows that depend on the separation between individuals. Moreover, we find that the accrued benefits in feeding current enhancement are typically asymmetric- individuals with slower solitary currents gain more from partnering than those with faster currents. We find that colony-formation is ephemeral in Stentor and individuals in colonies are highly dynamic unlike other colony-forming organisms like Volvox carteri 3. Our results demonstrate benefits endowed by the colonial organization in a simple unicellular organism and can potentially provide fundamental insights into the selective forces favoring early evolution of multicellular organization.

The mechanisms underlying both the establishment of mirror symmetry and deviations from it in development of bilateral multicellular organisms remain insufficiently understood. Actin cytoskeletons of individual cells exhibit intrinsic chirality, and a strong correlation exists between single-cell actin fibres chiral organisation and the collective alignment of cells confined to rectangular adhesive islands (2D-microtissues). Here, we demonstrate how multicellular chiral patterns can be inferred from the chiral behaviour of actin fibres in individual cells. By analysing chiral actin systems in cells with elliptical and semi-circular shapes, representing inner and boundary positions within 2D-microtissues, we defined the rules of chiral motile behaviour and formulated two models of cell alignment: (i) chiral rotation of inner cells and (ii) chiral tilting of boundary cells relative to island edges. In both models, neighbouring cells are also mutually aligned. Systematic variation of island area and aspect ratio, combined with dynamic observations, revealed the primary role of boundary cells. Chiral order first emerged at tissue boundaries and then propagated inward. This outside-in mechanism also explains why mirror-symmetric cell groups are difficult to achieve with uniformly chiral cells, but can be obtained either by reversing chirality in one subgroup or by enlarging the microtissue to minimise boundary influence.

Biological materials, like epithelial tissues, exhibit remarkable adaptability to mechanical stresses, dynamically remodeling their structure in response to external and internal forces. A key challenge is understanding how these tissues store a memory of past mechanical stimuli. Here, we investigate this memory using an active Vertex Model of epithelial sheets incorporating a local, mechanosensitive tension-remodeling rule where junctional tension updates depend on strain, acting as a slow, history-dependent variable. We demonstrate three hallmark mechanical consequences of this memory mechanism. First, a localized, short contractile cue permanently reprograms the global shear modulus, with the direction of change (stiffening or softening) controlled by the tension remodeling rate. Second, the tissue stores a long-range mechanical memory: a prior stimulus at one site modulates the tissues response to a subsequent, distant stimulus, mediated by coupling across the entire junctional network. Finally, we show that simple cyclic bulk deformation acts as a training protocol that autonomously tunes the tissues constitutive properties, including programming the Poisson ratio to auxetic (negative) values. These findings position epithelial mechanics within the framework of unsupervised physical learning, identifying the mechanosensitive remodeling rates as powerful control parameters for designing programmable tissue-scale rheology.

Effective wound healing relies on thousands of cells collectively migrating to close the gap. It is increasingly clear that part of this migration is driven by endogenous electrochemical fields which point towards the center of skin wounds and guide collective cell migration through "electrotaxis". Mounting evidence suggests exogenously applied electric fields can accelerate healing, but progress in this field has been limited by the use of brute-force, global stimulation strategies that are wound agnostic and reflect neither the collective nature of the healing process nor the dynamic nature of the wound geometry. Here, we develop an experimental system to study how monolayer mouse skin responds to spatiotemporally patterned electric fields. We first show that local electrical stimulation can produce near global cell migration responses arising from cell-cell mechanical adhesion. We apply this strategy to 2D circular wounds using a local ring electric field near the wound edge and reveal how to tune the timing of local stimulation to avoid cellular jamming. Finally, we integrate our findings with a biophysics-informed optimal control strategy to tune both when and where the electric field should be applied in time, resulting in dramatic improvements to healing.

Many living cells actively migrate in their environment to perform key biological functions - from unicellular organisms looking for food to single cells such as fibroblasts, leukocytes or cancer cells that can shape, patrol or invade tissues. Cell migration results from complex intracellular processes that enable cell self-propulsion 1,2, and has been shown to also integrate various chemical or physical extracellular signals 3,4,5. While it is established that cells can modify their environment by depositing biochemical signals or mechanically remodeling the extracellular matrix, the impact of such self-induced environmental perturbations on cell trajectories at various scales remains unexplored. Here, we show that cells remember their path: by confining cells on 1D and 2D micropatterned surfaces, we demonstrate that motile cells leave long-lived physicochemical footprints along their way, which determine their future path. On this basis, we argue that cell trajectories belong to the general class of self-interacting random walks, and show that self-interactions can rule large scale exploration by inducing long-lived ageing, subdiffusion and anomalous first-passage statistics. Altogether, our joint experimental and theoretical approach points to a generic coupling between motile cells and their environment, which endows cells with a spatial memory of their path and can dramatically change their space exploration.

Gradients of extracellular signals organise cells in tissues. Although there are several models for how gradients can pattern cell behaviour, it is not clear how cells react to gradients when the population is undergoing 3D morphogenesis, in which cell-cell and cell-signal interactions are continually changing. Dictyostelium cells follow gradients of their nutritional source to feed and maintain their undifferentiated state. Using light sheet imaging to simultaneously monitor signaling, single cell and population dynamics, we show that the cells migrate towards nutritional gradients in swarms. As swarms advance, they deposit clumps of cells at the rear, triggering differentiation. Clump deposition is explained by a physical model in which cell swarms behave as active droplets: cells proliferate within the swarm, with clump shedding occurring at a critical population size, at which cells at the rear no longer perceive the gradient and are not retained by the emergent surface tension of the swarm. The droplet model predicts vortex motion of the cells within the swarm emerging from the local transfer of propulsion forces, a prediction validated by 3D tracking of single cells. This active fluid behaviour reveals a developmental mechanism we term "musical chairs" decision-making, in which the decision to proliferate or differentiate is determined by the position of a cell within the group as it bifurcates.

Unlike in mice, frogs, and fish, left-right (L/R) body axis formation in avian embryos does not arise from the chiral beat of cilia. Instead, a counter-clockwise tissue rotation around Hensens node, the organizer of amniote development, repositions cells expressing L/R sidedness genes. Yet, the physical origin of this rotation remains elusive. Here, we provide evidence that in quail embryos, the node tissue generates an active chiral torque of [~]6{micro}N{micro}m to drive the chiral tissue flow. Microsurgery experiments reveal that this torque depends on actomyosin molecular activity, is generated within the dorsal node tissue, and requires the underlying ventral meso-/endoderm to act as a mechanical substrate sustaining the counter-torque. We conclude that a dorsoventrally oriented tissue-scale active torque dipole at the node translates cell-scale chirality to organismal L/R asymmetry, adding a mechanical dimension to the canonical function of embryonic organizers as signaling hubs.

For tissue development, cells must generate contractile forces which are transmitted to their surrounding matrix or neighbouring cells via adhesion complexes. It is often envisaged that a simple linear counterbalance of cell generated stress with extracellular matrix (ECM) traction forces exists. However, experimental evidence indicates that modulating cell-ECM attachment does not necessarily lead to expected reciprocal changes in intercellular stresses. As ECM composition or mechanical properties are rarely uniform, it is important to understand the complexity of how focal adhesions alter stress transmission and the force-balance of a tissue. To address this, we confined monolayers on adhesive patterns altering focal adhesion distribution. Traction force microscopy and laser ablations of cell-cell junctions were used to examine stresses across epithelial monolayers whilst modulating substrate stiffness. We show that monolayers reach different force-balance states depending on focal adhesion distribution. Using an active matter model and confirmed experimentally, we reveal that a force-balance is generated by non-uniform patterns of cell contractility linked to adhesion patterning. This work highlights the importance of integrating the position of cell-ECM attachments into our vision of the mechanical landscape of living tissues. TeaserTo infer a tissues force-balance, positional information of focal adhesion distribution needs to be integrated due to the emergence of non-uniform patterns of cell contractility.

Most animals coordinate behaviour using neural computations. Yet, single-celled organisms also exhibit stimulus-responsive, even cognitive, actions. To understand how a single cell can coordinate and drive complex behaviours without any neural encoding, we study an algal protist - a motile cell with four extremely long cilia. The organism displays a surprisingly rich locomotor repertoire, emerging from the intricate dynamics of the cilia, which form a tight bundle when swimming. We leverage high-speed quantitative live imaging to extract the spectrum of possible ciliary beating patterns, and derive a dispersion relation coupling the temporal frequency and spatial wavelength of cilia oscillations. We further reconstruct the attractor manifold embedded in the behavioural space, showing that despite the range and complexity of ciliary beating modes, the underlying behavioural manifold is intrinsically low-dimensional with elaborate topological structure. Dynamic and excitable transitions in motility behaviour are encoded as trajectories in this space.

While tumor malignancy has been extensively studied under the prism of genetic and epigenetic heterogeneity, tumor cell states also critically depend on reciprocal interactions with the microenvironment. This raises the hitherto untested possibility that heterogeneity of the untransformed tumor stroma can actively fuel malignant progression. As biological heterogeneity is inherently difficult to control, we adopted a reductionist approach and let tumor cells invade micro-engineered environments harboring obstacles with precision-controlled geometry. We find that not only the presence of obstacles, but more surprisingly their spatial disorder, causes a drastic shift from a collective to a single-cell mode of invasion - comparable in strength to cadherin loss. Combining live-imaging and perturbation experiments with minimal biophysical modeling, we demonstrate that cell detachments result both from local geometrical constraints and a global integration of spatial disorder over time. We show that different types of microenvironments map onto different universality classes of invasion dynamics - homogeneous substrates follow Kardar-Parisi-Zhang (KPZ) scaling, while disordered ones exhibit exponents consistent with KPZ with quenched disorder (KPZq). Our findings highlight generic physical principles for how the mode of cancer cell invasion depends on environmental heterogeneity, with potential implications to understand tumor evolution in vivo.

The intestinal epithelium in vertebrates has a characteristic architecture of protruding villi and receding crypts that enables nutrient absorption and cellular turnover. Intestinal organoids recapitulate its development and can be used as disease models, but the underlying mechanical processes are not fully understood yet. Here we combine advanced image processing and the bubbly vertex model for epithelial cell shape to achieve a fully three-dimensional reconstruction of cell shapes and forces during the development of mouse intestinal organoids. We show that the transition to budded morphologies is caused by a global increase in apical tension, which however is not maintained after budding, suggesting ratchet-like non-reversibility. We further demonstrate that luminal pressure decreases and basal line tensions increase during development, thus facilitating budding on the apical side, but at the same time mechanically stabilizing the system at the basal side, for example against cell extrusion. Our approach demonstrates how one can achieve a complete mechanical analysis of a complex tissue-like system.

In wild environments, physical and biochemical interactions between intermixed motile and sessile microorganisms give rise to spatial organization that is key for the functioning and ecology of complex communities. However, how motility-driven physical interactions contribute to shaping multispecies communities remains little understood. To address this gap, we investigated model binary mixtures of motile and non-motile Escherichia coli bacteria. We discovered a new type of non-equilibrium self-organization, wherein large-scale density fluctuations of non-motile bacteria emerge when mixed with motile ones under physiologically relevant conditions. Systematically exploring the phase diagram in microfluidics experiments and combining them with modeling and simulations, we uncovered the two-pronged physical mechanism of emergence: Circular swimming of motile cells close to surfaces generates recirculating hydrodynamic flows that advect non-motile cells, while sedimentation, by breaking the vertical symmetry, is essential for their local accumulation. This active self-organization behavior in mixed bacterial populations appears crucial for complex microbial community structuration.

Rigidity transitions govern tissue organization in ways reminiscent of inert materials. Yet, living tissues are composed of active units with autonomous timing mechanisms, raising the question whether microscopic cellular timing distribution influences collective mechanical states. Here we identify heterogeneity in cellular timescales as a heritable parameter, regulating rigidity transitions in embryonic tissues. Lineage tracking and quantitative mechanical analysis reveal that zebrafish morphogenesis starts with a tissue rigidity collapse occurring at maximal cell cycle length heterogeneity. This heterogeneity arises from size-dependent stochastic differences in resource allocation, with resource availability defining the cell cycle length. Such differences are inherited across generations, amplifying and structuring tissue-wide cell cycle length heterogeneity. Experiments and large-scale 3D simulations identify an optimum level of cellular timing variability at which cell-cell contact remodelling is spatially coordinated driving timely and robustly the rigidity transition. These findings demonstrate that embryos exploit microscopic temporal disorder for timing and tuning tissue morphogenesis.

Biomolecular condensates formed by phase separation are key players in cellular organization, yet their interfacial mechanics remain poorly understood. Here, we show that both synthetic and endogenous nuclear condensates exhibit critical-like interfacial behaviors near the phase boundary, including enhanced capillary fluctuations, critical slowing down, and reduced surface tension. By combining optogenetic control with sub-micron-resolution fluctuation spectroscopy, we quantitatively estimate surface tension, bending rigidity, and effective viscosity. Surface tension diminishes as the system approaches the critical composition, consistent with classical theories of phase separation. Notably, bending elasticity emerges as an unexpected feature of these nuclear liquid-like structures, suggesting the formation of structured interfacial layers that progressively weaken near criticality. Among these condensates, the nucleolus displayed exceptionally high viscosity, which may arise in part from viscoelastic coupling to the surrounding perinucleolar heterochromatin, effectively increasing the apparent viscosity in the long-time fluctuation regime. This non-invasive approach enables probing condensate mechanics in living cells and may provide a basis for diagnosing or modulating condensates in biomedical contexts.