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SUMMARYHow biological form emerges from cell fate decisions and tissue remodelling is a fundamental question in development biology. However, an understanding of how these processes operate side-by-side to set precise and robust patterns is largely missing. Here, we investigate this interplay during the process of vein refinement in theDrosophilapupal wing. By following reporters of signalling activity dynamically, together with tissue flows, we show that longitudinal vein refinement arises from a combination of local tissue deformation and cell fate adjustments controlled by a signalling network involving Notch, Dpp, and EGFR. Perturbing large-scale convergence and extension tissue flows does not affect vein refinement, showing that pre-patterned vein domains are able to intrinsically refine to the correct width. A minimal biophysical description taking into account key signalling interactions recapitulates the intrinsic tissue ability to establish a thin, regular vein independently of large-scale tissue flows. Supporting this prediction, artificial proveins optogenetically generated orthogonal to the axis of wing elongation refine against large-scale flows. Overall, we find that signalling-mediated updating of cell fate is a key contributor to reproducible patterning.

Segmenting cells within cellular aggregates in 3D is a growing challenge in cell biology, due to improvements in capacity and accuracy of microscopy techniques. Here we describe a pipeline to segment images of cell aggregates in 3D. The pipeline combines neural network segmentations with active meshes. We apply our segmentation method to cultured mouse mammary duct organoids imaged over 24 hours with oblique plane microscopy, a high-throughput light-sheet fluorescence microscopy technique. We show that our method can also be applied to images of mouse embryonic stem cells imaged with a spinning disc microscope. We segment individual cells based on nuclei and cell membrane fluorescent markers, and track cells over time. We describe metrics to quantify the quality of the automated segmentation. Our segmentation pipeline involves a Fiji plugin which implement active meshes deformation and allows a user to create training data, automatically obtain segmentation meshes from original image data or neural network prediction, and manually curate segmentation data to identify and correct mistakes. Our active meshes-based approach facilitates segmentation postprocessing, correction, and integration with neural network prediction.Statement of significanceIn vitro culture of organ-like structures derived from stem cells, so-called organoids, allows to image tissue morphogenetic processes with high temporal and spatial resolution. Three-dimensional segmentation of cell shape in timelapse movies of these developing organoids is however a significant challenge. In this work, we propose an image analysis pipeline for cell aggregates that combines deep learning with active contour segmentations. This combination offers a flexible and efficient way to segment three-dimensional cell images, which we illustrate with by segmenting datasets of growing mammary gland organoids and mouse embryonic stem cells.

Steroid hormones control various cellular activities in a context-dependent manner. For example, ecdysone, which acts through a type II nuclear receptor, has seemingly opposite effects in Drosophila wing precursors, promoting proliferation during larval stages, and triggering proliferation arrest at pupariation. We find that wing precursors proliferate normally in the complete absence of the ecdysone receptor (EcR), whether ecdysone is present or not, suggesting that ecdysone overrides a default antiproliferative activity of the receptor. By contrast, termination of proliferation by high concentration of 20E at the end of larval life involves conventional gene regulation by the ligand-receptor complex. The switch from one mode of regulation to the other is determined by ligand level, as measured with a calibrated EcR transcriptional reporter andex vivoproliferation assays. Accordingly, RNA Seq analysis uncovers distinct transcriptional responses to different doses of ecdysone. Some genes are only activated at high doses (high threshold targets) and likely to comprise genes that stop proliferation at pupariation, when ecdysone titres are high. We find that other target genes respond to all physiological concentrations of ecdysone. Some of these genes are known to promote proliferation and could therefore contribute to the pro-proliferation activity of low-level ecdysone. Finally, we show mathematically and with synthetic reporters that relatively simple combinations of regulatory elements can recapitulate the behaviour of both types of target genes.

Cell mechanical interactions play a fundamental role in the self-organisation of organisms. How these interactions drive coordinated cell movement in three-dimensions remains unclear. Here we report that cell doublets embedded in a 3D extracellular matrix undergo spontaneous rotations and we investigate the rotation mechanism using live cell imaging, quantitative measurements, mechanical perturbations, and theory. We find that rotation is driven by a polarized distribution of myosin within cell cortices. The mismatched orientation of this polarized distribution breaks the doublet mirror symmetry. In addition, cells adhere at their interface through adherens junctions and with the extracellular matrix through focal contacts near myosin clusters. Using a physical theory describing the doublet as two interacting active surfaces, we find that rotation is driven by myosin-generated gradients of active tension, whose profiles are dictated by interacting cell polarity axes. We show that interface three-dimensional shapes can be understood from the Curie principle: shapes symmetries are related to broken symmetries of myosin distribution in cortices. To test for the rotation mechanism, we suppress myosin clusters using laser ablation and we generate new myosin clusters by optogenetics. Our work clarifies how polarity-oriented active mechanical forces drive collective cell motion in three dimensions.

Entropy production plays a fundamental role in the study of non-equilibrium systems by offering a quantitative handle on the degree of time-reversal symmetry breaking. It depends crucially on the degree of freedom considered as well as on the scale of description. It was hitherto unknown how the entropy production at one resolution of the degrees of freedom is related to the entropy production at another resolution. This relationship is of particular relevance to coarse grained and continuum descriptions of a given phenomenon. In this work, we derive the scaling of the entropy production under iterative coarse graining on the basis of the correlations of the underlying microscopic transition rates. Our approach unveils a natural criterion to distinguish equilibrium-like and genuinely non-equilibrium macroscopic phenomena based on the sign of the scaling exponent of the entropy production per mesostate., Comment: 21 pages, 6 figures

Proper orientation of the mitotic spindle plays a crucial role in embryos, during tissue development, and in adults, where it functions to dissipate mechanical stress to maintain tissue integrity and homeostasis. While mitotic spindles have been shown to reorient in response to external mechanical stresses, the subcellular cues that mediate spindle reorientation remain unclear. Here, we have used a combination of optogenetics and computational modelling to better understand how mitotic spindles respond to inhomogeneous tension within the actomyosin cortex. Strikingly, we find that the optogenetic activation of RhoA only influences spindle orientation when it is induced at both poles of the cell. Under these conditions, the sudden local increase in cortical tension induced by RhoA activation reduces pulling forces exerted by cortical regulators on astral microtubules. This leads to a perturbation of the torque balance exerted on the spindle, which causes it to rotate. Thus, spindle rotation in response to mechanical stress is an emergent phenomenon arising from the interaction between the spindle positioning machinery and the cell cortex.

As organs and tissues approach their normal size during development or regeneration, growth slows down, and cell proliferation progressively comes to a halt. Among the various processes suggested to contribute to growth termination,1-10 mechanical feedback, perhaps via adherens junctions, has been suggested to play a role.11-14 However, since adherens junctions are only present in a narrow plane of the subapical region, other structures are likely needed to sense mechanical stresses along the apical-basal (A-B) axis, especially in a thick pseudostratified epithelium. This could be achieved by nuclei, which have been implicated in mechanotransduction in tissue culture.15 In addition, mechanical constraints imposed by nuclear crowding and spatial confinement could affect interkinetic nuclear migration (IKNM),16 which allows G2 nuclei to reach the apical surface, where they normally undergo mitosis.17-25 To explore how mechanical constraints affect IKNM, we devised an individual-based model that treats nuclei as deformable objects constrained by the cell cortex and the presence of other nuclei. The model predicts changes in the proportion of cell-cycle phases during growth, which we validate with the cell-cycle phase reporter FUCCI (Fluorescent Ubiquitination-based Cell Cycle Indicator).26 However, this model does not preclude indefinite growth, leading us to postulate that nuclei must migrate basally to access a putative basal signal required for S phase entry. With this refinement, our updated model accounts for the observed progressive slowing down of growth and explains how pseudostratified epithelia reach a stereotypical thickness upon completion of growth.

Collective motions of epithelial cells in vivo are essential for morphogenesis in developmental biology. Tissues elongate, contract, flow, and oscillate, thus sculpting embryos. These tissue level dynamics are known, but the physical mechanisms at the cellular level are unclear, with various behaviors depending on the tissues and species. Moreover, investigations on in vitro tissue behavior usually focus on only one type of cell dynamics and use diverse theoretical approaches, making systematic comparisons between studies challenging. Here, we show that a single epithelial monolayer of Madin Darby Canine Kidney (MDCK) cells can exhibit two types of local tissue kinematics, pulsations and long range coherent flows. We analyzed these distinct motions by using quantitative live imaging. We also report that these motions can be controlled with internal and external cues such as specific inhibitors, and friction modulation of the substrate by microcontact printing method. We further demonstrate with a unified vertex model that both behaviors depend on the competition between velocity alignment and random diffusion of cell polarization. When alignment and diffusion are comparable, a pulsatile flow emerges, whereas the tissue undergoes long-range flows when velocity alignment dominates. We propose that environmental friction, acto-myosin distributions, and cell polarization kinetics are important in regulating the dynamics of tissue morphogenesis.

We provide a perturbative framework to calculate extreme events of non-Markovian processes, by mapping the stochastic process to a two-species reaction diffusion process in a Doi-Peliti field theory combined with the Martin-Siggia-Rose formalism. This field theory treats interactions and the effect of external, possibly self-correlated noise in a perturbation about a Markovian process, thereby providing a systematic, diagrammatic approach to extreme events. We apply the formalism to Brownian Motion and calculate its survival probability distribution subject to self-correlated noise., 24 pages, 4 figures; added figures, comments and references

Tissue elongation is a central morphogenetic event occurring in all organisms in development1,2. During this process, symmetry of cells and tissues is broken by different mechanisms, such as neighbor exchange3,4, cell elongation5,6 and oriented cell division7. While the phenomenon is known to involve remodeling of adherens junctions4 and acto-myosin4,8 at the molecular level, mesoscopic mechanisms leading to distinct morphogenesis processes are poorly understood. This is partly because inputs from morphogen gradients9 or from neighboring tissues10,11 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.

During development, multicellular organisms undergo stereotypical patterns of tissue growth in space and time, but how this is orchestrated remains unclear. Drosophila histoblast nests are small clusters of progenitor epithelial cells that undergo extensive growth to give rise to the abdominal epidermis and are amenable to live-imaging. Our quantitative analysis of the temporal dynamics of abdomen growth reveals that histoblasts display oscillatory cell division rates and that growth termination occurs through the rapid emergence of arrested cells rather than a gradual increase in cell cycle time. By examining tissue mechanics throughout histoblast expansion, we uncover an increase in tissue stress together with a decrease in extracellular matrix resistance as the histoblasts expand. The decrease in resistance is driven by timely extracellular matrix remodeling, which is necessary to trigger tissue expansion. Thus, changes in the tissue microenvironment, and a rapid cell cycle exit, control the formation of the adult Drosophila abdomen.

SummaryDuring development, multicellular organisms undergo stereotypical patterns of tissue growth to yield organs of highly reproducible sizes and shapes. How this process is orchestrated remains unclear. Analysis of the temporal dynamics of tissue growth in theDrosophilaabdomen reveals that cell cycle times are spatially correlated and that growth termination occurs through the rapid emergence of a population of arrested cells rather than a gradual slowing down of cell cycle time. Reduction in apical tension associated with tissue crowding has been proposed as a developmental growth termination mechanism. Surprisingly, we find that growth arrest in the abdomen occurs while apical tension increases, showing that in this tissue a reduction in tension does not underlie the mechanism of growth arrest. However, remodeling of the extracellular matrix is necessary for tissue expansion. Thus, changes in the tissue microenvironment, and a rapid exit from proliferation, control the formation of the adultDrosophilaabdomen.

Morphogen gradients provide positional information during development. To uncover the minimal requirements for morphogen gradient formation, we have engineered a synthetic morphogen in Drosophila wing primordia. We show that an inert protein, green fluorescent protein (GFP), can form a detectable diffusion-based gradient in the presence of surface-associated anti-GFP nanobodies, which modulate the gradient by trapping the ligand and limiting leakage from the tissue. We next fused anti-GFP nanobodies to the receptors of Dpp, a natural morphogen, to render them responsive to extracellular GFP. In the presence of these engineered receptors, GFP could replace Dpp to organize patterning and growth in vivo. Concomitant expression of glycosylphosphatidylinositol (GPI)–anchored nonsignaling receptors further improved patterning, to near–wild-type quality. Theoretical arguments suggest that GPI anchorage could be important for these receptors to expand the gradient length scale while at the same time reducing leakage.

Asbtract Lumen formation plays an essential role in the morphogenesis of tissues during development. Here we review the physical principles that play a role in the growth and coarsening of lumens. Solute pumping by the cell, hydraulic flows driven by differences of osmotic and hydrostatic pressures, balance of forces between extracellular fluids and cell-generated cytoskeletal forces, and electro-osmotic effects have been implicated in determining the dynamics and steady-state of lumens. We use the framework of linear irreversible thermodynamics to discuss the relevant force, time and length scales involved in these processes. We focus on order of magnitude estimates of physical parameters controlling lumen formation and coarsening.

During embryonic development, mechanical forces are essential for cellular rearrangements driving tissue morphogenesis. Here, we show that in the early zebrafish embryo, friction forces are generated at the interface between anterior axial mesoderm (prechordal plate, ppl) progenitors migrating towards the animal pole and neurectoderm progenitors moving in the opposite direction towards the vegetal pole of the embryo. These friction forces lead to global rearrangement of cells within the neurectoderm and determine the position of the neural anlage. Using a combination of experiments and simulations, we show that this process depends on hydrodynamic coupling between neurectoderm and ppl as a result of E-cadherin-mediated adhesion between those tissues. Our data thus establish the emergence of friction forces at the interface between moving tissues as a critical force-generating process shaping the embryo.

In developing tissues, cells estimate their spatial position by sensing graded concentrations of diffusible signaling proteins called morphogens. Morphogen-sensing pathways exhibit diverse molecular architectures, whose roles in controlling patterning dynamics and precision remain unclear. Here, combining cell-based in vitro gradient reconstitution, genetic re-wiring, and mathematical modeling, we systematically analyzed the unique architectural features of the Sonic Hedgehog pathway. The combination of double-negative regulatory logic and negative feedback through the PTCH receptor accelerates gradient formation and improves robustness to variation in the morphogen production rate compared to alternative designs. The ability to isolate morphogen patterning from concurrent developmental processes, and to compare the patterning behaviors of alternative, re-wired, pathway architectures offers a powerful way to understand and engineer multicellular patterning.

During development, cell-generated forces induce tissue-scale deformations to shape the organism [1,2]. The pattern and extent of these deformations depend not solely on the temporal and spatial profile of the generated force fields but also on the mechanical properties of the tissues that the forces act on. It is thus conceivable that, much like the cell-generated forces, the mechanical properties of tissues are modulated during development in order to drive morphogenesis toward specific developmental endpoints. Although many approaches have recently emerged to assess effective mechanical parameters of tissues [3-8], they could not quantitatively relate spatially localized force induction to tissue-scale deformations in vivo. Here, we present a method that overcomes this limitation. Our approach is based on the application of controlled forces on a single microparticle embedded in an individual cell of an embryo. Combining measurements of bead displacement with the analysis of induced deformation fields in a continuum mechanics framework, we quantify material properties of the tissue and follow their changes over time. In particular, we uncover a rapid change in tissue response occurring during Drosophila cellularization, resulting from a softening of the blastoderm and an increase of external friction. We find that the microtubule cytoskeleton is a major contributor to epithelial mechanics at this stage. We identify developmentally controlled modulations in perivitelline spacing that can account for the changes in friction. Overall, our method allows for the measurement of key mechanical parameters governing tissue-scale deformations and flows occurring during morphogenesis. The research leading to these results has received funding from the Spanish Ministry of Economy and Competitiveness (MEIC) to the EMBL partnership, Plan Nacional, BFU2010-16546 and BFU2015-68754, and “Centro de Excelencia Severo Ochoa 2013–2017,” SEV-2012-0208. We acknowledge the support of the CERCA Programme/Generalitat de Catalunya. G.S. is supported by the Francis Crick Institute, which receives its core funding from Cancer Research UK (FC001317), UK Medical Research Council (FC001317), and Wellcome Trust (FC001317)

In embryonic development or tumor evolution, cells often migrate collectively within confining tracks defined by their microenvironment 1,2. In some of these situations, the displacements within a cell strand are antiparallel 3, giving rise to shear flows. However, the mechanisms underlying these spontaneous flows remain poorly understood. Here, we show that an ensemble of spindle-shaped cells plated in a well-defined stripe spontaneously develop a shear flow whose characteristics depend on the width of the stripe. On wide stripes, the cells self-organize in a nematic phase with a director at a well-defined angle with the stripe’s direction, and develop a shear flow close to the stripe’s edges. However, on stripes narrower than a critical width, the cells perfectly align with the stripe’s direction and the net flow vanishes. A hydrodynamic active gel theory provides an understanding of these observations and identifies the transition between the non-flowing phase oriented along the stripe and the tilted phase exhibiting shear flow as a Fréedericksz transition driven by the activity of the cells. This physical theory is grounded in the active nature of the cells and based on symmetries and conservation laws, providing a generic mechanism to interpret in vivo antiparallel cell displacements.

We present a new folded dual-view oblique plane microscopy (OPM) technique termed dOPM that enables two orthogonal views of the sample to be obtained by translating a pair of tilted mirrors in refocussing space. Using a water immersion 40× 1.15 NA primary objective, deconvolved image volumes of 200 nm beads were measured to have full width at half maxima (FWHM) of 0.35 ± 0.04 µm and 0.39 ± 0.02 µm laterally and 0.81 ± 0.07 µm axially. The measured z-sectioning value was 1.33 ± 0.45 µm using light-sheet FWHM in the frames of the two views of 4.99 ± 0.58 µm and 4.89 ± 0.63 µm. To qualitatively demonstrate that the system can reduce shadow artefacts while providing a more isotropic resolution, a multi-cellular spheroid approximately 100 µm in diameter was imaged.

International audience; Proliferating animal cells are able to orient their mitotic spindles along their interphase cell axis, setting up the axis of cell division, despite rounding up as they enter mitosis. This has previously been attributed to molecular memory and, more specifically, to the maintenance of adhesions and retraction fibers in mitosis [1-6], which are thought to act as local cues that pattern cortical Gαi, LGN, and nuclear mitotic apparatus protein (NuMA) [3, 7-18]. This cortical machinery then recruits and activates Dynein motors, which pull on astral microtubules to position the mitotic spindle. Here, we reveal a dynamic two-way crosstalk between the spindle and cortical motor complexes that depends on a Ran-guanosine triphosphate (GTP) signal [12], which is sufficient to drive continuous monopolar spindle motion independently of adhesive cues in flattened human cells in culture. Building on previous work [1, 12, 19-23], we implemented a physical model of the system that recapitulates the observed spindle-cortex interactions. Strikingly, when this model was used to study spindle dynamics in cells entering mitosis, the chromatin-based signal was found to preferentially clear force generators from the short cell axis, so that cortical motors pulling on astral microtubules align bipolar spindles with the interphase long cell axis, without requiring a fixed cue or a physical memory of interphase shape. Thus, our analysis shows that the ability of chromatin to pattern the cortex during the process of mitotic rounding is sufficient to translate interphase shape into a cortical pattern that can be read by the spindle, which then guides the axis of cell division.

Cell division and death can be regulated by the mechanical forces within a tissue. We study the consequences for the stability and roughness of a propagating interface, by analysing a model of mechanically-regulated tissue growth in the regime of small driving forces. For an interface driven by homeostatic pressure imbalance or leader-cell motility, long and intermediate-wavelength instabilities arise, depending respectively on an effective viscosity of cell number change, and on substrate friction. A further mechanism depends on the strength of directed motility forces acting in the bulk. We analyse the fluctuations of a stable interface subjected to cell-level stochasticity, and find that mechanical feedback can help preserve reproducibility at the tissue scale. Our results elucidate mechanisms that could be important for orderly interface motion in developing tissues., Final author version, In press at Phys. Rev. Lett., supplement is available at: https://www.dropbox.com/s/p5mwzdxlw40eglv/supplement_28Nov.pdf?dl=0 or contact authors

Summary Epithelial tissues can elongate in two dimensions by polarized cell intercalation, oriented cell division, or cell shape change, owing to local or global actomyosin contractile forces acting in the plane of the tissue. In addition, epithelia can undergo morphogenetic change in three dimensions. We show that elongation of the wings and legs of Drosophila involves a columnar-to-cuboidal cell shape change that reduces cell height and expands cell width. Remodeling of the apical extracellular matrix by the Stubble protease and basal matrix by MMP1/2 proteases induces wing and leg elongation. Matrix remodeling does not occur in the haltere, a limb that fails to elongate. Limb elongation is made anisotropic by planar polarized Myosin-II, which drives convergent extension along the proximal-distal axis. Subsequently, Myosin-II relocalizes to lateral membranes to accelerate columnar-to-cuboidal transition and isotropic tissue expansion. Thus, matrix remodeling induces dynamic changes in actomyosin contractility to drive epithelial morphogenesis in three dimensions., Graphical Abstract, Highlights • Apical and basal extracellular matrices are degraded to elongate Drosophila limbs • Apical matrix is degraded by the Stubble protease and basal matrix by MMPs • Limbs elongate via convergent extension and cell flattening, driven by Myosin-II • In the haltere, Ultrabithorax prevents matrix remodeling and tissue elongation, Diaz-de-la-Loza et al. show that morphogenetic elongation of Drosophila limbs occurs via both convergent extension and columnar-to-cuboidal cell shape change. These processes are spatially organized by Myosin-II and temporally organized by remodeling of the extracellular matrix, including both apical (ZP-domain-containing) and basal (Collagen IV/Laminin/Perlecan-containing) matrices.

International audience; In embryonic development or tumor evolution, cells often migrate collectively within confining tracks defined by their microenvironment 1,2. In some of these situations, the displacements within a cell strand are antiparallel 3, giving rise to shear flows. However, the mechanisms underlying these spontaneous flows remain poorly understood. Here, we show that an ensemble of spindle-shaped cells plated in a well-defined stripe spontaneously develop a shear flow whose characteristics depend on the width of the stripe. On wide stripes, the cells self-organize in a nematic phase with a director at a well-defined angle with the stripe's direction, and develop a shear flow close to the stripe's edges. However, on stripes narrower than a critical width, the cells perfectly align with the stripe's direction and the net flow vanishes. A hydrodynamic active gel theory provides an understanding of these observations and identifies the transition between the non-flowing phase oriented along the stripe and the tilted phase exhibiting shear flow as a Fréedericksz transition driven by the activity of the cells. This physical theory is grounded in the active nature of the cells and based on symmetries and conservation laws, providing a generic mechanism to interpret in vivo antiparallel cell displacements. Collective cell migration is classically associated with adhesive cell-cell contacts that can ensure large velocity correlation lengths 3-12. However, cells lacking stable cell-cell adhesions such as fibroblasts have been shown to collectively orient in nematic phases 13-16 and move in "streams" in dense monolayers. Interestingly, such streams have been reported in vivo in embryonic development 1 and cancer 2. They are often accompanied by bidirectional flows of cells within the same strand. In particular, cancer cells migrating collectively in vivo in effective "channels" formed between collagen fibers have been Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use

We review the general hydrodynamic theory of active soft materials that is motivated in particular by biological matter. We present basic concepts of irreversible thermodynamics of spatially extended multicomponent active systems. Starting from the rate of entropy production, we identify conjugate thermodynamic fluxes and forces and present generic constitutive equations of polar active fluids and active gels. We also discuss angular momentum conservation which plays a role in the the physics of active chiral gels. The irreversible thermodynamics of active gels provides a general framework to discuss the physics that underlies a wide variety of biological processes in cells and in multicellular tissues.

Summary During epithelial contraction, cells generate forces to constrict their surface and, concurrently, fine-tune the length of their adherens junctions to ensure force transmission. While many studies have focused on understanding force generation, little is known on how junctional length is controlled. Here, we show that, during amnioserosa contraction in Drosophila dorsal closure, adherens junctions reduce their length in coordination with the shrinkage of apical cell area, maintaining a nearly constant junctional straightness. We reveal that junctional straightness and integrity depend on the endocytic machinery and on the mechanosensitive activity of the actomyosin cytoskeleton. On one hand, upon junctional stretch and decrease in E-cadherin density, actomyosin relocalizes from the medial area to the junctions, thus maintaining junctional integrity. On the other hand, when junctions have excess material and ruffles, junction removal is enhanced, and high junctional straightness and tension are restored. These two mechanisms control junctional length and integrity during morphogenesis., Graphical Abstract, Highlights • Adherens junction length is controlled by actomyosin and junctional recycling • Upon stretch, actomyosin is enriched at junctions, and contractile pulses are arrested • Excess junctional length enhances junctional removal, restoring high straightness • E-cadherin levels control cell actomyosin distribution and contractile pulses, The control of adherens junction length is essential for tissue contraction. Sumi et al. show that junctional length is controlled by actomyosin and junctional recycling. When junctions are stretched, actomyosin relocalizes to junctions to reduce their length. When junctions are ruffled, junctional removal is enhanced to restore high junction straightness.

SummaryBiological tissues must generate forces to shape organs and achieve proper development. Such forces often result from the contraction of an apical acto-myosin meshwork. Here we describe an alternative mechanism for tissue contraction, based on individual cell volume change. We show that during Drosophila dorsal closure (DC), a wound healing-related process, the contraction of the amnioserosa (AS) is associated with a major reduction of the volume of its cells, triggered by caspase activation at the onset of the apoptotic program of AS cells. Cell volume decrease results in a contractile force that promotes tissue shrinkage. Estimating mechanical tensions with laser dissection and using 3D biophysical modeling, we show that the cell volume decrease acts together with the contraction of the actin cable surrounding the tissue to govern DC kinetics. Our study identifies a mechanism by which tissues generate forces and movements by modulating individual cell volume during development.

When cells move using integrin-based focal adhesions, they pull in the direction of motion with large, ∼100 Pa, stresses that contract the substrate. Integrin-mediated adhesions, however, are not required for in vivo confined migration. During focal adhesion-free migration, the transmission of propelling forces, and their magnitude and orientation, are not understood. Here, we combine theory and experiments to investigate the forces involved in adhesion-free migration. Using a non-adherent blebbing cell line as a model, we show that actin cortex flows drive cell movement through nonspecific substrate friction. Strikingly, the forces propelling the cell forward are several orders of magnitude lower than during focal-adhesion-based motility. Moreover, the force distribution in adhesion-free migration is inverted: it acts to expand, rather than contract, the substrate in the direction of motion. This fundamentally different mode of force transmission may have implications for cell-cell and cell-substrate interactions during migration in vivo.

During development, cell-generated forces induce tissue-scale deformations to shape the organism. Here, we present a method that allows to quantitatively relate such tissue-scale deformations to spatially localized forces and measure mechanical properties of epitheliain vivo. Our approach is based on the application of controlled forces on microparticles embedded in individual cells of an embryo. Combining measurements of the bead displacement with the analysis of induced deformation fields in a continuum mechanics framework, we can quantify tissue material properties and follow their change over time. In particular, we uncover a rapid change in tissue response occurring duringDrosophilacellularization, resulting from a softening of the blastoderm and an increase of external friction. Pharmacological treatments reveal that in addition to actomyosin, the microtubule cytoskeleton is a major contributor to epithelial mechanics at that stage. Overall, our method allows for measuring essential mechanical parameters governing tissue-scale deformations and flows occurring during morphogenesis.

In this article, we propose a general framework to study the dynamics and topology of cellular networks that capture the geometry of cell packings in two-dimensional tissues. Such epithelia undergo large-scale deformation during morphogenesis of a multicellular organism. Large-scale deformations emerge from many individual cellular events such as cell shape changes, cell rearrangements, cell divisions, and cell extrusions. Using a triangle-based representation of cellular network geometry, we obtain an exact decomposition of large-scale material deformation. Interestingly, our approach reveals contributions of correlations between cellular rotations and elongation as well as cellular growth and elongation to tissue deformation. Using this Triangle Method, we discuss tissue remodeling in the developing pupal wing of the fly Drosophila melanogaster., Comment: 26 pages, 18 figures

We derive a fully covariant theory of the mechanics of active surfaces. This theory provides a framework for the study of active biological or chemical processes at surfaces, such as the cell cortex, the mechanics of epithelial tissues, or reconstituted active systems on surfaces. We introduce forces and torques acting on a surface, and derive the associated force balance conditions. We show that surfaces with in-plane rotational symmetry can have broken up-down, chiral or planar-chiral symmetry. We discuss the rate of entropy production in the surface and write linear constitutive relations that satisfy the Onsager relations. We show that the bending modulus, the spontaneous curvature and the surface tension of a passive surface are renormalised by active terms. Finally, we identify novel active terms which are not found in a passive theory and discuss examples of shape instabilities that are related to active processes in the surface.