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Significance Cells migrating within the body perform vital functions in development and for defense and repair of tissues. In this dense environment, cells encounter mechanical forces and constraints not experienced when moving under buffer, and, accordingly, many change how they move. We find that gentle squashing, which mimics mechanical resistance, causes cells to move using blebs—a form of projection driven by fluid pressure—rather than pseudopods. This behavior depends on the Piezo stretch-operated ion channel in the cell membrane and calcium fluxes into the cell. Piezo is highly conserved and is required for light touch sensation; this work extends its functions into migrating cells., Blebs and pseudopods can both power cell migration, with blebs often favored in tissues, where cells encounter increased mechanical resistance. To investigate how migrating cells detect and respond to mechanical forces, we used a “cell squasher” to apply uniaxial pressure to Dictyostelium cells chemotaxing under soft agarose. As little as 100 Pa causes a rapid (

The process of making blood smears is common in both research and clinical settings, for investigating the health of blood cells and the presence of blood-borne parasites. It is very often carried out manually. We focus here on smears for malaria diagnosis and research which are frequently analyzed by optical microscopy and require a high quality. Automating the smear preparation promises to increase throughput and to improve the quality and consistency of the smears. We present here two devices (manual and motorized) designed to aid in the making of blood smears. These are fully documented, open-source hardware, and an important principle was to make them easily fabricated locally anywhere. Designs and assembly instructions are freely available under an open license. We also describe an image analysis pipeline for characterizing the quality of smears, and use it to optimize the settings and tunable parameters in the two devices. The devices perform as well as expert human operators, while not requiring a trained operator and offering potential advantages in reproducibility and standardization across facilities.

Funder: Mechanobiology Institute, Singapore; doi: http://dx.doi.org/10.13039/501100007672, During gastrulation of the zebrafish embryo, the cap of blastoderm cells organizes into the axial body plan of the embryo with left–right symmetry and head–tail, dorsal–ventral polarities. Our labs have been interested in the mechanics of early development and have investigated whether these large-scale cell movements can be described as tissue-level mechanical strain by a tectonics-based approach. The first step is to image the positions of all nuclei from mid-epiboly to early segmentation by digital sheet light microscopy, organize the surface of the embryo into multi-cell spherical domains, construct velocity fields from the movements of these domains and extract strain rate maps from the change in density of the domains. During gastrulation, tensile/expansive and compressive strains in the axial and equatorial directions are detected as anterior and posterior expansion along the anterior–posterior axis and medial–lateral compression across the dorsal–ventral axis and corresponds to the well characterized morphological movements of convergence and extension. Following gastrulation strain is represented by localized medial expansion at the onset of segmentation and anterior expansion at the onset of neurulation. In addition to linear strain, symmetric patterns of rotation/curl are first detected in the animal hemispheres at mid-epiboly and then the vegetal hemispheres by the end of gastrulation. In embryos treated with C59, a Wnt inhibitor that inhibits head and tail extension, the axial extension and vegetal curl are absent. By analysing the temporal sequence of large-scale movements, deformations across the embryo can be attributed to a combination of epiboly and dorsal convergence-extension.

Microbial-Induced Carbonate Precipitation (MICP) is an innovative ground improvement technique which can enhance the strength and stiffness of soils, and can also control their hydraulic conductivity. These engineering properties of MICP-treated soils are affected by particle-scale behaviour of the precipitated carbonate, i.e. composition, amount and distribution, which are controlled by the MICP process occurring at the particle-scale. In this study, we designed and fabricated a microfluidic chip to improve our understanding of MICP at particle-scale by observing the behaviour of bacteria and CaCO3 crystals during this process. We found that bacteria became evenly distributed throughout the microfluidic chip after the injection of bacterial suspension, grew during bacterial settling, and detached during the injection of cementation solution. Bacteria aggregated during the cementation solution injection, and CaCO3 crystals formed at narrow pore throats or open pore bodies either during or after cementation solution injections., Comment: Accepted for publication in Geotechnique. 20 page, 9 figures

Microbial-induced carbonate precipitation (MICP) has been explored for more than a decade as a promising soil improvement technique. However, it is still challenging to predict and control ...

The directed migration of cell collectives is essential in various physiological processes, such as epiboly, intestinal epithelial turnover, and convergent extension during morphogenesis as well as during pathological events like wound healing and cancer metastasis 1,2 . Collective cell migration leads to the emergence of coordinated movements over multiple cells. Our current understanding emphasizes that these movements are mainly driven by large-scale transmission of signals through adherens junctions 3,4 . In this study, we show that collective movements of epithelial cells can be triggered by polarity signals at the single cell level through the establishment of coordinated lamellipodial protrusions. We designed a minimalistic model system to generate one-dimensional epithelial trains confined in ring shaped patterns that recapitulate rotational movements observed in vitro in cellular monolayers and in vivo in genitalia or follicular cell rotation 5–7 . Using our system, we demonstrated that cells follow coordinated rotational movements after the establishment of directed Rac1-dependent polarity over the entire monolayer. Our experimental and numerical approaches show that the maintenance of coordinated migration requires the acquisition of a front-back polarity within each single cell but does not require the maintenance of cell-cell junctions. Taken together, these unexpected findings demonstrate that collective cell dynamics in closed environments as observed in multiple in vitro and in vivo situations 5,6,8,9 can arise from single cell behavior through a sustained memory of cell polarity.

BackgroundGlioblastoma (GBM) is a highly aggressive incurable brain tumor. The main cause of mortality in GBM patients is the invasive rim of cells migrating away from the main tumor mass and invading healthy parts of the brain. Although motion is driven by forces, our current understanding of the physical factors involved in glioma infiltration remains limited. This study aims to investigate the adhesion properties within and between patients’ tumors on a cellular level and test whether these properties correlate with cell migration.MethodsNine tissue samples were taken from spatially separated sections during 5-aminolevulinic acid (5-ALA) fluorescence guided surgery. Navigated biopsy samples were collected from strongly fluorescent tumor cores, a weak fluorescent tumor rim, and non-fluorescent tumor margins. A microfluidics device was built to induce controlled shear forces to detach cells from monolayer cultures. Cells were cultured on low modulus polydimethylsiloxane representative of the stiffness of brain tissue. Cell migration and morphology were then obtained using time lapse microscopy.ResultsGBM cell populations from different tumor fractions of the same patient exhibited different migratory and adhesive behaviors. These differences were associated with sampling location and amount of 5-ALA fluorescence. Cells derived from weak- and non-fluorescent tumor tissue were smaller, adhered less well, and migrated quicker than cells derived from strongly fluorescent tumor mass.ConclusionGBM tumors are biomechanically heterogeneous. Selecting multiple populations and broad location sampling are therefore important to consider for drug testing.Key pointsGBM tumors are biomechanically heterogeneousGBM cell migration is inversely correlated with cell-matrix adhesion strength5-ALA fluorescence intensity during surgery correlates with the motility properties of GBM cellsImportance of the studyThis is the first study to compare single cell migration and cell-matrix adhesion strength of GBM, using cell lines derived from different tumors and from different regions within the same tumor. Not accounting for internal sampling location within each tumor obscures differences in cell morphology, motility and adhesion properties between patients. Peripherical and marginal tumor cells have different adhesion profiles and are highly migratory compared to those found in the core of the tumor. Aggressive regions of the tumor (highly motile) are linked to the spatial distribution of adhesion strength and are strongly associated with 5-ALA fluorescence intensity. Preclinical tests aimed at developing a treatment for GBM using anti-invasive drugs or adhesion inhibitors, would benefit from using cell lines derived from the tumor periphery (with low 5-ALA intensity) rather than cell lines derived from the tumor core.

Soft materials often exhibit a distinctive power-law viscoelastic response arising from broad distribution of time-scales present in their complex internal structure. A promising tool to accurately describe the rheological behaviour of soft materials is fractional calculus. However, its use in the scientific community remains limited due to the unusual notation and non-trivial properties of fractional operators. This review aims to provide a clear and accessible description of fractional viscoelastic models for a broad audience, and to demonstrate the ability of these models to deliver a unified approach for the characterisation of power-law materials. The use of a consistent framework for the analysis of rheological data would help classify the empirical behaviours of soft and biological materials, and better understand their response., Comment: 28 pages, 12 figures. Soft Matter (2020)

The directed migration of cell collectives is essential in various physiological processes, such as epiboly, intestinal epithelial turnover, and convergent extension during morphogenesis as well as during pathological events like wound healing and cancer metastasis1,2. Collective cell migration leads to the emergence of coordinated movements over multiple cells. Our current understanding emphasizes that these movements are mainly driven by large-scale transmission of signals through adherens junctions3,4. In this study, we show that collective movements of epithelial cells can be triggered by polarity signals at the single cell level through the establishment of coordinated lamellipodial protrusions. We designed a minimalistic model system to generate one-dimensional epithelial trains confined in ring shaped patterns that recapitulate rotational movements observed in vitro in cellular monolayers and in vivo in genitalia or follicular cell rotation5–7. Using our system, we demonstrated that cells follow coordinated rotational movements after the establishment of directed Rac1-dependent polarity over the entire monolayer. Our experimental and numerical approaches show that the maintenance of coordinated migration requires the acquisition of a front-back polarity within each single cell but does not require the maintenance of cell-cell junctions. Taken together, these unexpected findings demonstrate that collective cell dynamics in closed environments as observed in multiple in vitro and in vivo situations5,6,8,9 can arise from single cell behavior through a sustained memory of cell polarity.

Microbial-induced calcium carbonate (CaCO3) precipitation (MICP) has been explored for its potential engineering applications such as soil stabilization, but current understanding of the fu...

Migrating cells face varied mechanical and physical barriers in physiological environments, but how they sense and respond to them remains to be fully understood. We used a custom-built ‘cell squasher’ to apply uniaxial pressure to Dictyostelium cells migrating under soft agarose. Within 10 seconds of application, loads of as little as 100 Pa cause cells to move using blebs instead of pseudopods. Cells lose volume and surface area under pressure and their actin dynamics are perturbed. Myosin-II is recruited to the cortex, potentially increasing contractility and so driving blebbing. The blebbing response depends on extra-cellular calcium, is accompanied by increased cytosolic calcium and largely abrogated in null mutants of the Piezo stretch-operated channel. We propose that migrating cells sense mechanical force through mechano-sensitive channels, leading to an influx of calcium and cortical recruitment of myosin, thus re-directing the motile apparatus to produce blebs rather than pseudopods.

Current technological trends in the field of microelectronics have highlighted the requirement to use cost-effective techniques for the precise deposition of highly resolved features. Laser-induced forward transfer (LIFT) meets these requirements and has already been applied for the direct printing of devices and components. However, in order to improve the process’ reproducibility and printing resolution, further research has to be conducted, regarding the rheological characteristics of the printable fluids and their jetting dynamics. In this work, we employ both pump-probe and high-speed imaging in order to investigate the formation and expansion of the liquid bubble, as well as the liquid jet's propagation. Newtonian as well as non-Newtonian fluids are studied and compared, over a wide range of viscosities. Furthermore, a computational model is utilized in order to gain more insight on the transfer mechanisms of the process. The simulation predictions are validated against experimental results, and found to be in good agreement, even in the case of non-Newtonian fluids. The results indicate that such accurate modelling can be developed as a new cost- and time-effective tool for the technique’s optimization.

The mechanical response of single cells and tissues exhibits a broad distribution of time-scales that often gives rise to a distinctive power-law rheology. Such complex behaviour cannot be easily captured by traditional rheological approaches, making material characterisation and predictive modelling very challenging. Here, we present a novel model combining conventional viscoelastic elements with fractional calculus that successfully captures the macroscopic relaxation response of epithelial monolayers. The parameters extracted from the fitting of the relaxation modulus allow prediction of the response of the same material to slow stretch and creep, indicating that the model captured intrinsic material properties. Two characteristic times, derived from the model parameters, delimit different regimes in the materials response. We compared the response of tissues with the behaviour of single cells as well as intra and extra-cellular components, and linked the power-law behaviour of the epithelium to the dynamics of the cell cortex. Such a unified model for the mechanical response of biological materials provides a novel and robust mathematical approach to consistently analyse experimental data and uncover similarities and differences in reported behaviour across experimental methods and research groups. It also sets the foundations for more accurate computational models of tissue mechanics.

Heterogeneity within tumour cell populations is commonly observed in most cancers. However, its impact on metastatic dissemination, one of the primary determinants of the disease prognosis, remains poorly understood. Working with a simplified numerical model of tumour spheroids, we investigated the impact of mechanical heterogeneity on the onset of tumour invasion into surrounding tissues. Our work establishes a positive link between tumour heterogeneity and metastatic dissemination, and recapitulates a number of invasion patterns identified in vivo , such as multicellular finger-like protrusions. Two complementary mechanisms are at play in heterogeneous tumours. A small proportion of stronger cells are able to initiate and lead the escape of cells, while collective effects in the bulk of the tumour provide the coordination required to sustain the invasive process through multicellular streaming. This suggests that the multicellular dynamics observed during metastasis is a generic feature of mechanically heterogeneous cell populations and might rely on a limited and generic set of attributes.

Epithelial monolayers are one-cell thick tissue sheets that separate internal and external environments. As part of their function, they withstand extrinsic mechanical stresses applied at high strain rate. However, little is known about how monolayers respond to mechanical deformations. In stress relaxation tests, monolayers respond in a biphasic manner and stress dissipation is accompanied by an increase in monolayer resting length, pointing to active remodelling of cell architecture during relaxation. Consistent with this, actomyosin remodels at a rate commensurate with mechanical relaxation and governs the rate of monolayer stress relaxation – as in single cells. By contrast, junctional complexes and intermediate filaments form stable connections between cells, enabling monolayers to behave rheologically as single cells. Together, these data show actomyosin cytoskeletal dynamics govern the rheological properties of monolayers by enabling active, ATP-dependent changes in the resting length. These findings have far-reaching consequences for our understanding of developmental morphogenesis and tissue response to mechanical stress.

Cell division plays an important role in animal tissue morphogenesis, which depends, critically, on the orientation of divisions. In isolated adherent cells, the orientation of mitotic spindles is sensitive to interphase cell shape and the direction of extrinsic mechanical forces. In epithelia, the relative importance of these two factors is challenging to assess. To do this, we used suspended monolayers devoid of ECM, where divisions become oriented following a stretch, allowing the regulation and function of epithelial division orientation in stress relaxation to be characterized. Using this system, we found that divisions align better with the long, interphase cell axis than with the monolayer stress axis. Nevertheless, because the application of stretch induces a global realignment of interphase long axes along the direction of extension, this is sufficient to bias the orientation of divisions in the direction of stretch. Each division redistributes the mother cell mass along the axis of division. Thus, the global bias in division orientation enables cells to act collectively to redistribute mass along the axis of stretch, helping to return the monolayer to its resting state. Further, this behavior could be quantitatively reproduced using a model designed to assess the impact of autonomous changes in mitotic cell mechanics within a stretched monolayer. In summary, the propensity of cells to divide along their long axis preserves epithelial homeostasis by facilitating both stress relaxation and isotropic growth without the need for cells to read or transduce mechanical signals.

Neural crest epithelial–mesenchymal transition (EMT) and collective cell migration rely on a solid-to-liquid-like transition triggered by internalization of N-cadherin downstream of lysophosphatidic acid receptor 2., Collective cell migration (CCM) and epithelial–mesenchymal transition (EMT) are common to cancer and morphogenesis, and are often considered to be mutually exclusive in spite of the fact that many cancer and embryonic cells that have gone through EMT still cooperate to migrate collectively. Here we use neural crest (NC) cells to address the question of how cells that have down-regulated cell–cell adhesions can migrate collectively. NC cell dissociation relies on a qualitative and quantitative change of the cadherin repertoire. We found that the level of cell–cell adhesion is precisely regulated by internalization of N-cadherin downstream of lysophosphatidic acid (LPA) receptor 2. Rather than promoting the generation of single, fully mesenchymal cells, this reduction of membrane N-cadherin only triggers a partial mesenchymal phenotype. This intermediate phenotype is characterized by an increase in tissue fluidity akin to a solid-like–to–fluid-like transition. This change of plasticity allows cells to migrate under physical constraints without abolishing cell cooperation required for collectiveness.

Directed migration of groups of cells is a critical aspect of tissue morphogenesis that ensures proper tissue organization and, consequently, function. Cells moving in groups, unlike single cells, must coordinate their migratory behavior to maintain tissue integrity. During directed migration, cells are guided by a combination of mechanical and chemical cues presented by neighboring cells and the surrounding extracellular matrix. One important class of signals that guide cell migration includes topographic cues. Although the contact guidance response of individual cells to topographic cues has been extensively characterized, little is known about the response of groups of cells to topographic cues, the impact of such cues on cell-cell coordination within groups, and the transmission of nonautonomous contact guidance information between neighboring cells. Here, we explore these phenomena by quantifying the migratory response of confluent monolayers of epithelial and fibroblast cells to contact guidance cues provided by grooved topography. We show that, in both sparse clusters and confluent sheets, individual cells are contact-guided by grooves and show more coordinated behavior on grooved versus flat substrates. Furthermore, we demonstrate both in vitro and in silico that the guidance signal provided by a groove can propagate between neighboring cells in a confluent monolayer, and that the distance over which signal propagation occurs is not significantly influenced by the strength of cell-cell junctions but is an emergent property, similar to cellular streaming, triggered by mechanical exclusion interactions within the collective system.

Rheology is the science of deformation and flow, with a focus on materials that do not exhibit simple linear elastic or viscous Newtonian behaviours. Rheology plays an important role in the empirical characterisation of soft viscoelastic materials commonly found in the food and cosmetics industry, as well as in biology and bioengineering. A broad range of theoretical tools exist to extract material parameters and interpret them thanks to data analysis and/or physical modelling. RHEOS (RHEology, Open-Source) is a software package designed to make the analysis of rheological data simpler, faster and more reproducible. RHEOS is currently limited to the broad family of linear viscoelastic models. A particular strength of the library is its ability to handle rheological models containing fractional derivatives which have demonstrable utility for the modelling of biological materials but have hitherto remained in relative obscurity-possibly due to their mathematical and computational complexity. RHEOS is written in Julia, which greatly assists achievement of our aims as it provides excellent computational efficiency and approachable syntax. RHEOS is fully documented and has extensive testing coverage. It should be noted that RHEOS is not an optimisation package. It builds on another optimisation package, NLopt, by adding a large number of abstractions and functionality specific to the exploration of viscoelastic data., Comment: 4 pages

The chemical, physical and mechanical properties of the extra-cellular environment have a strong effect on cell migration. Aspects such as pore-size or stiffness of the matrix influence the selection of the mechanism used by cells to propel themselves, including pseudopod or blebbing. How a cell perceives its environment, and how such a cue triggers a change in behaviour are largely unknown, but mechanics is likely to be involved. Because mechanical conditions are often controlled by modifying the composition of the environment, separating chemical and physical contributions is difficult and requires multiple controls. Here we propose a simple method to impose a mechanical compression on individual cells without altering the composition of the gel. Live imaging during compression provides accurate information about the cell’s morphology and migratory phenotype. UsingDictyosteliumas a model, we observe that a compression of the order of 500 Pa flattens the cells under gel by up to 50%. This uniaxial compression directly triggers a transition in the mode of migration, from primarily pseudopodial to bleb driven, in less than 30 sec. This novel device is therefore capable of influencing cell migration in real time and offers a convenient approach to systematically study mechanotransduction in confined environments.

Summary Under conditions of homeostasis, dynamic changes in the length of individual adherens junctions (AJs) provide epithelia with the fluidity required to maintain tissue integrity in the face of intrinsic and extrinsic forces. While the contribution of AJ remodeling to developmental morphogenesis has been intensively studied, less is known about AJ dynamics in other circumstances. Here, we study AJ dynamics in an epithelium that undergoes a gradual increase in packing order, without concomitant large-scale changes in tissue size or shape. We find that neighbor exchange events are driven by stochastic fluctuations in junction length, regulated in part by junctional actomyosin. In this context, the developmental increase of isotropic junctional actomyosin reduces the rate of neighbor exchange, contributing to tissue order. We propose a model in which the local variance in tension between junctions determines whether actomyosin-based forces will inhibit or drive the topological transitions that either refine or deform a tissue., Graphical Abstract, Highlights • Fluctuations in junction length cause neighbor exchange events without morphogenesis • The variance in junction tension determines how actomyosin influences T1 rates • Globally increasing junctional actomyosin levels inhibits neighbor exchange • A developmental increase in isotropic junction tension refines cellular packing, Curran et al. investigate adherens junction remodeling in the fly notum, where differences in actomyosin levels drive fluctuations in junction length and neighbor exchange, but not morphogenesis. A developmental increase in global junctional tension reduces, rather than drives, neighbor exchange to promote tissue ordering.

A method is described for imposing mechanical compression on individual cells while monitoring their morphology and migratory phenotype. A compression of the order of 500 Pa flattens the cells by up to 50% and triggers a transition in the mode of migration. This approach is convenient for studying mechanotransduction in confined environments., The chemical, physical, and mechanical properties of the extracellular environment have a strong effect on cell migration. Aspects such as pore size or stiffness of the matrix influence the selection of the mechanism used by cells to propel themselves, including by pseudopods or blebbing. How a cell perceives its environment and how such a cue triggers a change in behavior are largely unknown, but mechanics is likely to be involved. Because mechanical conditions are often controlled by modifying the composition of the environment, separating chemical and physical contributions is difficult and requires multiple controls. Here we propose a simple method to impose a mechanical compression on individual cells without altering the composition of the matrix. Live imaging during compression provides accurate information about the cell's morphology and migratory phenotype. Using Dictyostelium as a model, we observe that a compression of the order of 500 Pa flattens the cells under gel by up to 50%. This uniaxial compression directly triggers a transition in the mode of migration from primarily pseudopodial to bleb driven in

Open source hardware has the potential to revolutionise the way we build scientific instruments; with the advent of readily-available 3D printers, mechanical designs can now be shared, improved and replicated faster and more easily than ever before. However, printed parts are typically plastic and often perform poorly compared to traditionally machined mechanisms. We have overcome many of the limitations of 3D printed mechanisms by exploiting the compliance of the plastic to produce a monolithic 3D printed flexure translation stage, capable of sub-micron-scale motion over a range of $8\times8\times4\,$mm. This requires minimal post-print clean-up, and can be automated with readily-available stepper motors. The resulting plastic composite structure is very stiff and exhibits remarkably low drift, moving less than $20\,\mu$m over the course of a week, without temperature stabilisation. This enables us to construct a miniature microscope with excellent mechanical stability, perfect for timelapse measurements in situ in an incubator or fume hood. The ease of manufacture lends itself to use in containment facilities where disposability is advantageous, and to experiments requiring many microscopes in parallel. High performance mechanisms based on printed flexures need not be limited to microscopy, and we anticipate their use in other devices both within the laboratory and beyond., Comment: 7 pages, 10 figures

Cellularised materials are composed of cells interfaced through specialised intercellular junctions that link the cytoskeleton of one cell to that of its neighbours allowing for transmission of forces. Cellularised materials are common in early development and adult tissues where they can be found in the form of cell sheets, cysts, or amorphous aggregates and in pathophysiological conditions such as cancerous tumours. Given the growing realisation that forces can regulate cell physiology and developmental processes, understanding how cellularised materials deform under mechanical stress or dissipate stress appear as key biological questions. In this review, we will discuss the dynamic mechanical properties of cellularised materials devoid of extracellular matrix.

One-cell-thick monolayers are the simplest tissues in multicellular organisms, yet they fulfill critical roles in development and normal physiology. In early development, embryonic morphogenesis results largely from monolayer rearrangement and deformation due to internally generated forces. Later, monolayers act as physical barriers separating the internal environment from the exterior and must withstand externally applied forces. Though resisting and generating mechanical forces is an essential part of monolayer function, simple experimental methods to characterize monolayer mechanical properties are lacking. Here, we describe a system for tensile testing of freely suspended cultured monolayers that enables the examination of their mechanical behavior at multi-, uni-, and subcellular scales. Using this system, we provide measurements of monolayer elasticity and show that this is two orders of magnitude larger than the elasticity of their isolated cellular components. Monolayers could withstand more than a doubling in length before failing through rupture of intercellular junctions. Measurement of stress at fracture enabled a first estimation of the average force needed to separate cells within truly mature monolayers, approximately ninefold larger than measured in pairs of isolated cells. As in single cells, monolayer mechanical properties were strongly dependent on the integrity of the actin cytoskeleton, myosin, and intercellular adhesions interfacing adjacent cells. High magnification imaging revealed that keratin filaments became progressively stretched during extension, suggesting they participate in monolayer mechanics. This multiscale study of monolayer response to deformation enabled by our device provides the first quantitative investigation of the link between monolayer biology and mechanics.

Many insects with smooth adhesive pads can rapidly enlarge their contact area by centripetal pulls on the legs, allowing them to cope with sudden mechanical perturbations such as gusts of wind or raindrops. The short time scale of this reaction excludes any neuromuscular control; it is thus more likely to be caused by mechanical properties of the pad's specialized cuticle. This soft cuticle contains numerous branched fibrils oriented almost perpendicularly to the surface. Assuming a fixed volume of the water-filled cuticle, we hypothesized that pulls could decrease the fibril angle, thereby helping the contact area to expand laterally and longitudinally. Three-dimensional fluorescence microscopy on the cuticle of smooth stick insect pads confirmed that pulls significantly reduced the fibril angle. However, the fibril angle variation appeared insufficient to explain the observed increase in contact area. Direct strain measurements in the contact zone demonstrated that pulls not only expand the cuticle laterally, but also add new contact area at the pad's outer edge.

Our understanding of the elasticity and rheology of disordered materials, such as granular piles, foams, emulsions or dense suspensions relies on improving experimental tools to characterise their behaviour at the particle scale. While 2D observations are now routinely carried out in laboratories, 3D measurements remain a challenge. In this paper, we use a simple model system, a packing of soft elastic spheres, to illustrate the capability of X-ray microtomography to characterise the internal structure and local behaviour of granular systems. Image analysis techniques can resolve grain positions, shapes and contact areas; this is used to investigate the material's microstructure and its evolution upon strain. In addition to morphological measurements, we develop a technique to quantify contact forces and estimate the internal stress tensor. As will be illustrated in this paper, this opens the door to a broad array of static and dynamical measurements in 3D disordered systems.

The combination of laterally activating and inhibiting feedbacks is well known to spontaneously generate spatial organisation. It was introduced by Gierer and Meinhardt as an extension of Turing's great insight, that two reacting and diffusing chemicals can spontaneously drive spatial morphogenesisper se. In this study, we develop an accessible nonlinear and discrete probabilistic model to study simple generalisations of lateral activation and inhibition. By doing so, we identify novel modes of morphogenesis beyond the familiar Turing-type modes; notably, beyond stripes, hexagonal nets, pores, and labyrinths, we identify labyrinthine highways, Kagome lattices, gyrating labyrinths, and multi-colour travelling waves and spirals. The results are discussed within the context of Turing's original motivating interest: the mechanisms which underpin the morphogenesis of living organisms.

Closure of wounds and gaps in tissues is fundamental for the correct development and physiology of multicellular organisms and, when misregulated, may lead to inflammation and tumorigenesis. To re-establish tissue integrity, epithelial cells exhibit coordinated motion into the void by active crawling on the substrate and by constricting a supracellular actomyosin cable. Coexistence of these two mechanisms strongly depends on the environment. However, the nature of their coupling remains elusive because of the complexity of the overall process. Here we demonstrate that epithelial gap geometry in both in vitro and in vivo regulates these collective mechanisms. In addition, the mechanical coupling between actomyosin cable contraction and cell crawling acts as a large-scale regulator to control the dynamics of gap closure. Finally, our computational modelling clarifies the respective roles of the two mechanisms during this process, providing a robust and universal mechanism to explain how epithelial tissues restore their integrity., Epithelial wound closure proceeds through both crawling into the wound and by constricting an actomyosin cable in a so-called purse-string mechanism. Here the authors show that the two mechanisms are mechanically coupled and the curvature of the wound regulates the overall dynamics of wound closure.

The forebrain has the most complex shape and structure of the vertebrate brain regions and the mechanisms of its formation remain obscure. Convergence and extension movements are characteristic of the posterior (spinal cord and hindbrain) neural plate (pNP) while tissue de- formations and underlying cellular dynamics during the early shaping of the forebrain neural plate (fNP) are undefined. Here, we apply live imaging, automated cell tracking and compu- tational analysis to quantitatively map cell behaviour in the zebrafish fNP. We demonstrate a novel mechanism in which planar cell rearrangements, with a passive signature, are orthogo- nal to those in the pNP, and cell divisions lacking planar-polarity facilitate thickening from two to three layers. We develop a mechanical model of the fNP in which polarised cell be- haviour arises from interactions with dissimilar bordering tissues rather than from intrinsical- ly polarised cells. The model unifies in vivo observations and provides a mechanistic under- standing of fNP morphogenesis.

Morphogenetic movement plays a key role in early embryogenesis in multicellular organisms. An important morphogenetic event during early embryo development is the convergence and extension at mid-gastrulation stage, where cells collectively move from the ventral side of the embryo towards the dorsal axis, underlying the neural tube formation as well as the somatogenesis. However, the mechanics of early development is intensively studied at the cellular level but due to the technical challenge for quantifying the force and deformation the global mechanics at the tissue level remains unknown. Here we combine in-toto imaging by light-sheet microscopy and computational methods to calculate the strain map of zebrafish development from 7-12 hpf, representing the tissue deformation, during mid-gastrulation. Key parameters for the dynamics include: 1. vector field of collective cell migrations, 2. mechanical curl of sheets of cells, which describes the domain rotation, and 3. strain, which measures the tissue deformation, have been calculated over the entire embryo. Furthermore, based on these results we have identified strain associated with convergent extension and pre-somatogenesis events. The strain maps are a first step in the quantitation of the mechanical forces that power morphogenesis.

Our current understanding of cell sorting relies on physical difference, either in the interfacial properties or motile force, between cell types. But is such asymmetry a prerequisite for cell sorting? We test this using a minimal model in which the two cell populations are identical with respect to their physical properties and differences in motility arise solely from how cells interact with their surroundings. The model resembles the Schelling model used in social sciences to study segregation phenomena at the scale of societies. Our results demonstrate that segregation can emerge solely from cell motility being a dynamic property that changes in response to the local environment of the cell, but that additional mechanisms are necessary to reproduce the envelopment behaviour observedin vitro. The time course of segregation follows a power law, in agreement with the scaling reported from experiment and in other models of motility-driven segregation.

The cells ability to adhere to the extracellular matrix (ECM) is a fundamental feature of many higher eukaryotic cells and is required for cell migration, proliferation, and differentiation. At the scale of single cells, the primary patterns of adhesion to the ECM are called focal adhesions (FAs). They are thought to serve as mechano-sensor units. Recent progress in super resolution techniques opened the door to study ultrastructure of focal adhesions. However the dynamics of their spatial structure has not been studied yet. Here, we combine structured illumination microscopy (SIM) with total internal reflection fluorescence microscopy (TIRF) to study focal adhesions at high spatial resolution in live cells. We studied REF52 fibroblasts spread on fibronectin disks to avoid the variability of focal adhesions properties due to variable cell morphology and migratory status. We observed the formation of many focal adhesions localized at the lamellipodium-lamellum interface. We further show that (i) focal adhesions are formed by filaments subunits which grow retrograde and then shrink in ∼ 20 mins, (ii) that their formation depends on the spreading area of the cell and (iii) that each filament connects to a single actin cable, therefore linking the internal structure of focal adhesions to the maturation process through actin pulling on the focal complex. Thus our study reveals the ultrastructure and dynamics of focal adhesion and is a first step towards a better understanding of the formation of FAs with respect to the cell mechanical state.Keywords: focal adhesions, filamentous unit, SIM, retrograde flow, spreading area, paxillin.

Embryonic stem cells (ESCs) self-renew in a state of naïve pluripotency in which they are competent to generate all somatic cells1. It has been hypothesized that, before irreversibly committing, ESCs pass through at least one metastable transition state2-4. This transition would represent a gateway for differentiation and reprogramming of somatic cells5,6. We sought a mechanical phenotype of transition by probing the nuclear response to compressive and tensile forces and found that, during transition, nuclei of ESCs are auxetic: they displayed a cross-sectional expansion when stretched and a cross-sectional contraction when compressed, and their stiffness increased under compression. We show that the auxetic phenotype of transition ESC nuclei is driven at least in part by global chromatin decondensation. Through the regulation of molecular turnover in the differentiating nucleus by external forces, auxeticity could be a key element in mechanotransduction. Our findings highlight the importance of nuclear structure in the regulation of differentiation and reprogramming.

A number of biological processes, such as embryo development, cancer metastasis or wound healing, rely on cells moving in concert. The mechanisms leading to the emergence of coordinated motion remain however largely unexplored. Although biomolecular signalling is known to be involved in most occurrences of collective migration, the role of physical and mechanical interactions has only been recently investigated. In this study, a versatile framework for cell motility is implementedin silicoin order to study the minimal requirements for the coordination of a group of epithelial cells. We find that cell motility and cell–cell mechanical interactions are sufficient to generate a broad array of behaviours commonly observedin vitroandin vivo. Cell streaming, sheet migration and susceptibility to leader cells are examples of behaviours spontaneously emerging from these simple assumptions, which might explain why collective effects are so ubiquitous in nature. The size of the population and its confinement appear, in particular, to play an important role in the coordination process. In all cases, the complex response of the population can be predicted from the knowledge of the correlation length of the velocity field measured in the bulk of the epithelial layer. This analysis provides also new insights into cancer metastasis and cell sorting, suggesting, in particular, that collective invasion might result from an emerging coordination in a system where single cells are mechanically unable to invade.

The role of geometrical confinement on collective cell migration has been recognized but has not been elucidated yet. Here, we show that the geometrical properties of the environment regulate the formation of collective cell migration patterns through cell–cell interactions. Using microfabrication techniques to allow epithelial cell sheets to migrate into strips whose width was varied from one up to several cell diameters, we identified the modes of collective migration in response to geometrical constraints. We observed that a decrease in the width of the strips is accompanied by an overall increase in the speed of the migrating cell sheet. Moreover, large-scale vortices over tens of cell lengths appeared in the wide strips whereas a contraction-elongation type of motion is observed in the narrow strips. Velocity fields and traction force signatures within the cellular population revealed migration modes with alternative pulling and/or pushing mechanisms that depend on extrinsic constraints. Force transmission through intercellular contacts plays a key role in this process because the disruption of cell–cell junctions abolishes directed collective migration and passive cell–cell adhesions tend to move the cells uniformly together independent of the geometry. Altogether, these findings not only demonstrate the existence of patterns of collective cell migration depending on external constraints but also provide a mechanical explanation for how large-scale interactions through cell–cell junctions can feed back to regulate the organization of migrating tissues.

When walking on wet sand, each footstep leaves behind a temporarily dry impression. This counterintuitive observation is the most common illustration of the Reynolds principle of dilatancy: that is, a granular packing tends to expand as it is deformed, therefore increasing the amount of porous space. Although widely called upon in areas such as soil mechanics and geotechnics, a deeper understanding of this principle is constrained by the lack of analytical tools to study this behavior. Using x-ray radiography, we track a broad variety of granular flow profiles and quantify their intrinsic dilatancy behavior. These measurements frame Reynolds dilatancy as a kinematic process. Closer inspection demonstrates, however, the practical importance of flow induced compaction which competes with dilatancy, leading more complex flow properties than expected.

The dynamic reshaping of tissues during morphogenesis results from a combination of individual cell behaviours and collective cell rearrangements. However, a comprehensive framework to unambiguously measure and link cell behaviour to tissue morphogenesis is lacking. Here we introduce such a kinematic framework, bridging cell and tissue behaviours at an intermediate, mesoscopic, level of cell clusters or domains. By measuring domain deformation in terms of the relative motion of cell positions and the evolution of their shapes, we characterize the basic invariant quantities that measure fundamental classes of cell behaviour, namely tensorial rates of cell shape change and cell intercalation. In doing so we introduce an explicit definition of cell intercalation as a continuous process. We demonstrate how spatiotemporal mapping of strain rates in three models of tissue morphogenesis leads to new insight into morphogenetic mechanisms. Our quantitative approach has broad relevance for the precise characterisation and comparison of morphogenetic phenotypes.

We study the linear and nonlinear elastic behavior of amorphous systems using a two-dimensional random network of harmonic springs as a model system. A natural characterization of these systems arises in terms of the network coordination (average number of springs per node) relative to that of a marginally rigid network $\ensuremath{\delta}z$: a floppy network has $\ensuremath{\delta}zl0$, while a stiff network has $\ensuremath{\delta}zg0$. Under the influence of an externally applied load, we observe that the response of both floppy and stiff networks is controlled by the critical point corresponding to the onset of rigidity. We use numerical simulations to compute the exponents which characterize the shear modulus, the heterogeneity of the response, and the network stiffening as a function of $\ensuremath{\delta}z$ and derive these theoretically, thus allowing us to predict aspects of the mechanical response of glasses and fibrous networks.

Real-time cardiac ultrasound allows monitoring the heart motion during intracardiac beating heart procedures. Our application assists atrial septal defect (ASD) closure techniques using real-time 3D ultrasound guidance. One major image processing challenge is the processing of information at high frame rate. We present an optimized block flow technique, which combines the probability-based velocity computation for an entire block with template matching. We propose adapted similarity constraints both from frame to frame, to conserve energy, and globally, to minimize errors. We show tracking results on eight in-vivo 4D datasets acquired from porcine beating-heart procedures. Computing velocity at the block level with an optimized scheme, our technique tracks ASD motion at 41 frames/s. We analyze the errors of motion estimation and retrieve the cardiac cycle in ungated images

An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a consequence, the measure of the pre-avalanche dynamics is unlikely to serve as a tool for predicting macroscopic avalanches.