Large-scale mechanical properties of Xenopus embryonic epithelium

An EpCAM/Trop2 mechanostat differentially regulates collective behaviour of human carcinoma cells

EpCAM and its close relative Trop2 are well-known cell surface markers of carcinoma, but their potential role in cancer metastasis remains unclear. They are known, however, to downregulate myosin-dependent contractility, a key parameter involved in adhesion and migration. We investigate here the morphogenetic impact of the high EpCAM and Trop2 levels typically found in epithelial breast cancer cells, using spheroids of MCF7 cells as an in vitro model. Intriguingly, EpCAM depletion stimulated spheroid cohesive spreading, while Trop2 depletion had the opposite effect. Combining cell biological and biophysical approaches, we demonstrate that while EpCAM and Trop2 both contribute to moderate cell contractility, their depletions differentially impact on the process of "wetting" a substrate, here both matrix and neighboring cells, by affecting the balance of cortical tension at cell and tissue interfaces. These distinct phenotypes can be explained by partial enrichment at specific interfaces. Our data are consistent with the EpCAM-Trop2 pair acting as a mechanostat that tunes adhesive and migratory behaviours.

Monitoring the Mechanical Evolution of Tissue During Neural Tube Closure of Chick Embryo

Neural tube closure (NTC) is a critical process during embryonic development. Failure in this process can lead to neural tube defects, causing congenital malformations or even mortality. NTC involves a series of mechanisms on genetic, molecular, and mechanical levels. While mechanical regulation has become an increasingly attractive topic in recent years, it remains largely unexplored due to the lack of suitable technology for conducting mechanical testing of 3D embryonic tissue in situ. In response, we have developed a protocol for quantifying the mechanical properties of chicken embryonic tissue in a non-contact and non-invasive manner. This is achieved by integrating a confocal Brillouin microscope with an on-stage incubation system. To probe tissue mechanics, a pre-cultured embryo is collected and transferred to an on-stage incubator for ex ovo culture. Simultaneously, the mechanical images of the neural plate tissue are acquired by the Brillouin microscope at different time points during development. This protocol includes detailed descriptions of sample preparation, the implementation of Brillouin microscopy experiments, and data post-processing and analysis. By following this protocol, researchers can study the mechanical evolution of embryonic tissue during development longitudinally.

Wrinkling Instability in Unsupported Epithelial Sheets

We investigate the elasticity of an unsupported epithelial monolayer and we discover that unlike a thin solid plate, which wrinkles if geometrically incompatible with the underlying substrate, the epithelium may do so even in the absence of the substrate. From a cell-based model, we derive an exact elasticity theory and discover wrinkling driven by the differential apico-basal surface tension. Our theory is mapped onto that for supported plates by introducing a phantom substrate whose stiffness is finite beyond a critical differential tension. This suggests a new mechanism for an autonomous control of tissues over the length scale of their surface patterns.

Cell cortex regulation by the planar cell polarity protein Prickle1

The planar cell polarity pathway regulates cell polarity, adhesion, and rearrangement. Its cytoplasmic core components Prickle (Pk) and Dishevelled (Dvl) often localize as dense puncta at cell membranes to form antagonizing complexes and establish cell asymmetry. In vertebrates, Pk and Dvl have been implicated in actomyosin cortex regulation, but the mechanism of how these proteins control cell mechanics is unclear. Here we demonstrate that in Xenopus prechordal mesoderm cells, diffusely distributed, cytoplasmic Pk1 up-regulates the F-actin content of the cortex. This counteracts cortex down-regulation by Dvl2. Both factors act upstream of casein kinase II to increase or decrease cortical tension. Thus, cortex modulation by Pk1 and Dvl2 is translated into mechanical force and affects cell migration and rearrangement during radial intercalation in the prechordal mesoderm. Pk1 also forms puncta and plaques, which are associated with localized depletion of cortical F-actin, suggesting opposite roles for diffuse and punctate Pk1.

Arrested coalescence of multicellular aggregates

Multicellular aggregates are known to exhibit liquid-like properties. The fusion process of two cell aggregates is commonly studied as the coalescence of two viscous drops. However, tissues are complex materials and can exhibit viscoelastic behaviour. It is known that elastic effects can prevent the complete fusion of two drops, a phenomenon known as arrested coalescence. Here we study this phenomenon in stem cell aggregates and provide a theoretical framework which agrees with the experiments. In addition, agent-based simulations show that active cell fluctuations can control a solid-to-fluid phase transition, revealing that arrested coalescence can be found in the vicinity of an unjamming transition. By analysing the dynamics of the fusion process and combining it with nanoindentation measurements, we obtain the effective viscosity, shear modulus and surface tension of the aggregates. More generally, our work provides a simple, fast and inexpensive method to characterize the mechanical properties of viscoelastic materials.

Characterization of convergent thickening, a major convergence force producing morphogenic movement in amphibians

The morphogenic process of convergent thickening (CT) was originally described as the mediolateral convergence and radial thickening of the explanted ventral involuting marginal zone (IMZ) of Xenopus gastrulae (Keller and Danilchik, 1988). Here, we show that CT is expressed in all sectors of the pre-involution IMZ, which transitions to expressing convergent extension (CE) after involution. CT occurs without CE and drives symmetric blastopore closure in ventralized embryos. Assays of tissue affinity and tissue surface tension measurements suggest CT is driven by increased interfacial tension between the deep IMZ and the overlying epithelium. The resulting minimization of deep IMZ surface area drives a tendency to shorten the mediolateral (circumblastoporal) aspect of the IMZ, thereby generating tensile force contributing to blastopore closure (Shook et al., 2018). These results establish CT as an independent force-generating process of evolutionary significance and provide the first clear example of an oriented, tensile force generated by an isotropic, Holtfreterian/Steinbergian tissue affinity change.

Mechanical control of tissue shape and morphogenetic flows during vertebrate body axis elongation

Shaping embryonic tissues into their functional morphologies requires cells to control the physical state of the tissue in space and time. While regional variations in cellular forces or cell proliferation have been typically assumed to be the main physical factors controlling tissue morphogenesis, recent experiments have revealed that spatial variations in the tissue physical (fluid/solid) state play a key role in shaping embryonic tissues. Here we theoretically study how the regional control of fluid and solid tissue states guides morphogenetic flows to shape the extending vertebrate body axis. Our results show that both the existence of a fluid-to-solid tissue transition along the anteroposterior axis and the tissue surface tension determine the shape of the tissue and its ability to elongate unidirectionally, with large tissue tensions preventing unidirectional elongation and promoting blob-like tissue expansions. We predict both the tissue morphogenetic flows and stresses that enable unidirectional axis elongation. Our results show the existence of a sharp transition in the structure of morphogenetic flows, from a flow with no vortices to a flow with two counter-rotating vortices, caused by a transition in the number and location of topological defects in the flow field. Finally, comparing the theoretical predictions to quantitative measurements of both tissue flows and shape during zebrafish body axis elongation, we show that the observed morphogenetic events can be explained by the existence of a fluid-to-solid tissue transition along the anteroposterior axis. These results highlight the role of spatiotemporally-controlled fluid-to-solid transitions in the tissue state as a physical mechanism of embryonic morphogenesis.

Capillarity and active cell movement at mesendoderm translocation in the Xenopus gastrula

During Xenopus gastrulation, leading edge mesendoderm (LEM) advances animally as a wedge-shaped cell mass over the vegetally moving blastocoel roof (BCR). We show that close contact across the BCR-LEM interface correlates with attenuated net advance of the LEM, which is pulled forward by tip cells while the remaining LEM frequently separates from the BCR. Nevertheless, lamellipodia persist on the detached LEM surface. They attach to adjacent LEM cells and depend on PDGF-A, cell-surface fibronectin and cadherin. We argue that active cell motility on the LEM surface prevents adverse capillary effects in the liquid LEM tissue as it moves by being pulled. It counters tissue surface-tension effects with oriented cell movement and bulges the LEM surface out to keep it close to the curved BCR without attaching to it. Proximity to the BCR is necessary, in turn, for the maintenance and orientation of lamellipodia that permit mass cell movement with minimal substratum contact. Together with a similar process in epithelial invagination, vertical telescoping, the cell movement at the LEM surface defines a novel type of cell rearrangement: vertical shearing.

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Ectoderm to mesoderm transition by down-regulation of actomyosin contractility

Collective migration of cohesive tissues is a fundamental process in morphogenesis and is particularly well illustrated during gastrulation by the rapid and massive internalization of the mesoderm, which contrasts with the much more modest movements of the ectoderm. In the Xenopus embryo, the differences in morphogenetic capabilities of ectoderm and mesoderm can be connected to the intrinsic motility of individual cells, very low for ectoderm, high for mesoderm. Surprisingly, we find that these seemingly deep differences can be accounted for simply by differences in Rho-kinases (Rock)-dependent actomyosin contractility. We show that Rock inhibition is sufficient to rapidly unleash motility in the ectoderm and confer it with mesoderm-like properties. In the mesoderm, this motility is dependent on two negative regulators of RhoA, the small GTPase Rnd1 and the RhoGAP Shirin/Dlc2/ArhGAP37. Both are absolutely essential for gastrulation. At the cellular and tissue level, the two regulators show overlapping yet distinct functions. They both contribute to decrease cortical tension and confer motility, but Shirin tends to increase tissue fluidity and stimulate dispersion, while Rnd1 tends to favor more compact collective migration. Thus, each is able to contribute to a specific property of the migratory behavior of the mesoderm. We propose that the "ectoderm to mesoderm transition" is a prototypic case of collective migration driven by a down-regulation of cellular tension, without the need for the complex changes traditionally associated with the epithelial-to-mesenchymal transition.

Cell Sorting in Hydra vulgaris Arises from Differing Capacities for Epithelialization between Cell Types

Hydra vulgaris exhibits a remarkable capacity to reassemble its body plan from a disordered aggregate of cells. Reassembly begins by sorting two epithelial cell types, endoderm and ectoderm, into inner and outer layers, respectively. The cellular features and behaviors that distinguish ectodermal and endodermal lineages to drive sorting have not been fully elucidated. To dissect this process, we use micromanipulation to position single cells of diverse lineages on the surface of defined multicellular aggregates and monitor sorting outcomes by live imaging. Although sorting has previously been attributed to intrinsic differences between the epithelial lineages, we find that single cells of all lineages sort to the interior of ectodermal aggregates, including single ectodermal cells. This reveals that cells of the same lineage can adopt opposing positions when sorting as individuals or a collective. Ectodermal cell collectives adopt their position at the aggregate exterior by rapidly reforming an epithelium that engulfs cells adhered to its surface through a collective spreading behavior. In contrast, aggregated endodermal cells persistently lose epithelial features. These non-epithelialized aggregates, like isolated cells of all lineages, are adherent passengers for engulfment by the ectodermal epithelium. We find that collective spreading of the ectoderm and persistent de-epithelialization in the endoderm also arise during local wounding in Hydra, suggesting that Hydra's wound-healing and self-organization capabilities may employ similar mechanisms. Together, our data suggest that differing propensities for epithelialization can sort cell types into distinct compartments to build and restore complex tissue architecture.

EpCAM as Modulator of Tissue Plasticity

The Epithelial Cell Adhesion Molecule or EpCAM is a well-known marker highly expressed in carcinomas and showing a strong correlation with poor cancer prognosis. While its name relates to its proposed function as a cell adhesion molecule, EpCAM has been shown to have various signalling functions. In particular, it has been identified as an important positive regulator of cell adhesion and migration, playing an essential role in embryonic morphogenesis as well as intestinal homeostasis. This activity is not due to its putative adhesive function, but rather to its ability to repress myosin contractility by impinging on a PKC signalling cascade. This mechanism confers EpCAM the unique property of favouring tissue plasticity. I review here the currently available data, comment on possible connections with other properties of EpCAM, and discuss the potential significance in the context of cancer invasion.

Tissue rheology in embryonic organization

Tissue morphogenesis in multicellular organisms is brought about by spatiotemporal coordination of mechanical and chemical signals. Extensive work on how mechanical forces together with the well-established morphogen signalling pathways can actively shape living tissues has revealed evolutionary conserved mechanochemical features of embryonic development. More recently, attention has been drawn to the description of tissue material properties and how they can influence certain morphogenetic processes. Interestingly, besides the role of tissue material properties in determining how much tissues deform in response to force application, there is increasing theoretical and experimental evidence, suggesting that tissue material properties can abruptly and drastically change in development. These changes resemble phase transitions, pointing at the intriguing possibility that important morphogenetic processes in development, such as symmetry breaking and self-organization, might be mediated by tissue phase transitions. In this review, we summarize recent findings on the regulation and role of tissue material properties in the context of the developing embryo. We posit that abrupt changes of tissue rheological properties may have important implications in maintaining the balance between robustness and adaptability during embryonic development.

Mechanics of Anteroposterior Axis Formation in Vertebrates

The vertebrate anteroposterior axis forms through elongation of multiple tissues during embryogenesis. This process is based on tissue-autonomous mechanisms of force generation and intertissue mechanical coupling whose failure leads to severe developmental anomalies such as body truncation and spina bifida. Similar to other morphogenetic modules, anteroposterior body extension requires both the rearrangement of existing materials-such as cells and extracellular matrix-and the local addition of new materials, i.e., anisotropic growth, through cell proliferation, cell growth, and matrix deposition. Numerous signaling pathways coordinate body axis formation via regulation of cell behavior during tissue rearrangements and/or volumetric growth. From a physical perspective, morphogenesis depends on both cell-generated forces and tissue material properties. As the spatiotemporal variation of these mechanical parameters has recently been explored in the context of vertebrate body elongation, the study of this process is likely to shed light on the cross talk between signaling and mechanics during morphogenesis.

Micro-indentation and optical coherence tomography for the mechanical characterization of embryos: Experimental setup and measurements on chicken embryos

The investigation of the mechanical properties of embryos is expected to provide valuable information on the phenomenology of morphogenesis. It is thus believed that, by mapping the viscoelastic features of an embryo at different stages of growth, it may be possible to shed light on the role of mechanics in embryonic development. To contribute to this field, we present a new instrument that can determine spatiotemporal distributions of mechanical properties of embryos over a wide area and with unprecedented accuracy. The method relies on combining ferrule-top micro-indentation, which provides local measurements of viscoelasticity, with Optical Coherence Tomography, which can reveal changes in tissue morphology and help the user identify the indentation point. To prove the working principle, we have collected viscoelasticity maps of fixed and live HH11-HH12 chicken embryos. Our study shows that the instrument can reveal correlations between tissue morphology and mechanical behavior. STATEMENT OF SIGNIFICANCE: Local mechanical properties of soft biological tissue play a crucial role in several biological processes, including cell differentiation, cell migration, and body formation; therefore, measuring tissue properties at high resolution is of great interest in biology and tissue engineering. To provide an efficient method for the biomechanical characterization of soft biological tissues, we introduce a new tool in which the combination of non-invasive Optical Coherence Tomography imaging and depth-controlled indentation measurements allows one to map the viscoelastic properties of biological tissue and investigate correlations between local mechanical features and tissue morphology with unprecedented resolution.

Using a continuum model to decipher the mechanics of embryonic tissue spreading from time-lapse image sequences: An approximate Bayesian computation approach

Advanced imaging techniques generate large datasets capable of describing the structure and kinematics of tissue spreading in embryonic development, wound healing, and the progression of many diseases. These datasets can be integrated with mathematical models to infer biomechanical properties of the system, typically identifying an optimal set of parameters for an individual experiment. However, these methods offer little information on the robustness of the fit and are generally ill-suited for statistical tests of multiple experiments. To overcome this limitation and enable efficient use of large datasets in a rigorous experimental design, we use the approximate Bayesian computation rejection algorithm to construct probability density distributions that estimate model parameters for a defined theoretical model and set of experimental data. Here, we demonstrate this method with a 2D Eulerian continuum mechanical model of spreading embryonic tissue. The model is tightly integrated with quantitative image analysis of different sized embryonic tissue explants spreading on extracellular matrix (ECM) and is regulated by a small set of parameters including forces on the free edge, tissue stiffness, strength of cell-ECM adhesions, and active cell shape changes. We find statistically significant trends in key parameters that vary with initial size of the explant, e.g., for larger explants cell-ECM adhesion forces are weaker and free edge forces are stronger. Furthermore, we demonstrate that estimated parameters for one explant can be used to predict the behavior of other similarly sized explants. These predictive methods can be used to guide further experiments to better understand how collective cell migration is regulated during development.

Physical control of tissue morphogenesis across scales

During embryogenesis, tissues and organs are progressively shaped into their functional morphologies. While the information about tissue and organ shape is encoded genetically, the sculpting of embryonic structures in the 3D space is ultimately a physical process. The control of physical quantities involved in tissue morphogenesis originates at cellular and subcellular scales, but it is their emergent behavior at supracellular scales that guides morphogenetic events. In this review, we highlight the physical quantities that can be spatiotemporally tuned at supracellular scales to sculpt tissues and organs during embryonic development of animal species, and connect them to the cellular and molecular mechanisms controlling them.

Multiscale analysis of architecture, cell size and the cell cortex reveals cortical F-actin density and composition are major contributors to mechanical properties during convergent extension

The large-scale movements that construct complex three-dimensional tissues during development are governed by universal physical principles. Fine-grained control of both mechanical properties and force production is crucial to the successful placement of tissues and shaping of organs. Embryos of the frog Xenopus laevis provide a dramatic example of these physical processes, as dorsal tissues increase in Young's modulus by six-fold to 80 Pascal over 8 h as germ layers and the central nervous system are formed. These physical changes coincide with emergence of complex anatomical structures, rounds of cell division, and cytoskeletal remodeling. To understand the contribution of these diverse structures, we adopt the cellular solids model to relate bulk stiffness of a solid foam to the unit size of individual cells, their microstructural organization, and their material properties. Our results indicate that large-scale tissue architecture and cell size are not likely to influence the bulk mechanical properties of early embryonic or progenitor tissues but that F-actin cortical density and composition of the F-actin cortex play major roles in regulating the physical mechanics of embryonic multicellular tissues.

Increasing complexity: Mechanical guidance and feedback loops as a basis for self-organization in morphogenesis

The article is devoted to physical views on embryo development as a combination of structurally stable dynamics and symmetry-breaking events in the general process of self-organization. The first corresponds to the deterministic aspect of embryo development. The second type of processes is associated with sudden increase of variability in the periods of symmetry-breaking, which manifests unstable dynamics. The biological basis under these considerations includes chemokinetics (a system of inductors, repressors, and interaction with their next surrounding) and morphomechanics (i.e. mechanotransduction, mechanosensing, and related feedback loops). Although the latter research area is evolving rapidly, up to this time the role of mechanical properties of embryonic tissues and mechano-dependent processes in them are integrated in the general picture of embryo development to a lesser extent than biochemical signaling. For this reason, the present article is mostly devoted to experimental data on morphomechanics in the process of embryo development, also including analysis of its limitations and possible contradictions. The general system of feedback-loops and system dynamics delineated in this review is in large part a repetition of the views of Lev Beloussov, who was one of the founders of the whole areas of morphomechanics and morphodynamics, and to whose memory this article is dedicated.

Stress-generating tissue deformations in Xenopus embryos: Long-range gradients and local cell displacements

BACKGROUND

Although the role of endogenous mechanical stresses in regulating morphogenetic movements and cell differentiation is now well established, many aspects of mechanical stress generation and transmission in developing embryos remain unclear and require quantitative studies.

RESULTS

By measuring stress-bearing linear deformations (caused by differences in cell movement rates) in the outer cell layer of blastula - early tail-bud Xenopus embryos, we revealed a set of long-term tension-generating gradients of cell movement rates, modulated by short-term cell-cell displacements much increasing the rates of local deformations. Experimental relaxation of tensions distorted the gradients but preserved and even enhanced local cell-cell displacements. During development, an incoherent mode of cell behavior, characterized by extensive cell-cell displacements and poorly correlated cell trajectories, was exchanged for a more coherent regime with the opposite characteristics. In particular, cell shifts became more synchronous and acquired a periodicity of several dozen minutes.

CONCLUSIONS

Morphogenetic movements in Xenopus embryos are mediated by mechanically stressed dynamic structures of two different levels: extended gradients and short-term cell-cell displacements. As development proceeds, the latter component decreases and cell trajectories become more correlated. In particular, they acquire common periodicities, making morphogenesis more coherent.

Large, long range tensile forces drive convergence during Xenopus blastopore closure and body axis elongation

Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms.

Cell volume changes contribute to epithelial morphogenesis in zebrafish Kupffer's vesicle

How epithelial cell behaviors are coordinately regulated to sculpt tissue architecture is a fundamental question in biology. Kupffer’s vesicle (KV), a transient organ with a fluid-filled lumen, provides a simple system to investigate the interplay between intrinsic cellular mechanisms and external forces during epithelial morphogenesis. Using 3-dimensional (3D) analyses of single cells we identify asymmetric cell volume changes along the anteroposterior axis of KV that coincide with asymmetric cell shape changes. Blocking ion flux prevents these cell volume changes and cell shape changes. Vertex simulations suggest cell shape changes do not depend on lumen expansion. Consistent with this prediction, asymmetric changes in KV cell volume and shape occur normally when KV lumen growth fails due to leaky cell adhesions. These results indicate ion flux mediates cell volume changes that contribute to asymmetric cell shape changes in KV, and that these changes in epithelial morphology are separable from lumen-generated forces.

Ingression-type cell migration drives vegetal endoderm internalisation in the Xenopus gastrula

During amphibian gastrulation, presumptive endoderm is internalised as part of vegetal rotation, a large-scale movement that encompasses the whole vegetal half of the embryo. It has been considered a gastrulation process unique to amphibians, but we show that at the cell level, endoderm internalisation exhibits characteristics reminiscent of bottle cell formation and ingression, known mechanisms of germ layer internalisation. During ingression proper, cells leave a single-layered epithelium. In vegetal rotation, the process occurs in a multilayered cell mass; we refer to it as ingression-type cell migration. Endoderm cells move by amoeboid shape changes, but in contrast to other instances of amoeboid migration, trailing edge retraction involves ephrinB1-dependent macropinocytosis and trans-endocytosis. Moreover, although cells are separated by wide gaps, they are connected by filiform protrusions, and their migration depends on C-cadherin and the matrix protein fibronectin. Cells move in the same direction but at different velocities, to rearrange by differential migration.

Evaluating biomechanical properties of murine embryos using Brillouin microscopy and optical coherence tomography

Embryogenesis is regulated by numerous changes in mechanical properties of the cellular microenvironment. Thus, studying embryonic mechanophysiology can provide a more thorough perspective of embryonic development, potentially improving early detection of congenital abnormalities as well as evaluating and developing therapeutic interventions. A number of methods and techniques have been used to study cellular biomechanical properties during embryogenesis. While some of these techniques are invasive or involve the use of external agents, others are compromised in terms of spatial and temporal resolutions. We propose the use of Brillouin microscopy in combination with optical coherence tomography (OCT) to measure stiffness as well as structural changes in a developing embryo. While Brillouin microscopy assesses the changes in stiffness among different organs of the embryo, OCT provides the necessary structural guidance.

Sorting at embryonic boundaries requires high heterotypic interfacial tension

The establishment of sharp boundaries is essential for segregation of embryonic tissues during development, but the underlying mechanism of cell sorting has remained unclear. Opposing hypotheses have been proposed, either based on global tissue adhesive or contractile properties or on local signalling through cell contact cues. Here we use ectoderm-mesoderm separation in Xenopus to directly evaluate the role of these various parameters. We find that ephrin-Eph-based repulsion is very effective at inducing and maintaining separation, whereas differences in adhesion or contractility have surprisingly little impact. Computer simulations support and generalise our experimental results, showing that a high heterotypic interfacial tension between tissues is key to their segregation. We propose a unifying model, in which conditions of sorting previously considered as driven by differential adhesion/tension should be viewed as suboptimal cases of heterotypic interfacial tension.The mechanisms that cause different cells to segregate into distinct tissues are unclear. Here the authors show in Xenopus that formation of a boundary between two tissues is driven by local tension along the interface rather than by global differences in adhesion or cortical contractility.

The Molecular Basis of Radial Intercalation during Tissue Spreading in Early Development

Radial intercalation is a fundamental process responsible for the thinning of multilayered tissues during large-scale morphogenesis; however, its molecular mechanism has remained elusive. Using amphibian epiboly, the thinning and spreading of the animal hemisphere during gastrulation, here we provide evidence that radial intercalation is driven by chemotaxis of cells toward the external layer of the tissue. This role of chemotaxis in tissue spreading and thinning is unlike its typical role associated with large-distance directional movement of cells. We identify the chemoattractant as the complement component C3a, a factor normally linked with the immune system. The mechanism is explored by computational modeling and tested in vivo, ex vivo, and in vitro. This mechanism is robust against fluctuations of chemoattractant levels and expression patterns and explains expansion during epiboly. This study provides insight into the fundamental process of radial intercalation and could be applied to a wide range of morphogenetic events.

Xenopus as a model for studies in mechanical stress and cell division

We exist in a physical world, and cells within biological tissues must respond appropriately to both environmental forces and forces generated within the tissue to ensure normal development and homeostasis. Cell division is required for normal tissue growth and maintenance, but both the direction and rate of cell division must be tightly controlled to avoid diseases of over-proliferation such as cancer. Recent studies have shown that mechanical cues can cause mitotic entry and orient the mitotic spindle, suggesting that physical force could play a role in patterning tissue growth. However, to fully understand how mechanics guides cells in vivo, it is necessary to assess the interaction of mechanical strain and cell division in a whole tissue context. In this mini-review we first summarise the body of work linking mechanics and cell division, before looking at the advantages that the Xenopus embryo can offer as a model organism for understanding: (1) the mechanical environment during embryogenesis, and (2) factors important for cell division. Finally, we introduce a novel method for applying a reproducible strain to Xenopus embryonic tissue and assessing subsequent cell divisions.

In vivo quantification of spatially varying mechanical properties in developing tissues

The mechanical properties of the cellular microenvironment and their spatiotemporal variations are thought to play a central role in sculpting embryonic tissues, maintaining organ architecture and controlling cell behavior, including cell differentiation. However, no direct in vivo and in situ measurement of mechanical properties within developing 3D tissues and organs has yet been performed. Here we introduce a technique that employs biocompatible, magnetically responsive ferrofluid microdroplets as local mechanical actuators and allows quantitative spatiotemporal measurements of mechanical properties in vivo. Using this technique, we show that vertebrate body elongation entails spatially varying tissue mechanics along the anteroposterior axis. Specifically, we find that the zebrafish tailbud is viscoelastic (elastic below a few seconds and fluid after just 1 min) and displays decreasing stiffness and increasing fluidity toward its posterior elongating region. This method opens new avenues to study mechanobiology in vivo, both in embryogenesis and in disease processes, including cancer.

Polarized cortical tension drives zebrafish epiboly movements

The principles underlying the biomechanics of morphogenesis are largely unknown. Epiboly is an essential embryonic event in which three tissues coordinate to direct the expansion of the blastoderm. How and where forces are generated during epiboly, and how these are globally coupled remains elusive. Here we developed a method, hydrodynamic regression (HR), to infer 3D pressure fields, mechanical power, and cortical surface tension profiles. HR is based on velocity measurements retrieved from 2D+T microscopy and their hydrodynamic modeling. We applied HR to identify biomechanically active structures and changes in cortex local tension during epiboly in zebrafish. Based on our results, we propose a novel physical description for epiboly, where tissue movements are directed by a polarized gradient of cortical tension. We found that this gradient relies on local contractile forces at the cortex, differences in elastic properties between cortex components and the passive transmission of forces within the yolk cell. All in all, our work identifies a novel way to physically regulate concerted cellular movements that might be instrumental for the mechanical control of many morphogenetic processes.

A toolbox to explore the mechanics of living embryonic tissues

The sculpting of embryonic tissues and organs into their functional morphologies involves the spatial and temporal regulation of mechanics at cell and tissue scales. Decades of in vitro work, complemented by some in vivo studies, have shown the relevance of mechanical cues in the control of cell behaviors that are central to developmental processes, but the lack of methodologies enabling precise, quantitative measurements of mechanical cues in vivo have hindered our understanding of the role of mechanics in embryonic development. Several methodologies are starting to enable quantitative studies of mechanics in vivo and in situ, opening new avenues to explore how mechanics contributes to shaping embryonic tissues and how it affects cell behavior within developing embryos. Here we review the present methodologies to study the role of mechanics in living embryonic tissues, considering their strengths and drawbacks as well as the conditions in which they are most suitable.

Quantitative Morphology of Epithelial Folds

The shape of spatially modulated epithelial morphologies such as villi and crypts is usually associated with the epithelium-stroma area mismatch leading to buckling. We propose an alternative mechanical model based on intraepithelial stresses generated by differential tensions of apical, lateral, and basal sides of cells as well as on the elasticity of the basement membrane. We use it to theoretically study longitudinal folds in simple epithelia and we identify four types of corrugated morphologies: compact, invaginated, evaginated, and wavy. The obtained tissue contours and thickness profiles are compared to epithelial folds observed in invertebrates and vertebrates, and for most samples, the agreement is within the estimated experimental error. Our model establishes the groove-crest modulation of tissue thickness as a morphometric parameter that can, together with the curvature profile, be used to estimate the relative differential apicobasal tension in the epithelium.

Modulation of apical constriction by Wnt signaling is required for lung epithelial shape transition

In lung development the apically constricted columnar epithelium forms numerous buds during the pseudoglandular stage and subsequently changes the shape into flat or cuboidal pneumocytes that compose the air sacs during the canalicular and saccular (canalicular-saccular) stages, yet the impact of cell shapes on tissue morphogenesis remains unclear. The expression of Wnt components were decreased in the canalicular-saccular stages, and genetically constitutive activation of Wnt signaling impaired air sac formation by inducing apical constriction in the epithelium as seen in the pseudoglandular stage. Organ culture models also demonstrated that Wnt signaling induces apical constriction through the apical actomyosin cytoskeletal organization. Mathematical modeling revealed that apical constriction induces bud formation and loss of apical constriction is required for the formation of an air sac-like structure. MAP/Microtubule affinity-regulating kinase (MARK1) was identified as a downstream molecule of Wnt signaling and required for the apical cytoskeletal organization and bud formation. These results suggest that Wnt signaling is required for bud formation by inducing apical constriction during the pseudoglandular stage, while loss of Wnt signaling is for air sac formation in the canalicular-saccular stages.

Mechanical Forces and Growth in Animal Tissues

Mechanical forces shape biological tissues. They are the effectors of the developmental programs that orchestrate morphogenesis. A lot of effort has been devoted to understanding morphogenetic processes in mechanical terms. In this review, we focus on the interplay between tissue mechanics and growth. We first describe how tissue mechanics affects growth, by influencing the orientation of cell divisions and the signaling pathways that control the rate of volume increase and proliferation. We then address how the mechanical state of a tissue is affected by the patterns of growth. The forward and reverse interactions between growth and mechanics must be investigated in an integrative way if we want to understand how tissues grow and shape themselves. To illustrate this point, we describe examples in which growth homeostasis is achieved by feedback mechanisms that use mechanical forces.

PAPC mediates self/non-self-distinction during Snail1-dependent tissue separation

In Xenopus and zebrafish gastrulae, PAPC attenuates planar cell polarity signaling and controls formation of an adhesive, yet flexible, contact at the ectoderm–mesoderm boundary.

Cleft-like boundaries represent a type of cell sorting boundary characterized by the presence of a physical gap between tissues. We studied the cleft-like ectoderm–mesoderm boundary in Xenopus laevis and zebrafish gastrulae. We identified the transcription factor Snail1 as being essential for tissue separation, showed that its expression in the mesoderm depends on noncanonical Wnt signaling, and demonstrated that it enables paraxial protocadherin (PAPC) to promote tissue separation through two novel functions. First, PAPC attenuates planar cell polarity signaling at the ectoderm–mesoderm boundary to lower cell adhesion and facilitate cleft formation. Second, PAPC controls formation of a distinct type of adhesive contact between mesoderm and ectoderm cells that shows properties of a cleft-like boundary at the single-cell level. It consists of short stretches of adherens junction–like contacts inserted between intermediate-sized contacts and large intercellular gaps. These roles of PAPC constitute a self/non–self-recognition mechanism that determines the site of boundary formation at the interface between PAPC-expressing and -nonexpressing cells.

Force production and mechanical accommodation during convergent extension

Forces generated within the embryo during convergent extension (CE) must overcome mechanical resistance to push the head away from the rear. As mechanical resistance increases more than eightfold during CE and can vary twofold from individual to individual, we have proposed that developmental programs must include mechanical accommodation in order to maintain robust morphogenesis. To test this idea and investigate the processes that generate forces within early embryos, we developed a novel gel-based sensor to report force production as a tissue changes shape; we find that the mean stress produced by CE is 5.0±1.6 Pascal (Pa). Experiments with the gel-based force sensor resulted in three findings. (1) Force production and mechanical resistance can be coupled through myosin contractility. The coupling of these processes can be hidden unless affected tissues are challenged by physical constraints. (2) CE is mechanically adaptive; dorsal tissues can increase force production up to threefold to overcome a stiffer microenvironment. These findings demonstrate that mechanical accommodation can ensure robust morphogenetic movements against environmental and genetic variation that might otherwise perturb development and growth. (3) Force production is distributed between neural and mesodermal tissues in the dorsal isolate, and the notochord, a central structure involved in patterning vertebrate morphogenesis, is not required for force production during late gastrulation and early neurulation. Our findings suggest that genetic factors that coordinately alter force production and mechanical resistance are common during morphogenesis, and that their cryptic roles can be revealed when tissues are challenged by controlled biophysical constraints.

The cellular basis of tissue separation

The subdivision of the embryo into physically distinct regions is one of the most fundamental processes in development. General hypotheses for tissue separation based on differential adhesion or tension have been proposed in the past, but with little experimental support. During the last decade, the field has experienced a strong revival, largely driven by renewed interest in biophysical modeling of development. Here, I will discuss the various models of boundary formation and summarize recent studies that have shifted our understanding of the process from the simple juxtaposition of global tissue properties to the characterization of local cellular reactions. Current evidence favors a model whereby separation is controlled by cell surface cues, which, upon cell-cell contact, generate acute changes in cytoskeletal and adhesive properties to inhibit cell mixing, and whereby the integration of multiple local cues may dictate both the global morphogenetic properties of a tissue and its separation from adjacent cell populations.

Variable combinations of specific ephrin ligand/Eph receptor pairs control embryonic tissue separation

Vertebrate embryonic cells recognize self from non-self, thus restricting repulsion at tissue boundaries, through a combination of multiple ephrins and Eph receptors, simply based on binding selectivity and asymmetric expression.

Ephrins and Eph receptors are involved in the establishment of vertebrate tissue boundaries. The complexity of the system is puzzling, however in many instances, tissues express multiple ephrins and Ephs on both sides of the boundary, a situation that should in principle cause repulsion between cells within each tissue. Although co-expression of ephrins and Eph receptors is widespread in embryonic tissues, neurons, and cancer cells, it is still unresolved how the respective signals are integrated into a coherent output. We present a simple explanation for the confinement of repulsion to the tissue interface: Using the dorsal ectoderm–mesoderm boundary of the Xenopus embryo as a model, we identify selective functional interactions between ephrin–Eph pairs that are expressed in partial complementary patterns. The combined repulsive signals add up to be strongest across the boundary, where they reach sufficient intensity to trigger cell detachments. The process can be largely explained using a simple model based exclusively on relative ephrin and Eph concentrations and binding affinities. We generalize these findings for the ventral ectoderm–mesoderm boundary and the notochord boundary, both of which appear to function on the same principles. These results provide a paradigm for how developmental systems may integrate multiple cues to generate discrete local outcomes.

How embryonic tissues separate from each other to shape the developing organism is a fundamental question in developmental biology. In vertebrates, this process relies on local repulsive reactions specifically generated at contacts between cells of different types. These reactions are triggered by typical repulsive cell surface cues, the ephrin ligands, and Eph receptors. However, the expression of multiple ephrins and the Eph receptors by each cell type represents a puzzle: Why is repulsion observed only at the tissue interface and not within the tissue itself? By studying three cases of separation in the early amphibian embryo, we uncover a surprisingly simple logic underlying this phenomenon, which can be explained by the selectivity of ligand–receptor interactions and by their asymmetric distribution. The system is set such that, despite generalized interactions throughout the tissues, it is only at contacts between different cell types that the overall repulsive output is sufficiently strong to overcome cell–cell adhesion. Our study may serve as paradigm for how systematic dissection of complex cellular systems can reduce them to simple laws and make them intelligible.

Tissue cohesion and the mechanics of cell rearrangement

Morphogenetic processes often involve the rapid rearrangement of cells held together by mutual adhesion. The dynamic nature of this adhesion endows tissues with liquid-like properties, such that large-scale shape changes appear as tissue flows. Generally, the resistance to flow (tissue viscosity) is expected to depend on the cohesion of a tissue (how strongly its cells adhere to each other), but the exact relationship between these parameters is not known. Here, we analyse the link between cohesion and viscosity to uncover basic mechanical principles of cell rearrangement. We show that for vertebrate and invertebrate tissues, viscosity varies in proportion to cohesion over a 200-fold range of values. We demonstrate that this proportionality is predicted by a cell-based model of tissue viscosity. To do so, we analyse cell adhesion in Xenopus embryonic tissues and determine a number of parameters, including tissue surface tension (as a measure of cohesion), cell contact fluctuation and cortical tension. In the tissues studied, the ratio of surface tension to viscosity, which has the dimension of a velocity, is 1.8 µm/min. This characteristic velocity reflects the rate of cell-cell boundary contraction during rearrangement, and sets a limit to rearrangement rates. Moreover, we propose that, in these tissues, cell movement is maximally efficient. Our approach to cell rearrangement mechanics links adhesion to the resistance of a tissue to plastic deformation, identifies the characteristic velocity of the process, and provides a basis for the comparison of tissues with mechanical properties that may vary by orders of magnitude.

Biomechanics and the thermotolerance of development

Successful completion of development requires coordination of patterning events with morphogenetic movements. Environmental variability challenges this coordination. For example, developing organisms encounter varying environmental temperatures that can strongly influence developmental rates. We hypothesized that the mechanics of morphogenesis would have to be finely adjusted to allow for normal morphogenesis across a wide range of developmental rates. We formulated our hypothesis as a simple model incorporating time-dependent application of force to a viscoelastic tissue. This model suggested that the capacity to maintain normal morphogenesis across a range of temperatures would depend on how both tissue viscoelasticity and the forces that drive deformation vary with temperature. To test this model we investigated how the mechanical behavior of embryonic tissue (Xenopus laevis) changed with temperature; we used a combination of micropipette aspiration to measure viscoelasticity, electrically induced contractions to measure cellular force generation, and confocal microscopy to measure endogenous contractility. Contrary to expectations, the viscoelasticity of the tissues and peak contractile tension proved invariant with temperature even as rates of force generation and gastrulation movements varied three-fold. Furthermore, the relative rates of different gastrulation movements varied with temperature: the speed of blastopore closure increased more slowly with temperature than the speed of the dorsal-to-ventral progression of involution. The changes in the relative rates of different tissue movements can be explained by the viscoelastic deformation model given observed viscoelastic properties, but only if morphogenetic forces increase slowly rather than all at once.

Ephrin-Eph signaling in embryonic tissue separation

The physical separation of the embryonic regions that give rise to the tissues and organs of multicellular organisms is a fundamental aspect of morphogenesis. Pioneer experiments by Holtfreter had shown that embryonic cells can sort based on "tissue affinities," which have long been considered to rely on differences in cell-cell adhesion. However, vertebrate embryonic tissues also express a variety of cell surface cues, in particular ephrins and Eph receptors, and there is now firm evidence that these molecules are systematically used to induce local repulsion at contacts between different cell types, efficiently preventing mixing of adjacent cell populations.

Soft matter models of developing tissues and tumors

Analogies with inert soft condensed matter--such as viscoelastic liquids, pastes, foams, emulsions, colloids, and polymers--can be used to investigate the mechanical response of soft biological tissues to forces. A variety of experimental techniques and biophysical models have exploited these analogies allowing the quantitative characterization of the mechanical properties of model tissues, such as surface tension, elasticity, and viscosity. The framework of soft matter has been successful in explaining a number of dynamical tissue behaviors observed in physiology and development, such as cell sorting, tissue spreading, or the escape of individual cells from a tumor. However, living tissues also exhibit active responses, such as rigidity sensing or cell pulsation, that are absent in inert soft materials. The soft matter models reviewed here have provided valuable insight in understanding morphogenesis and cancer invasion and have set bases for using tissue engineering within medicine.

Cadherin-dependent mechanotransduction depends on ligand identity but not affinity

This study investigates the relationship between classical cadherin binding affinities and mechanotransduction through cadherin-mediated adhesions. The mechanical properties of cadherin-dependent intercellular junctions are generally attributed to differences in the binding affinities of classical cadherin subtypes that contribute to cohesive energies between cells. However, cell mechanics and mechanotransduction may also regulate intercellular contacts. We used micropipette measurements to quantify the two-dimensional affinities of cadherins at the cell surface, and two complementary mechanical measurements to assess ligand-dependent mechanotransduction through cadherin adhesions. At the cell surface, the classical cadherins investigated in this study form both homophilic and heterophilic bonds with two-dimensional affinities that differ by less than threefold. In contrast, mechanotransduction through cadherin adhesions is strongly ligand dependent such that homophilic, but not heterophilic ligation mediates mechanotransduction, independent of the cadherin binding affinity. These findings suggest that ligand-selective mechanotransduction may supersede differences in cadherin binding affinities in regulating intercellular contacts.

Active reinforcement of externally imposed folding in amphibians embryonic tissues

Although the folding of epithelial layers is one of the most common morphogenetic events, the underlying mechanisms of this process are still poorly understood. We aimed to determine whether an artificial bending of an embryonic cell sheet, which normally remains flat, is reinforced and stabilized by intrinsic cell transformations. We observed both reinforcement and stabilization in double explants of blastocoel roof tissue from Xenopus early gastrula embryos. The reinforcement of artificial bending occurred over the course of a few hours and was driven by the gradual apical constriction and radial elongation of previously compressed cells situated at the bending arch of the concave layer of explant. Apical constriction was associated with actomyosin contraction and endocytosis-mediated engulfing of the apical cell membranes. Cooperative apical constrictions of the concave layer of cells produced a tensile force that extended over the entire surface of the explant and correlated with apical contraction of the concave side cells. In the explants taken from the anterior regions of the embryo, this reinforcement was more stable and the bending better expressed than in those taken from suprablastoporal areas. The morphogenetic role of cell responses to the bending force is discussed.

Cadherin-dependent differential cell adhesion in Xenopus causes cell sorting in vitro but not in the embryo

Adhesion differences between cell populations are in principle a source of strong morphogenetic forces promoting cell sorting, boundary formation and tissue positioning, and cadherins are main mediators of cell adhesion. However, a direct link between cadherin expression, differential adhesion and morphogenesis has not yet been determined for a specific process in vivo. To identify such a connection, we modulated the expression of C-cadherin in the Xenopus laevis gastrula, and combined this with direct measurements of cell adhesion-related parameters. Our results show that gastrulation is surprisingly tolerant of overall changes in adhesion. Also, as expected, experimentally generated, cadherin-based adhesion differences promote cell sorting in vitro. Importantly, however, such differences do not lead to the sorting of cells in the embryo, showing that differential adhesion is not sufficient to drive morphogenesis in this system. Compensatory recruitment of cadherin protein to contacts between cadherin-deprived and -overexpressing cells could contribute to the prevention of sorting in vivo.

Cadherin function during Xenopus gastrulation

Xenopus gastrulation consists of the orderly deformation of a single, multilayered cell sheet that resembles a multilayered epithelium, and flexible cell-cell adhesion has to provide tissue cohesion while allowing for cell rearrangements that drive gastrulation. A few classic cadherins are expressed in the Xenopus early embryo. The prominent C-cadherin is essential for the cohesion of the animal part of the gastrula including ectoderm and chordamesoderm, and it contributes to the adhesion of endoderm and anterior mesoderm in the vegetal moiety. The cadherin/catenin complex is expressed in a graded pattern which is stable during early development. Regional differences in cell adhesion conform to the graded cadherin/catenin expression pattern. However, although the cadherin/catenin pattern seems to be actively maintained, and cadherin function is modulated to reinforce differential adhesiveness, it is not clear how regional differences in tissue cohesion affect gastrulation. Manipulating cadherin expression or function does not induce cell sorting or boundary formation in the embryo. Moreover, known boundary formation mechanisms in the gastrula are based on active cell repulsion. Cell rearrangement is also compatible with variable tissue cohesion. Thus, identifying roles for differential adhesion in the Xenopus gastrula remains a challenge.

A material's point of view on recent developments of polymeric biomaterials: control of mechanical and biochemical properties

Cells respond to a variety of stimuli, including biochemical, topographical and mechanical signals originating from their micro-environment. Cell responses to the mechanical properties of their substrates have been increasingly studied for about 14 years. To this end, several types of materials based on synthetic and natural polymers have been developed. Presentation of biochemical ligands to the cells is also important to provide additional functionalities or more selectivity in the details of cell/material interaction. In this review article, we will emphasize the development of synthetic and natural polymeric materials with well-characterized and tunable mechanical properties. We will also highlight how biochemical signals can be presented to the cells by combining them with these biomaterials. Such developments in materials science are not only important for fundamental biophysical studies on cell/material interactions but also for the design of a new generation of advanced and highly functional biomaterials.