Mechanical Coupling between Endoderm Invagination and Axis Extension in Drosophila

Confinement promotes nematic alignment of spindle-shaped cells during Drosophila embryogenesis

Tissue morphogenesis is often controlled by actomyosin networks pulling on adherens junctions (AJs), but junctional myosin levels vary. At an extreme, the Drosophila embryo amnioserosa forms a horseshoe-shaped strip of aligned, spindle-shaped cells lacking junctional myosin. What are the bases of amnioserosal cell interactions and alignment? Compared with surrounding tissue, we find that amnioserosal AJ continuity has lesser dependence on α-catenin, the mediator of AJ-actomyosin association, and greater dependence on Bazooka/Par-3, a junction-associated scaffold protein. Microtubule bundles also run along amnioserosal AJs and support their long-range curvilinearity. Amnioserosal confinement is apparent from partial overlap of its spindle-shaped cells, its outward bulging from surrounding tissue and from compressive stress detected within the amnioserosa. Genetic manipulations that alter amnioserosal confinement by surrounding tissue also result in amnioserosal cells losing alignment and gaining topological defects characteristic of nematically ordered systems. With Bazooka depletion, confinement by surrounding tissue appears to be relatively normal and amnioserosal cells align despite their AJ fragmentation. Overall, the fully elongated amnioserosa appears to form through tissue-autonomous generation of spindle-shaped cells that nematically align in response to confinement by surrounding tissue.

E-cadherin tunes tissue mechanical behavior before and during morphogenetic tissue flows

Adhesion between epithelial cells enables the remarkable mechanical behavior of epithelial tissues during morphogenesis. However, it remains unclear how cell-cell adhesion influences mechanics in static as well as in dynamically flowing epithelial tissues. Here, we systematically modulate E-cadherin-mediated adhesion in the Drosophila embryo and study the effects on the mechanical behavior of the germband epithelium before and during dramatic tissue remodeling and flow associated with body axis elongation. Before axis elongation, we find that increasing E-cadherin levels produces tissue comprising more elongated cells and predicted to be more fluid-like, providing reduced resistance to tissue flow. During axis elongation, we find that the dominant effect of E-cadherin is tuning the speed at which cells proceed through rearrangement events, revealing potential roles for E-cadherin in generating friction between cells. Before and during axis elongation, E-cadherin levels influence patterns of actomyosin-dependent forces, supporting the notion that E-cadherin tunes tissue mechanics in part through effects on actomyosin. Taken together, these findings reveal dual-and sometimes opposing-roles for E-cadherin-mediated adhesion in controlling tissue structure and dynamics in vivo that result in unexpected relationships between adhesion and flow.

Polarised cell intercalation during Drosophila axis extension is robust to an orthogonal pull by the invaginating mesoderm

As tissues grow and change shape during animal development, they physically pull and push on each other, and these mechanical interactions can be important for morphogenesis. During Drosophila gastrulation, mesoderm invagination temporally overlaps with the convergence and extension of the ectodermal germband; the latter is caused primarily by Myosin II-driven polarised cell intercalation. Here, we investigate the impact of mesoderm invagination on ectoderm extension, examining possible mechanical and mechanotransductive effects on Myosin II recruitment and polarised cell intercalation. We find that the germband ectoderm is deformed by the mesoderm pulling in the orthogonal direction to germband extension (GBE), showing mechanical coupling between these tissues. However, we do not find a significant change in Myosin II planar polarisation in response to mesoderm invagination, nor in the rate of junction shrinkage leading to neighbour exchange events. We conclude that the main cellular mechanism of axis extension, polarised cell intercalation, is robust to the mesoderm invagination pull. We find, however, that mesoderm invagination slows down the rate of anterior-posterior cell elongation that contributes to axis extension, counteracting the tension from the endoderm invagination, which pulls along the direction of GBE.

Signatures of structural disorder in developing epithelial tissues

Epithelial cells generate functional tissues in developing embryos through collective movements and shape changes. In some morphogenetic events, a tissue dramatically reorganizes its internal structure - often generating high degrees of structural disorder - to accomplish changes in tissue shape. However, the origins of structural disorder in epithelia and what roles it might play in morphogenesis are poorly understood. We study this question in the Drosophila germband epithelium, which undergoes dramatic changes in internal structure as cell rearrangements drive elongation of the embryo body axis. Using two order parameters that quantify volumetric and shear disorder, we show that structural disorder increases during body axis elongation and is strongly linked with specific developmental processes. Both disorder metrics begin to increase around the onset of axis elongation, but then plateau at values that are maintained throughout the process. Notably, the disorder plateau values for volumetric disorder are similar to those for random cell packings, suggesting this may reflect a limit on tissue behavior. In mutant embryos with disrupted external stresses from the ventral furrow, both disorder metrics reach wild-type maximum disorder values with a delay, correlating with delays in cell rearrangements. In contrast, in mutants with disrupted internal stresses and cell rearrangements, volumetric disorder is reduced compared to wild type, whereas shear disorder depends on specific external stress patterns. Together, these findings demonstrate that internal and external stresses both contribute to epithelial tissue disorder and suggest that the maximum values of disorder in a developing tissue reflect physical or biological limits on morphogenesis.

EGFR-dependent actomyosin patterning coordinates morphogenetic movements between tissues

The movements that give rise to the body's structure are powered by cell shape changes and rearrangements that are coordinated at supracellular scales. How such cellular coordination arises and integrates different morphogenetic programs is unclear. Using quantitative imaging, we found a complex pattern of adherens junction (AJ) levels in the ectoderm prior to gastrulation onset in Drosophila. AJ intensity exhibited a double-sided gradient, with peaks at the dorsal midline and ventral neuroectoderm. We show that this dorsal-ventral AJ pattern is regulated by epidermal growth factor (EGF) signaling and that this signal is required for ectoderm cell movement during mesoderm invagination and axis extension. We identify AJ levels and junctional actomyosin as downstream effectors of EGFR signaling. Overall, our study demonstrates a mechanism of coordination between tissue folding and convergent extension that facilitates embryo-wide gastrulation movements.

Serotonin signaling regulates actomyosin contractility during morphogenesis in evolutionarily divergent lineages

Serotonin is a neurotransmitter that signals through 5-HT receptors to control key functions in the nervous system. Serotonin receptors are also ubiquitously expressed in various organs and have been detected in embryos of different organisms. Potential morphogenetic functions of serotonin signaling have been proposed based on pharmacological studies but a mechanistic understanding is still lacking. Here, we uncover a role of serotonin signaling in axis extension of Drosophila embryos by regulating Myosin II (MyoII) activation, cell contractility and cell intercalation. We find that serotonin and serotonin receptors 5HT2A and 5HT2B form a signaling module that quantitatively regulates the amplitude of planar polarized MyoII contractility specified by Toll receptors and the GPCR Cirl. Remarkably, serotonin signaling also regulates actomyosin contractility at cell junctions, cellular flows and epiblast morphogenesis during chicken gastrulation. This phylogenetically conserved mechanical function of serotonin signaling in regulating actomyosin contractility and tissue flow reveals an ancestral role in morphogenesis of multicellular organisms.

Maintaining symmetry during body axis elongation

Bilateral symmetry defines much of the animal kingdom and is crucial for numerous functions of bilaterian organisms. Genetic approaches have discovered highly conserved patterning networks that establish bilateral symmetry in early embryos,1 but how this symmetry is maintained throughout subsequent morphogenetic events remains largely unknown.2 Here we show that the terminal patterning system-which relies on Ras/ERK signaling through activation of the Torso receptor by its ligand Trunk3-is critical for preserving bilateral symmetry during Drosophila body axis elongation, a process driven by cell rearrangements in the two identical lateral regions of the embryo and specified by the dorsal-ventral and anterior-posterior patterning systems.4 We demonstrate that fluctuating asymmetries in this rapid convergent-extension process are attenuated in normal embryos over time, possibly through noise-dissipating forces from the posterior midgut invagination and movement. However, when Torso signaling is attenuated via mutation of Trunk or RNAi directed against downstream Ras/ERK pathway components, body axis elongation results in a characteristic corkscrew phenotype,5 which reflects dramatic reorganization of global tissue flow and is incompatible with viability. Our results reveal a new function downstream of the Drosophila terminal patterning system in potentially active control of bilateral symmetry and should motivate systematic search for similar symmetry-preserving regulatory mechanisms in other bilaterians.

Curvature gradient drives polarized tissue flow in the Drosophila embryo

Tissue flow during morphogenesis is commonly driven by local constriction of cell cortices, which is caused by the activation of actomyosin contractility. This can lead to long-range flows due to tissue viscosity. However, in the absence of cell-intrinsic polarized forces or polarity in forces external to the tissue, these flows must be symmetric and centered around the region of contraction. Polarized tissue flows have been previously demonstrated to arise from the coupling of such contractile flows to points of increased friction or adhesion to external structures. However, we show with experiments and modeling that the onset of polarized tissue flow in early Drosophila morphogenesis occurs independent of adhesion and is instead driven by a geometric coupling of apical actomyosin contractility to tissue curvature. Particularly, the onset of polarized flow is driven by a mismatch between the position of apical myosin activation and the position of peak curvature at the posterior pole of the embryo. Our work demonstrates how genetic and geometric information inherited from the mother interact to create polarized flow during embryo morphogenesis.

How dynamic prestress governs the shape of living systems, from the subcellular to tissue scale

Cells and tissues change shape both to carry out their function and during pathology. In most cases, these deformations are driven from within the systems themselves. This is permitted by a range of molecular actors, such as active crosslinkers and ion pumps, whose activity is biologically controlled in space and time. The resulting stresses are propagated within complex and dynamical architectures like networks or cell aggregates. From a mechanical point of view, these effects can be seen as the generation of prestress or prestrain, resulting from either a contractile or growth activity. In this review, we present this concept of prestress and the theoretical tools available to conceptualize the statics and dynamics of living systems. We then describe a range of phenomena where prestress controls shape changes in biopolymer networks (especially the actomyosin cytoskeleton and fibrous tissues) and cellularized tissues. Despite the diversity of scale and organization, we demonstrate that these phenomena stem from a limited number of spatial distributions of prestress, which can be categorized as heterogeneous, anisotropic or differential. We suggest that in addition to growth and contraction, a third type of prestress-topological prestress-can result from active processes altering the microstructure of tissue.

Patterned mechanical feedback establishes a global myosin gradient

Morphogenesis, the coordinated execution of developmental programs that shape embryos, raises many fundamental questions at the interface between physics and biology. In particular, how the dynamics of active cytoskeletal processes are coordinated across the surface of entire embryos to generate global cell flows is poorly understood. Two distinct regulatory principles have been identified: genetic programs and dynamic response to mechanical stimuli. Despite progress, disentangling these two contributions remains challenging. Here, we combine in toto light sheet microscopy with genetic and optogenetic perturbations of tissue mechanics to examine theoretically predicted dynamic recruitment of non-muscle myosin II to cell junctions during Drosophila embryogenesis. We find dynamic recruitment has a long-range impact on global myosin configuration, and the rate of junction deformation sets the rate of myosin recruitment. Mathematical modeling and high frequency analysis reveal myosin fluctuations on junctions around a mean value set by mechanical feedback. Our model accounts for the early establishment of the global myosin pattern at 80% fidelity. Taken together our results indicate spatially modulated mechanical feedback as a key regulatory input in the establishment of long-range gradients of cytoskeletal configurations and global tissue flow patterns.

Different temporal requirements for tartan and wingless in the formation of contractile interfaces at compartmental boundaries

Compartmental boundaries physically separate developing tissues into distinct regions, which is fundamental for the organisation of the body plan in both insects and vertebrates. In many examples, this physical segregation is caused by a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. We performed an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, from the start of germband extension at gastrulation and throughout the germband extended stages (stages 6 to 11). First, we find that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared with wild-type and ftz mutant embryos, we show that Tartan is required during germband extension but not beyond. We identify candidate genes that could take over from Tartan at PSBs and confirm that at germband extended stages, actomyosin enrichment at PSBs requires Wingless signalling.

Engineered kinases as a tool for phosphorylation of selected targets in vivo

Reversible protein phosphorylation by kinases controls a plethora of processes essential for the proper development and homeostasis of multicellular organisms. One main obstacle in studying the role of a defined kinase–substrate interaction is that kinases form complex signaling networks and most often phosphorylate multiple substrates involved in various cellular processes. In recent years, several new approaches have been developed to control the activity of a given kinase. However, most of them fail to regulate a single protein target, likely hiding the effect of a unique kinase–substrate interaction by pleiotropic effects. To overcome this limitation, we have created protein binder-based engineered kinases that permit a direct, robust, and tissue-specific phosphorylation of fluorescent fusion proteins in vivo. We show the detailed characterization of two engineered kinases based on Rho-associated protein kinase (ROCK) and Src. Expression of synthetic kinases in the developing fly embryo resulted in phosphorylation of their respective GFP-fusion targets, providing for the first time a means to direct the phosphorylation to a chosen and tagged target in vivo. We presume that after careful optimization, the novel approach we describe here can be adapted to other kinases and targets in various eukaryotic genetic systems to regulate specific downstream effectors.

Morphogenetic forces planar polarize LGN/Pins in the embryonic head during Drosophila gastrulation

Spindle orientation is often achieved by a complex of Partner of Inscuteable (Pins)/LGN, Mushroom Body Defect (Mud)/Nuclear Mitotic Apparatus (NuMa), Gαi, and Dynein, which interacts with astral microtubules to rotate the spindle. Cortical Pins/LGN recruitment serves as a critical step in this process. Here, we identify Pins-mediated planar cell polarized divisions in several of the mitotic domains of the early Drosophila embryo. We found that neither planar cell polarity pathways nor planar polarized myosin localization determined division orientation; instead, our findings strongly suggest that Pins planar polarity and force generated from mesoderm invagination are important. Disrupting Pins polarity via overexpression of a myristoylated version of Pins caused randomized division angles. We found that disrupting forces through chemical inhibitors, depletion of an adherens junction protein, or blocking mesoderm invagination disrupted Pins planar polarity and spindle orientation. Furthermore, directional ablations that separated mesoderm from mitotic domains disrupted spindle orientation, suggesting that forces transmitted from mesoderm to mitotic domains can polarize Pins and orient division during gastrulation. To our knowledge, this is the first in vivo example where mechanical force has been shown to polarize Pins to mediate division orientation.

In toto light sheet fluorescence microscopy live imaging datasets of Ceratitis capitata embryonic development

The Mediterranean fruit fly (medfly), Ceratitis capitata, is an important model organism in biology and agricultural research with high economic relevance. However, information about its embryonic development is still sparse. We share nine long-term live imaging datasets acquired with light sheet fluorescence microscopy (484.5 h total recording time, 373 995 images, 256 Gb) with the scientific community. Six datasets show the embryonic development in toto for about 60 hours at 30 minutes intervals along four directions in three spatial dimensions, covering approximately 97% of the entire embryonic development period. Three datasets focus on germ cell formation and head involution. All imaged embryos hatched morphologically intact. Based on these data, we suggest a two-level staging system that functions as a morphogenetic framework for upcoming studies on medfly. Our data supports research on wild-type or aberrant morphogenesis, quantitative analyses, comparative approaches to insect development as well as studies related to pest control. Further, they can be used to test advanced image processing approaches or to train machine learning algorithms and/or neuronal networks.

Embryo-scale epithelial buckling forms a propagating furrow that initiates gastrulation

Cell apical constriction driven by actomyosin contraction forces is a conserved mechanism during tissue folding in embryo development. While much is now understood of the molecular mechanism responsible for apical constriction and of the tissue-scale integration of the ensuing in-plane deformations, it is still not clear if apical actomyosin contraction forces are necessary or sufficient per se to drive tissue folding. To tackle this question, we use the Drosophila embryo model system that forms a furrow on the ventral side, initiating mesoderm internalization. Past computational models support the idea that cell apical contraction forces may not be sufficient and that active or passive cell apico-basal forces may be necessary to drive cell wedging leading to tissue furrowing. By using 3D computational modelling and in toto embryo image analysis and manipulation, we now challenge this idea and show that embryo-scale force balance at the tissue surface, rather than cell-autonomous shape changes, is necessary and sufficient to drive a buckling of the epithelial surface forming a furrow which propagates and initiates embryo gastrulation.

Drosophila mesoderm invagination begins with the formation of a furrow. Here they show that a long-range mechanism, powered by actomyosin contraction between the embryo polar caps, works like a ‘cheese-cutter wire’ indenting the tissue surface and folding it into a propagating furrow.

The Lateral Epidermis Actively Counteracts Pulling by the Amnioserosa During Dorsal Closure

Dorsal closure is a prominent morphogenetic process during Drosophila embryogenesis, which involves two epithelial tissues, that is, the squamous amnioserosa and the columnar lateral epidermis. Non-muscle myosin II-driven constriction in the amnioserosa leads to a decrease in the apical surface area and pulls on the adjacent lateral epidermis, which subsequently moves dorsally. The pull by the amnioserosa becomes obvious in an elongation of the epidermal cells, especially of those in the first row. The contribution of the epidermal cell elongation has remained unclear to dorsal closure. Cell elongation may be a mere passive consequence or an active response to the pulling by the amnioserosa. Here, we found that the lateral epidermis actively responds. We analyzed tensions within tissues and cell junctions by laser ablation before and during dorsal closure, the elliptical and dorsal closure stages, respectively. Furthermore, we genetically and optochemically induced chronic and acute cell contraction, respectively. In this way, we found that tension in the epidermis increased during dorsal closure. A correspondingly increased tension was not observed at individual junctions, however. Junctional tension even decreased during dorsal closure in the epidermis. We strikingly observed a strong increase of the microtubule amount in the epidermis, while non-muscle myosin II increased in both tissues. Our data suggest that the epidermis actively antagonizes the pull from the amnioserosa during dorsal closure and the increased microtubules might help the epidermis bear part of the mechanical force.

Optogenetic inhibition of actomyosin reveals mechanical bistability of the mesoderm epithelium during Drosophila mesoderm invagination

Apical constriction driven by actin and non-muscle myosin II (actomyosin) provides a well-conserved mechanism to mediate epithelial folding. It remains unclear how contractile forces near the apical surface of a cell sheet drive out-of-the-plane bending of the sheet and whether myosin contractility is required throughout folding. By optogenetic-mediated acute inhibition of actomyosin, we find that during Drosophila mesoderm invagination, actomyosin contractility is critical to prevent tissue relaxation during the early, ‘priming’ stage of folding but is dispensable for the actual folding step after the tissue passes through a stereotyped transitional configuration. This binary response suggests that Drosophila mesoderm is mechanically bistable during gastrulation. Computer modeling analysis demonstrates that the binary tissue response to actomyosin inhibition can be recapitulated in the simulated epithelium that undergoes buckling-like deformation jointly mediated by apical constriction in the mesoderm and in-plane compression generated by apicobasal shrinkage of the surrounding ectoderm. Interestingly, comparison between wild-type and snail mutants that fail to specify the mesoderm demonstrates that the lateral ectoderm undergoes apicobasal shrinkage during gastrulation independently of mesoderm invagination. We propose that Drosophila mesoderm invagination is achieved through an interplay between local apical constriction and mechanical bistability of the epithelium that facilitates epithelial buckling.

Intertissue mechanical interactions shape the olfactory circuit in zebrafish

While the chemical signals guiding neuronal migration and axon elongation have been extensively studied, the influence of mechanical cues on these processes remains poorly studied in vivo. Here, we investigate how mechanical forces exerted by surrounding tissues steer neuronal movements and axon extension during the morphogenesis of the olfactory placode in zebrafish. We mainly focus on the mechanical contribution of the adjacent eye tissue, which develops underneath the placode through extensive evagination and invagination movements. Using quantitative analysis of cell movements and biomechanical manipulations, we show that the developing eye exerts lateral traction forces on the olfactory placode through extracellular matrix, mediating proper morphogenetic movements and axon extension within the placode. Our data shed new light on the key participation of intertissue mechanical interactions in the sculpting of neuronal circuits.

Forced into shape: Mechanical forces in Drosophila development and homeostasis

Mechanical forces play a central role in shaping tissues during development and maintaining epithelial integrity in homeostasis. In this review, we discuss the roles of mechanical forces in Drosophila development and homeostasis, starting from the interplay of mechanics with cell growth and division. We then discuss several examples of morphogenetic processes where complex 3D structures are shaped by mechanical forces, followed by a closer look at patterning processes. We also review the role of forces in homeostatic processes, including cell elimination and wound healing. Finally, we look at the interplay of mechanics and developmental robustness and discuss open questions in the field, as well as novel approaches that will help tackle them in the future.

From heterogeneous morphogenetic fields to homogeneous regions as a step towards understanding complex tissue dynamics

Within developing tissues, cell proliferation, cell motility and other cell behaviors vary spatially, and this variability gives a complexity to the morphogenesis. Recently, novel formalisms have been developed to quantify tissue deformation and underlying cellular processes. A major challenge for the study of morphogenesis now is to objectively define tissue sub-regions exhibiting different dynamics. Here, we propose a method to automatically divide a tissue into regions where the local deformation rate is homogeneous. This was achieved by several steps including image segmentation, clustering and region boundary smoothing. We illustrate the use of the pipeline using a large dataset obtained during the metamorphosis of the Drosophila pupal notum. We also adapt it to determine regions in which the time evolution of the local deformation rate is homogeneous. Finally, we generalize its use to find homogeneous regions for cellular processes such as cell division, cell rearrangement, or cell size and shape changes. We also illustrate it on wing blade morphogenesis. This pipeline will contribute substantially to the analysis of complex tissue shaping, and the biochemical and biomechanical regulations driving tissue morphogenesis.

Correct regionalization of a tissue primordium is essential for coordinated morphogenesis

During organ development, tubular organs often form from flat epithelial primordia. In the placodes of the forming tubes of the salivary glands in the Drosophila embryo, we previously identified spatially defined cell behaviors of cell wedging, tilting, and cell intercalation that are key to the initial stages of tube formation. Here, we address what the requirements are that ensure the continuous formation of a narrow symmetrical tube from an initially asymmetrical primordium whilst overall tissue geometry is constantly changing. We are using live-imaging and quantitative methods to compare wild-type placodes and mutants that either show disrupted cell behaviors or an initial symmetrical placode organization, with both resulting in severe impairment of the invagination. We find that early transcriptional patterning of key morphogenetic transcription factors drives the selective activation of downstream morphogenetic modules, such as GPCR signaling that activates apical-medial actomyosin activity to drive cell wedging at the future asymmetrically placed invagination point. Over time, transcription of key factors expands across the rest of the placode and cells switch their behavior from predominantly intercalating to predominantly apically constricting as their position approaches the invagination pit. Misplacement or enlargement of the initial invagination pit leads to early problems in cell behaviors that eventually result in a defective organ shape. Our work illustrates that the dynamic patterning of the expression of transcription factors and downstream morphogenetic effectors ensures positionally fixed areas of cell behavior with regards to the invagination point. This patterning in combination with the asymmetric geometrical setup ensures functional organ formation.

Using optogenetics to link myosin patterns to contractile cell behaviors during convergent extension

Distinct patterns of actomyosin contractility are often associated with particular epithelial tissue shape changes during development. For example, a planar-polarized pattern of myosin II localization regulated by Rho1 signaling during Drosophila body axis elongation is thought to drive cell behaviors that contribute to convergent extension. However, it is not well understood how specific aspects of a myosin pattern influence the multiple cell behaviors, including cell intercalation, cell shape changes, and apical cell area fluctuations, that simultaneously occur during morphogenesis. Here, we developed two optogenetic tools, optoGEF and optoGAP, to activate or deactivate Rho1 signaling, respectively. We used these tools to manipulate myosin patterns at the apical side of the germband epithelium during Drosophila axis elongation and analyzed the effects on contractile cell behaviors. We show that uniform activation or inactivation of Rho1 signaling across the apical surface of the germband is sufficient to disrupt the planar-polarized pattern of myosin at cell junctions on the timescale of 3-5 min, leading to distinct changes in junctional and medial myosin patterns in optoGEF and optoGAP embryos. These two perturbations to Rho1 activity both disrupt axis elongation and cell intercalation but have distinct effects on cell area fluctuations and cell packings that are linked with changes in the medial and junctional myosin pools. These studies demonstrate that acute optogenetic perturbations to Rho1 activity are sufficient to rapidly override the endogenous planar-polarized myosin pattern in the germband during axis elongation. Moreover, our results reveal that the levels of Rho1 activity and the balance between medial and junctional myosin play key roles not only in organizing the cell rearrangements that are known to directly contribute to axis elongation but also in regulating cell area fluctuations and cell packings, which have been proposed to be important factors influencing the mechanics of tissue deformation and flow.

The cell in the age of the genomic revolution: Cell Regulatory Networks

Over the last few years an intense activity in the areas of advanced microscopy and quantitative cell biology has put the focus on the morphogenetic events that shape embryos. The interest in these processes is taking place against the backdrop of genomic studies, particularly of global patterns of gene expression at the level of single cells, which cannot fully account for the way cells build tissues and organs. Here we discuss the need to integrate the activity of genes with that of cells and propose the need to develop a framework, based on cellular processes and cell interactions, that parallels that which has been created for gene activity in the form of Gene Regulatory Networks (GRNs). We begin to do this by suggesting elements for building Cell Regulatory Networks (CRNs). In the same manner that GRNs create schedules of gene expression that result in the emergence of cell fates over time, CRNs create tissues and organs i.e. space. We also suggest how GRNs and CRNs might interact in the building of embryos through feedback loops involving mechanics and tissue tectonics.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

Scribble mutation disrupts convergent extension and apical constriction during mammalian neural tube closure

Morphogenesis of the vertebrate neural tube occurs by elongation and bending of the neural plate, tissue shape changes that are driven at the cellular level by polarized cell intercalation and cell shape changes, notably apical constriction and cell wedging. Coordinated cell intercalation, apical constriction, and wedging undoubtedly require complex underlying cytoskeletal dynamics and remodeling of adhesions. Mutations of the gene encoding Scribble result in neural tube defects in mice, however the cellular and molecular mechanisms by which Scrib regulates neural cell behavior remain unknown. Analysis of Scribble mutants revealed defects in neural tissue shape changes, and live cell imaging of mouse embryos showed that the Scrib mutation results in defects in polarized cell intercalation, particularly in rosette resolution, and failure of both cell apical constriction and cell wedging. Scrib mutant embryos displayed aberrant expression of the junctional proteins ZO-1, Par3, Par6, E- and N-cadherins, and the cytoskeletal proteins actin and myosin. These findings show that Scribble has a central role in organizing the molecular complexes regulating the morphomechanical neural cell behaviors underlying vertebrate neurulation, and they advance our understanding of the molecular mechanisms involved in mammalian neural tube closure.

Roadmap for the multiscale coupling of biochemical and mechanical signals during development

The way in which interactions between mechanics and biochemistry lead to the emergence of complex cell and tissue organization is an old question that has recently attracted renewed interest from biologists, physicists, mathematicians and computer scientists. Rapid advances in optical physics, microscopy and computational image analysis have greatly enhanced our ability to observe and quantify spatiotemporal patterns of signalling, force generation, deformation, and flow in living cells and tissues. Powerful new tools for genetic, biophysical and optogenetic manipulation are allowing us to perturb the underlying machinery that generates these patterns in increasingly sophisticated ways. Rapid advances in theory and computing have made it possible to construct predictive models that describe how cell and tissue organization and dynamics emerge from the local coupling of biochemistry and mechanics. Together, these advances have opened up a wealth of new opportunities to explore how mechanochemical patterning shapes organismal development. In this roadmap, we present a series of forward-looking case studies on mechanochemical patterning in development, written by scientists working at the interface between the physical and biological sciences, and covering a wide range of spatial and temporal scales, organisms, and modes of development. Together, these contributions highlight the many ways in which the dynamic coupling of mechanics and biochemistry shapes biological dynamics: from mechanoenzymes that sense force to tune their activity and motor output, to collectives of cells in tissues that flow and redistribute biochemical signals during development.

Programmed and self-organized flow of information during morphogenesis

How the shape of embryos and organs emerges during development is a fundamental question that has fascinated scientists for centuries. Tissue dynamics arise from a small set of cell behaviours, including shape changes, cell contact remodelling, cell migration, cell division and cell extrusion. These behaviours require control over cell mechanics, namely active stresses associated with protrusive, contractile and adhesive forces, and hydrostatic pressure, as well as material properties of cells that dictate how cells respond to active stresses. In this Review, we address how cell mechanics and the associated cell behaviours are robustly organized in space and time during tissue morphogenesis. We first outline how not only gene expression and the resulting biochemical cues, but also mechanics and geometry act as sources of morphogenetic information to ultimately define the time and length scales of the cell behaviours driving morphogenesis. Next, we present two idealized modes of how this information flows - how it is read out and translated into a biological effect - during morphogenesis. The first, akin to a programme, follows deterministic rules and is hierarchical. The second follows the principles of self-organization, which rests on statistical rules characterizing the system's composition and configuration, local interactions and feedback. We discuss the contribution of these two modes to the mechanisms of four very general classes of tissue deformation, namely tissue folding and invagination, tissue flow and extension, tissue hollowing and, finally, tissue branching. Overall, we suggest a conceptual framework for understanding morphogenetic information that encapsulates genetics and biochemistry as well as mechanics and geometry as information modules, and the interplay of deterministic and self-organized mechanisms of their deployment, thereby diverging considerably from the traditional notion that shape is fully encoded and determined by genes.

Planar Cell Polarity and E-Cadherin in Tissue-Scale Shape Changes in Drosophila Embryos

Planar cell polarity and anisotropic cell behavior play critical roles in large-scale epithelial morphogenesis, homeostasis, wound repair, and regeneration. Cell-Cell communication and mechano-transduction in the second to minute scale mediated by E-cadherin complexes play a central role in the coordination and self-organization of cellular activities, such as junction dynamics, cell shape changes, and cell rearrangement. Here we review the current understanding in the interplay of cell polarity and cell dynamics during body axis elongation and dorsal closure in Drosophila embryos with a focus on E-cadherin dynamics in linking cell and tissue polarization and tissue-scale shape changes.

Mechanics as a Means of Information Propagation in Development

New research demonstrates that mechanics can serve as a means of information propagation in developing embryos. Historically, the study of embryonic development has had a dichotomy between morphogens and pattern formation on the one hand and morphogenesis and mechanics on the other. Secreted signals are the preeminent means of information propagation between cells and used to control cell fate, while physical forces act downstream or in parallel to shape tissue morphogenesis. However, recent work has blurred this division of function by demonstrating that mechanics can serve as a means of information propagation. Adhesive or repulsive interactions can propagate through a tissue as a wave. These waves are rapid and directional and can be used to control the flux of cells through a developmental trajectory. Here, two examples are reviewed in which mechanics both guides and mediates morphogenesis and two examples in which mechanics intertwines with morphogens to regulate cell fate.

Neuronal immunoglobulin superfamily cell adhesion molecules in epithelial morphogenesis: insights from Drosophila

In this review, we address the function of immunoglobulin superfamily cell adhesion molecules (IgCAMs) in epithelia. Work in the Drosophila model system in particular has revealed novel roles for calcium-independent adhesion molecules in the morphogenesis of epithelial tissues. We review the molecular composition of lateral junctions with a focus on their IgCAM components and reconsider the functional roles of epithelial lateral junctions. The epithelial IgCAMs discussed in this review have well-defined roles in the nervous system, particularly in the process of axon guidance, suggesting functional overlap and conservation in mechanism between that process and epithelial remodelling. We expand on the hypothesis that epithelial occluding junctions and synaptic junctions are compositionally equivalent and present a novel hypothesis that the mechanism of epithelial cell (re)integration and synaptic junction formation are shared. We highlight the importance of considering non-cadherin-based adhesion in our understanding of the mechanics of epithelial tissues and raise questions to direct future work. This article is part of the discussion meeting issue 'Contemporary morphogenesis'.

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

Mechanical Coupling Coordinates the Co-elongation of Axial and Paraxial Tissues in Avian Embryos

Tissues undergoing morphogenesis impose mechanical effects on one another. How developmental programs adapt to or take advantage of these effects remains poorly explored. Here, using a combination of live imaging, modeling, and microsurgical perturbations, we show that the axial and paraxial tissues in the forming avian embryonic body coordinate their rates of elongation through mechanical interactions. First, a cell motility gradient drives paraxial presomitic mesoderm (PSM) expansion, resulting in compression of the axial neural tube and notochord; second, elongation of axial tissues driven by PSM compression and polarized cell intercalation pushes the caudal progenitor domain posteriorly; finally, the axial push drives the lateral movement of midline PSM cells to maintain PSM growth and cell motility. These interactions form an engine-like positive feedback loop, which sustains a shared elongation rate for coupled tissues. Our results demonstrate a key role of inter-tissue forces in coordinating distinct body axis tissues during their co-elongation.

Orchestrating morphogenesis: building the body plan by cell shape changes and movements

During embryonic development, a simple ball of cells re-shapes itself into the elaborate body plan of an animal. This requires dramatic cell shape changes and cell movements, powered by the contractile force generated by actin and myosin linked to the plasma membrane at cell-cell and cell-matrix junctions. Here, we review three morphogenetic events common to most animals: apical constriction, convergent extension and collective cell migration. Using the fruit fly Drosophila as an example, we discuss recent work that has revealed exciting new insights into the molecular mechanisms that allow cells to change shape and move without tearing tissues apart. We also point out parallel events at work in other animals, which suggest that the mechanisms underlying these morphogenetic processes are conserved.

Gastrulation in Drosophila melanogaster: Genetic control, cellular basis and biomechanics

Gastrulation is generally understood as the morphogenetic processes that result in the spatial organization of the blastomere into the three germ layers, ectoderm, mesoderm and endoderm. This review summarizes our current knowledge of the morphogenetic mechanisms in Drosophila gastrulation. In addition to the events that drive mesoderm invagination and germband elongation, we pay particular attention to other, less well-known mechanisms including midgut invagination, cephalic furrow formation, dorsal fold formation, and mesoderm layer formation. This review covers topics ranging from the identification and functional characterization of developmental and morphogenetic control genes to the analysis of the physical properties of cells and tissues and the control of cell and tissue mechanics of the morphogenetic movements in the gastrula.

Anisotropy links cell shapes to tissue flow during convergent extension

Within developing embryos, tissues flow and reorganize dramatically on timescales as short as minutes. This includes epithelial tissues, which often narrow and elongate in convergent extension movements due to anisotropies in external forces or in internal cell-generated forces. However, the mechanisms that allow or prevent tissue reorganization, especially in the presence of strongly anisotropic forces, remain unclear. We study this question in the converging and extending Drosophila germband epithelium, which displays planar-polarized myosin II and experiences anisotropic forces from neighboring tissues. We show that, in contrast to isotropic tissues, cell shape alone is not sufficient to predict the onset of rapid cell rearrangement. From theoretical considerations and vertex model simulations, we predict that in anisotropic tissues, two experimentally accessible metrics of cell patterns-the cell shape index and a cell alignment index-are required to determine whether an anisotropic tissue is in a solid-like or fluid-like state. We show that changes in cell shape and alignment over time in the Drosophila germband predict the onset of rapid cell rearrangement in both wild-type and snail twist mutant embryos, where our theoretical prediction is further improved when we also account for cell packing disorder. These findings suggest that convergent extension is associated with a transition to more fluid-like tissue behavior, which may help accommodate tissue-shape changes during rapid developmental events.

Assembly of a persistent apical actin network by the formin Frl/Fmnl tunes epithelial cell deformability

Tissue remodelling during Drosophila embryogenesis is notably driven by epithelial cell contractility. This behaviour arises from the Rho1-Rok-induced pulsatile accumulation of non-muscle myosin II pulling on actin filaments of the medioapical cortex. While recent studies have highlighted the mechanisms governing the emergence of Rho1-Rok-myosin II pulsatility, little is known about how F-actin organization influences this process. Here, we show that the medioapical cortex consists of two entangled F-actin subpopulations. One exhibits pulsatile dynamics of actin polymerization in a Rho1-dependent manner. The other forms a persistent and homogeneous network independent of Rho1. We identify the formin Frl (also known as Fmnl) as a critical nucleator of the persistent network, since modulating its level in mutants or by overexpression decreases or increases the network density. Absence of this network yields sparse connectivity affecting the homogeneous force transmission to the cell boundaries. This reduces the propagation range of contractile forces and results in tissue-scale morphogenetic defects.

Dynamic morphoskeletons in development

Morphogenetic flows in developmental biology are characterized by the coordinated motion of thousands of cells that organize into tissues, naturally raising the question of how this collective organization arises. Using only the kinematics of tissue deformation, which naturally integrates local and global mechanisms along cell paths, we identify the dynamic morphoskeletons behind morphogenesis, i.e., the evolving centerpieces of multicellular trajectory patterns. These features are model- and parameter-free, frame-invariant, and robust to measurement errors and can be computed from unfiltered cell-velocity data. We reveal the spatial attractors and repellers of the embryo by quantifying its Lagrangian deformation, information that is inaccessible to simple trajectory inspection or Eulerian methods that are local and typically frame-dependent. Computing these dynamic morphoskeletons in wild-type and mutant chick and fly embryos, we find that they capture the early footprint of known morphogenetic features, reveal new ones, and quantitatively distinguish between different phenotypes.

The Physical Mechanisms of Drosophila Gastrulation: Mesoderm and Endoderm Invagination

A critical juncture in early development is the partitioning of cells that will adopt different fates into three germ layers: the ectoderm, the mesoderm, and the endoderm. This step is achieved through the internalization of specified cells from the outermost surface layer, through a process called gastrulation. In Drosophila, gastrulation is achieved through cell shape changes (i.e., apical constriction) that change tissue curvature and lead to the folding of a surface epithelium. Folding of embryonic tissue results in mesoderm and endoderm invagination, not as individual cells, but as collective tissue units. The tractability of Drosophila as a model system is best exemplified by how much we know about Drosophila gastrulation, from the signals that pattern the embryo to the molecular components that generate force, and how these components are organized to promote cell and tissue shape changes. For mesoderm invagination, graded signaling by the morphogen, Spätzle, sets up a gradient in transcriptional activity that leads to the expression of a secreted ligand (Folded gastrulation) and a transmembrane protein (T48). Together with the GPCR Mist, which is expressed in the mesoderm, and the GPCR Smog, which is expressed uniformly, these signals activate heterotrimeric G-protein and small Rho-family G-protein signaling to promote apical contractility and changes in cell and tissue shape. A notable feature of this signaling pathway is its intricate organization in both space and time. At the cellular level, signaling components and the cytoskeleton exhibit striking polarity, not only along the apical-basal cell axis, but also within the apical domain. Furthermore, gene expression controls a highly choreographed chain of events, the dynamics of which are critical for primordium invagination; it does not simply throw the cytoskeletal "on" switch. Finally, studies of Drosophila gastrulation have provided insight into how global tissue mechanics and movements are intertwined as multiple tissues simultaneously change shape. Overall, these studies have contributed to the view that cells respond to forces that propagate over great distances, demonstrating that cellular decisions, and, ultimately, tissue shape changes, proceed by integrating cues across an entire embryo.

Pulsed actomyosin contractions in morphogenesis

Cell and tissue shape changes are the fundamental elements of morphogenesis that drive normal development of embryos into fully functional organisms. This requires a variety of cellular processes including establishment and maintenance of polarity, tissue growth and apoptosis, and cell differentiation, rearrangement, and migration. It is widely appreciated that the cytoskeletal networks play an important role in regulating many of these processes and, in particular, that pulsed actomyosin contractions are a core cellular mechanism driving cell shape changes and cell rearrangement. In this review, we discuss the role of pulsed actomyosin contractions during developmental morphogenesis, advances in our understanding of the mechanisms regulating actomyosin pulsing, and novel techniques to probe the role of pulsed actomyosin processes in in vivo model systems.

Cellular, molecular, and biophysical control of epithelial cell intercalation

Convergent extension is a conserved mechanism for elongating tissues. In the Drosophila embryo, convergent extension is driven by planar polarized cell intercalation and is a paradigm for understanding the cellular, molecular, and biophysical mechanisms that establish tissue structure. Studies of convergent extension in Drosophila have provided key insights into the force-generating molecules that promote convergent extension in epithelial tissues, as well as the global systems of spatial information that systematically organize these cell behaviors. A general framework has emerged in which asymmetrically localized proteins involved in cytoskeletal tension and cell adhesion direct oriented cell movements, and spatial signals provided by the Toll, Tartan, and Teneurin receptor families break planar symmetry to establish and coordinate planar cell polarity throughout the tissue. In this chapter, we describe the cellular, molecular, and biophysical mechanisms that regulate cell intercalation in the Drosophila embryo, and discuss how research in this system has revealed conserved biological principles that control the organization of multicellular tissues and animal body plans.

The plakin domain of C. elegans VAB-10/plectin acts as a hub in a mechanotransduction pathway to promote morphogenesis

Mechanical forces can elicit a mechanotransduction response through junction-associated proteins. In contrast to the wealth of knowledge available for focal adhesions and adherens junctions, much less is known about mechanotransduction at hemidesmosomes. Here, we focus on the C. elegans plectin homolog VAB-10A, the only evolutionary conserved hemidesmosome component. In C. elegans, muscle contractions induce a mechanotransduction pathway in the epidermis through hemidesmosomes. We used CRISPR to precisely remove spectrin repeats (SRs) or a partially hidden Src homology 3 (SH3) domain within the VAB-10 plakin domain. Deleting the SH3 or SR8 domains in combination with mutations affecting mechanotransduction, or just the part of SR5 shielding the SH3 domain, induced embryonic elongation arrest because hemidesmosomes collapse. Notably, recruitment of GIT-1, the first mechanotransduction player, requires the SR5 domain and the hemidesmosome transmembrane receptor LET-805. Furthermore, molecular dynamics simulations confirmed that forces acting on VAB-10 could make the central SH3 domain, otherwise in contact with SR4, available for interaction. Collectively, our data strongly indicate that the plakin domain plays a central role in mechanotransduction and raise the possibility that VAB-10/plectin might act as a mechanosensor.

Setting up for gastrulation: D. melanogaster

Drosophila melanogaster embryos develop initially as a syncytium of totipotent nuclei and subsequently, once cellularized, undergo morphogenetic movements associated with gastrulation to generate the three somatic germ layers of the embryo: mesoderm, ectoderm, and endoderm. In this chapter, we focus on the first phase of gastrulation in Drosophila involving patterning of early embryos when cells differentiate their gene expression programs. This patterning process requires coordination of multiple developmental processes including genome reprogramming at the maternal-to-zygotic transition, combinatorial action of transcription factors to support distinct gene expression, and dynamic feedback between this genetic patterning by transcription factors and changes in cell morphology. We discuss the gene regulatory programs acting during patterning to specify the three germ layers, which involve the regulation of spatiotemporal gene expression coupled to physical tissue morphogenesis.

The tricellular vertex-specific adhesion molecule Sidekick facilitates polarised cell intercalation during Drosophila axis extension

In epithelia, tricellular vertices are emerging as important sites for the regulation of epithelial integrity and function. Compared to bicellular contacts, however, much less is known. In particular, resident proteins at tricellular vertices were identified only at occluding junctions, with none known at adherens junctions (AJs). In a previous study, we discovered that in Drosophila embryos, the adhesion molecule Sidekick (Sdk), well-known in invertebrates and vertebrates for its role in the visual system, localises at tricellular vertices at the level of AJs. Here, we survey a wide range of Drosophila epithelia and establish that Sdk is a resident protein at tricellular AJs (tAJs), the first of its kind. Clonal analysis showed that two cells, rather than three cells, contributing Sdk are sufficient for tAJ localisation. Super-resolution imaging using structured illumination reveals that Sdk proteins form string-like structures at vertices. Postulating that Sdk may have a role in epithelia where AJs are actively remodelled, we analysed the phenotype of sdk null mutant embryos during Drosophila axis extension using quantitative methods. We find that apical cell shapes are abnormal in sdk mutants, suggesting a defect in tissue remodelling during convergence and extension. Moreover, adhesion at apical vertices is compromised in rearranging cells, with apical tears in the cortex forming and persisting throughout axis extension, especially at the centres of rosettes. Finally, we show that polarised cell intercalation is decreased in sdk mutants. Mathematical modelling of the cell behaviours supports the notion that the T1 transitions of polarised cell intercalation are delayed in sdk mutants, in particular in rosettes. We propose that this delay, in combination with a change in the mechanical properties of the converging and extending tissue, causes the abnormal apical cell shapes in sdk mutant embryos.

This study identifies the adhesion molecule Sidekick as a resident protein of tricellular vertices between cells, at the level of adherens junctions. A combination of quantitative methods and modelling provides evidence that Sidekick facilitates polarised cell intercalation during Drosophila axis extension.

Convergent extension in mammalian morphogenesis

Convergent extension is a fundamental morphogenetic process that underlies not only the generation of the elongated vertebrate body plan from the initially radially symmetrical embryo, but also the specific shape changes characteristic of many individual tissues. These tissue shape changes are the result of specific cell behaviors, coordinated in time and space, and affected by the physical properties of the tissue. While mediolateral cell intercalation is the classic cellular mechanism for producing tissue convergence and extension, other cell behaviors can also provide similar tissue-scale distortions or can modulate the effects of mediolateral cell intercalation to sculpt a specific shape. Regulation of regional tissue morphogenesis through planar polarization of the variety of underlying cell behaviors is well-recognized, but as yet is not well understood at the molecular level. Here, we review recent advances in understanding the cellular basis for convergence and extension and its regulation.

Cellular defects resulting from disease-related myosin II mutations in Drosophila

The nonmuscle myosin II motor protein produces forces that are essential to driving the cell movements and cell shape changes that generate tissue structure. Mutations in myosin II that are associated with human diseases are predicted to disrupt critical aspects of myosin function, but the mechanisms that translate altered myosin activity into specific changes in tissue organization and physiology are not well understood. Here we use the Drosophila embryo to model human disease mutations that affect myosin motor activity. Using in vivo imaging and biophysical analysis, we show that engineering human MYH9-related disease mutations into Drosophila myosin II produces motors with altered organization and dynamics that fail to drive rapid cell movements, resulting in defects in epithelial morphogenesis. In embryos that express the Drosophila myosin motor variants R707C or N98K and have reduced levels of wild-type myosin, myosin motors are correctly planar polarized and generate anisotropic contractile tension in the tissue. However, expression of these motor variants is associated with a cellular-scale reduction in the speed of cell intercalation, resulting in a failure to promote full elongation of the body axis. In addition, these myosin motor variants display slowed turnover and aberrant aggregation at the cell cortex, indicating that mutations in the motor domain influence mesoscale properties of myosin organization and dynamics. These results demonstrate that disease-associated mutations in the myosin II motor domain disrupt specific aspects of myosin localization and activity during cell intercalation, linking molecular changes in myosin activity to defects in tissue morphogenesis.

Experimental validation of force inference in epithelia from cell to tissue scale

Morphogenesis relies on the active generation of forces, and the transmission of these forces to surrounding cells and tissues. Hence measuring forces directly in developing embryos is an essential task to study the mechanics of development. Among the experimental techniques that have emerged to measure forces in epithelial tissues, force inference is particularly appealing. Indeed it only requires a snapshot of the tissue, as it relies on the topology and geometry of cell contacts, assuming that forces are balanced at each vertex. However, establishing force inference as a reliable technique requires thorough validation in multiple conditions. Here we performed systematic comparisons of force inference with laser ablation experiments in four epithelial tissues from two animals, the fruit fly and the quail. We show that force inference accurately predicts single junction tension, tension patterns in stereotyped groups of cells, and tissue-scale stress patterns, in wild type and mutant conditions. We emphasize its ability to capture the distribution of forces at different scales from a single image, which gives it a critical advantage over perturbative techniques such as laser ablation. Overall, our results demonstrate that force inference is a reliable and efficient method to quantify the mechanical state of epithelia during morphogenesis, especially at larger scales when inferred tensions and pressures are binned into a coarse-grained stress tensor.

An actin-based viscoplastic lock ensures progressive body-axis elongation

Body-axis elongation constitutes a key step in animal development, laying out the final form of the entire animal. It relies on the interplay between intrinsic forces generated by molecular motors^1-3, extrinsic forces exerted by adjacent cells^4-7 and mechanical resistance forces due to tissue elasticity or friction^8-10. Understanding how mechanical forces influence morphogenesis at the cellular and molecular level remains a challenge¹. Recent work has outlined how small incremental steps power cell-autonomous epithelial shape changes^1-3, which suggests the existence of specific mechanisms that stabilize cell shapes and counteract cell elasticity. Beyond the twofold stage, embryonic elongation in Caenorhabditis elegans is dependent on both muscle activity⁷ and the epidermis; the tension generated by muscle activity triggers a mechanotransduction pathway in the epidermis that promotes axis elongation⁷. Here we identify a network that stabilizes cell shapes in C. elegans embryos at a stage that involves non-autonomous mechanical interactions between epithelia and contractile cells. We searched for factors genetically or molecularly interacting with the p21-activating kinase homologue PAK-1 and acting in this pathway, thereby identifying the α-spectrin SPC-1. Combined absence of PAK-1 and SPC-1 induced complete axis retraction, owing to defective epidermal actin stress fibre. Modelling predicts that a mechanical viscoplastic deformation process can account for embryo shape stabilization. Molecular analysis suggests that the cellular basis for viscoplasticity originates from progressive shortening of epidermal microfilaments that are induced by muscle contractions relayed by actin-severing proteins and from formin homology 2 domain-containing protein 1 (FHOD-1) formin bundling. Our work thus identifies an essential molecular lock acting in a developmental ratchet-like process.