Molecular model for force production and transmission during vertebrate gastrulation

PCP-dependent polarized mechanics in the cortex of individual cells during convergent extension

Convergent extension (CE) is a key process for tissue elongation during vertebrate development and is driven by polarized cell behaviors. Here, we used a novel image-based technique to investigate the mechanical properties of individual cells undergoing CE. Our results suggest a PCP- and Septin-dependent mechanical gradient, where cortical tension is higher at the anterior face of the cells compared with their posterior face. Disruption of PCP protein Vangl2 or its downstream effector Septin7 eliminates this mechanical polarity. These findings demonstrate a link between actin organization, PCP signaling, and mechanical polarization, providing new avenues into the mechanochemical regulation of cellular behaviors during CE.

The first embryo, the origin of cancer and animal phylogeny. V. Cancer stem cells as the unifying biomechanical principle between embryology and oncology

The role of embryology in metazoan evolution is rooted deeply in the history of science. Viewing Neoplasia as an evolutionary engine provides a scientific basis for reexamining the disease cancer. Once the embryo is understood as a benign tumor with a pivotal role in the evolution of all animal forms, there will be an immediate paradigm shift in the search for cancer cure, potentially revealing insights that may be buried within the great developmental transitions of metazoans. This article discusses one of the unifying principles between embryology and oncology, namely cancer stem cells. Some considerations are also provided on the central role of physics and biomechanics in the assembly of the first embryo, which can be regarded as a differentiated benign tumor. Mechanical impregnation of the nucleus of a stem cell, culminating in a totipotent/multipotent cell, was a major event safeguarding the success of embryogenesis throughout evolution. Germ cells in the earliest ctenophore embryos underwent delayed differentiation, subsequent to the mechanical assembly of the embryo. Finally, a discussion is presented on the concept that cancer and embryogenesis (cancer and healthy stem cells) are two sides of the same coin, that is, of the same process. The only difference is that cancer stem cells reveal themselves in inappropriate contexts. Neoplasia is a free force, whereas cancer is a force contained by animal organization.

The role of the node in maintaining axial progenitors

The production of the vertebrate body axis involves the coordinating activity of the organizer, which coincides in amniotes with the node in the gastrulating embryo. This organizer orchestrates nearby axial progenitor populations that produce the spinal cord and musculoskeleton. Various findings, discussed further in this review, suggest that some of these axial progenitors exhibit stem cell-like properties as they display maintenance behaviour such as self-renewal and sustained contribution to derivative tissues. We consider how the node acts to maintain and regulate these progenitor populations by providing mechanical forces and a niche-like signalling environment.

Inferring active and passive mechanical drivers of epithelial convergent extension

What can we learn about the mechanical processes that shape tissues by simply watching? Several schemes suggest that static cell morphology or junctional connectivity can reveal where chains of cells transmit force or where force asymmetries drive cellular rearrangements. We hypothesize that dynamic cell shape changes from time lapse sequences can be used to distinguish specific mechanisms of tissue morphogenesis. Convergent extension (CE) is a crucial developmental motif wherein a planar tissue narrows in one direction and lengthens in the other. It is tempting to assume that forces driving CE reside within cells of the deforming tissue, as CE may reflect a variety of active processes or passive responses to forces generated by adjacent tissues. In this work, we first construct a simple model of epithelial cells capable of passive CE in response to external forces. We adapt this framework to simulate CE from active anisotropic processes in three different modes: crawling, contraction, and capture. We develop an image analysis pipeline for analysis of morphogenetic changes in both live cells and simulated cells using a panel of mechanical and statistical approaches. Our results allow us to identify how each simulated mechanism uniquely contributes to tissue morphology and provide insight into how force transmission is coordinated. We construct a MEchanism Index (MEI) to quantify how similar live cells are to simulated passive and active cells undergoing CE. Applying these analyses to live cell data of Xenopus neural CE reveals features of both passive motion and active forces. Furthermore, we find spatial variation across the neural plate. We compare the inferred mechanisms in the frog midline to tissues undergoing CE in both the mouse and fly. We find that distinct active modes may have different prevalences depending on the model system. Our modeling framework allows us to gain insight from tissue timelapse images and assess the relative contribution of specific cellular mechanisms to observed tissue phenotypes. This approach can be used to guide further experimental inquiry into how mechanics influences the shaping of tissues and organs during development.

Distinct mechanisms regulate ventricular and atrial chamber wall formation

Tissues undergo distinct morphogenetic processes to achieve similarly shaped structures. In the heart, cardiomyocytes in both the ventricle and atrium build internal structures for efficient contraction. Ventricular wall formation (trabeculation) is initiated by cardiomyocyte delamination. How cardiomyocytes build the atrial wall is poorly understood. Using longitudinal imaging in zebrafish, we found that at least 25% of the atrial cardiomyocytes elongate along the long axis of the heart. These cell shape changes result in cell intercalation and convergent thickening, leading to the formation of the internal muscle network. We tested factors important for ventricular trabeculation including Nrg/ErbB and Notch signaling and found no evidence for their role in atrial muscle network formation. Instead, our data suggest that atrial cardiomyocyte elongation is regulated by Yap, which has not been implicated in trabeculation. Altogether, these data indicate that distinct cellular and molecular mechanisms build the internal muscle structures in the atrium and ventricle.

Cell contacts and pericellular matrix in the Xenopus gastrula chordamesoderm

Convergent extension of the chordamesoderm is the best-examined gastrulation movement in Xenopus. Here we study general features of cell-cell contacts in this tissue by combining depletion of adhesion factors C-cadherin, Syndecan-4, fibronectin, and hyaluronic acid, the analysis of respective contact width spectra and contact angles, and La3+ staining of the pericellular matrix. We provide evidence that like in other gastrula tissues, cell-cell adhesion in the chordamesoderm is largely mediated by different types of pericellular matrix. Specific glycocalyx structures previously identified in Xenopus gastrula tissues are absent in chordamesoderm but other contact types like 10-20 nm wide La3+ stained structures are present instead. Knockdown of any of the adhesion factors reduces the abundance of cell contacts but not the average relative adhesiveness of the remaining ones: a decrease of adhesiveness at low contact widths is compensated by an increase of contact widths and an increase of adhesiveness proportional to width. From the adhesiveness-width relationship, we derive a model of chordamesoderm cell adhesion that involves the interdigitation of distinct pericellular matrix units. Quantitative description of pericellular matrix deployment suggests that reduced contact abundance upon adhesion factor depletion is correlated with excessive accumulation of matrix material in non-adhesive gaps and the loss of some contact types.

PCP and Septins govern the polarized organization of the actin cytoskeleton during convergent extension

Convergent extension (CE) requires the coordinated action of the planar cell polarity (PCP) proteins1,2 and the actin cytoskeleton,3,4,5,6 but this relationship remains incompletely understood. For example, PCP signaling orients actomyosin contractions, yet actomyosin is also required for the polarized localization of PCP proteins.7,8 Moreover, the actin-regulating Septins play key roles in actin organization9 and are implicated in PCP and CE in frogs, mice, and fish5,6,10,11,12 but execute only a subset of PCP-dependent cell behaviors. Septin loss recapitulates the severe tissue-level CE defects seen after core PCP disruption yet leaves overt cell polarity intact.5 Together, these results highlight the general fact that cell movement requires coordinated action by distinct but integrated actin populations, such as lamella and lamellipodia in migrating cells13 or medial and junctional actin populations in cells engaged in apical constriction.14,15 In the context of Xenopus mesoderm CE, three such actin populations are important, a superficial meshwork known as the "node-and-cable" system,4,16,17,18 a contractile network at deep cell-cell junctions,6,19 and mediolaterally oriented actin-rich protrusions, which are present both superficially and deeply.4,19,20,21 Here, we exploited the amenability of the uniquely "two-dimensional" node and cable system to probe the relationship between PCP proteins, Septins, and the polarization of this actin network. We find that the PCP proteins Vangl2 and Prickle2 and Septins co-localize at nodes, and that the node and cable system displays a cryptic, PCP- and Septin-dependent anteroposterior (AP) polarity in its organization and dynamics.

New tools reveal PCP-dependent polarized mechanics in the cortex and cytoplasm of single cells during convergent extension

Understanding biomechanics of biological systems is crucial for unraveling complex processes like tissue morphogenesis. However, current methods for studying cellular mechanics in vivo are limited by the need for specialized equipment and often provide limited spatiotemporal resolution. Here we introduce two new techniques, Tension by Transverse Fluctuation (TFlux) and in vivo microrheology, that overcome these limitations. They both offer time-resolved, subcellular biomechanical analysis using only fluorescent reporters and widely available microscopes. Employing these two techniques, we have revealed a planar cell polarity (PCP)-dependent mechanical gradient both in the cell cortex and the cytoplasm of individual cells engaged in convergent extension. Importantly, the non-invasive nature of these methods holds great promise for its application for uncovering subcellular mechanical variations across a wide array of biological contexts.

Summary

Non-invasive imaging-based techniques providing time-resolved biomechanical analysis at subcellular scales in developing vertebrate embryos.

Blastopore gating mechanism to regulate extracellular fluid excretion

Fluid uptake and efflux play roles in early embryogenesis as well as in adult homeostasis. Multicellular organisms have two main pathways for fluid movement: cellular-level, such as transcellular and paracellular pathways, and tissue-level, involving muscle contraction. Interestingly, early Xenopus embryos with immature functional muscles excrete archenteron fluid via a tissue-level mechanism that opens the blastopore through a gating mechanism that is unclear. Using microelectrodes, we show that the archenteron has a constant fluid pressure and as development progress the blastopore pressure resistance decreases. Combining physical perturbations and imaging analyses, we found that the pushing force exerted by the circumblastoporal collars (CBCs) at the slit periphery regulates pressure resistance. We show that apical constriction at the blastopore dorsoventral ends contributes to this pushing force, and relaxation of ventral constriction causes fluid excretion. These results indicate that actomyosin contraction mediates temporal control of tissue-level blastopore opening and fluid excretion in early Xenopus embryos.

ccl19 and ccl21 affect cell movements and differentiation in early Xenopus development

We characterized Xenopus laevis C-C motif chemokine ligand 19.L (ccl19.L) and C-C motif chemokine ligand 21.L (ccl21.L) during early Xenopus embryogenesis. The temporal and spatial expression patterns of ccl19.L and ccl21.L tended to show an inverse correlation, except that the expression level was higher in the dorsal side at the gastrula stage. For example, even at the dorsal sector of the gastrulae, ccl19.L was expressed in the axial region and ccl21.L was expressed in the paraxial region. Dorsal overexpression of ccl19.L and ccl21.L and knockdown of Ccl19.L and Ccl21.L inhibited gastrulation, but their functions were different in cell behaviors during morphogenesis. Observation of Keller sandwich explants revealed that overexpression of both ccl19.L and ccl21.L and knockdown of Ccl21.L inhibited the convergent extension movements, while knockdown of Ccl19.L did not. ccl19.L-overexpressing explants attracted cells at a distance and ccl21.L-overexpressing explants attracted neighboring cells. Ventral overexpression of ccl19.L and ccl21.L induced secondary axis-like structures and chrd.1 expression at the ventral side. Upregulation of chrd.1 was induced by ligand mRNAs through ccr7.S. Knockdown of Ccl19.L and Ccl21.L inhibited gastrulation and downregulated chrd.1 expression at the dorsal side. The collective findings indicate that ccl19.L and ccl21.L might play important roles in morphogenesis and dorsal-ventral patterning during early embryogenesis in Xenopus.

Defined Microenvironments Trigger In Vitro Gastrulation in Human Pluripotent Stem Cells

Gastrulation is a stage in embryo development where three germ layers arise to dictate the human body plan. In vitro models of gastrulation have been demonstrated by treating pluripotent stem cells with soluble morphogens to trigger differentiation. However, in vivo gastrulation is a multistage process coordinated through feedback between soluble gradients and biophysical forces, with the multipotent epiblast transforming to the primitive streak followed by germ layer segregation. Here, the authors show how constraining pluripotent stem cells to hydrogel islands triggers morphogenesis that mirrors the stages preceding in vivo gastrulation, without the need for exogenous supplements. Within hours of initial seeding, cells display a contractile phenotype at the boundary, which leads to enhanced proliferation, yes-associated protein (YAP) translocation, epithelial to mesenchymal transition, and emergence of SRY-box transcription factor 17 (SOX17)+ T/BRACHYURY+ cells. Molecular profiling and pathway analysis reveals a role for mechanotransduction-coupled wingless-type (WNT) signaling in orchestrating differentiation, which bears similarities to processes observed in whole organism models of development. After two days, the colonies form multilayered aggregates, which can be removed for further growth and differentiation. This approach demonstrates how materials alone can initiate gastrulation, thereby providing in vitro models of development and a tool to support organoid bioengineering efforts.

Dorsal lip maturation and initial archenteron extension depend on Wnt11 family ligands

Wnt11 family proteins are ligands that activate a type of Dishevelled-mediated, non-canonical Wnt signaling pathway. Loss of function causes defects in gastrulation and/or anterior-posterior axis extension in all vertebrates. Non-mammalian vertebrate genomes encode two Wnt11 family proteins whose distinct functions have been unclear. We knocked down Wnt11b and Wnt11, separately and together, in Xenopus laevis. Single morphants exhibited very similar phenotypes of delayed blastopore closure, but they had different phenotypes during the tailbud period. In response to their very similar gastrulation phenotypes, we chose to characterize dual morphants. Using dark field illuminated time-lapse imaging and kymograph analysis, we identified a failure of dorsal blastopore lip maturation that correlated with slower blastopore closure and failure to internalize the endoderm at the dorsal blastopore lip. We connected these externally visible phenotypes to cellular events in the internal tissues by imaging intact fixed embryos stained for anillin and microtubules. We found that the initial extension of the archenteron is correlated with blastopore lip maturation, and archenteron extension is dramatically disrupted by decreased Wnt11 family signaling. We were aided in our interpretation of the immunofluorescence by the novel, membrane proximal location of the cleavage furrow protein anillin in the epithelium of the blastopore lip and early archenteron.

Dishevelled controls bulk cadherin dynamics and the stability of individual cadherin clusters during convergent extension

Animals are shaped through the movement of large cellular collectives. Such morphogenetic processes require cadherin-based cell adhesion to maintain tissue cohesion and planar cell polarity to coordinate movement. Despite a vast literature surrounding cadherin-based adhesion and planar cell polarity, it is unclear how these molecular networks interface. Here we investigate the relationship between cadherins and planar cell polarity during gastrulation cell movements in Xenopus laevis. We first assessed bulk cadherin localization and found that cadherins were enriched at a specific subset of morphogenetically active cell-cell junctions. We then found that cadherin and actin had coupled temporal dynamics and that disruption of planar cell polarity uncoupled these dynamics. Next, using superresolution time-lapse microscopy and quantitative image analysis, we were able to measure the lifespan and size of individual cadherin clusters. Finally, we show that planar cell polarity not only controls the size of cadherin clusters but, more interestingly, regulates cluster stability. These results reveal an intriguing link between two essential cellular properties, adhesion and planar polarity, and provide insight into the molecular control of morphogenetic cell movements.

ccr7 affects both morphogenesis and differentiation during early Xenopus embryogenesis

Chemokines play important roles in early embryogenesis, including morphogenesis and cell differentiation, before the immune system is established. We characterized Xenopus laevis CC-type chemokine receptor 7 S (ccr7.S) to clarify its role during early development. ccr7 transcripts were detected ubiquitously in early embryos. Dorsal overexpression of ccr7.S inhibited gastrulation, and ccr7.S mRNA-injected embryos had short axes and widely opened neural folds. Because the Keller sandwich explants of the injected embryos elongated well, ccr7.S might affect cell migration, but not convergent extension movements. Ventral ccr7.S overexpression induced secondary axes and chrd.1 upregulation in gastrula-stage embryos. Animal cap assays showed increased expression of neural and cement gland marker genes at later stages. Ccr7.S knockdown reduced chrd.1 expression and inhibited gastrulation at the dorsal side. Our findings suggest that ccr7.S plays important roles in morphogenetic movement and cell differentiation.

Convergent extension requires adhesion-dependent biomechanical integration of cell crawling and junction contraction

Convergent extension (CE) is an evolutionarily conserved collective cell movement that elongates several organ systems during development. Studies have revealed two distinct cellular mechanisms, one based on cell crawling and the other on junction contraction. Whether these two behaviors collaborate is unclear. Here, using live-cell imaging, we show that crawling and contraction act both independently and jointly but that CE is more effective when they are integrated via mechano-reciprocity. We thus developed a computational model considering both crawling and contraction. This model recapitulates the biomechanical efficacy of integrating the two modes and further clarifies how the two modes and their integration are influenced by cell adhesion. Finally, we use these insights to understand the function of an understudied catenin, Arvcf, during CE. These data are significant for providing interesting biomechanical and cell biological insights into a fundamental morphogenetic process that is implicated in human neural tube defects and skeletal dysplasias.

ARVCF catenin controls force production during vertebrate convergent extension

The design of an animal's body plan is encoded in the genome, and the execution of this program is a mechanical progression involving coordinated movement of proteins, cells, and whole tissues. Thus, a challenge to understanding morphogenesis is connecting events that occur across various length scales. Here, we describe how a poorly characterized adhesion effector, Arvcf catenin, controls Xenopus head-to-tail axis extension. We find that Arvcf is required for axis extension within the intact organism but not within isolated tissues. We show that the organism-scale phenotype results from a defect in tissue-scale force production. Finally, we determine that the force defect results from the dampening of the pulsatile recruitment of cell adhesion and cytoskeletal proteins to membranes. These results provide a comprehensive understanding of Arvcf function during axis extension and produce an insight into how a cellular-scale defect in adhesion results in an organism-scale failure of development.

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 regulation of early vertebrate embryogenesis

Embryonic cells grow in environments that provide a plethora of physical cues, including mechanical forces that shape the development of the entire embryo. Despite their prevalence, the role of these forces in embryonic development and their integration with chemical signals have been mostly neglected, and scrutiny in modern molecular embryology tilted, instead, towards the dissection of molecular pathways involved in cell fate determination and patterning. It is now possible to investigate how mechanical signals induce downstream genetic regulatory networks to regulate key developmental processes in the embryo. Here, we review the insights into mechanical control of early vertebrate development, including the role of forces in tissue patterning and embryonic axis formation. We also highlight recent in vitro approaches using individual embryonic stem cells and self-organizing multicellular models of human embryos, which have been instrumental in expanding our understanding of how mechanics tune cell fate and cellular rearrangements during human embryonic development.

Preparation of three-notochord explants for imaging analysis of the cell movements of convergent extension during early Xenopus morphogenesis

We describe a method of generating three-notochord explants to analyze the cell movements of convergent extension (CE) during Xenopus laevis gastrulation and neurulation. This method uses standard microsurgical techniques under a fluorescence stereomicroscope to combine notochordal sectors of gastrulae, side by side (lateral surfaces apposed) into a single explant. Three-notochord explants cultured on bovine serum albumin (BSA)-coated glass converged mediolaterally and extended in the anterior-posterior direction. The individual notochordal cells showed the mediolaterally oriented, bipolar tractional motility and the resulting mediolaterally oriented cell intercalation characteristic of CE, thereby reproducing both the in vivo tissue and the cell movements in an explant. Image analysis of three-notochord explants reveals the effects of overexpressions or knockdowns of genes, of manipulation of the extracellular matrix, and of exposure to chemical reagents on morphogenesis during gastrulation and neurulation, compared with control explants. Moreover, since three-notochord explants provide two zones of cell intercalation between notochords, individual cell behaviors between notochords of different characteristics and experimental treatments can be observed at the same time.

The role of cellular active stresses in shaping the zebrafish body axis

Tissue remodelling and organ shaping during morphogenesis are products of mechanical forces generated at the cellular level. These cell-scale forces can be coordinated across the tissue via information provided by biochemical and mechanical cues. Such coordination leads to the generation of complex tissue shape during morphogenesis. In this short review, we elaborate the role of cellular active stresses in vertebrate axis morphogenesis, primarily using examples from postgastrulation development of the zebrafish embryo.

Twinfilin1 controls lamellipodial protrusive activity and actin turnover during vertebrate gastrulation

The dynamic control of the actin cytoskeleton is a key aspect of essentially all animal cell movements. Experiments in single migrating cells and in vitro systems have provided an exceptionally deep understanding of actin dynamics. However, we still know relatively little of how these systems are tuned in cell-type-specific ways, for example in the context of collective cell movements that sculpt the early embryo. Here, we provide an analysis of the actin-severing and depolymerization machinery during vertebrate gastrulation, with a focus on Twinfilin1 (Twf1) in Xenopus. We find that Twf1 is essential for convergent extension, and loss of Twf1 results in a disruption of lamellipodial dynamics and polarity. Moreover, Twf1 loss results in a failure to assemble polarized cytoplasmic actin cables, which are essential for convergent extension. These data provide an in vivo complement to our more-extensive understanding of Twf1 action in vitro and provide new links between the core machinery of actin regulation and the specialized cell behaviors of embryonic morphogenesis.

Mechanical heterogeneity along single cell-cell junctions is driven by lateral clustering of cadherins during vertebrate axis elongation

Morphogenesis is governed by the interplay of molecular signals and mechanical forces across multiple length scales. The last decade has seen tremendous advances in our understanding of the dynamics of protein localization and turnover at subcellular length scales, and at the other end of the spectrum, of mechanics at tissue-level length scales. Integrating the two remains a challenge, however, because we lack a detailed understanding of the subcellular patterns of mechanical properties of cells within tissues. Here, in the context of the elongating body axis of Xenopus embryos, we combine tools from cell biology and physics to demonstrate that individual cell-cell junctions display finely-patterned local mechanical heterogeneity along their length. We show that such local mechanical patterning is essential for the cell movements of convergent extension and is imparted by locally patterned clustering of a classical cadherin. Finally, the patterning of cadherins and thus local mechanics along cell-cell junctions are controlled by Planar Cell Polarity signaling, a key genetic module for CE that is mutated in diverse human birth defects.

Stage-dependent sequential organization of nascent smooth muscle cells and its implications for the gut coiling morphogenesis in Xenopus larva

In vertebrates, gut coiling proceeds left-right asymmetrically during expansion of the gastrointestinal tract with highly organized muscular structures facilitating peristalsis. In this report, we explored the mechanisms of larval gut coiling morphogenesis relevant to its nascent smooth muscle cells using highly transparent Xenopus early larvae. First, to visualize the dynamics of intestinal smooth muscle cells, whole-mount specimens were immunostained with anti-smooth muscle-specific actin (SM-actin) antibody. We found that the nascent gut of Xenopus early larvae gradually expands the SM-actin-positive region in a stage-dependent manner. Transverse orientation of smooth muscle cells was first established, and next, the cellular longitudinal orientation along the gut axis was followed to make a meshwork of the contractile cells. Finally, anisotropic torsion by the smooth muscle cells was generated in the center of gut coiling, suggesting that twisting force might be involved in the late phase of coiling morphogenesis of the gut. Administration of S-(-)-Blebbistatin to attenuate the actomyosin contraction in vivo resulted in cancellation of coiling of the gut. Development of decapitation embryos, trunk 'torso' explants, and gut-only explants revealed that initial coiling of the gut proceeds without interactions with the other parts of the body including the central nervous system. We newly developed an in vitro model to assess the gut coiling morphogenesis, indicating that coiling pattern of the nascent Xenopus gut is partially gut-autonomous. Using this gut explant culture technique, inhibition of actomyosin contraction was performed by administrating either actin polymerization inhibitor, myosin light chain kinase inhibitor, or calmodulin antagonist. All of these reagents decreased the extent of gut coiling morphogenesis in vitro. Taken together, these results suggest that the contraction force generated by actomyosin-rich intestinal smooth muscle cells during larval stages is essential for the normal coiling morphogenesis of this muscular tubular organ.

From biomechanics to mechanobiology: Xenopus provides direct access to the physical principles that shape the embryo

Features of amphibian embryos that have served so well to elucidate the genetics of vertebrate development also enable detailed analysis of the physics that shape morphogenesis and regulate development. Biophysical tools are revealing how genes control mechanical properties of the embryo. The same tools that describe and control mechanical properties are being turned to reveal how dynamic mechanical information and feedback regulate biological programs of development. In this review we outline efforts to explore the various roles of mechanical cues in guiding cilia biology, axonal pathfinding, goblet cell regeneration, epithelial-to-mesenchymal transitions in neural crest, and mesenchymal-to-epithelial transitions in heart progenitors. These case studies reveal the power of Xenopus experimental embryology to expose pathways integrating mechanical cues with programs of development, organogenesis, and regeneration.

Cellular processes driving gastrulation in the avian embryo

Gastrulation consists in the dramatic reorganisation of the epiblast, a one-cell thick epithelial sheet, into a multilayered embryo. In chick, the formation of the internal layers requires the generation of a macroscopic convection-like flow, which involves up to 50,000 epithelial cells in the epiblast. These cell movements locate the mesendoderm precursors into the midline of the epiblast to form the primitive streak. There they acquire a mesenchymal phenotype, ingress into the embryo and migrate outward to populate the inner embryonic layers. This review covers what is currently understood about how cell behaviours ultimately cause these morphogenetic events and how they are regulated. We discuss 1) how the biochemical patterning of the embryo before gastrulation creates compartments of differential cell behaviours, 2) how the global epithelial flows arise from the coordinated actions of individual cells, 3) how the cells delaminate individually from the epiblast during the ingression, and 4) how cells move after the ingression following stereotypical migration routes. We conclude by exploring new technical advances that will facilitate future research in the chick model system.

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.

The notochord gene regulatory network in chordate evolution: Conservation and divergence from Ciona to vertebrates

The notochord is a structure required for support and patterning of all chordate embryos, from sea squirts to humans. An increasing amount of information on notochord development and on the molecular strategies that ensure its proper morphogenesis has been gleaned through studies in the sea squirt Ciona. This invertebrate chordate offers a fortunate combination of experimental advantages, ranging from translucent, fast-developing embryos to a compact genome and impressive biomolecular resources. These assets have enabled the rapid identification of numerous notochord genes and cis-regulatory regions, and provide a rather unique opportunity to reconstruct the gene regulatory network that controls the formation of this developmental and evolutionary chordate landmark. This chapter summarizes the morphogenetic milestones that punctuate notochord formation in Ciona, their molecular effectors, and the current knowledge of the gene regulatory network that ensures the accurate spatial and temporal orchestration of these processes.

Tissue mechanics drives regeneration of a mucociliated epidermis on the surface of Xenopus embryonic aggregates

Injury, surgery, and disease often disrupt tissues and it is the process of regeneration that aids the restoration of architecture and function. Regeneration can occur through multiple strategies including stem cell expansion, transdifferentiation, or proliferation of differentiated cells. We have identified a case of regeneration in Xenopus embryonic aggregates that restores a mucociliated epithelium from mesenchymal cells. Following disruption of embryonic tissue architecture and assembly of a compact mesenchymal aggregate, regeneration first restores an epithelium, transitioning from mesenchymal cells at the surface of the aggregate. Cells establish apico-basal polarity within 5 hours and a mucociliated epithelium within 24 hours. Regeneration coincides with nuclear translocation of the putative mechanotransducer YAP1 and a sharp increase in aggregate stiffness, and regeneration can be controlled by altering stiffness. We propose that regeneration of a mucociliated epithelium occurs in response to biophysical cues sensed by newly exposed cells on the surface of a disrupted mesenchymal tissue.

Convergent extension in the amphibian, Xenopus laevis

This review is a comprehensive analysis of the cell biology and biomechanics of Convergent Extension in Xenopus.

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.

PCP-dependent transcellular regulation of actomyosin oscillation facilitates convergent extension of vertebrate tissue

Oscillatory flows of actomyosin play a key role in the migration of single cells in culture and in collective cell movements in Drosophila embryos. In vertebrate embryos undergoing convergent extension (CE), the Planar Cell Polarity (PCP) pathway drives the elongation of the body axis and shapes the central nervous system, and mutations of the PCP genes predispose humans to various malformations including neural tube defects. However, the spatiotemporal patterns of oscillatory actomyosin contractions during vertebrate CE and how they are controlled by the PCP signaling remain unknown. Here, we address these outstanding issues using a combination of in vivo imaging and mathematical modeling. We find that effective execution of CE requires alternative oscillations of cortical actomyosin across cell membranes of neighboring cells within an optimal frequency range. Intriguingly, temporal and spatial clustering of the core PCP protein Prickle 2 (Pk2) is correlated to submembranous accumulations of F-actin, and depletion of Pk2 perturbs the oscillation of actomyosin contractions. These findings shed light on the significance of temporal regulation of actomyosin contraction by the PCP pathway during CE, in addition to its well-studied spatial aspects.

Emergent mechanics of actomyosin drive punctuated contractions and shape network morphology in the cell cortex

Filamentous actin (F-actin) and non-muscle myosin II motors drive cell motility and cell shape changes that guide large scale tissue movements during embryonic morphogenesis. To gain a better understanding of the role of actomyosin in vivo, we have developed a two-dimensional (2D) computational model to study emergent phenomena of dynamic unbranched actomyosin arrays in the cell cortex. These phenomena include actomyosin punctuated contractions, or "actin asters" that form within quiescent F-actin networks. Punctuated contractions involve both formation of high intensity aster-like structures and disassembly of those same structures. Our 2D model allows us to explore the kinematics of filament polarity sorting, segregation of motors, and morphology of F-actin arrays that emerge as the model structure and biophysical properties are varied. Our model demonstrates the complex, emergent feedback between filament reorganization and motor transport that generate as well as disassemble actin asters. Since intracellular actomyosin dynamics are thought to be controlled by localization of scaffold proteins that bind F-actin or their myosin motors we also apply our 2D model to recapitulate in vitro studies that have revealed complex patterns of actomyosin that assemble from patterning filaments and motor complexes with microcontact printing. Although we use a minimal representation of filament, motor, and cross-linker biophysics, our model establishes a framework for investigating the role of other actin binding proteins, how they might alter actomyosin dynamics, and makes predictions that can be tested experimentally within live cells as well as within in vitro models.

Recent genetic and mechanical studies of embryonic development reveal a critical role for intracellular scaffolds in generating the shape of the embryo and constructing internal organs. In this paper we developed computer simulations of these scaffolds, composed of filamentous actin (F-actin), a rod-like protein polymer, and mini-thick filaments, composed of non-muscle myosin II, forming a two headed spring-like complex of motor proteins that can walk on, and remodel F-actin networks. Using simulations of these dynamic interactions, we can carry out virtual experiments where we change the physics and chemistry of F-actin polymers, their associated myosin motors, and cross-linkers and observe the changes in scaffolds that emerge. For example, by modulating the motor stiffness of the myosin motors in our model we can observe the formation or loss of large aster-like structures. Such fine-grained control over the physical properties of motors or filaments within simulations are not currently possible with biological experiments, even where mutant proteins or small molecule inhibitors can be targeted to specific sites on filaments or motors. Our approach reflects a growing adoption of simulation methods to investigate microscopic features that shape actomyosin arrays and the mesoscale effects of molecular scale processes. We expect predictions from these models will drive more refined experimental approaches to expose the many roles of actomyosin in development.

Coming to Consensus: A Unifying Model Emerges for Convergent Extension

Cell motility is a widespread biological property that is best understood in the context of individual cell migration. Indeed, studies of migration in culture have provided tremendous insight into the signals and mechanics involved and have laid the foundation for our understanding of similar migrations by larger cellular collectives. By contrast, our understanding of another flavor of movement, cell intercalation during convergent extension, is only now emerging. Here, we integrate divergent findings related to intercalation in different settings into a unifying model, paying attention to how this model does and does not resemble current models for directed cell migration.

Tension, contraction and tissue morphogenesis

D'Arcy Thompson was a proponent of applying mathematical and physical principles to biological systems, an approach that is becoming increasingly common in developmental biology. Indeed, the recent integration of quantitative experimental data, force measurements and mathematical modeling has changed our understanding of morphogenesis - the shaping of an organism during development. Emerging evidence suggests that the subcellular organization of contractile cytoskeletal networks plays a key role in force generation, while on the tissue level the spatial organization of forces determines the morphogenetic output. Inspired by D'Arcy Thompson's On Growth and Form, we review our current understanding of how biological forms are created and maintained by the generation and organization of contractile forces at the cell and tissue levels. We focus on recent advances in our understanding of how cells actively sculpt tissues and how forces are involved in specific morphogenetic processes.

Mechanical and signaling roles for keratin intermediate filaments in the assembly and morphogenesis of Xenopus mesendoderm tissue at gastrulation

The coordination of individual cell behaviors is a crucial step in the assembly and morphogenesis of tissues. Xenopus mesendoderm cells migrate collectively along a fibronectin (FN) substrate at gastrulation, but how the adhesive and mechanical forces required for these movements are generated and transmitted is unclear. Traction force microscopy (TFM) was used to establish that traction stresses are limited primarily to leading edge cells in mesendoderm explants, and that these forces are balanced by intercellular stresses in follower rows. This is further reflected in the morphology of these cells, with broad lamellipodial protrusions, mature focal adhesions and a gradient of activated Rac1 evident at the leading edge, while small protrusions, rapid turnover of immature focal adhesions and lack of a Rac1 activity gradient characterize cells in following rows. Depletion of keratin (krt8) with antisense morpholinos results in high traction stresses in follower row cells, misdirected protrusions and the formation of actin stress fibers anchored in streak-like focal adhesions. We propose that maintenance of mechanical integrity in the mesendoderm by keratin intermediate filaments is required to balance stresses within the tissue to regulate collective cell movements.

Models of convergent extension during morphogenesis

Convergent extension (CE) is a fundamental and conserved collective cell movement that forms elongated tissues during embryonic development. Thus far, studies have demonstrated two different mechanistic models of collective cell movements during CE. The first, termed the crawling mode, was discovered in the process of notochord formation in Xenopus laevis embryos, and has been the established model of CE for decades. The second model, known as the contraction mode, was originally reported in studies of germband extension in Drosophila melanogaster embryos and was recently demonstrated to be a conserved mechanism of CE among tissues and stages of development across species. This review summarizes the two modes of CE by focusing on the differences in cytoskeletal behaviors and relative expression of cell adhesion molecules. The upstream molecules regulating these machineries are also discussed. There are abundant studies of notochord formation in X. laevis embryos, as this was one of the pioneering model systems in this field. Therefore, the present review discusses these findings as an approach to the fundamental biological question of collective cell regulation. WIREs Dev Biol 2018, 7:e293. doi: 10.1002/wdev.293

This article is categorized under: 1

Early Embryonic Development > Gastrulation and Neurulation

2

Comparative Development and Evolution > Model Systems

Cell migration, intercalation and growth regulate mammalian cochlear extension

Developmental remodeling of the sensory epithelium of the cochlea is required for the formation of an elongated, tonotopically organized auditory organ, but the cellular processes that mediate these events are largely unknown. We used both morphological assessments of cellular rearrangements and time-lapse imaging to visualize cochlear remodeling in mouse. Analysis of cell redistribution showed that the cochlea extends through a combination of radial intercalation and cell growth. Live imaging demonstrated that concomitant cellular intercalation results in a brief period of epithelial convergence, although subsequent changes in cell size lead to medial-lateral spreading. Supporting cells, which retain contact with the basement membrane, exhibit biased protrusive activity and directed movement along the axis of extension. By contrast, hair cells lose contact with the basement membrane, but contribute to continued outgrowth through increased cell size. Regulation of cellular protrusions, movement and intercalation within the cochlea all require myosin II. These results establish, for the first time, many of the cellular processes that drive the distribution of sensory cells along the tonotopic axis of the cochlea.

Regulation of gastrulation movements by emergent cell and tissue interactions

It is during gastrulation that the primordial germ layers are specified, embryonic axes become morphologically manifest, and the embryonic body plan begins to take shape. As morphogenetic movements push and pull nascent tissues into position within the gastrula, new interactions are established between neighboring cells and tissues. These interactions represent an emergent property within gastrulating embryos, and serve to regulate and promote ensuing morphogenesis that establishes the next set of cell/tissue contacts, and so on. Several recent studies demonstrate the critical roles of such interactions during gastrulation, including those between germ layers, along embryonic axes, and at tissue boundaries. Emergent tissue interactions result from - and result in - morphogen signaling, cell contacts, and mechanical forces within the gastrula. Together, these comprise a dynamic and complex regulatory cascade that drives gastrulation morphogenesis.

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.

Mechanical design in embryos: mechanical signalling, robustness and developmental defects

Embryos are shaped by the precise application of force against the resistant structures of multicellular tissues. Forces may be generated, guided and resisted by cells, extracellular matrix, interstitial fluids, and how they are organized and bound within the tissue's architecture. In this review, we summarize our current thoughts on the multiple roles of mechanics in direct shaping, mechanical signalling and robustness of development. Genetic programmes of development interact with environmental cues to direct the composition of the early embryo and endow cells with active force production. Biophysical advances now provide experimental tools to measure mechanical resistance and collective forces during morphogenesis and are allowing integration of this field with studies of signalling and patterning during development. We focus this review on concepts that highlight this integration, and how the unique contributions of mechanical cues and gradients might be tested side by side with conventional signalling systems. We conclude with speculation on the integration of large-scale programmes of development, and how mechanical responses may ensure robust development and serve as constraints on programmes of tissue self-assembly.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Epithelial-mesenchymal transition (EMT): A biological process in the development, stem cell differentiation, and tumorigenesis

The lineage transition between epithelium and mesenchyme is a process known as epithelial-mesenchymal transition (EMT), by which polarized epithelial cells lose their adhesion property and obtain mesenchymal cell phenotypes. EMT is a biological process that is often involved in embryogenesis and diseases, such as cancer invasion and metastasis. The EMT and the reverse process, mesenchymal-epithelial transition (MET), also play important roles in stem cell differentiation and de-differentiation (or reprogramming). In this review, we will discuss current research progress of EMT in embryonic development, cellular differentiation and reprogramming, and cancer progression, all of which are representative models for researches of stem cell biology in normal and in diseases. Understanding of EMT and MET may help to identify specific markers to distinguish normal stem cells from cancer stem cells in future.

Actomyosin Pulsing in Tissue Integrity Maintenance during Morphogenesis

The actomyosin cytoskeleton is responsible for many changes in cell and tissue shape. For a long time, the actomyosin cytoskeleton has been known to exhibit dynamic contractile behavior. Recently, discrete actomyosin assembly/disassembly cycles have also been observed in cells. These so-called actomyosin pulses have been observed in a variety of contexts, including cell polarization and division, and in epithelia, where they occur during tissue contraction, folding, and extension. In epithelia, evidence suggests that actomyosin pulsing, and more generally, actomyosin turnover, is required to maintain tissue integrity during contractile processes. This review explores possible functions for pulsing in the many instances during which pulsing has been observed, and also highlights proposed molecular mechanisms that drive pulsing.

Mechano-sensitive regulation of gene expression during the embryonic development

Cell movements during embryogenesis produce mechanical tensions that shape the embryo and can also regulate gene expression, thereby affecting cell differentiation. Increasing evidence indicates that mechanosensitive regulation of gene expression plays important roles during embryogenesis by coupling the processes of morphogenesis and differentiation. However, the molecular mechanisms of this phenomenon remain poorly understood. This review focuses on the molecular mechanisms that "translate" mechanical stimuli into gene expression.

Cell-cell adhesion interface: rise of the lateral membrane

The lateral membrane plays an important role in the mechanical stability of epithelial cell sheet in steady state. In addition, the lateral membrane is continuously remodeled during dynamic processes such as cell extrusion, cytokinesis, and intercellular cell movement. In wound healing, the lateral membrane must be built from flat and spread cells that had crawled into the area of the wound. Thus, forming the lateral membrane is a phenomenon that occurs not only in development but also during homeostatic maintenance and regeneration of differentiated epithelial tissues.

MRCK-1 Drives Apical Constriction in C. elegans by Linking Developmental Patterning to Force Generation

Apical constriction is a change in cell shape that drives key morphogenetic events including gastrulation and neural tube formation. Apical force-producing actomyosin networks drive apical constriction by contracting while connected to cell-cell junctions. The mechanisms by which developmental patterning regulates these actomyosin networks and associated junctions with spatial precision are not fully understood. Here we identify a myosin light-chain kinase MRCK-1 as a key regulator of C. elegans gastrulation that integrates spatial and developmental patterning information. We show that MRCK-1 is required for activation of contractile actomyosin dynamics and elevated cortical tension in the apical cell cortex of endoderm precursor cells. MRCK-1 is apically localized by active Cdc42 at the external, cell-cell contact-free surfaces of apically constricting cells, downstream of cell fate determination mechanisms. We establish that the junctional components α-catenin, β-catenin, and cadherin become highly enriched at the apical junctions of apically constricting cells and that MRCK-1 and myosin activity are required in vivo for this enrichment. Taken together, our results define mechanisms that position a myosin activator to a specific cell surface where it both locally increases cortical tension and locally enriches junctional components to facilitate apical constriction. These results reveal crucial links that can tie spatial information to local force generation to drive morphogenesis.

Filopodial-Tension Model of Convergent-Extension of Tissues

In convergent-extension (CE), a planar-polarized epithelial tissue elongates (extends) in-plane in one direction while shortening (converging) in the perpendicular in-plane direction, with the cells both elongating and intercalating along the converging axis. CE occurs during the development of most multicellular organisms. Current CE models assume cell or tissue asymmetry, but neglect the preferential filopodial activity along the convergent axis observed in many tissues. We propose a cell-based CE model based on asymmetric filopodial tension forces between cells and investigate how cell-level filopodial interactions drive tissue-level CE. The final tissue geometry depends on the balance between external rounding forces and cell-intercalation traction. Filopodial-tension CE is robust to relatively high levels of planar cell polarity misalignment and to the presence of non-active cells. Addition of a simple mechanical feedback between cells fully rescues and even improves CE of tissues with high levels of polarity misalignments. Our model extends easily to three dimensions, with either one converging and two extending axes, or two converging and one extending axes, producing distinct tissue morphologies, as observed in vivo.

The development of an embryo from a fertilized egg to an adult organism requires not only cell proliferation and differentiation, but also numerous types of tissue restructuring. The development of a relatively round initial embryo into one elongated along its rostral-caudal axis involves coordinated tissue elongation and cell reorganization in one or more groups of cells or tissues. Counterintuitively, in many organisms, cells in elongating tissues elongate and increase their protrusive activity in the direction perpendicular to the axis of elongation (convergent extension). Experimental and theoretical studies have not determined how this cell-level oriented protrusive activity leads to observed tissue-level changes in morphology. We propose a filopodial-tension model that shows how tension from oriented cell protrusions leads to observed patterns of tissue CE.