Embryonic wound healing by apical contraction and ingression in Xenopus laevis

Gears of life: A primer on the simple machines that shape the embryo

A simple machine is a basic of device that takes mechanical advantage to apply force. Animals and plants self-assemble through the operation of a wide variety of simple machines. Embryos of different species actuate these simple machines to drive the geometric transformations that convert a disordered mass of cells into organized structures with discrete identities and function. These transformations are intrinsically coupled to sequential and overlapping steps of self-organization and self-assembly. The processes of self-organization have been explored through the molecular composition of cells and tissues and their information networks. By contrast, efforts to understand the simple machines underlying self-assembly must integrate molecular composition with the physical principles of mechanics. This primer is concerned with effort to elucidate the operation of these machines, focusing on the "problem" of morphogenesis. Advances in understanding self-assembly will ultimately connect molecular-, subcellular-, cellular- and meso-scale functions of plants and animals and their ability to interact with larger ecologies and environmental influences.

Should I shrink or should I grow: cell size changes in tissue morphogenesis

Cells change shape, move, divide, and die to sculpt tissues. Common to all these cell behaviours are cell size changes, which have recently emerged as key contributors to tissue morphogenesis. Cells can change their mass-the number of macromolecules they contain-or their volume-the space they encompass. Changes in cell mass and volume occur through different molecular mechanisms and at different timescales, slow for changes in mass and rapid for changes in volume. Therefore, changes in cell mass and cell volume, which are often linked, contribute to the development and shaping of tissues in different ways. Here, we review the molecular mechanisms by which cells can control and alter their size, and we discuss how changes in cell mass and volume contribute to tissue morphogenesis. The role that cell size control plays in developing embryos is only starting to be elucidated. Research on the signals that control cell size will illuminate our understanding of the cellular and molecular mechanisms that drive tissue morphogenesis.

Embryos assist morphogenesis of others through calcium and ATP signaling mechanisms in collective teratogen resistance

Information for organismal patterning can come from a variety of sources. We investigate the possibility that instructive influences for normal embryonic development are provided not only at the level of cells within the embryo, but also via interactions between embryos. To explore this, we challenge groups of embryos with disruptors of normal development while varying group size. Here, we show that Xenopus laevis embryos are much more sensitive to a diverse set of chemical and molecular-biological perturbations when allowed to develop alone or in small groups, than in large groups. Keeping per-embryo exposure constant, we find that increasing the number of exposed embryos in a cohort increases the rate of survival while incidence of defects decreases. This inter-embryo assistance effect is mediated by short-range diffusible signals and involves the P2 ATP receptor. Our data and computational model emphasize that morphogenesis is a collective phenomenon not only at the level of cells, but also of whole bodies, and that cohort size is a crucial variable in studies of ecotoxicology, teratogenesis, and developmental plasticity.

Rap1 coordinates cell-cell adhesion and cytoskeletal reorganization to drive collective cell migration in vivo

Collective cell movements contribute to tissue development and repair and spread metastatic disease. In epithelia, cohesive cell movements require reorganization of adherens junctions and the actomyosin cytoskeleton. However, the mechanisms that coordinate cell-cell adhesion and cytoskeletal remodeling during collective cell migration in vivo are unclear. We investigated the mechanisms of collective cell migration during epidermal wound healing in Drosophila embryos. Upon wounding, the cells adjacent to the wound internalize cell-cell adhesion molecules and polarize actin and the motor protein non-muscle myosin II to form a supracellular cable around the wound that coordinates cell movements. The cable anchors at former tricellular junctions (TCJs) along the wound edge, and TCJs are reinforced during wound closure. We found that the small GTPase Rap1 was necessary and sufficient for rapid wound repair. Rap1 promoted myosin polarization to the wound edge and E-cadherin accumulation at TCJs. Using embryos expressing a mutant form of the Rap1 effector Canoe/Afadin that cannot bind Rap1, we found that Rap1 signals through Canoe for adherens junction remodeling, but not for actomyosin cable assembly. Instead, Rap1 was necessary and sufficient for RhoA/Rho1 activation at the wound edge. The RhoGEF Ephexin localized to the wound edge in a Rap1-dependent manner, and Ephexin was necessary for myosin polarization and rapid wound repair, but not for E-cadherin redistribution. Together, our data show that Rap1 coordinates the molecular rearrangements that drive embryonic wound healing, promoting actomyosin cable assembly through Ephexin-Rho1, and E-cadherin redistribution through Canoe, thus enabling rapid collective cell migration in vivo.

Purse-string contraction guides mechanical gradient-dictated heterogeneous migration of epithelial monolayer

Mechanical heterogeneity has been recognized as an important role in mediating collective cell migration, yet the related mechanism has not been elucidated. Herein, we fabricate heterogeneous stiffness gradients by leveraging microelastically-patterned hydrogels with varying periodic distance. We observe that a decrease in the periodic distance of the mechanical heterogeneity is accompanied by an overall increase in the velocity and directionality of the migrating monolayer. Moreover, inhibition of ROCK- and myosin ⅡA- but not Rac1-mediated contraction reduces monolayer migration on the mechanically heterogeneous substrates. Furthermore, we find that F-actin and myosin ⅡA form purse-string at the leading edge on the mechanically heterogeneous substrates. Together, these findings not only show that the orientational cell-cell contraction promotes collective cell migration under the mechanical heterogeneity, but also demonstrate that the mechanosensation arising from large-scale cell-cell interactions through purse-string formation mediated cell-cell orientational contraction can feed back to regulate the reorganization of epithelial tissues. STATEMENT OF SIGNIFICANCE: By detecting the links between heterogenous rigidity and collective cell migration behavior at the molecular level, we reveal that collective cell migration in the mechanical heterogeneity is driven by ROCK- and myosin-ⅡA-dependent cytoskeletal tension. We confirm that cytoskeletal tension across the epithelial tissue is holistically linked through F-actin and myosin-ⅡA, which cooperate to form purse-string structures for modulating collective tissue behavior on the exogenous matrix with mechanical heterogeneity. Mechanical heterogeneity initiates tissue growth, remodelling, and morphogenesis by orientating cell contractility. Therefore, tensional homeostasis across large-scale cell interactions appears to be necessary and sufficient to trigger collective tissue behavior. Overall, these findings shed light on the role of mechanical heterogeneity in tissue microenvironment for reorganization and morphogenesis.

Contractile and expansive actin networks in Drosophila: Developmental cell biology controlled by network polarization and higher-order interactions

Actin networks are central to shaping and moving cells during animal development. Various spatial cues activate conserved signal transduction pathways to polarize actin network assembly at sub-cellular locations and to elicit specific physical changes. Actomyosin networks contract and Arp2/3 networks expand, and to affect whole cells and tissues they do so within higher-order systems. At the scale of tissues, actomyosin networks of epithelial cells can be coupled via adherens junctions to form supracellular networks. Arp2/3 networks typically integrate with distinct actin assemblies, forming expansive composites which act in conjunction with contractile actomyosin networks for whole-cell effects. This review explores these concepts using examples from Drosophila development. First, we discuss the polarized assembly of supracellular actomyosin cables which constrict and reshape epithelial tissues during embryonic wound healing, germ band extension, and mesoderm invagination, but which also form physical borders between tissue compartments at parasegment boundaries and during dorsal closure. Second, we review how locally induced Arp2/3 networks act in opposition to actomyosin structures during myoblast cell-cell fusion and cortical compartmentalization of the syncytial embryo, and how Arp2/3 and actomyosin networks also cooperate for the single cell migration of hemocytes and the collective migration of border cells. Overall, these examples show how the polarized deployment and higher-order interactions of actin networks organize developmental cell biology.

p38-mediated cell growth and survival drive rapid embryonic wound repair

Embryos repair wounds rapidly, with no inflammation or scarring, in a process that involves polarization of the actomyosin cytoskeleton. Actomyosin polarization results in the assembly of a contractile cable around the wound that drives wound closure. Here, we demonstrate that a contractile actomyosin cable is not sufficient for rapid wound repair in Drosophila embryos. We show that wounding causes activation of the serine/threonine kinase p38 mitogen-activated protein kinase (MAPK) in the cells adjacent to the wound. p38 activation reduces the levels of wound-induced reactive oxygen species in the cells around the wound, limiting wound size. In addition, p38 promotes an increase in volume in the cells around the wound, thus facilitating the collective cell movements that drive rapid wound healing. Our data indicate that p38 regulates cell volumes through the sodium-potassium-chloride cotransporter NKCC1. Our work reveals cell growth and cell survival as cell behaviors critical for embryonic wound repair.

Multiscale In Vivo Imaging of Collective Cell Migration in Drosophila Embryos

Coordinated cell movements drive embryonic development and tissue repair, and can also spread disease. Time-lapse microscopy is an integral part in the study of the cell biology of collective cell movements. Advances in imaging techniques enable monitoring dynamic cellular and molecular events in real time within living animals. Here, we demonstrate the use of spinning disk confocal microscopy to investigate coordinated cell movements and epithelial-to-mesenchymal-like transitions during embryonic wound closure in Drosophila. We describe image-based metrics to quantify the efficiency of collective cell migration. Finally, we show the application of super-resolution radial fluctuation microscopy to obtain multidimensional, super-resolution images of protrusive activity in collectively moving cells in vivo. Together, the methods presented here constitute a toolkit for the modern analysis of collective cell migration in living animals.

The pulse of morphogenesis: actomyosin dynamics and regulation in epithelia

Actomyosin networks are some of the most crucial force-generating components present in developing tissues. The contractile forces generated by these networks are harnessed during morphogenesis to drive various cell and tissue reshaping events. Recent studies of these processes have advanced rapidly, providing us with insights into how these networks are initiated, positioned and regulated, and how they act via individual contractile pulses and/or the formation of supracellular cables. Here, we review these studies and discuss the mechanisms that underlie the construction and turnover of such networks and structures. Furthermore, we provide an overview of how ratcheted processivity emerges from pulsed events, and how tissue-level mechanics are the coordinated output of many individual cellular behaviors.

Actin and myosin dynamics are independent during Drosophila embryonic wound repair

Collective cell movements play a central role in embryonic development, tissue repair, and metastatic disease. Cell movements are often coordinated by supracellular networks formed by the cytoskeletal protein actin and the molecular motor nonmuscle myosin II. During wound closure in the embryonic epidermis, the cells around the wound migrate collectively into the damaged region. In Drosophila embryos, mechanical tension stabilizes myosin at the wound edge, facilitating the assembly of a supracellular myosin cable around the wound that coordinates cell migration. Here, we show that actin is also stabilized at the wound edge. However, loss of tension or myosin activity does not affect the dynamics of actin at the wound margin. Conversely, pharmacological stabilization of actin does not affect myosin levels or dynamics around the wound. Together, our data suggest that actin and myosin are independently regulated during embryonic wound closure, thus conferring robustness to the embryonic wound healing response.

Structural Redundancy in Supracellular Actomyosin Networks Enables Robust Tissue Folding

Tissue morphogenesis is strikingly robust. Yet, how tissues are sculpted under challenging conditions is unknown. Here, we combined network analysis, experimental perturbations, and computational modeling to determine how network connectivity between hundreds of contractile cells on the ventral side of the Drosophila embryo ensures robust tissue folding. We identified two network properties that mechanically promote robustness. First, redundant supracellular cytoskeletal network paths ensure global connectivity, even with network degradation. By forming many more connections than are required, morphogenesis is not disrupted by local network damage, analogous to the way redundancy guarantees the large-scale function of vasculature and transportation networks. Second, directional stiffening of edges oriented orthogonal to the folding axis promotes furrow formation at lower contractility levels. Structural redundancy and directional network stiffening ensure robust tissue folding with proper orientation.

Tissue Fluidity Promotes Epithelial Wound Healing

The collective behaviour of cells in epithelial tissues is dependent on their mechanical properties. However, the contribution of tissue mechanics to wound healing in vivo remains poorly understood. Here we investigate the relationship between tissue mechanics and wound healing in live Drosophila wing imaginal discs and show that by tuning epithelial cell junctional tension, we can systematically alter the rate of wound healing. Coincident with the contraction of an actomyosin purse string, we observe cells flowing past each other at the wound edge by intercalating, reminiscent of molecules in a fluid, resulting in seamless wound closure. Using a cell-based physical model, we predict that a reduction in junctional tension fluidises the tissue through an increase in intercalation rate and corresponding reduction in bulk viscosity, in the manner of an unjamming transition. The resultant fluidisation of the tissue accelerates wound healing. Accordingly, when we experimentally reduce tissue tension in wing discs, intercalation rate increases and wounds repair in less time.

Forceful closure: cytoskeletal networks in embryonic wound repair

Embryonic tissues heal wounds rapidly and without scarring, in a process conserved across species and driven by collective cell movements. The mechanisms of coordinated cell movement during embryonic wound closure also drive tissue development and cancer metastasis; therefore, embryonic wound repair has received considerable attention as a model of collective cell migration. During wound closure, a supracellular actomyosin cable at the wound edge coordinates cells, while actin-based protrusions contribute to cell crawling and seamless wound healing. Other cytoskeletal networks are reorganized during wound repair: microtubules extend into protrusions and along cell-cell boundaries as cells stretch into damaged regions, septins accumulate at the wound margin, and intermediate filaments become polarized in the cells adjacent to the wound. Thus, diverse cytoskeletal networks work in concert to maintain tissue structure, while also driving and organizing cell movements to promote rapid repair. Understanding the signals that coordinate the dynamics of different cytoskeletal networks, and how adhesions between cells or with the extracellular matrix integrate forces across cells, will be important to elucidate the mechanisms of efficient embryonic wound healing and may have far-reaching implications for developmental and cancer cell biology.

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

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

Septin-dependent remodeling of cortical microtubule drives cell reshaping during epithelial wound healing

Wounds in embryos heal rapidly through contraction of the wound edges. Despite well-recognized significance of the actomyosin purse string for wound closure, roles for other cytoskeletal components are largely unknown. Here, we report that the septin cytoskeleton cooperates with actomyosin and microtubules to coordinate circumferential contraction of the wound margin and concentric elongation of wound-proximal cells in Xenopus laevis embryos. Microtubules reoriented radially, forming bundles along lateral cell cortices in elongating wound-proximal cells. Depletion of septin 7 (Sept7) slowed wound closure by attenuating the wound edge contraction and cell elongation. ROCK/Rho-kinase inhibitor-mediated suppression of actomyosin contractility enhanced the Sept7 phenotype, whereas the Sept7 depletion did not affect the accumulation of actomyosin at the wound edge. The cortical microtubule bundles were reduced in wound-proximal cells in Sept7 knockdown (Sept7-KD) embryos, but forced bundling of microtubules mediated by the microtubule-stabilizing protein Map7 did not rescue the Sept7-KD phenotype. Nocodazole-mediated microtubule depolymerization enhanced the Sept7-KD phenotype, suggesting that Sept7 is required for microtubule reorganization during cell elongation. Our findings indicate that septins are required for the rapid wound closure by facilitating cortical microtubule reorganization and the concentric elongation of surrounding cells.

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

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

Coordinating cell movements in vivo: junctional and cytoskeletal dynamics lead the way

Collective cell movements drive embryonic development and tissue repair, and can cause disease. However, the mechanisms that coordinate the migration of groups of cells in vivo are unclear. Cells generate, transmit and sense mechanical forces to align their movements. Therefore, the machinery used by cells to generate force (cytoskeleton) and to transmit and sense mechanical signals (cell-cell adhesion) is critical for collective movement. Here, we review the components and organization of the cytoskeletal and cell-cell adhesive machineries, and how they are organized to promote collective cell movements in living animals. We discuss the signals that orchestrate molecular rearrangements necessary for coordinated cell motility, and we provide specific examples of movements both in the plane of the tissue (wound healing) and perpendicular to that plane (apical constriction).

Shape of my heart: Cell-cell adhesion and cytoskeletal dynamics during Drosophila cardiac morphogenesis

The fruit fly Drosophila melanogaster has recently emerged as an excellent system to investigate the genetics of cardiovascular development and disease. Drosophila provides an inexpensive and genetically-tractable in vivo system with a large number of conserved features. In addition, the Drosophila embryo is transparent, and thus amenable to time-lapse fluorescence microscopy, as well as biophysical and pharmacological manipulations. One of the conserved aspects of heart development from Drosophila to humans is the initial assembly of a tube. Here, we review the cellular behaviours and molecular dynamics important for the initial steps of heart morphogenesis in Drosophila, with particular emphasis on the cell-cell adhesion and cytoskeletal networks that cardiac precursors use to move, coordinate their migration, interact with other tissues and eventually sculpt a beating heart.

Tension regulates myosin dynamics during Drosophila embryonic wound repair

Embryos repair epithelial wounds rapidly in a process driven by collective cell movements. Upon wounding, actin and the molecular motor non-muscle myosin II are redistributed in the cells adjacent to the wound, forming a supracellular purse string around the lesion. Purse string contraction coordinates cell movements and drives rapid wound closure. By using fluorescence recovery after photobleaching in Drosophila embryos, we found that myosin turns over as the purse string contracts. Myosin turnover at the purse string was slower than in other actomyosin networks that had a lower level of contractility. Mathematical modelling suggested that myosin assembly and disassembly rates were both reduced by tension at the wound edge. We used laser ablation to show that tension at the purse string increased as wound closure progressed, and that the increase in tension was associated with reduced myosin turnover. Reducing purse string tension by laser-mediated severing resulted in increased turnover and loss of myosin. Finally, myosin motor activity was necessary for its stabilization around the wound and for rapid wound closure. Our results indicate that mechanical forces regulate myosin dynamics during embryonic wound repair.

Spina Bifida: Pathogenesis, Mechanisms, and Genes in Mice and Humans

Spina bifida is among the phenotypes of the larger condition known as neural tube defects (NTDs). It is the most common central nervous system malformation compatible with life and the second leading cause of birth defects after congenital heart defects. In this review paper, we define spina bifida and discuss the phenotypes seen in humans as described by both surgeons and embryologists in order to compare and ultimately contrast it to the leading animal model, the mouse. Our understanding of spina bifida is currently limited to the observations we make in mouse models, which reflect complete or targeted knockouts of genes, which perturb the whole gene(s) without taking into account the issue of haploinsufficiency, which is most prominent in the human spina bifida condition. We thus conclude that the need to study spina bifida in all its forms, both aperta and occulta, is more indicative of the spina bifida in surviving humans and that the measure of deterioration arising from caudal neural tube defects, more commonly known as spina bifida, must be determined by the level of the lesion both in mouse and in man.

Tension (re)builds: Biophysical mechanisms of embryonic wound repair

Embryonic tissues display an outstanding ability to rapidly repair wounds. Epithelia, in particular, serve as protective layers that line internal organs and form the skin. Thus, maintenance of epithelial integrity is of utmost importance for animal survival, particularly at embryonic stages, when an immune system has not yet fully developed. Rapid embryonic repair of epithelial tissues is conserved across species, and involves the collective migration of the cells around the wound. The migratory cell behaviours associated with wound repair require the generation and transmission of mechanical forces, not only for the cells to move, but also to coordinate their movements. Here, we review the forces involved in embryonic wound repair. We discuss how different force-generating structures are assembled at the molecular level, and the mechanisms that maintain the balance between force-generating structures as wounds close. Finally, we describe the mechanisms that cells use to coordinate the generation of mechanical forces around the wound. Collective cell movements and their misregulation have been associated with defective tissue repair, developmental abnormalities and cancer metastasis. Thus, we propose that understanding the role of mechanical forces during embryonic wound closure will be crucial to develop therapeutic interventions that promote or prevent collective cell movements under pathological conditions.

The cellular and molecular mechanisms of tissue repair and regeneration as revealed by studies in Xenopus

Survival of any living organism critically depends on its ability to repair and regenerate damaged tissues and/or organs during its lifetime following injury, disease, or aging. Various animal models from invertebrates to vertebrates have been used to investigate the molecular and cellular mechanisms of wound healing and tissue regeneration. It is hoped that such studies will form the framework for identifying novel clinical treatments that will improve the healing and regenerative capacity of humans. Amongst these models, Xenopus stands out as a particularly versatile and powerful system. This review summarizes recent findings using this model, which have provided fundamental knowledge of the mechanisms responsible for efficient and perfect tissue repair and regeneration.

Mechanics of epithelial tissues during gap closure

The closure of gaps is crucial to maintaining epithelium integrity during developmental and repair processes such as dorsal closure and wound healing. Depending on biochemical as well as physical properties of the microenvironment, gap closure occurs through assembly of multicellular actin-based contractile cables and/or protrusive activity of cells lining the gap. This review discusses the relative contributions of 'purse-string' and cell crawling mechanisms regulated by cell-substrate and cell-cell interactions, cellular mechanics and physical constraints from the environment.

Polarized E-cadherin endocytosis directs actomyosin remodeling during embryonic wound repair

Clathrin, dynamin, and ARF6 accumulate around wounds in Drosophila embryos in a calcium- and actomyosin-dependent manner and drive polarized E-cadherin endocytosis, which is necessary for actomyosin remodeling during wound repair.

Embryonic epithelia have a remarkable ability to rapidly repair wounds. A supracellular actomyosin cable around the wound coordinates cellular movements and promotes wound closure. Actomyosin cable formation is accompanied by junctional rearrangements at the wound margin. We used in vivo time-lapse quantitative microscopy to show that clathrin, dynamin, and the ADP-ribosylation factor 6, three components of the endocytic machinery, accumulate around wounds in Drosophila melanogaster embryos in a process that requires calcium signaling and actomyosin contractility. Blocking endocytosis with pharmacological or genetic approaches disrupted wound repair. The defect in wound closure was accompanied by impaired removal of E-cadherin from the wound edge and defective actomyosin cable assembly. E-cadherin overexpression also resulted in reduced actin accumulation around wounds and slower wound closure. Reducing E-cadherin levels in embryos in which endocytosis was blocked rescued actin localization to the wound margin. Our results demonstrate a central role for endocytosis in wound healing and indicate that polarized E-cadherin endocytosis is necessary for actomyosin remodeling during embryonic wound repair.

Understanding early organogenesis using a simplified in situ hybridization protocol in Xenopus

Organogenesis is the study of how organs are specified and then acquire their specific shape and functions during development. The Xenopuslaevis embryo is very useful for studying organogenesis because their large size makes them very suitable for identifying organs at the earliest steps in organogenesis. At this time, the primary method used for identifying a specific organ or primordium is whole mount in situ hybridization with labeled antisense RNA probes specific to a gene that is expressed in the organ of interest. In addition, it is relatively easy to manipulate genes or signaling pathways in Xenopus and in situ hybridization allows one to then assay for changes in the presence or morphology of a target organ. Whole mount in situ hybridization is a multi-day protocol with many steps involved. Here we provide a simplified protocol with reduced numbers of steps and reagents used that works well for routine assays. In situ hybridization robots have greatly facilitated the process and we detail how and when we utilize that technology in the process. Once an in situ hybridization is complete, capturing the best image of the result can be frustrating. We provide advice on how to optimize imaging of in situ hybridization results. Although the protocol describes assessing organogenesis in Xenopus laevis, the same basic protocol can almost certainly be adapted to Xenopus tropicalis and other model systems.

Forces driving epithelial wound healing

A fundamental feature of multicellular organisms is their ability to self-repair wounds through the movement of epithelial cells into the damaged area. This collective cellular movement is commonly attributed to a combination of cell crawling and "purse-string" contraction of a supracellular actomyosin ring. Here we show by direct experimental measurement that these two mechanisms are insufficient to explain force patterns observed during wound closure. At early stages of the process, leading actin protrusions generate traction forces that point away from the wound, showing that wound closure is initially driven by cell crawling. At later stages, we observed unanticipated patterns of traction forces pointing towards the wound. Such patterns have strong force components that are both radial and tangential to the wound. We show that these force components arise from tensions transmitted by a heterogeneous actomyosin ring to the underlying substrate through focal adhesions. The structural and mechanical organization reported here provides cells with a mechanism to close the wound by cooperatively compressing the underlying substrate.

Apical constriction: themes and variations on a cellular mechanism driving morphogenesis

Apical constriction is a cell shape change that promotes tissue remodeling in a variety of homeostatic and developmental contexts, including gastrulation in many organisms and neural tube formation in vertebrates. In recent years, progress has been made towards understanding how the distinct cell biological processes that together drive apical constriction are coordinated. These processes include the contraction of actin-myosin networks, which generates force, and the attachment of actin networks to cell-cell junctions, which allows forces to be transmitted between cells. Different cell types regulate contractility and adhesion in unique ways, resulting in apical constriction with varying dynamics and subcellular organizations, as well as a variety of resulting tissue shape changes. Understanding both the common themes and the variations in apical constriction mechanisms promises to provide insight into the mechanics that underlie tissue morphogenesis.

ERK and phosphoinositide 3-kinase temporally coordinate different modes of actin-based motility during embryonic wound healing

Embryonic wound healing provides a perfect example of efficient recovery of tissue integrity and homeostasis, which is vital for survival. Tissue movement in embryonic wound healing requires two functionally distinct actin structures: a contractile actomyosin cable and actin protrusions at the leading edge. Here, we report that the discrete formation and function of these two structures is achieved by the temporal segregation of two intracellular upstream signals and distinct downstream targets. The sequential activation of ERK and phosphoinositide 3-kinase (PI3K) signalling divides Xenopus embryonic wound healing into two phases. In the first phase, activated ERK suppresses PI3K activity, and is responsible for the activation of Rho and myosin-2, which drives actomyosin cable formation and constriction. The second phase is dominated by restored PI3K signalling, which enhances Rac and Cdc42 activity, leading to the formation of actin protrusions that drive migration and zippering. These findings reveal a new mechanism for coordinating different modes of actin-based motility in a complex tissue setting, namely embryonic wound healing.

Wounded cells drive rapid epidermal repair in the early Drosophila embryo

Epithelial tissues are protective barriers that display a remarkable ability to repair wounds. Wound repair is often associated with an accumulation of actin and nonmuscle myosin II around the wound, forming a purse string. The role of actomyosin networks in generating mechanical force during wound repair is not well understood. Here we investigate the mechanisms of force generation during wound repair in the epidermis of early and late Drosophila embryos. We find that wound closure is faster in early embryos, where, in addition to a purse string around the wound, actomyosin networks at the medial cortex of the wounded cells contribute to rapid wound repair. Laser ablation demonstrates that both medial and purse-string actomyosin networks generate contractile force. Quantitative analysis of protein localization dynamics during wound closure indicates that the rapid contraction of medial actomyosin structures during wound repair in early embryos involves disassembly of the actomyosin network. By contrast, actomyosin purse strings in late embryos contract more slowly in a mechanism that involves network condensation. We propose that the combined action of two force-generating structures--a medial actomyosin network and an actomyosin purse string--contributes to the increased efficiency of wound repair in the early embryo.

Computational and experimental study of the mechanics of embryonic wound healing

Wounds in the embryo show a remarkable ability to heal quickly without leaving a scar. Previous studies have found that an actomyosin ring (purse string) forms around the wound perimeter and contracts to close the wound over the course of several dozens of minutes. Here, we report experiments that reveal an even faster mechanism which remarkably closes wounds by more than 50% within the first 30s. Circular and elliptical wounds (~100μm in size) were made in the blastoderm of early chick embryos and allowed to heal, with wound area and shape characterized as functions of time. The closure rate displayed a biphasic behavior, with rapid constriction lasting about a minute, followed by a period of more gradual closure to complete healing. Fluorescent staining suggests that both healing phases are driven by actomyosin contraction, with relatively rapid contraction of fibers at cell borders within a relatively thick ring of tissue (several cells wide) around the wound followed by slower contraction of a thin supracellular actomyosin ring along the margin, consistent with a purse string mechanism. Finite-element modeling showed that this idea is biophysically plausible, with relatively isotropic contraction within the thick ring giving way to tangential contraction in the thin ring. In addition, consistent with experimental results, simulated elliptical wounds heal with little change in aspect ratio, and decreased membrane tension can cause these wounds to open briefly before going on to heal. These results provide new insight into the healing mechanism in embryonic epithelia.

Inositol kinase and its product accelerate wound healing by modulating calcium levels, Rho GTPases, and F-actin assembly

Wound healing is essential for survival. We took advantage of the Xenopus embryo, which exhibits remarkable capacities to repair wounds quickly and efficiently, to investigate the mechanisms responsible for wound healing. Previous work has shown that injury triggers a rapid calcium response, followed by the activation of Ras homolog (Rho) family guanosine triphosphatases (GTPases), which regulate the formation and contraction of an F-actin purse string around the wound margin. How these processes are coordinated following wounding remained unclear. Here we show that inositol-trisphosphate 3-kinase B (Itpkb) via its enzymatic product inositol 1,3,4,5-tetrakisphosphate (InsP4) plays an essential role during wound healing by modulating the activity of Rho family GTPases and F-actin ring assembly. Furthermore, we show that Itpkb and InsP4 modulate the speed of the calcium wave, which propagates from the site of injury into neighboring uninjured cells. Strikingly, both overexpression of itpkb and exogenous application of InsP4 accelerate the speed of wound closure, a finding that has potential implications in our quest to find treatments that improve wound healing in patients with acute or chronic wounds.

Epithelial machines of morphogenesis and their potential application in organ assembly and tissue engineering

Sheets of embryonic epithelial cells coordinate their efforts to create diverse tissue structures such as pits, grooves, tubes, and capsules that lead to organ formation. Such cells can use a number of cell behaviors including contractility, proliferation, and directed movement to create these structures. By contrast, tissue engineers and researchers in regenerative medicine seeking to produce organs for repair or replacement therapy can combine cells with synthetic polymeric scaffolds. Tissue engineers try to achieve these goals by shaping scaffold geometry in such a way that cells embedded within these scaffold self-assemble to form a tissue, for instance aligning to synthetic fibers, and assembling native extracellular matrix to form the desired tissue-like structure. Although self-assembly is a dominant process that guides tissue assembly both within the embryo and within artificial tissue constructs, we know little about these critical processes. Here, we compare and contrast strategies of tissue assembly used by embryos to those used by engineers during epithelial morphogenesis and highlight opportunities for future applications of developmental biology in the field of tissue engineering.

Computational models for mechanics of morphogenesis

In the developing embryo, tissues differentiate, deform, and move in an orchestrated manner to generate various biological shapes driven by the complex interplay between genetic, epigenetic, and environmental factors. Mechanics plays a key role in regulating and controlling morphogenesis, and quantitative models help us understand how various mechanical forces combine to shape the embryo. Models allow for the quantitative, unbiased testing of physical mechanisms, and when used appropriately, can motivate new experimentaldirections. This knowledge benefits biomedical researchers who aim to prevent and treat congenital malformations, as well as engineers working to create replacement tissues in the laboratory. In this review, we first give an overview of fundamental mechanical theories for morphogenesis, and then focus on models for specific processes, including pattern formation, gastrulation, neurulation, organogenesis, and wound healing. The role of mechanical feedback in development is also discussed. Finally, some perspectives aregiven on the emerging challenges in morphomechanics and mechanobiology.

Drosophila embryos close epithelial wounds using a combination of cellular protrusions and an actomyosin purse string

The repair of injured tissue must occur rapidly to prevent microbial invasion and maintain tissue integrity. Epithelial tissues in particular, which serve as a barrier against the external environment, must repair efficiently in order to restore their primary function. Here we analyze the effect of different parameters on the epithelial wound repair process in the late stage Drosophila embryo using in vivo wound assays, expression of cytoskeleton and membrane markers, and mutant analysis. We define four distinct phases in the repair process, expansion, coalescence, contraction and closure, and describe the molecular dynamics of each phase. Specifically, we find that myosin, E-cadherin, Echinoid, the plasma membrane, microtubules and the Cdc42 small GTPase respond dynamically during wound repair. We demonstrate that perturbations of each of these components result in specific impairments to the wound healing process. Our results show that embryonic epithelial wound repair is mediated by two simultaneously acting mechanisms: crawling driven by cellular protrusions and actomyosin ring contraction along the leading edge of the wound.

Transcriptional regulation of Profilin during wound closure in Drosophila larvae

Injury is an inevitable part of life, making wound healing essential for survival. In postembryonic skin, wound closure requires that epidermal cells recognize the presence of a gap and change their behavior to migrate across it. In Drosophila larvae, wound closure requires two signaling pathways [the Jun N-terminal kinase (JNK) pathway and the Pvr receptor tyrosine kinase signaling pathway] and regulation of the actin cytoskeleton. In this and other systems, it remains unclear how the signaling pathways that initiate wound closure connect to the actin regulators that help execute wound-induced cell migrations. Here, we show that chickadee, which encodes the Drosophila Profilin, a protein important for actin filament recycling and cell migration during development, is required for the physiological process of larval epidermal wound closure. After injury, chickadee is transcriptionally upregulated in cells proximal to the wound. We found that JNK, but not Pvr, mediates the increase in chic transcription through the Jun and Fos transcription factors. Finally, we show that chic-deficient larvae fail to form a robust actin cable along the wound edge and also fail to form normal filopodial and lamellipodial extensions into the wound gap. Our results thus connect a factor that regulates actin monomer recycling to the JNK signaling pathway during wound closure. They also reveal a physiological function for an important developmental regulator of actin and begin to tease out the logic of how the wound repair response is organized.

Exposure to external environment of low ion concentrations is the trigger for rapid wound closure in Xenopus laevis embryos

Wounds in Xenopus laevis embryos close rapidly, as previously described. In this study, we examined the dependence on extracellular Na(+) and/or Cl(-) ion concentrations of the closure of wounds in Xenopus embryos inflicted by thermal injury. Wound closure did not occur in normal amphibian medium (100% NAM), while wound areas remarkably decreased either in 10-50% NAM or in 100% NAM lacking Na(+) or Cl(-). Similarly, wound areas did not change in a set of Na(+) and Cl(-) ion concentrations equivalent to those of the humoral fluids of intact Xenopus embryos, but rapid wound closure was induced by decreasing the concentration of either of the two ions. A tangential accumulation of actin cytoskeleton along the wound edge was associated with wound closure. However, a similar actin alignment formed even under the 100% NAM condition, in which wounds did not close, as stated above. The epidermis around the wound edge exhibited ellipse-shaped hypertrophy, and the marginal cells centripetally elongated during wound closure. On the other hand, no distinct morphological changes occurred in 100% NAM, although the epidermis was somewhat thickened. Thus, the morphological changes in the epidermis specific to the low ionic environment most likely play active roles in the wound closure of Xenopus laevis embryos, whereas the tangential actin alignment alone may be insufficient. Taken together, we propose that the wound closure in Xenopus embryos is triggered by a decline in either the extracellular Na(+) or Cl(-) ion concentration, and that this process is required for the abovementioned changes in the shape of the marginal cells.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Low-magnification live imaging of Xenopus embryos for cell and developmental biology

Biological processes occur dynamically, so dynamic analyses are essential to further our understanding of them. Embryos of the frog Xenopus laevis are an ideal model system for time-lapse imaging of biological processes. Xenopus embryos are especially amenable to in vivo imaging of whole embryos, individual cells, and subcellular processes. Low-magnification imaging of intact Xenopus embryos can be combined effectively with manipulations of gene function and has provided key insights into the control of morphogenetic movements during gastrulation and neural tube closure. The utility of this approach is not limited to tissue movements, but rather extends to studies of cytokinesis, wound healing, and other dynamic events. This protocol describes methods for low-magnification time-lapse imaging of intact Xenopus embryos.

Live-cell imaging and quantitative analysis of embryonic epithelial cells in Xenopus laevis

Embryonic epithelial cells serve as an ideal model to study morphogenesis where multi-cellular tissues undergo changes in their geometry, such as changes in cell surface area and cell height, and where cells undergo mitosis and migrate. Furthermore, epithelial cells can also regulate morphogenetic movements in adjacent tissues(1). A traditional method to study epithelial cells and tissues involve chemical fixation and histological methods to determine cell morphology or localization of particular proteins of interest. These approaches continue to be useful and provide "snapshots" of cell shapes and tissue architecture, however, much remains to be understood about how cells acquire specific shapes, how various proteins move or localize to specific positions, and what paths cells follow toward their final differentiated fate. High resolution live imaging complements traditional methods and also allows more direct investigation into the dynamic cellular processes involved in the formation, maintenance, and morphogenesis of multicellular epithelial sheets. Here we demonstrate experimental methods from the isolation of animal cap tissues from Xenopus laevis embryos to confocal imaging of epithelial cells and simple measurement approaches that together can augment molecular and cellular studies of epithelial morphogenesis.

Experimental control of excitable embryonic tissues: three stimuli induce rapid epithelial contraction

Cell generated contractility is a major driver of morphogenesis during processes such as epithelial bending and epithelial-to-mesenchymal transitions. Previous studies of contraction in embryos have relied on developmentally programmed cell shape changes such as those that accompany ventral furrow formation in Drosophila, bottle cell formation in Xenopus, ingression in amniote embryos, and neurulation in vertebrate embryos. We have identified three methods to reproducibly and acutely induce contraction in embryonic epithelial sheets: laser activation, electrical stimulation, and nano-perfusion with chemicals released by wounding. Contractions induced by all three methods occur over a similar time-scale (1 to 2 min) and lead to reorganization of the F-actin cytoskeleton. By combining induced contractions with micro-aspiration we can simultaneously measure the stiffness of the tissue and the force and work done by contractions. Laser activation allows real-time visualization of F-actin remodeling during contraction. Perfusion with cell lysate suggests that these three stimuli activate physiologically relevant pathways that maintain epithelial tension or trigger epithelial morphogenesis. Our methods provide the means to control and study cellular contractility and will allow dissection of molecular mechanisms and biomechanics of cellular contractility.

Genetic screen in Drosophila melanogaster uncovers a novel set of genes required for embryonic epithelial repair

The wound healing response is an essential mechanism to maintain the integrity of epithelia and protect all organisms from the surrounding milieu. In the "purse-string" mechanism of wound closure, an injured epithelial sheet cinches its hole closed via an intercellular contractile actomyosin cable. This process is conserved across species and utilized by both embryonic as well as adult tissues, but remains poorly understood at the cellular level. In an effort to identify new players involved in purse-string wound closure we developed a wounding strategy suitable for screening large numbers of Drosophila embryos. Using this methodology, we observe wound healing defects in Jun-related antigen (encoding DJUN) and scab (encoding Drosophila alphaPS3 integrin) mutants and performed a forward genetics screen on the basis of insertional mutagenesis by transposons that led to the identification of 30 lethal insertional mutants with defects in embryonic epithelia repair. One of the mutants identified is an insertion in the karst locus, which encodes Drosophila beta(Heavy)-spectrin. We show beta(Heavy)-spectrin (beta(H)) localization to the wound edges where it presumably exerts an essential function to bring the wound to normal closure.

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

The involvement of lethal giant larvae and Wnt signaling in bottle cell formation in Xenopus embryos

Lethal giant larvae (Lgl) plays a critical role in establishment of cell polarity in epithelial cells. While Frizzled/Dsh signaling has been implicated in the regulation of the localization and activity of Lgl, it remains unclear whether specific Wnt ligands are involved. Here we show that Wnt5a triggers the release of Lgl from the cell cortex into the cytoplasm with the concomitant decrease in Lgl stability. The observed changes in Lgl localization were independent of atypical PKC (aPKC), which is known to influence Lgl distribution. In ectodermal cells, both Wnt5a and Lgl triggered morphological and molecular changes characteristic of apical constriction, whereas depletion of their functions prevented endogenous and ectopic bottle cell formation. Furthermore, Lgl RNA partially rescued bottle cell formation in embryos injected with a dominant negative Wnt5a construct. These results suggest a molecular link between Wnt5a and Lgl that is essential for apical constriction during vertebrate gastrulation.

Apical constriction: a cell shape change that can drive morphogenesis

Biologists have long recognized that dramatic bending of a cell sheet may be driven by even modest shrinking of the apical sides of cells. Cell shape changes and tissue movements like these are at the core of many of the morphogenetic movements that shape animal form during development, driving processes such as gastrulation, tube formation, and neurulation. The mechanisms of such cell shape changes must integrate developmental patterning information in order to spatially and temporally control force production-issues that touch on fundamental aspects of both cell and developmental biology and on birth defects research. How does developmental patterning regulate force-producing mechanisms, and what roles do such mechanisms play in development? Work on apical constriction from multiple systems including Drosophila, Caenorhabditis elegans, sea urchin, Xenopus, chick, and mouse has begun to illuminate these issues. Here, we review this effort to explore the diversity of mechanisms of apical constriction, the diversity of roles that apical constriction plays in development, and the common themes that emerge from comparing systems.

Integration of single and multicellular wound responses

Single cells and multicellular tissues rapidly heal wounds. These processes are considered distinct, but one mode of healing--Rho GTPase-dependent formation and closure of a purse string of actin filaments (F-actin) and myosin-2 around wounds--occurs in single cells and in epithelia. Here, we show that wounding of one cell in Xenopus embryos elicits Rho GTPase activation around the wound and at the nearest cell-cell junctions in the neighbor cells. F-actin and myosin-2 accumulate at the junctions and around the wound itself, and as the resultant actomyosin array closes over the wound site, junctional F-actin and myosin-2 become mechanically integrated with the actin and myosin-2 around the wound, forming a hybrid purse string. When cells are ablated rather than wounded, Rho GTPase activation and F-actin accumulation occur at cell-cell junctions surrounding the ablated cell, and the purse string closes the hole in the epithelium. Elevation of intracellular free calcium, an essential upstream signal for the single-cell wound response, also occurs at the cell-cell contacts and in neighbor cells. Thus, the single and multicellular purse string wound responses represent points on a signaling and mechanical continuum that are integrated by cell-cell junctions.

Inca: a novel p21-activated kinase-associated protein required for cranial neural crest development

Inca (induced in neural crest by AP2) is a novel protein discovered in a microarray screen for genes that are upregulated in Xenopus embryos by the transcriptional activator protein Tfap2a. It has no significant similarity to any known protein, but is conserved among vertebrates. In Xenopus, zebrafish and mouse embryos, Inca is expressed predominantly in the premigratory and migrating neural crest (NC). Knockdown experiments in frog and fish using antisense morpholinos reveal essential functions for Inca in a subset of NC cells that form craniofacial cartilage. Cells lacking Inca migrate successfully but fail to condense into skeletal primordia. Overexpression of Inca disrupts cortical actin and prevents formation of actin "purse strings", which are required for wound healing in Xenopus embryos. We show that Inca physically interacts with p21-activated kinase 5 (PAK5), a known regulator of the actin cytoskeleton that is co-expressed with Inca in embryonic ectoderm, including in the NC. These results suggest that Inca and PAK5 cooperate in restructuring cytoskeletal organization and in the regulation of cell adhesion in the early embryo and in NC cells during craniofacial development.

Measurement of in vivo stress resultants in neurulation-stage amphibian embryos

In order to obtain the first quantitative measurements of the in vivo stresses in early-stage amphibian embryos, we developed a novel instrument that uses a pair of parallel wires that are glued to the surface of an embryo normal to the direction in which the stress is to be determined. When a slit is made parallel to the wires and between them, tension in the surrounding tissue causes the slit to open. Under computer control, one of the wires is moved so as to restore the original wire spacing, and the steady-state closure force is determined from the degree of wire flexure. A cell-level finite element model is used to convert the wire bending force to an in-plane stress since the wire force is not proportional to the slit length. The device was used to measure stress resultants (force carried per unit of slit length) on the dorsal, ventral and lateral aspects of neurulation-stage axolotl (Ambystoma mexicanum) embryos. The resultants were anisotropic and varied with location and developmental stage, with values ranging from -0.17 mN/m to 1.92 mN/m. In general, the resultants could be decomposed into patterns associated with internal pressure in the embryo, bending of the embryo along its mid-sagittal plane and neural tube closure. The patterns of stress revealed by the experiments support a number of current theories about the mechanics of neurulation.