Cell adhesion in amphibian gastrulation

Blastocoel morphogenesis: A biophysics perspective

The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.

Cell-cell contact landscapes in Xenopus gastrula tissues

Molecular and structural facets of cell-cell adhesion have been extensively studied in monolayered epithelia. Here, we perform a comprehensive analysis of cell-cell contacts in a series of multilayered tissues in the Xenopus gastrula model. We show that intercellular contact distances range from 10 to 1,000 nm. The contact width frequencies define tissue-specific contact spectra, and knockdown of adhesion factors modifies these spectra. This allows us to reconstruct the emergence of contact types from complex interactions of the factors. We find that the membrane proteoglycan Syndecan-4 plays a dominant role in all contacts, including narrow C-cadherin-mediated junctions. Glypican-4, hyaluronic acid, paraxial protocadherin, and fibronectin also control contact widths, and unexpectedly, C-cadherin functions in wide contacts. Using lanthanum staining, we identified three morphologically distinct forms of glycocalyx in contacts of the Xenopus gastrula, which are linked to the adhesion factors examined and mediate cell-cell attachment. Our study delineates a systematic approach to examine the varied contributions of adhesion factors individually or in combinations to nondiscrete and seemingly amorphous intercellular contacts.

Ectoderm to mesoderm transition by down-regulation of actomyosin contractility

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

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.

Polarity and morphogenesis of the eye epithelium requires the adhesion junction associated adaptor protein Traf4

During development, neuroepithelial progenitors acquire apico-basal polarity and adhere to one another via apically located tight and adherens junction complexes. This polarized neuroepithelium must continue to integrate cells arising through cell divisions and intercalation, and allow for cell movements, at the same time as undergoing morphogenesis. Cell proliferation, migration and intercalation all occur in the morphing embryonic eye. To understand how eye development might depend on dynamic epithelial adhesion, we investigated the function of a known regulator of junctional plasticity, Tumour necrosis factor receptor-associated factor 4 (Traf4). traf4a mRNA is expressed in the developing eye vesicle over the period of optic cup morphogenesis, and Traf4a loss leads to disrupted evagination and elongation of the eye vesicles, and aberrant organization and apico-basal polarity of the eye epithelium. We propose a model whereby Traf4a regulates apical junction plasticity in nascent eye epithelium, allowing for its polarization and morphogenesis. Symbols and Abbreviations: AB: apico-basal; aPKC: atypical protein kinase-C; CRISPR: clustered regularly-interspaced short palindromic repeats; GFP: green fluorescent protein; hpf: hours post-fertilization; MO: antisense morpholino oligonucleotide; pHH3: phospho histone H3; ss: somite stage; Traf4: Tumour necrosis factor receptor-associated factor 4; ZO-1: zona occludens-1.

PDGF-A suppresses contact inhibition during directional collective cell migration

The leading-edge mesendoderm (LEM) of the Xenopus gastrula moves as an aggregate by collective migration. However, LEM cells on fibronectin in vitro show contact inhibition of locomotion by quickly retracting lamellipodia upon mutual contact. We found that a fibronectin-integrin-syndecan module acts between p21-activated kinase 1 upstream and ephrin B1 downstream to promote the contact-induced collapse of lamellipodia. To function in this module, fibronectin has to be present as puncta on the surface of LEM cells. To overcome contact inhibition in LEM cell aggregates, PDGF-A deposited in the endogenous substratum of LEM migration blocks the fibronectin-integrin-syndecan module at the integrin level. This stabilizes lamellipodia preferentially in the direction of normal LEM movement and supports cell orientation and the directional migration of the coherent LEM cell mass.

Cell migration in the Xenopus gastrula

Xenopus gastrulation movements are in large part based on the rearrangement of cells by differential cell-on-cell migration within multilayered tissues. Different patterns of migration-based cell intercalation drive endoderm and mesoderm internalization and their positioning along their prospective body axes. C-cadherin, fibronectin, integrins, and focal contact components are expressed in all gastrula cells and play putative roles in cell-on-cell migration, but their actual functions in this respect are not yet understood. The gastrula can be subdivided into two motility domains, and in the vegetal, migratory domain, two modes of cell migration are discerned. Vegetal endoderm cells show ingression-type migration, a variant of amoeboid migration characterized by the lack of locomotory protrusions and by macropinocytosis as a mechanism of trailing edge resorption. Mesendoderm and prechordal mesoderm cells use lamellipodia in a mesenchymal mode of migration. Gastrula cell motility can be dissected into traits, such as cell polarity, adhesion, mobility, or protrusive activity, which are controlled separately yet in complex, combinatorial ways. Cells can instantaneously switch between different combinations of traits, showing plasticity as they respond to substratum properties. This article is categorized under: Early Embryonic Development > Gastrulation and Neurulation.

Modulation of cellular polarization and migration by ephrin/Eph signal-mediated boundary formation

Compartment boundaries are essential for ensuring proper cell organization during embryo development and in adult tissues, yet the mechanisms underlying boundary establishment are not completely understood. A number of mechanisms, including (i) differential adhesion, (ii) differential tension, and (iii) cell signaling-mediated cell repulsion, are known to contribute and likely a context-dependent balance of each of these dictates boundary implementation. The ephrin/Eph signaling pathway is known to impact boundary formation in higher animals. In different contexts, ephrin/Eph signaling is known to modulate adhesive properties and migratory behavior of cells. Furthermore it has been proposed that ephrin/Eph signaling may modulate cellular tensile properties, leading to boundary implementation. It remains unclear however, whether, in different contexts, ephrin/Eph act through distinct dominant action modes (e.g. differential adhesion vs. cell repulsion), or whether ephrin/Eph signaling elicits multiple cellular changes simultaneously. Here, using micropatterning of cells over-expressing either EphB3 or ephrinB1, we assess the contribution of each these factors in one model. We show that in this system ephrinB1/EphB3-mediated boundaries are accompanied by modulation of tissue-level architecture and polarization of cell migration. These changes are associated with changes in cell shape and cytoskeletal organization also suggestive of altered cellular tension.

Sorting at embryonic boundaries requires high heterotypic interfacial tension

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

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

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

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

Phosphorylation-dependent ubiquitination of paraxial protocadherin (PAPC) controls gastrulation cell movements

Paraxial protocadherin (PAPC) has been shown to be involved in gastrulation cell movements during early embryogenesis. It is first expressed in the dorsal marginal zone at the early gastrula stage and subsequently restricted to the paraxial mesoderm in Xenopus and zebrafish. Using Xenopus embryos, we found that PAPC is also regulated at the protein level and is degraded and excluded from the plasma membrane in the axial mesoderm by the late gastrula stage. Regulation of PAPC requires poly-ubiquitination that is dependent on phosphorylation. PAPC is phosphorylated by GKS3 in the evolutionarily conserved cytoplasmic domain, and this in turn is necessary for poly-ubiquitination by an E3 ubiquitin ligase β-TrCP. We also show that precise control of PAPC by phosphorylation/ubiquitination is essential for normal Xenopus gastrulation cell movements. Taken together, our findings unveil a novel mechanism of regulation of a cell adhesion protein and show that this system plays a crucial role in vertebrate embryogenesis.

An in vitro model of tissue boundary formation for dissecting the contribution of different boundary forming mechanisms

Separation of phenotypically distinct cell populations is necessary to ensure proper organization and function of tissues and organs therefore understanding fundamental mechanisms that drive this cell segregation is important. In this work, authors present an in vivo model system that accurately recapitulates important aspects of cell segregation in vivo and allows dissection of cell behaviours driving cell segregation.

Mechanisms of boundary formation by Eph receptor and ephrin signaling

The formation of sharp borders, across which cell intermingling is restricted, has a crucial role in the establishment and maintenance of organized tissues. Signaling of Eph receptors and ephrins underlies formation of a number of boundaries between and within tissues during vertebrate development. Eph-ephrin signaling can regulate several types of cell response-adhesion, repulsion and tension-that can in principle underlie the segregation of cells and formation of sharp borders. Recent studies have implicated each of these cell responses as having important roles at different boundaries: repulsion at the mesoderm-ectoderm border, decreased adhesion at the notochord-presomitic mesoderm border, and tension at boundaries within the hindbrain and forebrain. These distinct responses to Eph receptor and ephrin activation may in part be due to the adhesive properties of the tissue.

The cellular basis of tissue separation

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

FAK is required for tension-dependent organization of collective cell movements in Xenopus mesendoderm

Collective cell movements are integral to biological processes such as embryonic development and wound healing and also have a prominent role in some metastatic cancers. In migrating Xenopus mesendoderm, traction forces are generated by cells through integrin-based adhesions and tension transmitted across cadherin adhesions. This is accompanied by assembly of a mechanoresponsive cadherin adhesion complex containing keratin intermediate filaments and the catenin-family member plakoglobin. We demonstrate that focal adhesion kinase (FAK), a major component of integrin adhesion complexes, is required for normal morphogenesis at gastrulation, closure of the anterior neural tube, axial elongation and somitogenesis. Depletion of zygotically expressed FAK results in disruption of mesendoderm tissue polarity similar to that observed when expression of keratin or plakoglobin is inhibited. Both individual and collective migrations of mesendoderm cells from FAK depleted embryos are slowed, cell protrusions are disordered, and cell spreading and traction forces are decreased. Additionally, keratin filaments fail to organize at the rear of cells in the tissue and association of plakoglobin with cadherin is diminished. These findings suggest that FAK is required for the tension-dependent assembly of the cadherin adhesion complex that guides collective mesendoderm migration, perhaps by modulating the dynamic balance of substrate traction forces and cell cohesion needed to establish cell polarity.

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

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

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

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

Ephrin-Eph signaling in embryonic tissue separation

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

A molecular base for cell sorting at embryonic boundaries: contact inhibition of cadherin adhesion by ephrin/ Eph-dependent contractility

The mechanism responsible for subdividing the embryo into individual tissues is a fundamental, yet still poorly understood, question in developmental biology. Various general hypotheses have been proposed, involving differences in cell adhesion, contractility, or contact-mediated repulsion. However, the key parameter in tissue separation, i.e., the regulation of cadherin-based adhesion at the boundary, has not yet been investigated. We show that cadherin clustering is specifically inhibited at the vertebrate notochord-presomitic mesoderm boundary, preventing formation of adhesive bonds between cells of the two different types. This local regulation depends on differentially expressed ephrins and Eph receptors, which increase cell contractility and generate a membrane blebbing-like behavior along the boundary. Inhibiting myosin activity is sufficient to induce cadherin clustering and formation of stable contacts across the boundary, causing notochord and presomitic tissues to fuse. Local inhibition of cadherin adhesion explains how sharp separation can be achieved in response to cell-cell contact signals.

Boundary formation in the development of the vertebrate hindbrain

The formation of a sharp interface of adjacent subdivisions is important for establishing the precision of tissue organization, and at specific borders it serves to organize key signaling centers. We discuss studies of vertebrate hindbrain development that have given important insights into mechanisms that underlie the formation and maintenance of sharp borders. The hindbrain is subdivided into a series of segments with distinct anteroposterior identity that underlies the specification of distinct neuronal cell types. During early stages of segmentation, cell identity switching contributes to the refinement of borders and enables homogenous territories to be maintained despite intermingling of cells between segments. At later stages, there is a specific restriction to cell intermingling between segments that is mediated by Eph receptor and ephrin signaling. Eph-ephrin signaling can restrict cell intermingling and sharpen borders through multiple mechanisms, including the regulation of cell adhesion and contact inhibition of cell migration.

Cadherin point mutations alter cell sorting and modulate GTPase signaling

This study investigated the impact of cadherin binding differences on both cell sorting and GTPase activation. The use of N-terminal domain point mutants of Xenopus C-cadherin enabled us to quantify binding differences and determine their effects on cadherin-dependent functions without any potential complications arising as a result of differences in cytodomain interactions. Dynamic cell-cell binding measurements carried out with the micropipette manipulation technique quantified the impact of these mutations on the two-dimensional binding affinities and dissociation rates of cadherins in the native context of the cell membrane. Pairwise binding affinities were compared with in vitro cell-sorting specificity and ligation-dependent GTPase signaling. Two-dimensional affinity differences greater than five-fold correlated with cadherin-dependent in vitro cell segregation, but smaller differences failed to induce cell sorting. Comparison of the binding affinities with GTPase signaling amplitudes further demonstrated that differential binding also proportionally modulates intracellular signaling. These results show that differential cadherin affinities have broader functional consequences than merely controlling cell-cell cohesion.

Gastrulation: making and shaping germ layers

Gastrulation is a fundamental phase of animal embryogenesis during which germ layers are specified, rearranged, and shaped into a body plan with organ rudiments. Gastrulation involves four evolutionarily conserved morphogenetic movements, each of which results in a specific morphologic transformation. During emboly, mesodermal and endodermal cells become internalized beneath the ectoderm. Epibolic movements spread and thin germ layers. Convergence movements narrow germ layers dorsoventrally, while concurrent extension movements elongate them anteroposteriorly. Each gastrulation movement can be achieved by single or multiple motile cell behaviors, including cell shape changes, directed migration, planar and radial intercalations, and cell divisions. Recent studies delineate cyclical and ratchet-like behaviors of the actomyosin cytoskeleton as a common mechanism underlying various gastrulation cell behaviors. Gastrulation movements are guided by differential cell adhesion, chemotaxis, chemokinesis, and planar polarity. Coordination of gastrulation movements with embryonic polarity involves regulation by anteroposterior and dorsoventral patterning systems of planar polarity signaling, expression of chemokines, and cell adhesion molecules.

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

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

Molecular mechanisms of cell segregation and boundary formation in development and tumorigenesis

The establishment and maintenance of precisely organized tissues requires the formation of sharp borders between distinct cell populations. The maintenance of segregated cell populations is also required for tissue homeostasis in the adult, and deficiencies in segregation underlie the metastatic spreading of tumor cells. Three classes of mechanisms that underlie cell segregation and border formation have been uncovered. The first involves differences in cadherin-mediated cell-cell adhesion that establishes interfacial tension at the border between distinct cell populations. A second mechanism involves the induction of actomyosin-mediated contraction by intercellular signaling, such that cortical tension is generated at the border. Third, activation of Eph receptors and ephrins can lead to both decreased adhesion by triggering cleavage of E-cadherin, and to repulsion of cells by regulation of the actin cytoskeleton, thus preventing intermingling between cell populations. These mechanisms play crucial roles at distinct boundaries during development, and alterations in cadherin or Eph/ephrin expression have been implicated in tumor metastasis.

The kidney and planar cell polarity

Planar cell polarity (PCP) or tissue polarity describes a coordinated polarity at the plane of the tissue where most or all cells within a tissue are polarized in one direction. It is perpendicular to the apical-basal polarity of the cell. PCP is manifested readily in the Drosophila wing and cuticle bristles, Drosophila eye ommatidia, and mammalian hair and inner ear hair bundles, and less evidently, in cellular processes such as in the coordinated, directional cell movements, and oriented cell divisions that are important for tissue morphogenesis. Several distinct molecular and cellular processes have been implicated in the regulation of PCP. Here, we review potential roles for PCP during mouse kidney development and maintenance, including ureteric bud branching morphogenesis, renal medulla elongation, tubule diameter establishment/maintenance, glomerulogenesis, and response to injury. The potential mechanisms underlying these processes, including oriented cell division and coordinated cell migration/cell intercalation, are discussed. In addition, we discuss some unaddressed research topics related to PCP in the kidney that we hope will spur further discussion and investigation.

Mesoderm layer formation in Xenopus and Drosophila gastrulation

During gastrulation, the mesoderm spreads out between ectoderm and endoderm to form a mesenchymal cell layer. Surprisingly the underlying principles of mesoderm layer formation are very similar in evolutionarily distant species like the fruit fly, Drosophila melanogaster, and the frog, Xenopus laevis, in which the molecular and the cellular basis of mesoderm layer formation have been extensively studied. Complementary expression of growth factors in the ectoderm and their receptors in the mesoderm act to orient cellular protrusive activities and direct cell movement, leading to radial cell intercalation and the spreading of the mesoderm layer. This mechanism is contrasted with generic physical mechanisms of tissue spreading that consider the adhesive and physical properties of the cells and tissues. Both mechanisms need to be integrated to orchestrate mesenchymal morphogenesis.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Budding behaviors: Growth of the limb as a model of morphogenesis

Questions regarding morphogenesis have played second fiddle to those pertaining to pattern formation among the limb development set for some time. A recent series of publications has reinvigorated the search for mechanisms by which the limb bud arises, elongates and acquires its peculiar shape. While there are stage-specific variations, the theme that resonates across these studies is that mesoderm and cartilage cells in the limb bud exhibit polarity that drives directional movement and oriented division. Noncanonical Wnt signalling is important for these cell behaviors at all stages of limb development. While the emerging morphogenetic mechanisms underlying limb bud outgrowth are partly analogous to those of other developing structures, insights from the limb have the potential to reveal intriguing new mechanisms by which three dimensional mesoderm changes shape.

EphrinB/EphB signaling controls embryonic germ layer separation by contact-induced cell detachment

BACKGROUND

The primordial organization of the metazoan body is achieved during gastrulation by the establishment of the germ layers. Adhesion differences between ectoderm, mesoderm, and endoderm cells could in principle be sufficient to maintain germ layer integrity and prevent intermixing. However, in organisms as diverse as fly, fish, or amphibian, the ectoderm-mesoderm boundary not only keeps these germ layers separated, but the ectoderm also serves as substratum for mesoderm migration, and the boundary must be compatible with repeated cell attachment and detachment.

PRINCIPAL FINDINGS

We show that localized detachment resulting from contact-induced signals at the boundary is at the core of ectoderm-mesoderm segregation. Cells alternate between adhesion and detachment, and detachment requires ephrinB/EphB signaling. Multiple ephrinB ligands and EphB receptors are expressed on each side of the boundary, and tissue separation depends on forward signaling across the boundary in both directions, involving partially redundant ligands and receptors and activation of Rac and RhoA.

CONCLUSION

This mechanism differs from a simple differential adhesion process of germ layer formation. Instead, it involves localized responses to signals exchanged at the tissue boundary and an attachment/detachment cycle which allows for cell migration across a cellular substratum.

Cell sorting in development

During the development of multicellular organisms, cell fate specification is followed by the sorting of different cell types into distinct domains from where the different tissues and organs are formed. Cell sorting involves both the segregation of a mixed population of cells with different fates and properties into distinct domains, and the active maintenance of their segregated state. Because of its biological importance and apparent resemblance to fluid segregation in physics, cell sorting was extensively studied by both biologists and physicists over the last decades. Different theories were developed that try to explain cell sorting on the basis of the physical properties of the constituent cells. However, only recently the molecular and cellular mechanisms that control the physical properties driving cell sorting, have begun to be unraveled. In this review, we will provide an overview of different cell-sorting processes in development and discuss how these processes can be explained by the different sorting theories, and how these theories in turn can be connected to the molecular and cellular mechanisms driving these processes.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.