The Spanish Society for Developmental Biology (SEBD) organized its 18th meeting in October 2024 (hereafter SEBD2024), coinciding with the society's 30th anniversary and serving as the stage for its celebrations. This article provides an overview of the event, including the speakers, scientific sessions and the different activities related to the anniversary.

The remodeling of epithelial tissues is a critical process in morphogenesis, often involving the apoptotic removal of individual cells while preserving tissue integrity. In Drosophila, the amnioserosa—a highly dynamic extra-embryonic tissue—undergoes extensive remodeling, culminating in its complete elimination at the end of dorsal closure. While apoptotic cell delaminations in the amnioserosa have been proposed to contribute to dorsal closure, the cellular mechanisms underlying this process remain poorly understood. In this study, we have investigated actomyosin dynamics during cell delaminations and analyzed the consequences of perturbing non-muscle Myosin activity globally in the entire tissue as well as locally in groups of cells. We found that Myosin plays an essential role in both triggering and executing cell delaminations, with high Myosin contractility promoting cell delamination via caspase activation. Additionally, our results suggest that cell delaminations are governed by both cell-autonomous Myosin dynamics and mechanical cues from the tissue environment. Together, these findings provide new insights into the regulation of epithelial cell removal and the complex interplay between apoptotic and mechanical signals during tissue remodeling.

Autonomous oscillatory dynamics are ubiquitous at every level in Biology. At the cellular level, one of the most relevant and well characterized examples of periodic behavior is the cyclic assembly and disassembly of actomyosin networks. In Drosophila , these oscillations induce the robust contraction and expansion of individual cells required for correct dorsal closure, while in the follicular epithelium that surrounds the germline, periodic contractions of the basal actomyosin network are required for proper elongation of the egg chamber. While some studies suggest that actomyosin oscillations are driven by upstream signaling or mechanochemical features, we have recently proposed that they arise as a systems property from the competition between two well characterized features of the actomyosin machinery: 1) cooperative assembly of actin networks mediated by Actin crosslinker proteins and 2) tension-induced disassembly of actin networks mediated by myosin motors. Here, we perform experiments in amnioserosa and in the follicle cells of drosophila and simulations using a coarse-grained model of the actomyosin cortex to characterize the properties of the oscillations and how they depend on different features of the system. We also compare model and experiments to study the dynamics of actomyosin flows and the effect of mechanical coupling between cells in the tissue. In conclusion, our model is a powerful tool to study key features of actomyosin oscillations, from the effect of the individual components to network properties and finally supra-cellular organization of the oscillations at the tissue level.

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

We review recent developments in the understanding of the biomechanics of apicomedial actomyosin and how its contractility can tense and deform tissue. Myosin pulses are driven by a biochemical oscillator but how they are modulated by the mechanical context remains unclear. On the other hand, the emergence of tissue behaviour is highly dependent on the material properties of actin, on how strongly components are connected and on the influence of neighbouring tissues. We further review the use of constitutive equations in exploring the mechanics of epithelial apices dominated by apicomedial Myosin contractility.

In this work, we combine genetic perturbation, time-lapse imaging and quantitative image analysis to investigate how pulsatile actomyosin contractility drives cell oscillations, apical cell contraction and tissue closure during morphogenesis of the amnioserosa, the main force-generating tissue during the dorsal closure in Drosophila. We show that Myosin activity determines the oscillatory and contractile behaviour of amnioserosa cells. Reducing Myosin activity prevents cell shape oscillations and reduces cell contractility. By contrast, increasing Myosin activity increases the amplitude of cell shape oscillations and the time cells spend in the contracted phase relative to the expanded phase during an oscillatory cycle, promoting cell contractility and tissue closure. Furthermore, we show that in AS cells, Rok controls Myosin foci formation and Mbs regulates not only Myosin phosphorylation but also adhesion dynamics through control of Moesin phosphorylation, showing that Mbs coordinates actomyosin contractility with cell-cell adhesion during amnioserosa morphogenesis.

We have investigated how cell contractility and adhesion are functionally integrated during epithelial morphogenesis. To this end, we have analysed the role of α-Catenin, a key molecule linking E-Cadherin-based adhesion and the actomyosin cytoskeleton, during Drosophila embryonic dorsal closure, by studying a newly developed allelic series. We find that α-Catenin regulates pulsatile apical contraction in the amnioserosa, the main force-generating tissue driving closure of the embryonic epidermis. α-Catenin controls actomyosin dynamics by stabilising and promoting the formation of actomyosin foci, and also stabilises DE-Cadherin (Drosophila E-Cadherin, also known as Shotgun) at the cell membrane, suggesting that medioapical actomyosin contractility regulates junction stability. Furthermore, we uncover a genetic interaction between α-Catenin and Vinculin, and a tension-dependent recruitment of Vinculin to amniosersoa apical cell membranes, suggesting the existence of a mechano-sensitive module operating in this tissue.

BACKGROUND:Force generation and the material properties of cells and tissues are central to morphogenesis but remain difficult to measure in vivo. Insight is often limited to the ratios of mechanical properties obtained through disruptive manipulation, and the appropriate models relating stress and strain are unknown. The Drosophila amnioserosa epithelium progressively contracts over 3 hours of dorsal closure, during which cell apices exhibit area fluctuations driven by medial myosin pulses with periods of 1.5-6 min. Linking these two timescales and understanding how pulsatile contractions drive morphogenetic movements is an urgent challenge.RESULTS:We present a novel framework to measure in a continuous manner the mechanical properties of epithelial cells in the natural context of a tissue undergoing morphogenesis. We show that the relationship between apicomedial myosin fluorescence intensity and strain during fluctuations is consistent with a linear behaviour, although with a lag. We thus used myosin fluorescence intensity as a proxy for active force generation and treated cells as natural experiments of mechanical response under cyclic loading, revealing unambiguous mechanical properties from the hysteresis loop relating stress to strain. Amnioserosa cells can be described as a contractile viscoelastic fluid. We show that their emergent mechanical behaviour can be described by a linear viscoelastic rheology at timescales relevant for tissue morphogenesis. For the first time, we establish relative changes in separate effective mechanical properties in vivo. Over the course of dorsal closure, the tissue solidifies and effective stiffness doubles as net contraction of the tissue commences. Combining our findings with those from previous laser ablation experiments, we show that both apicomedial and junctional stress also increase over time, with the relative increase in apicomedial stress approximately twice that of other obtained measures.CONCLUSIONS:Our results show that in an epithelial tissue undergoing net contraction, stiffness and stress are coupled. Dorsal closure cell apical contraction is driven by the medial region where the relative increase in stress is greater than that of stiffness. At junctions, by contrast, the relative increase in the mechanical properties is the same, so the junctional contribution to tissue deformation is constant over time. An increase in myosin activity is likely to underlie, at least in part, the change in medioapical properties and we suggest that its greater effect on stress relative to stiffness is fundamental to actomyosin systems and confers on tissues the ability to regulate contraction rates in response to changes in external mechanics.

Pulsatile actomyosin contractility driving cell shape oscillations is a common feature of actomyosin networks present in a variety of tissues undergoing morphogenetic processes. The origin of this oscillatory dynamics, how it is stabilized over time to give rise to net cell shape changes and how it is spatially coordinated across a tissue, are questions that have being extensively investigated in recent years. In this work, I review how genetics, cell biology, and quantitative and theoretical approaches have started to give a comprehensive understanding of these problems revealing that both biochemical and mechanical regulation play an important role in the emergence, coordination and stabilization of this activity. Developmental Dynamics 245:268–275, 2016. © 2015 Wiley Periodicals, Inc.

Mechanics has an important role during morphogenesis, both in the generation of forces driving cell shape changes and in determining the effective material properties of cells and tissues. Drosophila dorsal closure has emerged as a reference model system for investigating the interplay between tissue mechanics and cellular activity. During dorsal closure, the amnioserosa generates one of the major forces that drive closure through the apical contraction of its constituent cells. We combined quantitation of live data, genetic and mechanical perturbation and cell biology, to investigate how mechanical properties and contraction rate emerge from cytoskeletal activity. We found that a decrease in Myosin phosphorylation induces a fluidization of amnioserosa cells which become more compliant. Conversely, an increase in Myosin phosphorylation and an increase in actin linear polymerization induce a solidification of cells. Contrary to expectation, these two perturbations have an opposite effect on the strain rate of cells during DC. While an increase in actin polymerization increases the contraction rate of amnioserosa cells, an increase in Myosin phosphorylation gives rise to cells that contract very slowly. The quantification of how the perturbation induced by laser ablation decays throughout the tissue revealed that the tissue in these two mutant backgrounds reacts very differently. We suggest that the differences in the strain rate of cells in situations where Myosin activity or actin polymerization is increased arise from changes in how the contractile forces are transmitted and coordinated across the tissue through ECadherin-mediated adhesion. Altogether, our results show that there is an optimal level of Myosin activity to generate efficient contraction and suggest that the architecture of the actin cytoskeleton and the dynamics of adhesion complexes are important parameters for the emergence of coordinated activity throughout the tissue.

Poster presentado en el X Meeting Spanish Society for Developmental Biology, celebrado en Madrid del 13 al 15 de octubre de 2014.

Oscillatory behaviour in force-generating systems is a pervasive phenomenon in cell biology. In this work, we investigate how oscillations in the actomyosin cytoskeleton drive cell shape changes during the process of Dorsal Closure (DC), a morphogenetic event in Drosophila embryo development whereby epidermal continuity is generated through the pulsatile apical area reduction of cells constituting the amnioserosa (AS) tissue. We present a theoretical model of AS cell dynamics by which the oscillatory behaviour arises due to a coupling between active myosin-driven forces, actin turnover and cell deformation. Oscillations in our model are cell-autonomous and are modulated by neighbour coupling, and our model accurately reproduces the oscillatory dynamics of AS cells and their amplitude and frequency evolution. A key prediction arising from our model is that the rate of actin turnover and Myosin contractile force must increase during DC in order to reproduce the decrease in amplitude and period of cell area oscillations observed in vivo. This prediction opens up new ways to think about the molecular underpinnings of AS cell oscillations and their link to net tissue contraction and suggests the form of future experimental measurements.

The fact that morphogenesis, the set of tissue movements and deformations that generate amazing and complex forms during embryonic development, results from the tight interplay between biochemical and mechanical processes, has only recently started to be assimilated by the developmental biology community. Although it is now accepted that the material properties of cells and the mechanical stresses generated inside them are important parameters for embryonic development, it remains largely unknown how and at what time- and length-scales the interplay between mechanical and biochemical activity takes place (1Davidson L. von Dassow M. Zhou J. Multi-scale mechanics from molecules to morphogenesis.Int. J. Biochem. Cell Biol. 2009; 41: 2147-2162Crossref PubMed Scopus (56) Google Scholar, 2Grill S.W. Growing up is stressful: biophysical laws of morphogenesis.Curr. Opin. Genet. Dev. 2011; 21: 647-652Crossref PubMed Scopus (25) Google Scholar). It is increasingly evident that answering this question requires a multidisciplinary effort that combines the more-classical approaches of developmental biology with quantitative measurements and physical modeling techniques. In a recent issue of Biophysical Journal, Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) go a big step in this direction by presenting a model of dorsal closure, a morphogenetic process of the Drosophila embryo, where the interplay between biochemical and mechanical inputs generates several features of the cell and tissue behavior (reviewed in Gorfinkiel et al. (4Gorfinkiel N. Schamberg S. Blanchard G.B. Integrative approaches to morphogenesis: lessons from dorsal closure.Genesis. 2011; 49: 522-533Crossref PubMed Scopus (44) Google Scholar)). Dorsal closure (DC) is a very well-studied morphogenetic process whereby interactions between two tissues, the amnioserosa (AS) and the epidermis, close a discontinuity at the dorsal side of the embryo to generate a continuous epidermis. Using innovative approaches, more than 10 years ago Dan Kiehart and colleagues uncovered the main tissue-level forces involved in this process: the resistive force of the epidermis is mainly countered by an AS contractile force, which is in turn aided by a supracellular actin cable formed at the interface between the two tissues that generates tension in the direction of closure (5Hutson M.S. Tokutake Y. Edwards G.S. et al.Forces for morphogenesis investigated with laser microsurgery and quantitative modeling.Science. 2003; 300: 145-149Crossref PubMed Scopus (388) Google Scholar, 6Kiehart D.P. Galbraith C.G. Montague R.A. et al.Multiple forces contribute to cell sheet morphogenesis for dorsal closure in Drosophila.J. Cell Biol. 2000; 149: 471-490Crossref PubMed Scopus (521) Google Scholar). In the last few years, work from several labs has started to unravel, with an unprecedented quantitative precision, the molecular and cellular mechanisms underlying the generation of such forces. Importantly, it has been shown that AS cells exhibit oscillations in their apical surface area driven by the transient activity of the actomyosin cytoskeleton. The frequency of these oscillations increase as DC progresses until it becomes undetectable, and this correlates with an increase in the rate of contraction of these cells (7Blanchard G.B. Murugesu S. Gorfinkiel N. et al.Cytoskeletal dynamics and supracellular organization of cell shape fluctuations during dorsal closure.Development. 2010; 137: 2743-2752Crossref PubMed Scopus (171) Google Scholar, 8David D.J. Tishkina A. Harris T.J. The PAR complex regulates pulsed actomyosin contractions during amnioserosa apical constriction in Drosophila.Development. 2010; 137: 1645-1655Crossref PubMed Scopus (146) Google Scholar, 9Solon J. Kaya-Copur A. Brunner D. et al.Pulsed forces timed by a ratchet-like mechanism drive directed tissue movement during dorsal closure.Cell. 2009; 137: 1331-1342Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar). Models of epithelial organization have successfully been applied to the understanding of how cell mechanical properties generate stable epithelial configurations and the formation of well-defined boundaries between cell populations (see, for example, Aliee et al. (10Aliee M. Roper J.C. Dahmann C. et al.Physical mechanisms shaping the Drosophila dorsoventral compartment boundary.Curr. Biol. 2012; 22: 967-976Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar) and Farhadifar et al. (11Farhadifar R. Röper J.C. Jülicher F. et al.The influence of cell mechanics, cell-cell interactions, and proliferation on epithelial packing.Curr. Biol. 2007; 17: 2095-2104Abstract Full Text Full Text PDF PubMed Scopus (750) Google Scholar)). In this type of approach, it is generally assumed that mechanical properties and other cell behaviors such as nonuniform cell proliferation, anisotropic cell division, and cell elongation, are previously set by signaling molecules operating in these cells. The model presented by Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) moves a step forward in the understanding of how the interplay between mechanical and cellular activity takes place and thus makes an important contribution to the field of epithelial morphogenesis. The model tackles two of the most relevant questions in the field. The first question refers to the nature of cell area and actomyosin oscillations exhibited by apically contracting cells. Such oscillatory behavior has also been observed in other tissues and in other organisms, suggesting it represents a fundamental property of cytoskeletal systems (12Gorfinkiel N. Blanchard G.B. Dynamics of actomyosin contractile activity during epithelial morphogenesis.Curr. Opin. Cell Biol. 2011; 23: 531-539Crossref PubMed Scopus (81) Google Scholar). The main innovation of the model is that oscillations emerge from the coupling of two different timescales: the timescale of myosin turnover, driven by a signaling molecule (itself oscillatory due to a myosin-dependent degradation rate), and the viscoelastic relaxation timescale of the cells due to their intrinsic viscoelasticity. It is this tight mechano-chemical coupling that generates sustained oscillations in certain regions of the parameter space, which, in the model presented by Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar), are chosen such that oscillations do not occur at the single cell level but emerge from neighbor-neighbor interactions. The second question that the authors explore is the mechanism underlying the net contraction of AS cells. The existence of a ratchetlike mechanism underlying apical cell contraction has been previously suggested in the literature, with two possible scenarios for the AS: an extracellular ratchet provided by the supracellular actin cable and an internal ratchet for each individual cell (7Blanchard G.B. Murugesu S. Gorfinkiel N. et al.Cytoskeletal dynamics and supracellular organization of cell shape fluctuations during dorsal closure.Development. 2010; 137: 2743-2752Crossref PubMed Scopus (171) Google Scholar, 9Solon J. Kaya-Copur A. Brunner D. et al.Pulsed forces timed by a ratchet-like mechanism drive directed tissue movement during dorsal closure.Cell. 2009; 137: 1331-1342Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 13Martin A.C. Kaschube M. Wieschaus E.F. Pulsed contractions of an actin-myosin network drive apical constriction.Nature. 2009; 457: 495-499Crossref PubMed Scopus (828) Google Scholar). Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) explore both these scenarios. The internal ratchet is implemented via the stepwise reduction of the rest length of cell edge and radial viscoelastic elements when the cell area reaches its minimum at each oscillatory cycle, thus providing a bona fide ratchet mechanism for the rest length around which fluctuations occur. Similarly, the external ratchet is realized by adding an elastic spring along the outer boundary of the simulated AS and by decreasing its rest-length each time the tissue area reaches a minimum. The theoretical analysis of these two ratchet mechanisms leads the authors to predict that the intracellular ratchet makes the more significant contribution to contraction. Models are a tool to explore possible mechanisms behind a specific process and, for experimentalists, models are a useful tool if they make predictions than can be tested. This is the case for the model presented by Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). One of the main predictions of the model is the existence of an oscillating signaling molecule, upstream of myosin activity. If this is the case, it is highly improbable that cell area and myosin oscillations arise from oscillations in the expression of this molecule, given the timescale of the oscillations. In fact, in the model by Wang et al., the signaling molecule is produced at a constant rate but its degradation depends on active myosin concentration, so that its levels oscillate. Whether it is the total levels or the active form of the molecule that oscillates, this is an important prediction of Wang’s model. An interesting possibility is that the activity of proteins of the Rho GTPase family, which control cytoskeletal dynamics in various systems, is oscillating. In migrating cells, it has been found that the activity of RhoA and Rac/Cdc42, measured through live Förster resonance energy transfer sensors, increases and decreases in synchrony with the protrusion and retraction cycle of the migrating cell (14Machacek M. Hodgson L. Danuser G. et al.Coordination of Rho GTPase activities during cell protrusion.Nature. 2009; 461: 99-103Crossref PubMed Scopus (710) Google Scholar), raising the possibility that oscillations in any of these proteins could in turn be driving myosin oscillations. However, at the molecular level, oscillations in cytoskeletal activity have also been reported and modeled using a different framework which invokes a dynamic instability in the force-velocity relationship of a collection of motors moving along a filament (15Kruse K. Jülicher F. Oscillations in cell biology.Curr. Opin. Cell Biol. 2005; 17: 20-26Crossref PubMed Scopus (148) Google Scholar). More recently, oscillations in cell shape and cytoskeletal activity during cytokinesis have also been modeled by means of coupling the turnover of the actin cortex with the viscoelastic relaxation timescale of the cell (16Sedzinski J. Biro M. Paluch E. et al.Polar actomyosin contractility destabilizes the position of the cytokinetic furrow.Nature. 2011; 476: 462-466Crossref PubMed Scopus (247) Google Scholar). The latter approach assumes a mechano-sensory role for the actin cytoskeleton, for which there is still no clear evidence. We expect interesting times ahead trying to solve the origin of such oscillations. Another important prediction of the model is the existence of an intracellular ratchet allowing net contraction to occur. In Wang’s model, this is simulated by a stepwise decrease in the rest-length of the radial spokes and edges of the cells that increases in strength as cells go through the different phases of DC. The authors suggest a plausible mechanism for this based on results from in vitro reconstituted networks that involves the buckling of cross-linked actin filaments onto which actomyosin foci move and the subsequent removal of the extra material (17Soares e Silva M. Depken M. Koenderink G.H. et al.Active multistage coarsening of actin networks driven by myosin motors.Proc. Natl. Acad. Sci. USA. 2011; 108: 9408-9413Crossref PubMed Scopus (166) Google Scholar). Whether such a mechanism underpins the establishment of the ratchet will require careful quantitative experiments measuring the levels of myosin and actin in AS cells as DC progresses through its distinct phases, as well as the function of putative actin cross-linkers. Of course the model cannot account for all the biological processes underlying DC, nor is this the intention of the authors. An important question that remains unexplored is that of the mechanisms driving the flow of actomyosin foci. Although some actomyosin foci coalesce and soon disassemble without exhibiting any significant movement, several actomyosin accumulations flow across the apical surface of AS cells, exhibiting a preferential direction of movement along the medio-lateral axis of cells. In the Caenorhabditis elegans oocyte, a gradient of contractility underlies such actomyosin flows (18Mayer M. Depken M. Grill S.W. et al.Anisotropies in cortical tension reveal the physical basis of polarizing cortical flows.Nature. 2010; 467: 617-621Crossref PubMed Scopus (337) Google Scholar). In epithelial cells, it has been suggested that a mechanical imbalance generated by the planar localization of ECadherin is at the basis of the actomyosin flows (19Rauzi M. Lenne P.F. Lecuit T. Planar polarized actomyosin contractile flows control epithelial junction remodeling.Nature. 2010; 468: 1110-1114Crossref PubMed Scopus (444) Google Scholar), but AS cells do not show any signs of planar polarization of adhesion proteins. Despite this, it is likely that adhesion complexes have an important role both in the oscillatory behavior and in the effective contraction of AS cells. Adherens junctions are active complexes connecting neighboring cells and transmitting the forces generated inside the cells across the tissue. Several reports show that adherens junctions feel the tension and can change accordingly, suggesting they may have an important role in feedback loops that increase tension both inside the cells and across the tissue (20Liu Z. Tan J.L. Chen C.S. et al.Mechanical tugging force regulates the size of cell-cell junctions.Proc. Natl. Acad. Sci. USA. 2010; 107: 9944-9949Crossref PubMed Scopus (528) Google Scholar, 21Yonemura S. Wada Y. Shibata M. et al.α-Catenin as a tension transducer that induces adherens junction development.Nat. Cell Biol. 2010; 12: 533-542Crossref PubMed Scopus (672) Google Scholar). In the model of Wang et al., cells are connected passively to each other, but future models of DC and other morphogenetic processes will probably incorporate this essential property of epithelial cells. Finally, although DC has been, until now, approached as a two-dimensional process, it is evident that cells live in three dimensions and there is some evidence showing that as cells contract, they become columnar (22Rugendorff A. Younossi-Hartenstein A. Hartenstein V. Embryonic origin and differentiation of the Drosophila heart.Roux’s Arch. Dev. Biol. 1994; 203: 266-280Crossref Scopus (159) Google Scholar). Although it is generally assumed that volume is conserved during the process, precise measurements of both changes in cell shape and of cytoskeletal activity in three dimensions are still lacking. It is likely that future models of DC will incorporate the third dimension. The model by Wang et al. (3Wang Q. Feng J.J. Pismen L.M. A cell-level biomechanical model of Drosophila Dorsal Closure.Biophys. J. 2012; 103: 2265-2274Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) makes an important contribution not only to the field of dorsal closure but also to the young multidisciplinary field emerging at the intersection between developmental biology and the other physical sciences. Lastly, it demonstrates that experiment and theory can be engaged in a feedback loop that advances our understanding of the basic principles underlying biological processes. I am deeply grateful to Pedro F. Machado and Guy B. Blanchard for discussions and for comments on the manuscript. I also thank Jerome Solon for discussions and for sharing unpublished results, and members of my lab for discussions. This work was supported by a Marie Curie Career Integration Grant (No. PCIG09-GA-2011-293479) and a grant from the Spanish Ministry of Science (No. BFU2011-25828).

Summary Dorsal closure (DC) is an essential step during Drosophila development whereby a hole is sealed in the dorsal epidermis and serves as a model for cell sheet morphogenesis and wound healing. It involves the orchestrated interplay of transcriptional networks and dynamic regulation of cell machinery to bring about shape changes, mechanical forces, and emergent properties. Here we provide insight into the regulation of dorsal closure by describing novel autonomous and non-autonomous roles for U-shaped (Ush) in the amnioserosa, the epidermis, and in mediation of communication between the tissues. We identified Ush by gene expression microarray analysis of Dpp signaling targets and show that Ush mediates some DC functions of Dpp. By selectively restoring Ush function in either the AS or the epidermis in ush mutants, we show that the AS makes a greater (Ush-dependent) contribution to closure than the epidermis. A signal from the AS induces epidermal cell elongation and JNK activation in the DME, while cable formation requires Ush on both sides of the leading edge, i.e. in both the AS and epidermis. Our study demonstrates that the amnioserosa and epidermis communicate at several steps during the process: sometimes the epidermis instructs the amnioserosa, other times the AS instructs the epidermis, and still other times they appear to collaborate.

Although developmental biology has been dominated by the genetic analysis of embryonic development, in recent years genetic tools have been combined with new approaches such as imaging of live processes, automated and quantitative image analysis, mechanical perturbation and mathematical modeling, to study the principles underlying the formation of organisms. Here we focus on recent work carried out on Dorsal Closure, a morphogenetic process during Drosophila embryogenesis, to illustrate how this multidisciplinary approach is yielding new and unexpected insights into how cells organize themselves through the activity of their molecular components to give rise to the stereotyped and macroscopic movements observed during development.

In the past few years, advances in microscopy and quantitative image analysis have lead to a completely new understanding of the processes underlying the cell shape changes and cell rearrangements that drive tissue morphogenesis. In a handful of tissues so far, though the number will surely increase rapidly, it has been shown that cell behaviour is not continuous but proceeds in pulses driven by the contractile activity of dynamic cortical actomyosin networks. The patterns and dynamics of temporary subcellular contractile foci, driven by local increases in actin and myosin, are remarkably similar in disparate tissues. Cells in all tissues display a similar range of intervals between contractions, with increasing frequencies associated with stronger tissue morphogenesis. Contractile foci appear to flow within cells with speeds that are consistent across tissues. We highlight the difference between contractile tension and stiffness, the latter being a requirement for any ratchet mechanism that stabilises contraction to produce effective tissue morphogenesis. At least two different types of ratchet mechanism are discussed, with the stiffness conferred either by a more stable actomyosin population at cell-cell junctions or through cortical actomyosin forming a quasi-stable supracellular network. Pulsatile contractions, polarized cell organization and various stiffening ratchet mechanisms combine to provide a rich variety of options for robust epithelial tissue remodelling.

During development tissue deformations are essential for the generation of organs and to provide the final form of an organism. These deformations rely on the coordination of individual cell behaviours which have their origin in the modulation of subcellular activities. Here we explore the role endocytosis and recycling on tissue deformations that occur during dorsal closure of the Drosophila embryo. During this process the AS contracts and the epidermis elongates in a coordinated fashion, leading to the closure of a discontinuity in the dorsal epidermis of the Drosophila embryo. We used dominant negative forms of Rab5 and Rab11 to monitor the impact on tissue morphogenesis of altering endocytosis and recycling at the level of single cells. We found different requirements for endocytosis (Rab5) and recycling (Rab11) in dorsal closure, furthermore we found that the two processes are differentially used in the two tissues. Endocytosis is required in the AS to remove membrane during apical constriction, but is not essential in the epidermis. Recycling is required in the AS at early stages and in the epidermis for cell elongation, suggesting a role in membrane addition during these processes. We propose that the modulation of the balance between endocytosis and recycling can regulate cellular morphology and tissue deformations during morphogenesis.

Hedgehog (Hh) moves from the producing cells to regulate the growth and development of distant cells in a variety of tissues. Here, we have investigated the mechanism of Hh release from the producing cells to form a morphogenetic gradient in the Drosophila wing imaginal disk epithelium. We describe that Hh reaches both apical and basolateral plasma membranes, but the apical Hh is subsequently internalized in the producing cells and routed to the basolateral surface, where Hh is released to form a long-range gradient. Functional analysis of the 12-transmembrane protein Dispatched, the glypican Dally-like (Dlp) protein, and the Ig-like and FNNIII domains of protein Interference Hh (Ihog) revealed that Dispatched could be involved in the regulation of vesicular trafficking necessary for basolateral release of Hh, Dlp, and Ihog. We also show that Dlp is needed in Hh-producing cells to allow for Hh release and that Ihog, which has been previously described as an Hh coreceptor, anchors Hh to the basolateral part of the disk epithelium.

Fluctuations in the shape of amnioserosa (AS) cells during Drosophila dorsal closure (DC) provide an ideal system with which to understand contractile epithelia, both in terms of the cellular mechanisms and how tissue behaviour emerges from the activity of individual cells. Using quantitative image analysis we show that apical shape fluctuations are driven by the medial cytoskeleton, with periodic foci of contractile myosin and actin travelling across cell apices. Shape changes were mostly anisotropic and neighbouring cells were often, but transiently, organised into strings with parallel deformations. During the early stages of DC, shape fluctuations with long cycle lengths produced no net tissue contraction. Cycle lengths shortened with the onset of net tissue contraction, followed by a damping of fluctuation amplitude. Eventually, fluctuations became undetectable as AS cells contracted rapidly. These transitions were accompanied by an increase in apical myosin, both at cell-cell junctions and medially, the latter ultimately forming a coherent, but still dynamic, sheet across cells. Mutants with increased myosin activity or actin polymerisation exhibited precocious cell contraction through changes in the subcellular localisation of myosin. thick veins mutant embryos, which exhibited defects in the actin cable at the leading edge, showed similar timings of fluctuation damping to the wild type, suggesting that damping is an autonomous property of the AS. Our results suggest that cell shape fluctuations are a property of cells with low and increasing levels of apical myosin, and that medial and junctional myosin populations combine to contract AS cell apices and drive DC.