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Seminar « Coordination of tissue size, shape, and pattern by intercellular signalling/ Coordination de la taille, la forme et le pattern des tissus par la communication entre cellules pendant le développement », organized by Jean-Paul Vincent, Senior Group Leader, The Francis Crick Institute Mill Hill Laboratory, London, UK, from 4 to 9 November 2019--SummaryThis meeting brought together developmental biologists and physicists/mathematicians to discuss how cell-to-cell signalling ensures the reliable formation of well-proportioned organs and tissues. We took a multidisciplinary view of current challenges in developmental biology including the coordination of patterning with growth, scaling, the role of time in spatial patterning, growth termination, and self-organisation of synthetic tissues. Throughout the meeting, participants were encouraged to debate and provide constructive criticisms to foster interactions and creative thinking. The participants included 16 senior scientists, 3 junior group leaders (less than three years as PIs) and one PhD student.--RésuméCe séminaire a rassemblé des biologistes, des physiciens et mathématiciens pour discuter des problèmes les plus saillants de la biologie du développement. Avec une approche multidisciplinaire, nous avons adressé les questions suivantes : Comment le patterning et la croissance sont-ils coordonnés de sorte que les différents types cellulaires se forment à la bonne position même lorsque la taille du tissue varie ? Quels sont les mécanismes qui permettent aux cellules de ‘connaître’ la taille du tissu dans lequel elles résident et donc de faire en sorte que la croissance se termine au bon moment ? Quels sont les mécanismes qui organisent la chorégraphie cellulaire qui engendre la forme des tissus et organes. Les organismes modèles tels que la mouche, le poisson zèbre et la souris continuent d’apporter des réponses à ces questions.  De plus, ces approches sont maintenant complémentées par la reconstitution in vitro et des modèles mathématiques. En encourageant un débat constructif entre ces disciplines le séminaire a facilité l’émergence d’idées nouvelles qui stimuleront la recherche future. Ont participé au séminaire 16 chercheurs établis, 3 jeunes chefs de groupe et un étudiant en thèse.--Compte-renduSignalling and patterning      Morphogens are secreted proteins that provide positional information in developing tissues.James Briscoe spoke about patterning of the vertebrate neural tube. He described how the gene regulatory network, comprising several reciprocally inhibiting transcription factors, converts a morphogen signal into accurately positioned domains of gene expression in the neural tube. In addition, he presented preliminary investigations into the difference in developmental tempo between mouse and human embryos that raised the possibility that kinetic properties of the proteins in the gene regulatory network are responsible for controlling the pace of development. Jean-Paul Vincent and Guillaume Salbreux then presented their efforts to engineer an artificial signal that can organise patterning and growth in the developing wing of the fruit fly. They also showed that the behaviour of this signal could be modelled by a modified diffusion equation that incorporates binding to artificial receptors. During normal development, multiple signals contribute to patterning information. Using a reaction and diffusion model of vein fate in the Drosophila wing, Marc de Gennes explained how signalling interactions turn a wide and rough pro-vein domain into a regular and thin vein with a well-defined width. Moreover, he described how variation in the initial pattern is buffered by the signalling network, which makes the pattern robust.Growth and PatterningTissues can vary in size according to environmental conditions and it is important that patterning information is adjusted accordingly (a process known as scaling). Indeed, in many instances, there seems to be cross-talk between growth and patterning. The primordium of the Drosophila wing has served as a paradigm to mechanistically characterize the role of morphogens in promoting patterning activities and growth from the signaling centers. The Wingless and Dpp morphogens are expressed in two orthogonal stripes that correspond to the compartment boundaries. While their gradients organize patterning by regulating the expression of well-defined target genes, the graded activity of these two morphogens is not an absolute requirement for wing growth. Marco Milán demonstrated that these two morphogens can promote anisotropic growth through distinct, non-interchangeable, molecular mechanisms.  In another instance of coordination between growth and patterning, Ani Kicheva talked about how the pattern of neural progenitor types is established in the growing spinal cord. She showed that opposing signalling gradients of BMP and Shh are decoded by a downstream transcriptional network and this mechanism can account for the precision of pattern. The rate of tissue growth affects the pattern by its influence on the anisotropic growth of different progenitor domains. The question of pattern scaling was also addressed by Arthur Lander using the fruit fly wing as an example.  He showed that a prevailing model is not supported by experiments, and proposed a new model in which the morphogen spreads by negatively regulating the production of the receptor that cells use to capture it.  He suggested that the system may be better viewed as one that couples morphogen scale to growth, so that each regulates the other, rather than one in which growth directs gradients to scale. Oscillations                                                                                                                           Although spatial signalling (e.g. by morphogens) is a key principle underlying developmental patterning. It is becoming clear that temporal control of gene expression plays an important role too. Nancy Papalopulu showed that transcription noise triggers oscillations in gene expression that are then used to direct cell state transitions during neurogenesis. Alexander Aulehla discussed novel top-down strategies to study signalling oscillations linked to the formation of somites, the precursors of vertebrae. His group is using entrainment/synchronization theory approaches that enable direct experimental control of oscillations dynamics. He presented evidence that relative phase-shift encoding between multiple oscillatory signals encodes spatial and temporal information for embryonic patterning. More generally, he discussed the use of theoretical abstraction and phenomenological modelling to gain insight into complex biological systems.                                                      Growth terminationHow tissue ‘know’ their size and stop growing accordingly remains an unsolved question in developmental biology. Jochen Rink reported about his lab’s recent manipulation of body size in planarian flatworms to understand how biological systems sense and respond to body size. He presented RNAseq experiments that demonstrate strong correlations between gene expression and body size. Preliminary results suggest that this phenomenon involves signals with intrinsic size-dependent activity that coordinate size-dependent gene expression responses in a wide range of tissues. Nic Tapon and Guillaume Salbreux combined live imaging and numerical simulations to study growth termination in the developing abdomen of Drosophila. During tissue expansion, cell divisions are initially synchronised, but this synchronisation gradually decreases due to the stochastic nature of cell division times. Rather than a gradual slowing down of cell cycle time, tissue growth arrest occurs through the rapid emergence of a population of quiescent cells within the tissue.Organoids / Embryoids            Major advances in tissue culture are opening the way to creating organs and embryos in a dish. Luciano Marcon presented a pioneering in-vitro system to study symmetry breaking during mouse gastrulation. Anne Grapin-Botton discussed how individual cells isolated from an organ can self-organize into miniature organ-like structures. She showed sequential events, starting by cell compaction and formation of small groups of cells which reform cell-cell junctions and polarize. Cells form these organoids more efficiently as more cells are assembled, likely via Notch-delta interactions fuelling growth and balancing it with differentiation. She also discussed that both cell number and the medium can control the number of lumen and their evolution into a single balloon-like lumen or a network of ducts. Finally, in silico modelling based on digitization of networks shows that fluid secretion in the ducts and its flow down to the duodenal outlet enables the evolution of this network into a tree. This phenomenon does not happen in organoids possibly due to the lack of an outlet.Eric Siggia talked about synthetic embryology using human embryonic stem cells to recreate aspects of gastrulation, the process by which the embryo creates its body axes, anterior-posterior, dorsal-ventral, and left-right. At the same time the pluripotent stem cells specialize to one of the three broad cell types from which the body is constructed: endoderm (gut), mesoderm (muscle, blood), and ectoderm (nerves, skin). Eric showed how his synthetic systems in two and three dimensions, could reveal the secreted signals, both activators and repressors, that are responsible for defining the body axes. In the summary discussion, Eric argued that simplified models could be predictive, and that dynamic experiments were the most informative for fitting and challenging such models.MorphogenesisThe ultimate output of development is the generation of organs and tissues of specific shape and function. This problem can be address in organoids (see above) but also in vivo, computationally, and from an evolutionary standpoint. Madhav Mani described a general method to quantify variations in shape. Using a live imaging approach, Caren Norden described the sophisticated choreography that cells undertake to generate the vertebrate retina, using the zebrafish as a model. Theoretical approaches to morphogenesis were also described. Dagmar Iber discussed how pattern scaling can be achieved in the Drosophila wing disc, and how the stereotypic positions of branch points in the mouse lung and kidney epithelial tree can arise from interactions between morphogens and their receptors. She also showed that the 3D packing of cells in epithelia can be accounted to the minimization of the lateral contact surface energy. General principles of epithelia morphogenesis were also discussed by Madhav Mani. Tissue shape is the target of evolutionary pressure since it determines function. Arkhat Abzhanov discussed how methods from mathematical modelling, geometric morphometrics, comparative embryology and experimental developmental genetics could and should be combined to study evolution of natural adaptive diversity using beaks of Darwin's finches and faces of the New World bats as case studies.Systemic size controlOrgans and tissues develop in the context of a whole organism and it is important that this is coordinated in time. Pierre Leopold presented unpublished data on the control of organ growth in the drosophila model. His work demonstrates the role of a relaxing-like hormone called Dilp8 in coordinating growth and developmental progression in response to local growth impairment. In addition to its function in tissue repair, Dilp8 ensures that final organ size is finely adjustment through a developmental checkpoint occurring at the end of the growth period.  Alberto Rosello-Diez presented studies on how inter-tissue communication underlies growth compensation in developing bones in mice. He showed that upon unilateral growth inhibition in limb bones, the interaction between bones and surrounding tissues, and between bones and other organs, is likely key to maintain intra-limb and inter-limb body proportions.--Communications Arkhat ABZHANOV (Imperial College London, UK) - How and why biological shapes change during evolution                            Alexandre AULHELA (EMBL   Heidelberg, Germany) - Oscillatory signaling dynamics during mesoderm patterning — a top-down entrainment approac                                         James BRISCOE (The Francis Crick Institute, London, UK) - The cellular and molecular logic of neural tube development Marc (de) GENNES (Francis Crick Institute, London, UK) Drawing lines on a deforming canvass                                                       Anne GRAPIN-BOTTON (Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany) - Emergence in pancreas organoids                            Dagmar IBER ( ETH Zurich, Bale, Switzerland) - From Networks to Function - Computational Models of Organogenesis                                             Anna KICHEVA (IST Austria, Klosterneuburg, Germany) - Coordination of progenitor specification and growth in the developing spinal cord        Arthur LANDER (University of California Irvine, UCLA, USA) - Feedback control of pattern, growth and scale Pierre LEOPOLD  (INSERM/ CNRS Paris, France) - Growth coordination and final size adjustment during Drosophila development                                             Madhav MANI (Northwestern University, Evanston (IL), USA) - A new approach to quantitative phenotyping and force inference                                  Luciano MARCON (CABD, Sevilla, Spain) - A bi-stable reaction-diffusion mechanism for size-independent symmetry breaking of mouse embryoid bodies                                            Marco MILAN (IRB Barcelona, Spain) - Control of anisotropic growth by two orthogonal morphogen gradients                                     Caren NORDEN (MPI-CBG, Dresden, Germany) - New organogenesis paradigms from fish to humans: The interplay of cells and tissue                                           Nancy PAPALOPULU (The University of Manchester, UK) - Understanding how single cells make cell state transitions in a tissue environment                                                    Jochen RINK (MPI-CBG/BPC, Dresden, Germany) - Size sensing during planarian growth / de-growth Alberto ROSELLO-DIEZ (Australian Regenerative Medicine Institute. Monash University, Clayton, Australia) - Going out on a limb to study organ growth regulation Guillaume SALBREUX (The Francis Crick Institute, London, UK) - Epithelial flows and patterning                                               Eric SIGGIA (Rockefeller University, New York, USA) - Embryonic stem cell assays for cell signaling                                             Nicolas TAPON (The Francis Crick Institute, London, UK) - Growth control in the Drosophila abdomen                                       Jean-Paul VINCENT (The Francis Crick Institute Mill Hill Laboratory, London, UK) - Generation of signaling landscapes&#160;&#160;