Mechanics of development

14:05-14:45 Active and passive forces in epithelial branching

Professor Celeste Nelson, Princeton University, USA

Abstract

The morphogenetic patterning that generates three-dimensional (3D) tissues requires dynamic concerted rearrangements of individual cells with respect to each other. The group has developed microfabrication- and lithographic tissue engineering-based approaches to investigate the mechanical forces and downstream signalling responsible for generating the airways of the lung and the milk ducts of the mammary gland. Professor Nelson will discuss how to combine these experimental techniques with computational models to uncover the physical forces that drive development of engineered tissues and tissues in vivo. Professor Nelson will also describe efforts to uncover and actuate the different physical mechanisms used to build the airways in lungs from birds, mammals, and reptiles.

14:45-15:30 Models and measurements of cortical folding

Dr Philip Bayly, University of Washington in St. Louis, USA

Abstract

Cortical folding, or gyrification, is critical to human brain development and function. The folded shape of the human brain allows the cerebral cortex, the thin outer layer of neurons and their associated processes, to attain a large surface area (~1600 cm²)  relative to brain volume (~1400 cm³). Abnormal cortical folding has been associated with severe cognitive and emotional disorders, including epilepsy, autism, and schizophrenia. Among different species the onset of cerebral cortical folding consistently takes place after the majority of cerebral cortical neurons have completed migration from their sites of origin (cell layers near the surface of the lateral ventricle) to the nascent cerebral cortex (the cortical plate). 

Despite decades of study the mechanical forces that lead to cortical folding remain incompletely understood. Leading hypotheses have focused on the roles of tangential growth, heterogeneities in the birth and migration of neurons, and internal tension in axons. Advances in the mathematical modelling of growth and morphogenesis, combined with new experimental data, promise to clarify the mechanical basis of cortical folding. Recent experimental studies have illuminated not only the fundamental cellular and molecular processes underlying cortical development, but also the stress state, mechanical properties, and spatiotemporal patterns of growth in the developing brain. The combination of mathematical modelling with these measurements allows us to evaluate hypothesized mechanisms objectively, and to ensure that they are consistent with physical law. Dr Bayly will review the neurobiology of cortical development, summarize pertinent experimental observations, and discuss the potential role of growth-induced instability in cortical folding.

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15:30-16:00 Coffee

16:00-17:00 From mechanobiology to developmentally-inspired engineering

Professor Donald Ingber, Wyss Institute at Harvard University, USA

Abstract

Professor Ingber will review work from his laboratory beginning with mechanistic studies relating to the biophysical basis of cell shape determination and tissue morphogenesis, and ending with demonstration of how the group has leveraged our understanding of the importance of mechanical forces as bioregulators to develop new medical devices and therapeutics. The work began with the hypothesis the mechanical forces are as important biological regulators as chemicals and genes, and that cells use tensegrity architecture to control their form and function. Testing this theory required development of new analytical tools, including magnetic cytometry, femtosecond laser nanosurgery, microcontact printing, and microfluidic culture systems. These studies led to confirmation of the critical role that physical forces play in developmental control, experimental confirmation of the cellular tensegrity model, and discovery that transmembrane integrins and cytoskeletal prestress mediate mechanotransduction and developmental control. Importantly, the group also leveraged these insights to develop new engineering innovations that are being translated into the clinic or commercial marketplace. Examples include: angiogenesis inhibitors for cancer therapy identified based on cell shape changes; ‘shrink wrap’-like polymer gels that trigger tissue and organ formation by physically inducing mesenchymal condensation; mechanotherapeutics that prevent pulmonary edema by inhibiting integrin-mediated mechanochemical signal transduction; shear stress-targeted drug delivery systems that are directed to sites of vascular occlusion and dissolve blood clots without systemic toxicity; and microfluidic cell culture devices lined by living human cells, known as ‘Organs-on-Chips’, which recapitulate organ-level structure and functions by providing appropriate mechanical cues as a way to replace animal testing for drug development, mechanistic discovery, and personalized medicine.

08:30-09:00 Uncovering the mechanistic basis of biomechanical input controlling skeletal development: exploring the interplay with Wnt signalling at the joint

Professor Paula Murphy, Trinity College Dublin, the University of Dublin, Ireland

Abstract

Embryo movement is essential to the formation of a functional skeleton. Using mouse and chick models, the group has previously shown that mechanical forces influence gene regulation and tissue patterning, particularly at developing joints. However, there remains a lack of knowledge of the molecular mechanisms that underpin the influence of mechanical signals. 

Wnt signalling is required during skeletal development and is altered under reduced mechanical stimulation. To explore Wnt signalling as a mediator of mechanical input, the expression of Wnt ligand and Fzd receptor genes in the developing skeletal rudiments was profiled. Canonical Wnt activity restricted to the developing joint is reduced under immobilization while over-expression of activated b-catenin or the Wnt antagonist Sfrp3 following electroporation of chick embryo limbs, supports the proposed role for Wnt signalling in mechanoresponsive joint patterning. Two key findings advance our understanding of the interplay between Wnt signalling and mechanical stimuli: firstly, the loss of canonical Wnt activity at the joint shows reciprocal, co-ordinated regulation of Wnt and BMP pathways under mechanical influence. Secondly, this occurs simultaneously with increased expression of several Wnt pathway component genes in a territory peripheral to the joint, identifying the importance of mechanical stimulation on a population of potential joint progenitor cells.

09:00-09:30 New players and concepts in muscoskeletal biomechanics

Professor Elazar Zelzer, Weizmann Institute of Science, Israel

Abstract

Muscle spindles and Golgi tendon organs (GTOs), proprioceptive mechanoreceptors located inside striated muscles and in myotendinous junctions, respectively, are components of the stretch reflex circuitry that control muscle activity. Using genetic mouse models, the group demonstrates the involvement of proprioception in regulating spine alignment and spontaneous realignment of fractured bones, dubbed natural reduction. Failure of the mechanism that maintains posture may result in spinal deformity as in adolescent idiopathic scoliosis. The group shows that null mutants for Runx3 transcription factor, which lack connectivity between proprioceptors and spinal cord, developed peripubertal scoliosis not preceded by vertebral dysplasia or muscle asymmetry. Deletion of Runx3 in the peripheral nervous system or specifically in peripheral sensory neurons, or of enhancer elements driving Runx3 expression in proprioceptive neurons, induced a similar phenotype. Egr3 knockout mice, lacking spindles but not GTOs, displayed a less severe phenotype, suggesting that both receptor types are required for this regulatory mechanism.

Fracture repair involves restoration of bone morphology. Comparison among mice of different ages revealed, surprisingly, that three-month-old mice exhibited more rapid and effective natural reduction than newborns. Fractured bones of Runx3-null mutants failed to realign properly. Blocking Runx3 expression in peripheral nervous system, but not in limb mesenchyme, recapitulated the null phenotype, as did inactivation of muscles flanking the fracture site. Egr3 knockout mice displayed a less severe phenotype, suggesting that both receptor types, as well as muscle contraction, are required for this regulatory mechanism. Overall, these findings uncover physiological roles for proprioception in non-autonomous regulation of skeletal integrity and repair. 

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09:30-10:00 The role of fetal movements in shaping the developing human skeleton

Dr Niamh Nowlan, Imperial College London, UK

Abstract

Mechanical stimulation generated by fetal kicking and movements is known to be important for prenatal musculoskeletal development, and there are a number of human conditions that emphasise the link between abnormal fetal movements and delayed or impaired skeletal development. The most common of these is developmental dysplasia of the hip (DDH), a relatively common joint shape abnormality (clinical incidence 1.3 in 1000), the risk of which is strongly associated with restricted fetal movement, such as fetal breech position. The group is using computational modelling approaches (finite element analysis and musculoskeletal modelling), combined with human fetal imaging data to try to understand how the biomechanical stimulation (stresses and strains) caused by fetal movements evolve in the prenatal developing hip joint. Furthermore, the group models a range of intra-uterine conditions and situations that increase the risk of DDH, such as fetal breech position and oligohydramnios (reduced amniotic fluid), in order to be able to understand how a range of factors affecting movement may impact on the developing hip joint.

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10:00-10:30 Mechanobiology of embryonic tendon development and regeneration

Professor Catherine K Kuo, University of Rochester, USA

Abstract

Tendons transmit muscle-generated forces to bones to enable skeletal movements and stabilize joint structures. Proper development and maintenance of these extracellular matrix-rich tissues is critical to their demanding physical roles throughout the body. Abnormal tendon formation during embryogenesis is associated with frequently occurring musculoskeletal deformities such as congenital tallipes equino varus. Furthermore, tendons injured postnatally fail to recapitulate development during healing, and instead heal with aberrant matrix composition and organization, resulting in reduced functionality and greater susceptibility to re-injury. The group is interested in understanding how tendon mechanical properties elaborate during normal embryonic development to inform the prevention of tendon-related musculoskeletal birth defects and the enhancement of regenerative postnatal healing. This talk will focus specifically on the novel approaches the group has utilized to characterize the mechanical properties of tendons during embryonic development, and crosslinking mechanisms that have been identified to be critical to this process. Furthermore, the talk will discuss the development of engineered hydrogel systems that mimic the embryonic tendon mechanical microenvironment, enabling study of how dynamic changes in tissue stiffness continuously influence cell behaviours (differentiation, extracellular matrix synthesis, etc.) during development. Finally, the talk will discuss the role of physical movements (eg, kicking) in regulating tendon formation during embryonic development. These findings provide new insights into the crucial roles of mechanics and tissue mechanical properties in new tendon formation.

10:30-11:00 Coffee

11:00-11:45 Notochord mechanics shape the vertebrate axis

Associate Professor Michel Bagnat, Duke University, USA

Abstract

The notochord is a conserved axial structure that serves as a hydrostatic scaffold for embryonic axis elongation and, later on, for proper spine assembly. The vertebrate notochord consists of a core of large vacuolated cells surrounded by an epithelial sheath that is encased by an elaborate extracellular matrix. Each vacuolated cell contains a single fluid filled vacuole that occupies most of the cellular volume. These cells are arranged in a stereotypical staircase pattern within the notochord. The group investigated the origin of this pattern and found that it can be achieved purely by physical principles. Moreover, the group is able to model the arrangement of vacuolated cells within the notochord using a physical model composed of silicone tubes and water absorbing beads, suggesting that the notochord is a self-organizing structure. Furthermore, it was found that the biological structure and the physical model can be accurately described by the theory developed for cylindrical foams. 

In spite of their large size and being under constant mechanical stress, vacuolated cells possess a remarkable structural integrity. This is in part due to the presence of numerous caveolae, omega shaped cell surface invaginations, at the plasma membrane of vacuolated cells, which buffer mechanical tension. In zebrafish that lack caveolae, vacuolated cells collapse under the strain of axial bending during locomotion. Then, release of nucleotides from collapsed vacuolated cell leads to the invasion and trans-differentiation of sheath cells into new vacuolated cells. This regenerative response restores the mechanical properties of the notochord, thus allowing normal spine development.

11:45-12:30 Haemodynamic regulation of heart valve development

Professor Mark Kahn, University of Pennsylvania, USA

Abstract

Recent studies using a variety of animal models support a central role for flow and fluid forces in the development of valves in the heart, blood vessels and lymphatic vessels. An outstanding question in this field is how information sensed by endothelial cells that are directly subject to fluid forces is converted to instructions to underlying cells that participate most directly in formation of valves. To address this question the group has investigated the role of the transcription factor Krupple-like Factor 2 (KLF2) in heart valve development. KLF2 expression is highly upregulated by fluid shear forces in endothelial cells in vitro and its expression in vivo corresponds closely to predicted endothelial fluid shear forces as well. The group finds that loss of endocardial KLF2 in mouse embryos results in mid-gestation death due to an inability to remodel the cardiac cushion to a mature valve as the embryo grows. Loss of KLF2 is accompanied by cell non-autonomous changes in mesenchymal cell proliferation and condensation, two key steps in valve development. Loss of KLF2 results in virtually complete loss of the secreted protein Wnt9b as well as a severe loss of canonical WNT signalling in the sub-endocardial mesenchymal cells that participate in valve formation. Endocardial loss of Wnt9b recapitulates the Klf2 deficient phenotype in mice and studies in zebrafish embryos performed in collaboration with the Vermot lab demonstrate that Wnt9b is strongly regulated by both fluid shear forces and klf2a in vivo. These studies provide one answer to the question of how the information generated by fluid forces at the blood-endothelial interface is transmitted to cells not in contact with blood to orchestrate the creation of heart valves.

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13:30-14:15 A mechanical coupling checkpoint required for mechanotransduction in endothelial cells

Professor Ellie Tzima, University of Oxford, UK

Abstract

The Tzima lab investigates the role of mechanotransduction in regulating cardiovascular function in health and disease. We focus on endothelial mechanosensing and how fluid shear stress affects vessel and cardiac morphogenesis and function. Over the last decade, we identified and systematically characterised one of the most comprehensive models of endothelial mechanotransduction available to date. Using state-of-the-art approaches we showed that PECAM is a part of a junctional mechanosensory complex that senses and transduces mechanical force that ultimately regulates flow-mediated vascular remodelling. PECAM does not act alone; it crosstalks with integrins, another established mechanotransduction site, to collectively determine downstream responses. Recent work from our group, identified the signalling adaptor protein Src homologous and collagen protein (Shc) as a major player in endothelial mechanotransduction. We showed that Shc is tyrosine phosphorylated in response to mechanical force and binds to both the junctional mechanosensory complex and integrins. We now present data showing that Shc tyrosine phosphorylation provides a mechanical coupling checkpoint that allows certain signals to proceed, while blocking others in endothelial cells.

14:15-15:00 Tissue mechanics and mechanical forces during cardiovascular development

Dr Julien Vermot, Institut de génétique et de biologie moléculaire et cellulaire, France

Abstract

Mechanical forces are fundamental to cardiovascular development and physiology. The interactions between mechanical forces and endothelial cells are mediated by mechanotransduction feedback loops. The group is interested in understanding how hemodynamic forces modulate cardiovascular function and morphogenesis. Overall, their recent work is unravelling the biological links between mechanical forces, mechanotransduction and endothelial cell responses. Here the talk will discuss the recent work aiming at addressing how mechanical forces are spreading into the embryonic heart at each contraction and how these forces are sensed to modulate cardiac valve development.

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15:00-15:30 Coffee

15:30-16:15 Tissue dynamics in development and repair

Dr Yanlan Mao, University College London, UK

Abstract

Actomyosin contractility is a key regulator of tissue dynamics. During development, tissue dynamics, such as cell intercalations and oriented cell divisions, are critical for shaping tissues and organs. However, less is known about how tissues regulate their dynamics during tissue homeostasis and repair, to maintain their shape after development. In this talk, Dr Mao will discuss how tissues respond to mechanical perturbations, such as stretching or wounding, by altering their actomyosin contractile structures, to change tissue dynamics, and thus preserve tissue shape and patterning.

16:15-17:00 Blood flow and heart formation

Professor Sandra Rugonyi, Oregon Health & Science University, USA

Abstract

Heart formation involves a complex progression of finely orchestrated biological and biophysical processes. Blood flow dynamics, which is established early during cardiac development, when the heart is a linear tube without valves or chambers, is indispensable for proper heart formation. Cardiac looping, septation and valve formation all happen under blood flow, and, in fact, perturbations in blood flow dynamics during tubular heart stages lead to structural heart malformations. Blood flow mechanics, sensed by cardiac cells, act as a feedback loop regulating heart development. Feedback from blood flow mechanical stimuli is needed so that cardiovascular development addresses the increasing requirements of the embryo, while ensuring proper cardiac function. However, the discovery of the mechanisms by which developing cells sense and respond to blood flow mechanical stimuli is only starting to emerge. Quantification of both mechanical stimuli and biological responses is needed to determine the magnitude of the response to blood flow, affected regulatory processes, and possible implications to congenital heart disease. This talk will discuss these results in quantifying blood flow stimuli and biological responses, as well as implications on heart malformation.

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09:00-09:45 Organoid development by design

Professor Matthias Lutolf, École polytechnique fédérale de Lausanne, Switzerland

Abstract

Over the past years, organoids have stepped into the limelight as unique in vitro models for studying organ development, function and disease, owing to the previously unmatched fidelity with which they approximate real organs. Organoids form through poorly understood morphogenetic processes in which initially homogeneous aggregates of stem cells spontaneously self-organize within three-dimensional extracellular matrices (ECM). Yet, the absence of any predefined patterning influences such as morphogen gradients or mechanical cues results in an extensive heterogeneity. Moreover, the current mismatch in shape, size and lifespan between native organs and their in vitro counterparts severely hinders their applicability. In this talk, Professor Lutolf will discuss some of the ongoing efforts in developing programmable organoids that are assembled by combining bioengineering approaches with insights from developmental biology. Specifically, using intestinal organoids (‘mini-guts’) as a model system, Professor Lutolf will show for example how it is possible to overcome the stochasticity in self-organization by controlling cell fate decisions through localized changes in ECM mechanics. The convergence of bioengineering and cellular self-organization may be broadly applicable to attain more physiological organoid sizes, shapes and function.

09:45-10:30 Physical organogenesis of the gut

Dr Nicolas R Chevalier, Laboratoire Matière et Systèmes Complexes; CNRS - Université Paris Diderot, France

Abstract

The intestine is our body’s longest and most anisotropic organ. In this talk, Dr Chevalier will present the physical factors (physiological static and dynamic forces, biomechanical properties) that drive anisotropic gut growth during embryonic development.

10:30-11:00 Coffee

11:00-12:00 Mechanobiology of mammalian epidermis

Professor Fiona Watt FMedSci FRS, King's College London, UK

Abstract

The ability to reconstitute human epidermis in culture has afforded the opportunity to examine stem cell-niche interactions at single cell resolution. Professor Watt will describe the responses of human epidermal stem cells to different topographical cues, ranging from the 1 to 100 micron scale. Professor Watt will describe the signal transduction pathways that lead to the onset of terminal differentiation and show how different cues trigger differentiation via different pathways.

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12:00-12:30 Summary of discussions and closing remarks