Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F50: Morphogenesis IIFocus
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Sponsoring Units: DBIO GSOFT GSNP Chair: Zi Chen, Dartmouth Coll Room: LACC 511B |
Tuesday, March 6, 2018 11:15AM - 11:51AM |
F50.00001: Excitable dynamics of the segmentation clock Invited Speaker: Olivier Pourquie The periodic segmentation of the vertebrate body axis into somites, |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F50.00002: Grain Boundaries Form Dynamically in the Zebrafish Cone Mosaic Hayden Nunley, Mikiko Nagashima, Kamirah Demouchet, Alcides Gonzalez, Pamela Raymond, David Lubensky In zebrafish retinae, cone photoreceptors self-organize by subtype into a crystalline lattice called the cone mosaic. This system is a striking example of ordered cell packing in a vertebrate epithelium. This lattice lies on the surface of the retinal hemisphere. The lattice grows in such a way that topological defects, called Y-junctions, must be inserted to maintain constant cell spacing. Y-junctions form grain boundaries, running in the polar direction. These grain boundaries are similar to those observed in physical crystals at a free energy minimum on a curved surface. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F50.00003: Geometry of wave propagation on active deformable surfaces Pearson Miller, Norbert Stoop, Jorn Dunkel Fundamental biological and biomimetic processes, from tissue morphogenesis to soft robotics, rely on the propagation of chemical and mechanical surface waves to signal and coordinate active force generation. The complex interplay between surface geometry and contraction wave dynamics remains poorly understood. Here, we couple a dispersive wave model to non-Euclidean shell mechanics to identify and characterize generic features of chemo-mechanical wave propagation on active deformable surfaces. Our theoretical framework is validated against recent data from contractile wave measurements on ascidian and starfish oocytes, producing good quantitative agreement in both cases. The theory is then applied to illustrate how geometry and preexisting discrete symmetries can be utilized to focus active elastic surface waves. Generalizing to the targeted design of active morphable materials, we conclude by demonstrating that a controlled cascade of spontaneous transitions between discrete symmetries can be induced on both the shell and the traveling wave through the careful tuning of material properties. Altogether, our results show how geometry, elasticity and chemical signaling can be harnessed to construct dynamically adaptable, autonomous mechanical surface wave guides. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F50.00004: Revealing Developmental Logic with Deep Neural Networks Jingxiang Shen, Chao Tang Embryonic pattern formation is a classic case of emergent phenomena, where macroscopic structures result from recursive interaction of simple identical units like cellular automata. Being fascinating these phenomena themselves, the reverse design problem is particularly challenging: given a pattern or function, what could the underlying microscopic logic be like? |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F50.00005: Collective behavior in 2D co-culture of cells modeled as active, mechano-adhesive discs Supravat Dey, Moumita Das In many biological processes, whether the formation of embryos or of tumors, cells dynamically organize in a context-dependent and spatiotemporal manner. These cells live and actively migrate in a heterogeneous environment of many cell types with different physical properties. For example, in many types of cancers such as colon, melanoma, prostrate and breast cancers, experiments have shown that the cancer cells are mechanically more deformable than the corresponding non-tumorigenic cells. It is also known that while non-cancerous (epithelial) cells tend to adhere to each other due to the adhesion protein E-cadherin and form a confluent tissue, in cancerous (mesenchymal) cells the expression of this protein is often heavily downregulated. Motivated by this, we study the organization of a binary population of two types of self-propelled discs (cells) with different stiffness and adhesion, where the elastic and adhesive cell-cell interaction is modeled by the Johnson-Kendall-Roberts force for two contacting soft spheres. We investigate the growth and structures of segregating clusters by studying MSDs and density structure factors over time. Our results may elucidate how changes in cell mechano-adhesive properties during tumor progression impact cellular organization in tumors. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F50.00006: Liquid crystal elastomer coatings with programmed response of surface profile Greta Babakhanova, Taras Turiv, Yubing Guo, Matthew Hendrix, Qihuo Wei, Albert Schenning, Dirk Broer, O Lavrentovich Stimuli-responsive liquid crystal elastomers (LCEs) with a strong coupling of orientational molecular order and rubber-like elasticity show a great potential as working elements in soft robotics, sensing, transport and propulsion systems. In this work, we demonstrate that the predesigned director patterns can be used to control a dynamic surface topography of LCE coatings. The director pattern is created by a photoalignment method. As the temperature changes, the coating responds by changing the topography of the free surface. The topography change is uniquely determined by the director pattern preprogrammed during the alignment and polymerization of LCEs. The deterministic dependence of the dynamic surface profile on the director pattern is explained by activation forces rooted in (i) stretching-contraction of the polymer networks driven by temperature; (ii) spatially varying orientation of the LCE. The activation force concept brings the responsive LCEs into the domain of active matter. The demonstrated relationship can be used to design programmable coatings with functionalities that mimic biological tissues such as skin. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F50.00007: The Physics of Blastoderm Flow during Early Gastrulation of Tribolium castaneum Stefan Muenster, Alexander Mietke, Akanksha Jain, Pavel Tomancak, Stephan Grill The early embryo of the red flour beetle, Tribolium castaneum, initially consists of a single-layered blastoderm covering the yolk uniformly that differentiates into an embryonic rudiment as well as extraembryonic amnion and serosa. The germband forms inside the egg when the embryonic rudiment condenses and folds along the ventral midline; this process is accompanied by large-scale flow and expansion of the serosa which ultimately covers the entire surface of the egg. The mechanical properties of these tissues and the forces governing gastrulation in Tribolium are poorly understood. Here, we present our findings on the dynamics of myosin in the early blastoderm of Tribolium using multiview lightsheet live imaging. We quantify the global distribution of myosin throughout the flow phase and present a physical description that couples the contractile forces generated by myosin to the mechanical properties of the blastoderm. In particular, we describe the overall tissue as a thin, actively contractile, viscous bulk medium that exhibits friction with the vitelline membrane. This description accurately captures the large-scale deformation the tissue undergoes during the initial stages of gastrulation, thus laying a foundation for the physical description of gastrulation in Tribolium. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F50.00008: An animal shape as an equilibrium Mengsha Gong, Michael Abrams, Chin-Lin Guo, Lea Goentoro A century ago, D’Arcy Thompson likened a jellyfish’s shape to the equilibrium form of a gelatinous drop. A recent finding that injured jellyfish reorganize and recover radial symmetry presented a tractable morphogenetic system in which to explore animal shape regulation after development. Performing grafting in Aurelia aurita, we explored how this system responds to different initial conditions. We found that slight changes in geometric initial condition led Aurelia to assume shapes such as circle, oval, trapezoid, square, and triangle, showing that radial symmetry is but one of many shapes this system can support. Further experiments and mathematical modeling suggested these shapes can be captured as equilibria of local force balancing, driven by muscle contractions and viscoelastic tissues. As the model predicts, increasing the rate of muscle contraction biases the solution toward radial symmetry. As expected for jellyfish shape as an equilibrium, Aurelia sequentially reorganized to rebalance new asymmetries. Maintaining animal shape as an equilibrium may facilitate evolvability. This finding may inspire self-organizing machinery that adapts to changing force landscapes. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F50.00009: Geometry-dependent pattern formation in active biological materials Tzer Han Tan, Manon Wigbers, Erwin Frey, Nikta Fakhri Robust and precise patterning of cytoskeletal dynamics such as actin polymerization and myosin activity is crucial for embryos to generate proper mechanical form. In turn, the mechanical stresses that arise can induce shape deformation or shear-induced flow, modulating the reaction-diffusion dynamics of biochemical processes inside these cells. The general mechanisms of such mechanochemical feedback, underlying the formation of self-organized spatiotemporal patterns important for development, have remained largely unexplored. Here, we use the Rho wave dynamics in meiotic starfish oocytes to study the coupling between geometry and biochemical regulation. We can modulate the speed and the wavelength of the Rho waves by manipulating the geometrical shape parameters of the oocytes confined in micro-fabricated PDMS chambers. By combining our experimental results with a mass-conserved reaction-diffusion theoretical model, we discover a close interplay between geometry and biochemical regulation in biological pattern formation. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F50.00010: Threshold Response to Stochasticity in Morphogenesis George Courcoubetis, Paul Marjoram, Sergey Nuzhdin, Stephan Haas During development of biological organisms, multiple complex structures are formed. In many instances, these structures must exhibit a high degree of order to be functional, although their constituents are intrinsically stochastic. Hence, it has been suggested that biological robustness ultimately relies on clean-up mechanisms. In the context of the Drosophila eye disc, multiple optical units, ommatidia, are positioned in a crystal-like fashion. During the larva-to-pupa-stage of the eye disc development, the centers of the ommatidia are first specified via differentiation of R8 cells by reaction and diffusion of morphogens. Subsequently, the spatial configuration of R8 cells is refined via diffusion of cell proliferation and apoptosis. In this talk, I present our mathematical modeling of these spatio-temporal processes, including measures that quantify order of the resulting patterns in the presence of noise. We observe an interesting universal sigmoidal response to increasing stochasticity: ordered patterns persist up to a threshold noise level in the model parameters. We argue that this sigmoidal dependence is indicative of cryptic variation, whereby underlying differences in gene expression between wild type organisms exist, but do not lead to different phenotypes. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F50.00011: Variations in Material Properties and Growth Patterns of Leaves Michal Sahaf, Eran Sharon Leaf growth is a complex process, which involves many biological and chemical cycles leading to the final shape of the leaf. We measure the growth patterns and mechanical properties of growing leaves, and study their growth process by viewing the leaf as an active material, a mechanical object which can change its properties in response to outer and inner signals. We identify changes triggered by physical stimuli such as mechanical stress and light, and interpreter them in the context of growth and morphogenesis. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F50.00012: Mechanical feedback controls heterogeneity in plant development Antoine Fruleux, Arezki Boudaoud How do organs form with consistent sizes and shapes, with substantial variability at the cellular level? |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F50.00013: Role of the Supracellular Actomyosin Cable during Epithelial Wound Healing Yanjun Yang, Herbert Levine The closure of wounds in epithelia is center to many physiological processes in both development and repair of multicellular organisms. Depending on the biochemical and mechanical environment as well as cell type, this process often involves cell crawling and the purse-string contraction of a supracellular actomyosin ring around the wound. However, how these two mechanisms, especially the supracellular actomyosin ring, contribute to the wound healing is still unclear. To decipher this complex process, we develop a particle-based model that includes purse-string contraction, cell crawling and other properties incorporated with monolayers of Madin–Darby canine kidney (MDCK) cells. Our model captures the traction force patterns under several different conditions in experiments. In addition to traction force pointing away from the wound on the leading edge, we observed patterns of traction force pointing towards the wound. We show this inward pointing force pattern is induced by the purse-string contraction. By tuning the strength of the purse-string contraction and cell crawling, we depict the role of this actomyosin cable during wound healing. Our model can also explain which regulators affect the efficiency of these two mechanisms and how they interplay with each other. |
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