Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A23: Non-Linear Deformations in BiologyFocus Session
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Sponsoring Units: DBIO Chair: Eva-Maria Collins, Swarthmore Coll Room: 304 |
Monday, March 2, 2020 8:00AM - 8:36AM |
A23.00001: Mechanics and Elasticity of dynamic cellular monolayers - How Volvox embryos turn inside-out Invited Speaker: Stephanie Hoehn Living tissues are intelligent materials that can change their mechanical properties while they develop. In spite of extensive studies in multiple model organisms we are only just beginning to understand these dynamic properties and their role in tissue development. Although many tissues are known to exhibit visco-elastic properties, it is unclear which properties dominate three-dimensional shape changes of cellular monolayers, such as epithelia. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A23.00002: A bilayer model of the non-linear elastodynamics of Hydra mouth opening Tapan Goel, Cassidy Tran, Ellen Adams, Patrick Henry Diamond, Eva-Maria Collins Mouth opening in Hydra involves extreme deformations, with radial cell strains of up to 30%. Hydra consists of two neuronally excitable, mechanically coupled epithelia-muscular cell layers, an outer ectoderm and an inner endoderm. The layers are coupled basally by a viscoelastic extracellular matrix, into which the cells extend contractile fibers (myonemes). In the head, ectodermal myonemes are arranged radially and endodermal myonemes in concentric rings. Experiments show that mouth opening proceeds through a series of radial ‘tugs’ – local myoneme contractions that must synchronize to produce a symmetric opening. We investigate if a bilayer model of viscoelastic sheets containing networks of radial and circular non-linear springs with Poisson distributed compressive forcing can capture mouth opening. Constraining the model using in vivo data, we test its ability to reproduce the time history of the mouth area during opening. We compare the kinematics generated by radially travelling contraction pulses with that for simultaneous contractions at all radii. Further, we explore the conditions required for space-time synchronization of ‘tugs’ into a quasi-symmetric opening and the dependence on the ratio of firing rate and the relaxation timescale of the viscoelastic sheet. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A23.00003: Investigation of Rice Root Tip Circumnutation Functions in Heterogeneous Environments Erin McCaskey, Isaiah Taylor, Kevin Lehner, Philip N Benfey, Daniel I Goldman Circumnutation is the pattern of oscillatory growth widespread among plants. Little is known about the function of circumnutation, particularly in root-heterogeneity interactions. Traits that allow for exploration may be advantageous, as roots can encounter heterogeneities in their environment that prevent productive growth. In this work we used a clear gel-based media to create a growth environment of two gel layers with varying stiffness, which can model soil horizons with varying compaction. A high-throughput automatic imaging system acquired images to visualize the root growth. Roots were grown in either a soft gel upper layer to a stiffer gel bottom layer, or the opposite. Our results show interesting differences in the ability of wild-type and non-circumnutating roots to penetrate the lower gel layer. These differences provide insights into the function of circumnutation. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A23.00004: Mechanics of behavior: Comprehensive search behavior encoded in cytoskeletal dynamics of single cell Lacrymaria olor Eliott Flaum, Deepak Krishnamurthy, Scott Coyle, Manu Prakash Complex animal behavior arises from interplay between actuators and sensors. Surprisingly, single eukaryotic cells such as protists are also capable of complex behavior, but how an algorithm such as search might be encoded in intrinsic dynamics of a single cell remains unknown. Here we elucidate the mechanics of search behavior in the predatory ciliate Lacrymaria olor, which has the ability to extend and contract its “neck” seven body lengths in a second while efficiently searching the space around it for prey in minutes. Our recent work establishes that L. olor’s search strategy is encoded in antagonistic active systems: subcellular structures that use surface cilia and the cortical cytoskeleton (Coyle et al, 2019). Here we reveal the underlying geometrical features of the cytoskeleton, and membrane and volume constraints, that together program extension and contraction dynamics of this active filament. Through force spectroscopy and membrane tension experiments in live cells, we reveal the role of this dynamic force landscape and how it shapes the search phase space. Our work combines theoretical active filament models with experimental data to unravel how active mechanics leads to emergent behavior in single cells. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A23.00005: Functional consequences of microscopic skin features on snake locomotion Jennifer Rieser, Tai-De Li, Daniel I Goldman, Joseph Mendelson Interactions between limbless animals and their natural environments, which are essential for movement, are mediated solely through skin contact. We used atomic force microscopy to investigate how surface textures on shed skins vary across snakes and environments. While most snakes have microfibrils oriented longitudinally, a few distantly-related snake species have convergently lost these features in favor of a more isotropic structure. We hypothesize that these microstructures affect the frictional interaction with the substrate and we use resistive force theory to model the effects of frictional anisotropy on snake locomotion. For lateral undulation, we predict that an anisotropic frictional interaction in movement along the body is favored over transverse movement, improving performance (measured in distance traveled per cycle), and that larger anisotropies produce larger displacements. In sidewinding locomotion, however, we predict the opposite trend: decreased frictional anisotropy improves performance. These predictions are consistent with our AFM measurements of small-scale features on snake skins and suggest a functional benefit for the convergent loss of structure shared by sidewinding vipers. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A23.00006: Bioelectrical signaling via domain wall migration Harold McNamara, Rajath Salegame, Ziad Al Tanoury, Haitan Xu, Shahinoor Begum, Gloria Ortiz, Olivier Pourquie, Adam Cohen Electrical signaling in biology is typically associated with action potentials, transient spikes in membrane voltage that return to baseline. The Hodgkin-Huxley equations of electrophysiology belong to a more general class of reaction-diffusion equations which could, in principle, support patterns of membrane voltage which are stable in time but structured in space. Here we show theoretically and experimentally that homogeneous or nearly homogeneous tissues can undergo spontaneous spatial symmetry breaking into domains with different resting potentials, separated by stable bioelectrical domain walls. Transitions from one resting potential to another can occur through long-range migration of these domain walls. We map bioelectrical domain wall motion using all-optical electrophysiology in an engineered cell line and in human iPSC-derived myoblasts. Bioelectrical domain wall migration may occur during embryonic development and during physiological signaling processes in polarized tissues. These results demonstrate a novel form of bioelectrical pattern formation and long-range signaling. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A23.00007: Modeling the Mechanosensitivity of Crawling Cells John Molina, Ryoichi Yamamoto In this work, we study the ability of cells to probe and dynamically adapt to the mechanical properties of their surroundings, i.e., their mechanosensitivity. Experimentally, this can be studied by observing the reorientation of crawling cells over cyclically stretched substrates. To understand the observed cell-specific reorientation, we have introduced a computational model that couples the cyclically stretched substrate to the sub-cellular elements responsible for cell shape and motility: cell membrane, actin cytoskeleton, and focal adhesions. Depending on which sub-cellular process is being probed, and the type of coupling with the substrate, our simulations predict either no reorientation, a bi-stability in the parallel and perpendicular directions, or a complete reorientation. In particular, we show that an asymmetry in the adhesion dynamics during the loading and unloading phases of the stretching can be used to selectively align the cells. Our results provide further evidence for the importance of focal adhesion dynamics in determining the mechanosensitive response of cells. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A23.00008: Geometry, Elasticity, Growth: The Connection Between Cowrie Growth Dynamics and Shell Form Michael Gabriel Levy, Michael Robert DeWeese Thin elastic sheets are currently a hot topic in soft matter physics. We propose a new model illustrating how the physics of bending and wrinkling sheets could underlie the geometry of Cowrie Seashells, offering both qualitative and quantitative insights. This work suggests generally new approaches to the mechanics underlying biological development. Despite both the cowry's import -- in monetary history and as a collector’s item -- and a rich history of seashell modeling, we are the first to mechanistically consider cowrie form at all, let alone couple growth, elasticity, and form to recapitulate the shape of the central spiral, the thickening of the shell base, and the ridge-like teeth which form at the shell opening. By conecting elasticity with the biological processes of shell repair and body growth, we suggest that most aspects of form are emergent from the developmental decision to extend the mantle and extrude it over the shell instead of standard volumetric growth. In addition to our theoretical results, we test our model against both published and reported data, suggest previously unreported scaling relations, and demonstrate methods for extracting geometrical information from both three-dimensional scans and two dimensional images. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A23.00009: Theoretical tool bridging cell polarities with development of robust morphologies Silas Boye Nissen, Steven Rønhild, Ala Trusina, Kim Sneppen Despite continual renewal and damages, a multicellular organism is able to maintain its complex morphology. How is this stability compatible with the complexity and diversity of living forms? Looking for answers at protein level may be limiting as diverging protein sequences can result in similar morphologies. Inspired by the progressive role of apical-basal and planar cell polarity in development, we propose that stability, complexity, and diversity are emergent properties in populations of proliferating polarized cells. We support our hypothesis by a theoretical approach, developed to effectively capture both types of polar cell adhesions. When applied to specific cases of development – gastrulation and the origins of folds and tubes – our theoretical tool suggests experimentally testable predictions pointing to the strength of polar adhesion, restricted directions of cell polarities, and the rate of cell proliferation to be major determinants of morphological diversity and stability. |
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