Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G44: Multi-cellular Processes and Development |
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Sponsoring Units: DBIO Chair: Karen Kasza, Sloan-Kettering Institute Room: Hilton Baltimore Holiday Ballroom 1 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G44.00001: The Mechanics of Angiogenesis in Collagen Tubes Jolie Breaux, Abigail De La Pena, Melanie Suaris, Steven Zehnder, Thomas Angelini Cells in all types of tissue are sensitive to their mechanical environment. Understanding cell mechanics in tissue growth can lead to advancements in important medical applications, like technologies that enhance angiogenesis during wound healing. Great progress has been made in understanding the mechanics of angiogenesis with assays performed in flat bottomed culture dishes. Here we present results from an in vitro study of collective endothelial cell mechanics in a 3D culture system that mimics the geometry of a real endothelium. Human Aortic Endothelial Cells were grown inside of a collagen tube supported by a rigid cylindrical scaffold. We developed a time-lapse small angle light scattering method to directly measure the radial distribution of cells in the 3D matrix over time. Accompanying live-cell time-lapse microscopy was performed to monitor the cells' collective movement and organization. We find that the cells generate sufficient contractile force to detach the collagen matrix from the support scaffold while maintaining a macroscopic cylindrical arrangement, creating a fiber. Cell sensitivity to scaffold material properties, curvature, and symmetry will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G44.00002: Instabilities and topology changes in planar polarized epithelial sheets David Lubensky Epithelia--sheets of cells joined together by specialized junctional structures--are one of the basic building blocks of tissues and organs in animals. In many epithelia, rotational symmetry is broken and cells become polarized in a particular direction in the plane of the sheet. Here, we study the interplay between such planar cell polarity and the shape and packing of individual cells. Using general symmetry arguments and simple phenomenological models, we give a classification of the instabilities that can occur in such a coupled system. In particular, we show that two routes to chiral symmetry breaking are possible, both of which require that cells first become elongated along one axis. We also consider the evolution of the cell packing after an initial instability, including how planar polarity affects T1 topological transitions. We close with possible applications of these results to development in \textit{Drosophila} and in zebrafish. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G44.00003: Imaging the Population Dynamics of Bacterial Communities in the Zebrafish Gut Matthew Jemielita, Michael Taormina, Adam Burns, W. Zac Stephens, Jennifer Hampton, Karen Guillemin, Raghuveer Parthasarathy The vertebrate gut is home to a diverse microbial ecosystem whose composition has a strong influence on the development and health of the host organism. While researchers are increasingly able to identify the constituent members of the microbiome, very little is known about the spatial and temporal dynamics of commensal microbial communities, including the mechanisms by which communities nucleate, grow, and interact. We address these issues using a model organism: the larval zebrafish (Danio rerio) prepared microbe-free and inoculated with controlled compositions of fluorophore-expressing bacteria. Live imaging with light sheet fluorescence microscopy enables visualization of individual bacterial cells as well as growing colonies over the entire volume of the gut over periods up to 24 hours. We analyze the structure and dynamics of imaged bacterial communities, uncovering correlations between population size, growth rates, and the timing of inoculations that suggest the existence of active changes in the host environment induced by early bacterial exposure. Our data provide the first visualizations of gut microbiota development over an extended period of time in a vertebrate. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G44.00004: In-plane video force microscopy of morphogenesis in epithelia M. Shane Hutson, David Mashburn, Eric Copenhaver, W. Tyler McCleery, Jim Veldhuis, Steven Kim, G. Wayne Brodland Video force microscopy (VFM) is a technique that takes segmented time-lapse images as input and makes least-squares estimates for the cell-edge tensions and cell-internal pressures needed to drive observed changes in cell shape. VFM has previously been used to estimate the cell-level forces that drive invagination during Drosophila gastrulation. Doing so required time-lapse images containing entire cross-sections of the embryo. Here, we extend video force microscopy to in-plane images of epithelia -- including examples in which the images cover only a small region of a larger epithelium. This extension requires imposition of constraints on the average cell-internal pressure and the average stress external to the observed patch. We will demonstrate successful estimation of forces in exact models, as well as anomalous cases that prevent successful force estimation. We will then show applications of this technique for inferring the forces driving Drosophila germband retraction and wound healing. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G44.00005: Substrate properties affect collective cell motion Adrian Pegoraro, Ming Guo, Allen Ehrlicher, David Weitz When cells move collectively, cooperative motion, which is characterized by long range correlations in cell movement, is necessary for migration. This collective cell motion is influenced by cell-cell interactions as well as by cell-substrate coupling. Furthermore, on soft substrates it is possible for cells to mechanically couple over long distances through the substrate itself. By changing the properties of the substrate, it is possible to decouple some of these contributions and better understand the role they play in collective cell motion. We vary both the substrate stiffness and adhesion protein concentration and find changes in the collective cell motion of the cells despite only small differences in total cell density and average cell size in the confluent layers. We test these changes on polyacrylamide and PDMS substrates as well as on structured substrates made of PDMS posts that prevent mechanical coupling through the substrate while still allowing stiffness to be varied. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G44.00006: Modeling Excitable Systems Coupled Through External Medium Javad Noorbakhsh, Pankaj Mehta Excitable systems are stable dynamical systems in which any input beyond a threshold results in a significant output. This behavior is ubiquitous in nature and is seen in biological systems such as Dictyostelium discoideum amoeba and neurons to oscillatory chemical reactions. In this work we will focus on transition to oscillation in populations of excitable systems coupled through an external medium and will study their synchronization. We will describe a mechanism to tune the frequency of oscillations using an external input and will study the effects of stochasticity and inhomogeneity on the collective behavior of the system. Furthermore we will include diffusion into the dynamics of the external medium and will study formation of spatial patterns, their characteristics and their robustness to different factors. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G44.00007: Volumetric Measurements of Amnioserosa Cells in Developing Drosophila David Mashburn, Aroshan Jayasinghe, Shane Hutson The behavior of cells in tissue in developing Drosophila melanogaster has become increasingly clearer over the past few decades, in large part due to advances in imaging techniques, genetic markers, predictive modeling, and micromanipulation (notably laser microsurgery). We now know apical contractions in amnioserosa cells are a significant factor in large scale processes like germ band retraction and dorsal closure. Also, laser microsurgery induces cellular recoil that strongly mimics a 2D elastic sheet. Still, what we know about these processes comes entirely from the apical surface where the standard fluorescent markers like cadherin are located, but many open questions exist concerning the remaining ``dark'' portion of cells. Does cell volume remain constant during contraction or do cells leak? Also, what shape changes do cells undergo? Do they bulge, wedge, contract prismatically, or something else? By using a marker that labels the entire membrane of amnioserosa cells (Resille, 117) and adapting our watershed segmentation routines for 4D datasets, we have been able to quantify the entire volumetric region of cells in tissue through time and compare changes in apical area and volume. Preliminary results suggest a fairly constant volume over the course of a contraction cycle. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G44.00008: The mechanics of retinal detachment Tom Chou, Michael Siegel We present a model of the mechanical and fluid forces associated with exudative retinal detachments where the retinal photoreceptor cells separate typically from the underlying retinal pigment epithelium (RPE). By computing the total fluid volume flow arising from transretinal, vascular, and retinal pigment epithelium (RPE) pump currents, we determine the conditions under which the subretinal fluid pressure exceeds the maximum yield stress holding the retina and RPE together, giving rise to an irreversible, extended retinal delamination. We also investigate localized, blister-like retinal detachments by balancing mechanical tension in the retina with both the retina-RPE adhesion energy and the hydraulic pressure jump across the retina. For detachments induced by traction forces, we find a critical radius beyond which the blister is unstable to growth. Growth of a detached blister can also be driven by inflamed tissue within which {\it e.g.}, the hydraulic conductivities of the retina or choroid increase, the RPE pumps fail, or the adhesion properties change. We determine the parameter regimes in which the blister either becomes unstable to growth, remains stable and finite-sized, or shrinks, allowing possible healing. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G44.00009: Direct micro-mechanical measurements on \textit{C. elegans} Matilda Backholm, William S. Ryu, Kari Dalnoki-Veress The millimeter-sized nematode \textit{Caenorhabditis elegans} provides an excellent biophysical system for both static and dynamic biomechanical studies. The undulatory motion exhibited by this model organism as it crawls or swims through a medium is ubiquitous in nature at scales from microns to meters. A successful description of this form of locomotion requires knowledge of the material properties of the crawler, as well as its force output as it moves. Here we present an experimental technique with which the material properties and dynamics of \textit{C. elegans} can be directly probed. By using the deflection of a flexible micropipette, the bending stiffness of \textit{C. elegans} has been measured at all stages of its life cycle, as well as along the body of the adult worm. The mechanical properties of the worm are modelled as a viscoelastic material which provides new insights into its material properties. The forces exerted by the worm during undulatory motion are also discussed. Direct experimental characterization of this model organism provides guidance for theoretical treatments of undulatory locomotion in general. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G44.00010: Emission of sound from the mammalian inner ear Tobias Reichenbach, Aleksandra Stefanovic, Fumiaki Nin, A.J. Hudspeth The mammalian inner ear, or cochlea, not only acts as a detector of sound but can also produce tones itself. These otoacoustic emissions are a striking manifestation of the mechanical active process that sensitizes the cochlea and sharpens its frequency discrimination. It remains uncertain how these signals propagate back to the middle ear, from which they are emitted as sound. Although reverse propagation might occur through waves on the cochlear basilar membrane, experiments suggest the existence of a second component in otoacoustic emissions. We have combined theoretical and experimental studies to show that mechanical signals can also be transmitted by waves on Reissner's membrane, a second elastic structure within the cochea [1]. We have developed a theoretical description of wave propagation on the parallel Reissner's and basilar membranes and its role in the emission of distortion products. By scanning laser interferometry we have measured traveling waves on Reissner's membrane in the gerbil, guinea pig, and chinchilla. The results accord with the theory and thus support a role for Reissner's membrane in otoacoustic emission.\\[4pt] [1] T. Reichenbach, A. Stefanovic, F. Nin, A. J. Hudspeth, Waves on Reissner's membrane: a mechanism for the propagation of otoacous [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G44.00011: Biofilms suck: how bacteria beat the diffusion limit Thomas Angelini, Wenbo Zhang, Steven Zehnder, Jolie Breaux Multicellular behavior in bacterial biofilms is intimately tied to the production of an extracellular polysaccharide (EPS) matrix that encases the cells and provides physical integrity to the colony as a whole. Recent work in \textit{Bacillus subtilis} biofilms shows that a sudden increase in EPS production generates osmotic stresses that cause the biofilm to expand. Moreover, EPS production is triggered by a nutrient depletion gradient that develops in the biofilm due to diffusive mass transport limitations. These polymer physics based biofilm behaviors suggest that EPS production may have evolved in biofilms to beat the diffusion limit of nutrient transport into the colony, though no direct observation of nutrient transport has been observed previously. Here we measure the rate of nutrient transport into \textit{b. subtilis} biofilms and find that when EPS production is up-regulated, the polymer sucks fluid into the colony with a characteristic time dependence like that of pressure driven flow. Preliminary data and analysis will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G44.00012: Inhomogeneous DNA replication kinetics is associated with immune system response John Bechhoefer, Michel G. Gauthier, Paolo Norio In eukaryotic organisms, DNA replication is initiated at ``origins,'' launching ``forks'' that spread bidirectionally to replicate the genome. The distribution and firing rate of these origins and the fork progression velocity form the ``replication program.'' Previous models of DNA replication in eukaryotes have assumed firing rates and replication fork velocities to be homogeneous across the genome. But large variations in origin activity and fork velocity do occur. Here, we generalize our replication model to allow for arbitrary spatial variation of initiation rates and fork velocities in a given region of the genome. We derive and solve rate equations for the forks and replication probability, to obtain the mean-field replication program. After testing the model on simulations, we analyze the changes in replication program that occur during B cell development in the mouse. B cells play a major role in the adaptive immune system by producing the antibodies. We show that the process of cell differentiation is associated with a change in replication program, where the zones of high origin initiation rates located in the immunoglobulin heavy-chain locus shift their position as the locus prepares to undergo the recombination events responsible for generating antibody specificity. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G44.00013: Effects of Viscosity on the Gravi-kinesis Responses of Swimming {\it Paramecia} Studied Using Manetic Force Buoyancy Variation Ilyong Jung, James M. Valles Previous studies have shown that {\it paramecia} exhibit negative gravi-kinesis. They exert a stronger propulsive force when swimming up than when swimming down. This behavior is very surprising since it suggests they sense their tiny apparent weight of only $\sim$ 80pN. In an effort to understand the mechanism of this sensing, we are testing how the viscosity of the swimming medium influences their gravi-kinetic response. We employ the technique of magnetic force buoyancy variation to simulate different effective gravity levels on swimming {\it Paramecia}. We are analyzing their swimming response employing a phenomenological model that relates the parameters describing their helical trajectories to the beating of their cilia.\\ This work was supported by NSF PHY0750360 and at the NHMFL by NSF DMR-0084173 [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G44.00014: Fundamental role of bistability in optimal homeostatic control Guanyu Wang Bistability is a fundamental phenomenon in nature and has a number of fine properties. However, these properties are consequences of bistability at the physiological level, which do not explain why it had to emerge during evolution. Using optimal homeostasis as the first principle and Pontryagin's Maximum Principle as the optimization approach, I find that bistability emerges as an indispensable control mechanism. Because the mathematical model is general and the result is independent of parameters, it is likely that most biological systems use bistability to control homeostasis. Glucose homeostasis represents a good example. It turns out that bistability is the only solution to a dilemma in glucose homeostasis: high insulin efficiency is required for rapid plasma glucose clearance, whereas an insulin sparing state is required to guarantee the brain's safety during fasting. This new perspective can illuminate studies on the twin epidemics of obesity and diabetes and the corresponding intervening strategies. For example, overnutrition and sedentary lifestyle may represent sudden environmental changes that cause the lose of optimality, which may contribute to the marked rise of obesity and diabetes in our generation. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G44.00015: Time-Dependent Kinematics of Complex Human Structures Saami J. Shaibani The human body can be arranged in numerous geometrical configurations, including many interesting scenarios from the sport of gymnastics. One particularly challenging analytical example among these is the forward flip with maximum separation from the ground at the apex of the flight. The temporal aspects of this move involve the evaluation of multiple different positions during the trajectory, which adds significantly to the effort required. When a forward flip was executed during a football game [1], ready access to the recording [2] of this allowed a detailed kinematic examination to be performed. Careful application of highly intricate protocols [3] produces results which are consistent with similar athletic environments. The emphasis in this research is to transcend standard approaches elsewhere, which are severely limited to generic athletes and/or generic circumstances. Pedagogical benefits of the rigorous methodology adopted here are explored beyond what was introduced in a recent related study [4].\\[4pt] [1] Cardinals at Bengals on 24/12/2011\\[0pt] [2] via popular video-sharing website\\[0pt] [3] OUEL reports 1426/82 \& 1427/82, 1982\\[0pt] [4] http://aapt.org/AbstractSearch/FullAbstract.cfm?KeyID=20973, 2012. [Preview Abstract] |
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