Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H40: Multi-cellular Processes and Development |
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Sponsoring Units: DBP Chair: Eva-Maria Schoetz, Princeton University Room: A122/123 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H40.00001: Dynamics of asexual reproduction in planarians Eva-Maria Schoetz, Bryan Lincoln, Sofia Quinodoz Planaria research is experiencing a resurgence due to the development of molecular tools, the Planarian genome project and database resources. Despite the resulting progress in planarian biology research, an extensive study of their physical properties remains to be undertaken. We developed a method to collect a large amount of data on the dynamics of clonal reproduction in the freshwater planarian S.mediterranea. The capability of planarians to regenerate an entire organism from a minuscule body part is based on a homogeneously distributed stem cell population that comprises 25-30{\%} of all cells. Due to this stem cell contingent, planarians can reproduce spontaneously by dividing into a larger head and a smaller tail piece, which then will rebuild the missing body parts, including a central nervous system, within about a week. Time-lapse imaging allows us to characterize the fission process in detail, revealing the stages of the process as well as capturing the nature of the rupture itself. A traction force measurement setup is being developed to allow us to quantify the forces planarians exert on the substrate during reproduction, a macroscopic analog to the Traction Force Microscopy setups used to determine local cellular forces. We are particularly interested in the molecular processes during division and the interplay between tissue mechanics and cell signaling. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H40.00002: Quantifying cell behaviors during embryonic wound healing David Mashburn, Xiaoyan Ma, Sarah Crews, Holley Lynch, W. Tyler McCleery, M. Shane Hutson During embryogenesis, internal forces induce motions in cells leading to widespread motion in tissues. We previously developed laser hole-drilling as a consistent, repeatable way to probe such epithelial mechanics. The initial recoil (less than 30s) gives information about physical properties (elasticity, force) of cells surrounding the wound, but the long-term healing process (tens of minutes) shows how cells adjust their behavior in response to stimuli. To study this biofeedback in many cells through time, we developed tools to quantify statistics of individual cells. By combining watershed segmentation with a powerful and efficient user interaction system, we overcome problems that arise in any automatic segmentation from poor image quality. We analyzed cell area, perimeter, aspect ratio, and orientation relative to wound for a wide variety of laser cuts in dorsal closure. We quantified statistics for different regions as well, i.e. cells near to and distant from the wound. Regional differences give a distribution of wound-induced changes, whose spatial localization provides clues into the physical/chemical signals that modulate the wound healing response. [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H40.00003: Keeping track of embryo development: new insights in the coupling between local and global changes Timon Idema, Philip Nelson, Andrea Liu, Julien Dubuis, Lisa Manning, Thomas Gregor Modern imaging techniques allow us to study biological systems such as Drosophila in vivo during early development. Between the ninth and fourteenth cell cycles of the Drosophila embryo, nuclei are positioned at the embryo's surface and are observed to divide at the end of each cycle in a highly synchronized fashion. We have implemented a new tracking technique that allows us to determine the shapes of the nuclei as they elongate and divide, and to follow their motion on the surface. We find that during each cycle, the nuclei shapes evolve with time in a consistent way from nucleus to nucleus. These shape changes spread as waves with a well-defined wave velocity through the embryo, coupling local (nucleus) and collective (entire embryo) development. The waves in turn induce collective motions of the nuclei, not just after division but also before it. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H40.00004: A cellular Potts model of germband retraction and dorsal closure M. Shane Hutson, Jason Rohner, Sarah Crews, W. Tyler McCleery, W. Bradley Robinson Germband retraction and dorsal closure are critical morphogenetic events in fruit fly embryogenesis. Both involve the coordinated reshaping of two epitheloid tissues -- germband (GB) and amnioserosa (AS). The GB is initially curled into a U-shape with the AS between the arms of the U. Retraction leaves the embryo's dorsal surface covered by AS cells which then contract to pull lateral parts of the GB up to cover the dorsal surface. We have simulated these events using a cellular Potts model. The model is 3D with several generalized cell types: a central yolk; a surrounding monolayer of AS and GB cells with epithelial polarization; and an outer vitelline membrane enclosing the cells and a perivitelline fluid. The model also incorporates several critical cell behaviors: polarized apical constriction of AS cells; controlled relaxation of stretched GB cells; and differentiation of GB cells at the GB-AS interface so that these cells then contract a supracellular purse-string and extend filopodia that reach across the AS and zip together the GB's approaching lateral flanks. We will discuss how all of these components are necessary to reproduce normal tissue motions and those observed during laser microsurgery experiments. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H40.00005: Developmental and Metabolite Transport Strategies to Optimize the Growth of Filamentous Cyanobacteria Aidan Brown, Andrew Rutenberg Individual cells of filamentous cyanobacteria share nutrients through cytoplasmic and/or periplasmic connections. Under conditions of low fixed-nitrogen some cells terminally differentiate into heterocysts, which fix nitrogen for the remaining photosynthetic vegetative cells. Heterocysts are observed to occur in a regular pattern separated by clusters of vegetative cells. Using a quantitative model of nitrogen uptake, consumption and transport together with vegetative cell growth and division, we explore how the overall growth rate of the filament depends on different heterocyst positioning patterns and on particular strategies of nitrogen transport. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H40.00006: Elasticity of developing cardiac tissue Stephanie Majkut, Joe Swift, Christine Krieger, Dennis Discher Proper development and function of the heart from the tissue to cellular scale depends on a compliant ECM. Here we study the maturation of embryonic cardiac tissue mechanics in parallel with the effects of extracellular mechanics on individual cardiomyocyte function throughout early development. We used micropipette aspiration to measure local and bulk elastic moduli (E) of embryonic avian heart tissue from days 2-12. We observe stiffening of the early heart tube from E = 1 kPa at day 1 to E = 2 kPa at day 4, reaching neonatal values by day 12. Treating heart tubes with blebbistatin led to 30{\%} decrease in E, indicating a significant but partial actomyosin contribution to mechanics at these stages. We performed a proteomic analysis of intact and decellularized 2-4 day heart tubes by mass spectrometry to quantify the ECM present at these stages. Isolated cardiomyocytes from 2-4 day chick embryos were cultured on collagen-coated PA gels of various stiffnesses. Beating magnitude was modulated by substrates with E = 1-2 kPa, similar to physiological E at those stages. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H40.00007: ECM ordering effects as a marker for early tissue formation on artificial substrates - a sum-frequency-generation spectroscopy study Patrick Koelsch, Mark-Oliver Diesner The in situ monitoring of the interphase between a substrate and a cellular layer is of great interest as it allows determination of changes in surface properties and extracellular matrix (ECM) organization. The latter is an early indicator of major cellular processes like migration, adhesion, proliferation, metastasis, tissue formation, and gain or loss of differentiation. We demonstrated recently that vibrational sum-frequency-generation (SFG) spectroscopy can be used to probe the layer between living cells and a solid substrate. In this contribution we will report on the investigation of ordering phenomena within the ECM of fibroblasts allowing to track early stages of tissue formation. SFG spectroscopy offers a unique way to observe these correlated changes of order in real-time without the need of labeling or disruption. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H40.00008: Distinguishing Pattern Formation Phenotypes: Applying Minkowski Functionals to Cell Biology Systems Erin Rericha, Can Guven, Carole Parent, Wolfgang Losert Spatial Clustering of proteins within cells or cells themselves frequently occur in cell biology systems. However quantifying the underlying order and determining the regulators of these cluster patterns have proved difficult due to the inherent high noise levels in the systems. For instance the patterns formed by wild type and cyclic-AMP regulatory mutant \textit{Dictyostelium} cells are visually distinctive, yet the large error bars in measurements of the fractal number, area, Euler number, eccentricity, and wavelength making it difficult to quantitatively distinguish between the patterns. We apply a spatial analysis technique based on Minkowski functionals and develop metrics which clearly separate wild type and mutant cell lines into distinct categories. Having such a metric facilitated the development of a computational model for cellular aggregation and its regulators. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H40.00009: Measuring the viscosity of embryonic epithelia \textit{in vivo} by magnetic tweezers Xiaoyan Ma, M. Paula Angarita, Mershard Frierson, Drew Sheldon, M. Shane Hutson During early development, sheets of epithelial cells are reshaped by cellular forces. Several recent investigations in fruit fly (\textit{Drosophila}) embryos have used laser microsurgery and video force microscopy to measure these forces; however, these measurements are actually limited to force/viscosity ratios because the effective viscosity of epithelial cells in a living embryo is largely unknown. This effective viscosity may vary spatially within the embryo and temporally as development progresses. To address this issue, we use microinjection, magnetic tweezers and confocal microscopy to measure the effective viscosity of epithelial cells in fruit fly embryos \textit{in vivo}. We inject fluorescent magnetic beads (2-$\mu $m diameter) into GFP-labeled embryos at the multi-nuclear syncytial blastoderm stage. The beads are pulled to embryo's surface by a permanent magnet and become engulfed by individual epithelial cells during cellularization. During later stages of development, we supply current pulses to an electromagnet to apply force pulses to the beads with a magnitude of $\sim $100 pN. The effective viscosity is inferred from the movement of these beads as tracked by confocal microscopy. We will report initial results on amnioserosa cells during dorsal closure. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H40.00010: Modeling of Endothelial Glyccalyx via Dissipative Particle Dynamics Mingge Deng, Haojun Liang, George Karniadakis We employ Dissipative Particle Dynamics (DPD) to simulate flow in small vessels with the endothelial glycocalyx attached to the wall. Of particular interest is the quantification of the slip velocity at the edge of glycocalyx and of the increased pressure drop at different crafting densities, stiffness and height of the glycocalyx. Results will be presented for capillaries and small arterioles, and interactions with discrete red blood cells will be included in the modeling. In addition to the physical insight gain for this important but relatively unexplored bioflow, simple models for the slip velocity will be proposed that can be used in continuum simulations of blood flow in micro-vessels. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H40.00011: An approach to collective behavior in cell cultures: modeling and analysis of ECIS data David Rabson, Evan Lafalce, Douglas Lovelady, Chun-Min Lo We review recent results in which statistical measures of noise in ECIS data distinguished healthy cell cultures from cancerous or poisoned ones: after subtracting the ``signal,'' the $1/f^\alpha$ noise in the healthy cultures shows longer short-time and long-time correlations. We discuss application of an artificial neural network to detect the cancer signal, and we demonstrate a computational model of cell-cell communication that produces signals similar to those of the experimental data. The simulation is based on the $q$-state Potts model with inspiration from the Bak-Tang-Wiesenfeld sand-pile model. We view the level of organization larger than cells but smaller than organs or tissues as a kind of ``mesoscopic'' biological physics, in which few-body interactions dominate, and the experiments and computational model as ways of exploring this regime. [Preview Abstract] |
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