Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W3: Multi-cellular Processes and Development |
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Sponsoring Units: DBIO Chair: Shane Hutson, Vanderbilt University Room: 107 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W3.00001: On Growth and Form of the Zebrafish Gut Microbiome Matthew Jemielita, Michael Taormina, Annah Rolig, Adam Burns, Jennifer Hampton, Karen Guillemin, Raghuveer Parthasarathy The vertebrate gut is home to a diverse microbial community whose composition has a strong influence on the development and health of the host organism. Researchers can identify the members of the microbiota, yet little is known about the spatial and temporal dynamics of these microbial communities, including the mechanisms guiding their nucleation, growth, and interactions. We address these issues using the larval zebrafish (\textit{Danio rerio}) as a model organism, which are raised microbe-free and then inoculated with controlled compositions of fluorophore-expressing bacteria. Live imaging using light sheet fluorescence microscopy enables visualization of the gut's entire microbial population over the first 24 hours of colonization. Image analysis allows us to quantify microbial populations that range from a few individuals to tens of thousands of microbes, and analyze the structure and growth kinetics of gut bacterial communities. We find that genetically-identical microbes can show surprisingly different growth rates and colonization abilities depending on their order of arrival. This demonstrates that knowing only the constituents of the gut community is insufficient to determine their dynamics; rather, the history of colonization matters. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W3.00002: Wave Propagation in Expanding Cell Layers Kazage J Christophe Utuje, Shiladitya Banerjee, M. Cristina Marchetti The coordinated migration of groups of cells drives important biological processes, such as wound healing and morphogenesis. In this talk we present a minimal continuum model of an expanding cell monolayer coupling elastic deformations to myosin-based activity in the cells. The myosin-driven contractile activity is quantified by the chemical potential difference for the process of ATP hydrolysis by myosin motors. A new ingredient of the model is a feedback of the local strain rate of the monolayer on contractility that naturally yields a mechanism for viscoelasticity of the cellular medium. By combining analytics and numerics we show that this simple model reproduces qualitatively many experimental findings, including the build-up of contractile stresses at the center of the cell monolayer, and the existence of traveling mechanical waves that control spreading dynamics and stress propagation in the cell monolayer. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W3.00003: Assessing ODE models of tumor growth Hana Dobrovolny, Hana Jaafari, Michael Ellis Mathematical models are often used to study and optimize treatment of cancer. In order to accurately predict the efficacy of a particular treatment, the model must correctly describe tumor growth. Over the years, several differential equation models of tumor growth have been proposed and independently fit to experimental data sets. While all the models provide reasonable fits to tumor growth data, the models have never been confronted with the same experimental data to determine whether any of the models provides a more accurate description of tumor growth. We collected tumor growth data from the literature and fit the various tumor growth models to the data to determine which model best describes tumor growth. Our results indicate that no single model can capture the variety of growth behavior captured in experiments. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W3.00004: Villification of the gut Tuomas Tallinen, Amy E. Shyer, Clifford J. Tabin, L. Mahadevan The villi of the human and chick gut are formed in similar stepwise progressions, wherein the mesenchyme and attached epithelium first fold into longitudinal ridges, then a zigzag pattern, and lastly individual villi. We combine biological manipulations and quantitative modeling to show that these steps of villification depend on the sequential differentiation of the distinct smooth muscle layers of the gut, which restrict the expansion of the growing endoderm and mesenchyme, generating compressive stresses that lead to their buckling and folding. Our computational model incorporates measured elastic properties and growth rates in the developing gut, recapitulating the morphological patterns seen during villification in a variety of species. Our study provides a mechanical basis for the genesis of these epithelial protrusions that are essential for providing sufficient surface area for nutrient absorption. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W3.00005: Energy barriers and cell migration in confluent tissues Dapeng Bi, J.H. Lopez, J.M. Schwarz, M. Lisa Manning Biological processes such as embryogensis, tumorigenesis and wound healing require cells to move within a tissue. While the migration of single cells has been extensively studied, it has remained unclear how single cell properties control migration through a confluent tissue. We develop numerical and theoretical models to calculate energy barriers to cell rearrangements, which govern cell motility. In contrast to sheared foams where energy barriers are power-law distributed, energy barriers in tissues are exponentially distributed and depend systematically on the cell's number of neighbors. Using simple extensions of `trap' and `Soft Glassy Rheology' models, we demonstrate that these energy barrier distributions give rise to glassy behavior and use the models to make testable predictions for two-time correlation functions and caging times. We incorporate these ideas into a continuum model that combines glassy rheology with active polarization to better understand collective migration in epithelial sheets. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W3.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W3.00007: Mechanical analysis of a heat-shock induced developmental defect Sarah M. Crews, W. Tyler McCleery, M. Shane Hutson Embryonic development in~\textit{Drosophila}~is a complex process involving coordinated movements of mechanically interacting tissues. Perturbing this system with a transient heat shock can result in a number of developmental defects. In particular, a heat shock applied during the earliest morphogenetic movements of gastrulation can lead to apparent recovery, but then subsequent~morphogenetic failure 5-6 hours later during~germ band retraction. The process of germ band retraction requires an intact amnioserosa -- a single layered extra-embryonic epithelial tissue -- and heat shock at gastrulation can induce the later opening of~holes in the amnioserosa. These holes are highly correlated with failures of germ band retraction. These holes could be caused by a combination of~mechanical weakness in the amnioserosa~or~local increases in mechanical~stress.~ Here, we assess the role of mechanical stress using confocal imaging to compare cell and tissue morphology in the amnioserosa of normal and heat-shocked embryos and laser hole drilling to map the stress field around the times and locations at which heat-shock induced holes open.~ [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W3.00008: Predictive modeling of multicellular structure formation by using Cellular Particle Dynamics simulations Matthew McCune, Ashkan Shafiee, Gabor Forgacs, Ioan Kosztin Cellular Particle Dynamics (CPD) is an effective computational method for describing and predicting the time evolution of biomechanical relaxation processes of multicellular systems. A typical example is the fusion of spheroidal bioink particles during post bioprinting structure formation. In CPD cells are modeled as an ensemble of cellular particles (CPs) that interact via short-range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through integration of their equations of motion. CPD was successfully applied to describe and predict the fusion of 3D tissue construct involving identical spherical aggregates. Here, we demonstrate that CPD can also predict tissue formation involving uneven spherical aggregates whose volumes decrease during the fusion process. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W3.00009: Charting the Vasculome: Uncovering the Principles of Vascular Organization Jacob Oppenheim, Marcelo Magnasco The efficient distribution of resources in any system requires a carefully designed architecture that is both space filling and efficient. While the principles of such networks are beginning to be uncovered in plants, they remain poorly elucidated in the case of higher animals. We have developed a high-throughput, easily implemented method of mapping vascular networks in mammalian tissue. By combining high resolution, rapid fluorescence blockface imaging with serial sectioning, we are able to map the vasculature of the rat liver at a resolution of 10 microns, revealing the structure above the level of the capillaries, constituting the largest vascular dataset yet assembled. We have developed algorithms for the efficient three-dimensional reconstruction from two-dimensional images, allowing skeletonization and investigation of its geometry and topology. We are able to calculate the scaling properties of these networks as well as the frequency of loops at each level. Using sophisticated topological tools, we are beginning to elucidate the principles of their organization. Ultimately, a greater understanding of vasculature is necessary for the success of efforts in synthetic and regenerative biology along with the better understanding of the growth and development of cancers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W3.00010: Characterizing Loopy Biological Distribution Networks in Three Dimensions Carl Modes, Eleni Katifori, Marcelo Magnasco In order to develop useful predictive models for vascular or other biological distribution networks that include the effects of network architecture, development, and topology some set of tools must be chosen to characterize vasculature in a physically relevant, mathematically compact way. Few such tools are extant. To address this issue we have generalized the existing two dimensional leaf venation characterization to a fully three dimensional setting, from whence it may be brought to bear on many problems in human and mammalian vasculature, particularly where that vasculature is extremely complex, as in the organs. The new algorithm rests on the abstraction of the physical `tiling' from the two dimensional case to an effective, statistical tiling of an abstract surface that the network may be thought to sit in. Generically these abstract surfaces are richer than the flat plane and as a result there are now two families of fundamental units that may aggregate upon cutting weakest links -- the plaquettes of the tiling and the longer `topological' cycles associated with the abstract surface. Upon sequential removal of these weakest links, two characterizing trees emerge that condense most of the relevant information from the full network. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W3.00011: Fundamental limits to the precision of multicellular sensing Andrew Mugler, Matthew D. Brennan, Andre Levchenko, Ilya Nemenman Recent experiments suggest that connected cells detect shallower chemical gradients together than alone, and that this enhanced sensitivity is lost when cell-to-cell communication is blocked. Here we derive the fundamental limits to the precision of gradient sensing by a chain of communicating cells. We show using linear response theory how precision is limited by the external diffusion of signaling molecules, by the exchange of messenger molecules between cells, and by the number of cells in the chain. We discuss how our predictions could be compared with ongoing experiments. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W3.00012: Noise Sensitivity in Force-inference Techniques David N. Mashburn, M. Shane Hutson, Jim H. Veldhuis, G. Wayne Brodland Forward finite-element modeling of developmental processes has vastly improved our understanding of morphogenesis; however, determining the model parameters necessary to reproduce in vivo behaviors typically requires either computationally-intensive parameter searches or somewhat arbitrary user selections. To bypass these difficulties, we previously developed an inverse technique called Video Force Microscopy (VFM) that takes positional information from microscopy data and infers the forces necessary to reproduce the observed dynamics. Applying VFM to the output of a forward model produces exact results, but the over-determined solutions for any real data will generally have a nonzero residual error. Applying VFM to real images requires converting the image into a segmented mesh of polygonal cells, a process in which the image resolution creates inherent positional noise and the choice of mesh nodes influences the angles used in the force balance equations. We have investigated the robustness and quality of VFM solutions by analyzing sensitivity to both the unavoidable positional noise and addition/removal of mesh nodes. We have also evaluated the accuracy of both the residual and the matrix condition numbers as predictors of the true error (as measured against a gold standard). [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W3.00013: A Mechanical and Biochemical Model of Intimal Atherosclerotic Lesions Pak-Wing Fok, Rebecca Vandiver We investigate a 1D axisymmetric model of intimal hyperplasia using hyperelasticity theory. Our model incorporates growth of the intima due to cell proliferation which in turn is driven by the release of a cytokine such as Platelet-Derived Growth Factor (PDGF). The main novelty of our model is that the growth rate is tied to local stresses and the local concentration of PDGF. The resulting system is a triple free boundary problem with different regions of the vessel wall having different homeostatic stress, depending on the local PDGF concentration. This system is coupled to force-balance equations that yield distributions for the stress and deformation. We find that rapid intimal thickening coupled to a quiescent media puts the intima in a state of compression and results in slow time scales of evolution. Our results are compared with intima-media thickness measurements of carotid arteries from previous clinical studies. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W3.00014: The Mechanics of Cell Intercalation Madhav Mani, Boris Shraiman, Thomas Lecuit Cell-intercalation involves the cytoskeleton-driven exchange of cellular neighbors. Developmental cues produce directional biases in the pattern of neighbor-exchanges, resulting in the alteration of tissue shape -- morphogenesis. Focusing on cell-intercalation during early fly development, I will address both static and dynamical aspects. A quantitative correspondence is drawn between cytoskeletal levels, stresses and geometry. This construction of a constitutive law, relies on a novel image analysis tool that infers mechanical features of the cellular lattice from live imaging (from the Lecuit Lab, Marseilles). Building on our understanding of these static aspects, we construct a phenomenological, and physically-motivated, model for cytoskeletal remodeling based on temporal correlation analyses. This model predicts the qualitative phases of junctional states, insights into the T1 event that mediates intercalation, and several of the collective properties of cell-intercalation that have remained unaddressed so far -- we go on to validate these predictions. We conclude with introducing the idea that tissue-wide anisotropies, central to morphogenesis and patterning in the embryo, can emerge as a consequences of the collective aspects of mechanical interactions. [Preview Abstract] |
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