Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W7: Advances in the Biological Physics of Morphogenesis |
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Sponsoring Units: DBP Chair: Timothy Newman, Arizona State University Room: LACC 408B |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W7.00001: Biological Morphogenesis as Multiscale Transformations of Soft Matter Invited Speaker: Living tissues are condensed materials, with inherent physical properties in common with their nonliving counterparts. Tissues are both viscoelastic materials and excitable media. Early-stage embryos have liquid-like properties due to the random mobility of their constituent cells. The shaping of embryonic tissues can be understood, in significant part, on the basis of these properties, as can the ``equilibrium'' configurations of mixtures of cells with different cell-cell adhesive strengths. Interaction of cell aggregates with their microenvironments give rise, in certain instances, to sheet-like arrangements of cells known as epithelia. Their elastic properties cause them to fold, bulge, and undergo other morphogenetic changes. The molecular composition of cells is regulated by networks of interacting genes whose collective ``expression'' exhibit multistable and oscillatory dynamics. Cell aggregates, moreover, produce their own microenvironments by secreting diffusible signal molecules and structurally complex extracellular matrices. Together, these properties cause tissue masses to exhibit a wide range of nonlinear and self-organizing behaviors which are integrated and fine-tuned by evolution to produce developmental systems: species-specific temporal sequences of organized patterns and arrangements of different cell types. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:42PM |
W7.00002: Using many-body theory to understand chemotactic movement in cellular systems, with application to the chick embryo Invited Speaker: Inter-cellular communication is essential for coordinated cell movement and spatio-temporal differentiation. Examples are collective behavior of unicellular organisms (such as Dictyostelium aggregation) and formation of structures in multi-cellular organisms (e.g. gastrulation in early embryos). Cells communicate with one another via short-range contact interactions and long-range interactions mediated by chemical signaling fields. In the examples given above the number of cells varies between hundreds to tens of thousands, and the cell population may have strong phenotypic heterogeneity. It is therefore important to develop a model framework which retains discrete cell identity, and allows a flexible description of cell-cell interactions. We present one such framework here, inspired by the many-body formulation of interacting systems, and constructed using approximations which are biologically plausible. We describe a perturbative analysis of chemotactic aggregation, which illustrates the importance of statistical correlations between cells. We also discuss the implementation of this framework as an optimized numerical algorithm, and show some early results on primitive streak formation in the chick embryo. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 4:18PM |
W7.00003: Tissue architecture, cell traction, deformable scaffolds, and the forces that shape the embryo during morphogenesis. Invited Speaker: Morphogenesis is the process of constucting form and shape. Morphogenesis during early development of the embryo involves orchestrated movements of cells and tissues. These morphogenetic movements establish the body plan and organs of the early embryo. The rates and trajectories of these movements depend on three physical features of the early embryo: 1) the forces generated by cells, 2) the mechanical properties of the tissues, and 3) the architecture of the tissues. These three mechanical features of the embryo are some of the earliest phenotypic features generated by the genome. We are taking an interdisciplinary approach combining biophysical, cell biological, and classical embryological techniques to understand the mechanics of morphogenesis. Using nanoNewton-sensitive force transducers we can apply forces and measure time dependent elastic modulii of tissue fragments 100 micrometers across. Using traction-force microscopy we can measure forces generated by cells on their environment. We use drugs and chimeric proteins to investigate the localization and function of molecular complexes responsible for force generation and the modulus. We use microsurgery to take-apart and construct novel tissues to investigate the role of geometry and architecture in the mechanics of morphogenesis. Together with simulation techniques these quantitative approaches will provide us with a practical nuts-and-bolts understanding of how the genome encodes the shapes and forms of life. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:54PM |
W7.00004: Computational Modeling of Biological Development Invited Speaker: The patterns of gene expression are only part of the complex set of processes that govern the formation of tissue structures during embryonic development. Cells need to differentiate and to migrate long distances through tissues. How do they know what to become and where to go? Cells secrete and follow gradients of diffusible chemicals (chemotaxis) and secrete non-diffusing extracellular matrix. In addition, variable adhesion molecules expressed on cells' surfaces help them to form coherent structures by differential adhesion. CompuCell is a public domain modeling environment which implements a simple, energy minimization framework to describe these and related morphogenetic processes. One attractive feature of this approach is that it can interface at small length scales with increasingly sophisticated models for genetic regulation and biochemistry inside individual cells and at large length scales with continuum Partial Differential Equation and Finite Element models. We provide examples of how this method applies to problems including the development of the bone structure in the avian wing, the life cycle of the simple organism, \textit{Dictyostelium discoideum} and to vascular development and show how it ``postdicts'' the results of VE-cadherin knock-out experiments on \textit{in vitro }vasculogenesis experiments. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:30PM |
W7.00005: Taming morphogenesis: employing the biophysical basis of tissue self-assembly to build living structures of prescribed geometry Invited Speaker: We exploit the fundamental morphogenetic capacity of cells and tissues to build three- dimensional organ-like modules of prescribed shape. Morphogenesis is under strict genetic control, but genes do not create form and shape: physical mechanisms do. As an example we discuss one such mechanism based on tissue liquidity. Specifically, we demonstrate that the in vitro fusion of embryonic heart cushion tissue explants matches the in vivo behavior of this tissue during the establishment of cardiac chambers, and proceeds both qualitatively and quantitatively in analogy with the coalescence of liquid drops. Based on this and similar findings we designed experiments and built models to show that homotypic or heterotypic multicellular aggregates behave as self-assembling single color or multicolor ``bio-ink'' particles. We illustrate this by the layer-by-layer printing of these bioink particles (either manually or using specifically designed bioprinters) into hydrogel-biopaper, according to pre-designed three-dimensional pattern, namely tubes. By appropriate tuning of the embedding gel's physical properties, the cellular bio-ink particles, due to their liquid-like properties, fuse into toroidal and lumen-containing vessel-like organ modules. [Preview Abstract] |
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