Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P7: System Biology II: The Physics of Morphogenesis |
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Sponsoring Units: DBP GSNP Chair: Shane Hutson, Vanderbilt University Room: Ballroom C3 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P7.00001: Contractile forces driving embryonic development Invited Speaker: Proper development of an organism requires an orchestrated interplay of large sets of components. Recent developments in live fluorescent imaging methods allow the visualization of many key proteins in cells and tissues. Developing quantitative image analysis methods to measure the dynamics of shape changes in individual cells is central for understanding how a tissue gets sculpted, what molecular machineries are driving this process, and what interactions between cells are regulating it. In this talk, I will present recent advances in our understanding of the dynamical processes during morphogenesis, focusing on the example of tissue folding and invagination at the beginning of gastrulation in Drosophila. I show that this process is driven by a contractile multicellular actomyosin meshwork that dynamically forms within a few minutes at the cell surfaces. In individual cells, contraction is pulsed, with phases of contraction interrupted by pauses in which the cell size is maintained, i.e. a ratchet type dynamics that reduces the surface area of cells incrementally. Measuring the dynamics of whole cell shape changes in 2-photon live imaging data reveals that contraction pulses drive cell lengthening and relocation of cell nuclei, two transformations that are essential for successful invagination of tissue. This analysis further shows that over subsequent stages of invagination, during which cells undergo an elaborate sequence of shape changes, the volume of individual cells is a preserved quantity. These results shed new light on the forces and cellular dynamics driving tissue morphogenesis and are a step towards a quantitative understanding of how an organism's shape and internal structure arises in development. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P7.00002: From Global Stresses to Local Cell Packing During Development Invited Speaker: To perform their functions, cells in epithelial tissues must often adopt highly regular packings. It is still not fully understood how these ordered arrangements of cells arise from disordered, proliferative epithelia during development. I will use experimental and theoretical studies on an attractive model system, the cone cell mosaic in fish retina, to illustrate some ways that mechanical forces and cell signaling can interact to produce this transformation. Experiments examining the response to surgical lesions suggest that the correct mechanical environment at the tissue scale is essential to induce cone cells to rearrange into a rectangular lattice. Starting from this observation, I will argue that large-scale mechanical stresses naturally couple to and orient cell polarization and that this coupling can lead cells to line up in regular rows, as observed in the fish retina. This model predicts that cells in the rows will adopt characteristic trapezoidal shapes and that fragments of rows will persist even in tissue where the mosaic pattern is disrupted by lesions; these predictions are borne out by an analysis of cell packings at the level of the zonula occludens in wildtype and lesioned retinas. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P7.00003: Emergence and Dynamics of Polar Order in Developing Epithelia Invited Speaker: Planar Cell Polarity (PCP) is a conserved process in many vertebrate and invertebrate tissues, and is fundamental for the coordination of cell behavior and patterning. A well-studied example is the orientational pattern of hairs in the wing of the adult fruit fly Drosophila, which is an important model organism in biology. The Drosophila wing is an epithelium, i.e., a two-dimensional sheet of cells, which grows from a few cells to thousands of cells during the course of development. In the wing epithelium, planar polarity is established by an anisotropic distribution of PCP proteins within cells. The distribution of these proteins in a given cell affects the polarity of neighboring cells, such that at the end of wing development a large-scale PCP orientational order emerges. Here we present a theoretical study of planar polarity in developing epithelia based on a vertex model, which takes into account cell mechanics, cell adhesion, and cell division, combined with experimental results obtained from time-lapse imaging of the wing development. We show that in experiment, polarity order does not develop de novo at the end of wing development, but rather cells are initially polarized at an angle with respect to their final polarity axis. During wing development, the polarity axes of cells reorient towards their final direction. We identify a basic mechanism to generate such a large-scale initial polarization, based on the growth of a small number of cells with an initially random PCP distribution. Finally, we study the effect of shear and oriented cell division on dynamics of PCP order, showing that these two processes can robustly reorient the polarity axes of cells. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P7.00004: Simple Physics in Diseases and Embryonic Development of the Eye Invited Speaker: While molecular-level regulation within cells during embryonic development is highly complex, the physical mechanisms which translate this intracellular information into multicellular physical structure at the tissue level are often surprisingly simple. I will discuss an example where regulation of cell-cell contact energies is primarily responsible for robust and evolvable regular patterns, the organization of the ommatidia and supporting cells into the regular tiling characteristic of the Drosophila eye and another example where adhesion failures in the human retina result in choroidal neovascularization leading to blindness. In both cases, simulations based on materials-science techniques can help us understand the patterning mechanisms and the reasons for their robustness and failures. Such simulations are easy to extend to other developmental phenomena and to development-related diseases like cancer. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P7.00005: Morphogenesis of walled cells Invited Speaker: Walled cells have the ability to remodel their shape while sustaining an internal turgor pressure that can reach values up to 10 atmospheres. This requires a tight and simultaneous regulation of cell wall assembly and mechanochemistry, but the underlying mechanisms by which this is achieved remain unclear. In this talk I will discuss the interplay between growth and mechanics in shaping a walled cell, in the particularly simple geometry of tip-growing cells, which elongate via the assembly and expansion of cell wall in the apical region of the cell. Using only conservation laws and describing the observed irreversible expansion of the cell wall during growth as the extension of an inhomogeneous viscous fluid shell under the action of turgor pressure, we determine theoretically the radius of the cell and its growth velocity in terms of the turgor pressure and the secretion rate and rheology of the cell wall material. Moreover, we derive simple scaling laws for the geometry of the cell and find that a single dimensionless parameter, which characterizes the relative roles of cell wall assembly and expansion, is sufficient to explain the observed variation in shapes of tip-growing cells. Our work shows that the physics of cell wall expansion tightly constrains cell shape, providing a unified explanation of the characteristic morphologies of tip-growing cells across species that span several kingdoms, even though their underlying molecular mechanisms of cell morphogenesis are very different. More generally, our description provides a general framework to understand cell growth and remodeling in plants (pollen tubes, root hairs, etc.), fungi (hyphal growth and fission and budding yeast) and some bacteria. [Preview Abstract] |
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