Elasticity and glocality: initiation of embryonic inversion in Volvox.

Elastic objects across a wide range of scales deform under local changes of their intrinsic properties, yet the shapes are glocal, set by a complicated balance between local properties and global geometric constraints. Here, we explore this interplay during the inversion process of the green alga Volvox, whose embryos must turn themselves inside out to complete their development. This process has recently been shown to be well described by the deformations of an elastic shell under local variations of its intrinsic curvatures and stretches, although the detailed mechanics of the process have remained unclear. Through a combination of asymptotic analysis and numerical studies of the bifurcation behaviour, we illustrate how appropriate local deformations can overcome global constraints to initiate inversion.We thank Stephanie H¨ohn, Aurelia R. HonerkampSmith and Philipp Khuc Trong for extensive discussions. This work was supported in part by an EPSRC studentship (PAH), an EPSRC Established Career Fellowship (REG), and a Wellcome Trust Senior Investigator Award (REG).This is the final version of the article. It first appeared from Royal Society Publishing via http://dx.doi.org/10.1098/rsif.2015.067

Nonlinear and nonlocal elasticity in coarse-grained differential-tension models of epithelia.

The shapes of epithelial tissues result from a complex interplay of contractile forces in the cytoskeleta of the cells in the tissue and adhesion forces between them. A host of discrete, cell-based models describe these forces by assigning different surface tensions to the apical, basal, and lateral sides of the cells. These differential-tension models have been used to describe the deformations of epithelia in different living systems, but the underlying continuum mechanics at the scale of the epithelium are still unclear. Here, we derive a continuum theory for a simple differential-tension model of a two-dimensional epithelial monolayer and study the buckling of this epithelium under imposed compression. The analysis reveals how the cell-level properties encoded in the differential-tension model lead to linear and nonlinear elastic as well as nonlocal, nonelastic behavior at the continuum level

Turing's Diffusive Threshold in Random Reaction-Diffusion Systems.

Turing instabilities of reaction-diffusion systems can only arise if the diffusivities of the chemical species are sufficiently different. This threshold is unphysical in most systems with N=2 diffusing species, forcing experimental realizations of the instability to rely on fluctuations or additional nondiffusing species. Here, we ask whether this diffusive threshold lowers for N>2 to allow "true" Turing instabilities. Inspired by May's analysis of the stability of random ecological communities, we analyze the probability distribution of the diffusive threshold in reaction-diffusion systems defined by random matrices describing linearized dynamics near a homogeneous fixed point. In the numerically tractable cases N⩽6, we find that the diffusive threshold becomes more likely to be smaller and physical as N increases, and that most of these many-species instabilities cannot be described by reduced models with fewer diffusing species.Gordon and Betty Moore Foundatio

Multicolor 3D-dSTORM Reveals Native-State Ultrastructure of Polysaccharides' Network during Plant Cell Wall Assembly.

The plant cell wall, a form of the extracellular matrix, is a complex and dynamic network of polymers mediating a plethora of physiological functions. How polysaccharides assemble into a coherent and heterogeneous matrix remains mostly undefined. Further progress requires improved molecular-level visualization methods that would gain a deeper understanding of the cell wall nanoarchitecture. dSTORM, a type of super-resolution microscopy, permits quantitative nanoimaging of the cell wall. However, due to the lack of single-cell model systems and the requirement of tissue-level imaging, its use in plant science is almost absent. Here we overcome these limitations; we compare two methods to achieve three-dimensional dSTORM and identify optimal photoswitching dyes for tissue-level multicolor nanoscopy. Combining dSTORM with spatial statistics, we reveal and characterize the ultrastructure of three major polysaccharides, callose, mannan, and cellulose, in the plant cell wall precursor and provide evidence for cellulose structural re-organization related to callose content

Pectin homogalacturonan nanofilament expansion drives morphogenesis in plant epidermal cells

The process by which plant cells expand and gain shape has presented a challenge for researchers. Current models propose that these processes are driven by turgor pressure acting on the cell wall. Using nanoimaging, we show that the cell wall contains pectin nanofilaments that possess an intrinsic expansion capacity. Additionally, we use growth models containing such structures to show that a complex plant cell shape can derive from chemically induced local and polarized expansion of the pectin nanofilaments without turgor-driven growth. Thus, the plant cell wall, outside of the cell itself, is an active participant in shaping plant cells. Extracellular matrix function may similarly guide cell shape in other kingdoms, including Animalia

Technical Data Package Independent Assessment

Excerpt from the Proceedings of the Nineteenth Annual Acquisition Research SymposiumThe Program Management Office (PMO) is responsible for the quality and integrity of the data associated with system delivery and performance. When competing a new system or system update, the Program Manager (PM) is required to complete acquisition planning activities for a specific procurement and to develop a well-conceived acquisition strategy. This activity includes developing and validating Technical Data Package (TDP) requirements. Often, the program staff are committed to managing the current system and do not have the resources to fully review and validate the TDP for a major competition. Therefore, the TDP may not represent the best product to industry. A PMO may consider having an outside entity conduct a review of their technical data to assess the readiness and viability of the TDP. An Independent Assessment (IA) may provide significant information for understanding what technical data is available, what data is missing for a competitive solicitation, and what needs to be purchased under a new or follow-on contract. An IA can assist with conducting the first steps in the competitive process of developing requirements and conducting market research. The purpose of an IA is to provide the PMO with additional validation that the TDP is sound for a solicitation and subsequent contract

Shape-Shifting Polyhedral Droplets

Cooled oil emulsion droplets in aqueous surfactant solution have been observed to flatten into a remarkable host of polygonal shapes with straight edges and sharp corners, but different driving mechanisms - (i) a partial phase transition of the liquid bulk oil into a plastic rotator phase near the droplet interface and (ii) buckling of the interfacially frozen surfactant monolayer enabled by drastic lowering of surface tension - have been proposed. Here, combining experiment and theory, we analyse the hitherto unexplored initial stages of the evolution of these 'shape-shifting' droplets, during which a polyhedral droplet flattens into a polygonal platelet under cooling and gravity. Using reflected-light microscopy, we reveal how icosahedral droplets evolve through an intermediate octahedral stage to flatten into hexagonal platelets. This behaviour is reproduced by a theoretical model of the phase transition mechanism, but the buckling mechanism can only reproduce the flattening if surface tension decreases by several orders of magnitude during cooling so that the flattening is driven by buoyancy. The analysis thus provides further evidence that the first mechanism underlies the 'shape-shifting' phenomena.Comment: 11 pages, 12 figure

Theory of shape-shifting droplets

Recent studies of cooled oil emulsion droplets uncovered transformations into a host of flattened shapes with straight edges and sharp corners, driven by a partial phase transition of the bulk liquid phase. Here, we explore theoretically the simplest geometric competition between this phase transition and surface tension in planar polygons and recover the observed sequence of shapes and their statistics in qualitative agreement with experiments. Extending the model to capture some of the three-dimensional structure of the droplets, we analyze the evolution of protrusions sprouting from the vertices of the platelets and the topological transition of a puncturing planar polygon

Technical Data Package Independent Assessment

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