984 research outputs found
A migrating epithelial monolayer flows like a Maxwell viscoelastic liquid
We perform a bidimensional Stokes experiment in an active cellular material:
an autonomously migrating monolayer of Madin-Darby Canine Kidney (MDCK)
epithelial cells flows around a circular obstacle within a long and narrow
channel, involving an interplay between cell shape changes and neighbour
rearrangements. Based on image analysis of tissue flow and coarse-grained cell
anisotropy, we determine the tissue strain rate, cell deformation and
rearrangement rate fields, which are spatially heterogeneous. We find that the
cell deformation and rearrangement rate fields correlate strongly, which is
compatible with a Maxwell viscoelastic liquid behaviour (and not with a
Kelvin-Voigt viscoelastic solid behaviour). The value of the associated
relaxation time is measured as ~min, is observed to be
independent of obstacle size and division rate, and is increased by inhibiting
myosin activity. In this experiment, the monolayer behaves as a flowing
material with a Weissenberg number close to one which shows that both elastic
and viscous effects can have comparable contributions in the process of
collective cell migration.Comment: 17 pages, 15 figure
Fast determination of coarse grained cell anisotropy and size in epithelial tissue images using Fourier transform
Mechanical strain and stress play a major role in biological processes such
as wound healing or morphogenesis. To assess this role quantitatively, fixed or
live images of tissues are acquired at a cellular precision in large fields of
views. To exploit these data, large numbers of cells have to be analyzed to
extract cell shape anisotropy and cell size. Most frequently, this is performed
through detailed individual cell contour determination, using so-called
segmentation computer programs, complemented if necessary by manual detection
and error corrections. However, a coarse grained and faster technique can be
recommended in at least three situations. First, when detailed information on
individual cell contours is not required, for instance in studies which require
only coarse-grained average information on cell anisotropy. Second, as an
exploratory step to determine whether full segmentation can be potentially
useful. Third, when segmentation is too difficult, for instance due to poor
image quality or too large a cell number. We developed a user-friendly, Fourier
transform-based image analysis pipeline. It is fast (typically cells per
minute with a current laptop computer) and suitable for time, space or ensemble
averages. We validate it on one set of artificial images and on two sets of
fully segmented images, one from a Drosophila pupa and the other from a chicken
embryo; the pipeline results are robust. Perspectives include \textit{in vitro}
tissues, non-biological cellular patterns such as foams, and stacks.Comment: 13 pages; 9 figure
Constructing irreducible polynomials recursively with a reverse composition method
We suggest a construction of the minimal polynomial of
over from the minimal polynomial for all positive integers whose prime factors divide . The
computations of our construction are carried out in . The key
observation leading to our construction is that for holds
where and
is a primitive -th root of unity in . The
construction allows to construct a large number of irreducible polynomials over
of the same degree. Since different applications require
different properties, this large number allows the selection of the candidates
with the desired properties
Screening in two-dimensional foams
Using the Surface Evolver software, we perform numerical simulations of
point-like deformations in a two-dimensional foam. We study perturbations which
are infinitesimal or finite, isotropic or anisotropic, and we either conserve
or do not conserve the number of bubbles. We measure the displacement fields
around the perturbation. Changes in pressure decrease exponentially with the
distance to perturbation, indicating a screening over a few bubble diameters
Chicken loaves prepared with broiler light and dark meat
O presente trabalho teve como objetivo específico a obtenção de fiambres com as carnes branca (do peito) e escura (das coxas e pernas) de frango, separadamente. Os cortes foram desossados manualmente e às carnes, após limpeza e corte, foram adicionados ingredientes não cárneos e aditivos, sendo elas moídas após 12 h. O processamento térmico foi realizado em defumador até a temperatura interna de 71°C. Foram obtidos fiambres curados e defumados com boa qualidade sensorial, adequado nível de proteína e baixo conteúdo calórico, do tipo semi-conserva. O rendimento foi maior na elaboração do fiambre de carne branca, o qual foi preferido na avaliação sensorial, em relação ao produto de carne escura.Chiken loaves were prepared with light and dark meat from broiler and deboned breasts and legs. The meat was cut, cured, ground, smoked and pasteurized (until 71°C). The yield was greater for the loaf prepared with light meat, which was also ranked first in sensory analysis. The light and the dark meat loaves had the following chemical compositions: moisture 70.6 - 71.0%; protein 22.0 - 20.2%; moisture/protein ratio 3.2 - 3.5; fat 4.0 - 4.5%; sodium chloride 2.4 - 2.5%; sodium nitrite 51.5 - 69.0 ppm; pH 6.1 - 6.25, respectively
Deformation of grain boundaries in polar ice
The ice microstructure (grain boundaries) is a key feature used to study ice
evolution and to investigate past climatic changes. We studied a deep ice core,
in Dome Concordia, Antarctica, which records past mechanical deformations. We
measured a "texture tensor" which characterizes the pattern geometry and
reveals local heterogeneities of deformation along the core. These results
question key assumptions of the current models used for dating
Simulations of viscous shape relaxation in shuffled foam clusters
We simulate the shape relaxation of foam clusters and compare them with the
time exponential expected for Newtonian fluid. Using two-dimensional Potts
Model simulations, we artificially create holes in a foam cluster and shuffle
it by applying shear strain cycles. We reproduce the experimentally observed
time exponential relaxation of cavity shapes in the foam as a function of the
number of strain steps. The cavity rounding up results from local rearrangement
of bubbles, due to the conjunction of both a large applied strain and local
bubble wall fluctuations
Cell adhesion and cortex contractility determine cell patterning in the Drosophila retina
Hayashi and Carthew (Nature 431 [2004], 647) have shown that the packing of
cone cells in the Drosophila retina resembles soap bubble packing, and that
changing E- and N-cadherin expression can change this packing, as well as cell
shape.
The analogy with bubbles suggests that cell packing is driven by surface
minimization. We find that this assumption is insufficient to model the
experimentally observed shapes and packing of the cells based on their cadherin
expression. We then consider a model in which adhesion leads to a surface
increase, balanced by cell cortex contraction. Using the experimentally
observed distributions of E- and N-cadherin, we simulate the packing and cell
shapes in the wildtype eye. Furthermore, by changing only the corresponding
parameters, this model can describe the mutants with different numbers of
cells, or changes in cadherin expression.Comment: revised manuscript; 8 pages, 6 figures; supplementary information not
include
- …