399 research outputs found
Inferring the flow properties of epithelial tissues from their geometry
Amorphous materials exhibit complex material properties with strongly nonlinear behaviors. Below a yield stress they behave as plastic solids, while they start to yield above a critical stress sigma(c). A key quantity controlling plasticity which is, however, hard to measure is the density P(x) of weak spots, where x is the additional stress required for local plastic failure. In the thermodynamic limit P(x) similar to x(theta) is singular at x = 0 in the solid phase below the yield stress sigma(c). This singularity is related to the presence of system spanning avalanches of plastic events. Here we address the question if the density of weak spots and the flow properties of a material can be determined from the geometry of an amorphous structure alone. We show that a vertex model for cell packings in tissues exhibits the phenomenology of plastic amorphous systems. As the yield stress is approached from above, the strain rate vanishes and the avalanches size S and their duration tau diverge. We then show that in general, in materials where the energy functional depends on topology, the value x is proportional to the length L of a bond that vanishes in a plastic event. For this class of models P(x) is therefore readily measurable from geometry alone. Applying this approach to a quantification of the cell packing geometry in the developing wing epithelium of the fruit fly, we find that in this tissue P(L) exhibits a power law with exponents similar to those found numerically for a vertex model in its solid phase. This suggests that this tissue exhibits plasticity and non-linear material properties that emerge from collective cell behaviors and that these material properties govern developmental processes. Our approach based on the relation between topology and energetics suggests a new route to outstanding questions associated with the yielding transition
Non-monotonous crossover between capillary condensation and interface localisation/delocalisation transition in binary polymer blends
Within self-consistent field theory we study the phase behaviour of a
symmetric binary AB polymer blend confined into a thin film. The film surfaces
interact with the monomers via short range potentials. One surface attracts the
A component and the corresponding semi-infinite system exhibits a first order
wetting transition. The surface interaction of the opposite surface is varied
as to study the crossover from capillary condensation for symmetric surface
fields to the interface localisation/delocalisation transition for
antisymmetric surface fields. In the former case the phase diagram has a single
critical point close to the bulk critical point. In the latter case the phase
diagram exhibits two critical points which correspond to the prewetting
critical points of the semi-infinite system. The crossover between these
qualitatively different limiting behaviours occurs gradually, however, the
critical temperature and the critical composition exhibit a non-monotonic
dependence on the surface field.Comment: to appear in Europhys.Let
Statistical Properties of Business Firms Structure and Growth
We analyze a database comprising quarterly sales of 55624 pharmaceutical
products commercialized by 3939 pharmaceutical firms in the period 1992--2001.
We study the probability density function (PDF) of growth in firms and product
sales and find that the width of the PDF of growth decays with the sales as a
power law with exponent . We also find that the average
sales of products scales with the firm sales as a power law with exponent
. And that the average number products of a firm scales
with the firm sales as a power law with exponent . We
compare these findings with the predictions of models proposed till date on
growth of business firms
Bulk and Interfacial Shear Thinning of Immiscible Polymers
Nonequilibrium molecular dynamics simulations are used to study the shear
thinning behavior of immiscible symmetric polymer blends. The phase separated
polymers are subjected to a simple shear flow imposed by moving a wall parallel
to the fluid-fluid interface. The viscosity begins to shear thin at much lower
rates in the bulk than at the interface. The entire shear rate dependence of
the interfacial viscosity is consistent with a shorter effective chain length
that also describes the width of the interface. This is independent
of chain length and is a function only of the degree of immiscibility of
the two polymers. Changes in polymer conformation are studied as a function of
position and shear rate.Shear thinning correlates more closely with a decrease
in the component of the radius of gyration along the velocity gradient than
with elongation along the flow. At the interface, this contraction of chains is
independent of and consistent with the bulk behavior for chains of length
. The distribution of conformational changes along chains is also studied.
Central regions begin to stretch at a shear rate that decreases with increasing
, while shear induced changes at the ends of chains are independent of .Comment: 8 pages, 8 figure
Long-term Outcome of Hirschsprung Disease: Impact on Quality of Life and Social Condition at Adult Age
BACKGROUND: Hirschsprung disease is a rare congenital disease typically requiring surgical treatment during childhood. Quality of life and social condition at adult age can be impaired by disease-specific sequelae.
OBJECTIVE: This study aimed to assess the quality of life and social outcome of adult patients operated on for Hirschsprung disease during childhood.
DESIGN: Patients operated on for Hirschsprung disease during childhood were identified and specific questionnaires were sent to them.
SETTINGS: Data from 2 referral centers were used.
PATIENTS: Patients who completed the questionnaires regarding quality of life and social condition were included.
MAIN OUTCOME MEASURES: The Hirschsprung\u27s Disease and Anorectal Malformations Quality of Life disease-specific questionnaire (8 dimensions explored; each scored from 0 to 100 maximum score) and a sociodemographic questionnaire were sent to identified patients. Sociodemographic data were compared with those of the French general population.
RESULTS: Thirty-four patients had Hirschsprung disease (men, 76%; mean age, 32 years) were included in the study. Mean total Hirschsprung\u27s Disease and Anorectal Malformations Quality of Life score was 611 of 800 (maximum score 800). The 2 most impaired dimensions were "physical symptoms" and "diarrhea" (62.9/100 and 73.6/100). Fecal continence was only marginally affected (mean score, 89/100). Patients with Hirschsprung disease achieved better educational levels than the French general population. Parental and marital status did not differ between the 2 groups.
LIMITATIONS: This study had the limitations inherent to a retrospective study.
CONCLUSION: The quality of life of adult patients with Hirschsprung disease sequelae is marginally impaired in this study. Despite the consequences of this congenital abnormality, the condition eventually achieved can be considered as satisfactory. See Video Abstract at http://links.lww.com/DCR/A917
Heat transport in model jammed solids
We calculate numerically the normal modes of vibrations in 3D jammed packings
of soft spheres as a function of the packing fraction and obtain the energy
diffusivity, a spectral measure of transport that controls sound propagation
and thermal conductivity. The crossover frequency between weak and strong
phonon scattering is controlled by the coordination and shifts to zero as the
system is decompressed towards the critical packing fraction at which rigidity
is lost. Below the crossover, the diffusivity displays a power-law divergence
with inverse frequency, which suggests that the vibrational modes are primarily
transverse waves, weakly scattered by disorder. Above it, a large number of
modes appear whose diffusivity plateaus at a nearly constant value independent
of the inter-particle potential, before dropping to zero above the Anderson
localization frequency. The thermal conductivity of a marginally jammed solid
just above the rigidity threshold is calculated and related to the one measured
experimentally at room temperature for most glasses.Comment: 15 pages, 7 figure
The impact of anorectal malformations on anorectal function and social integration in adulthood: report from a national database.
AIM: The impact of anorectal malformation (ARM) on bowel function and social, educational and occupational end-points was investigated in adult patients entered on a national database.
METHOD: Data from a national database of adult patients operated on between 1962 and 1999 for ARM were analysed. The database Malformations Ano-rectales et Pelviennes rares (MAREP) was part of a common information system, CEMARA, on rare congenital disorders. A self-administered questionnaire regarding bowel function, academic qualifications, employment and family status was mailed to patients. The type of ARM, subsequent follow-up and management including surgical interventions were retrospectively retrieved from medical records.
RESULTS: Of 210 adult patients on the registry since 2008, 68 were included in this study. Only three (8.5%) had had regular follow-up. All reported some disturbance in bowel function. The fertility rate of 1.5 children per woman did not differ from the general population.
CONCLUSION: Anorectal malformation ARM often leads to suboptimal bowel function in adulthood. This has an impact on social integration
Dynactin1 depletion leads to neuromuscular synapse instability and functional abnormalities.
Dynactin subunit 1 is the largest subunit of the dynactin complex, an activator of the molecular motor protein complex dynein. Reduced levels of DCTN1 mRNA and protein have been found in sporadic amyotrophic lateral sclerosis (ALS) patients, and mutations have been associated with disease, but the role of this protein in disease pathogenesis is still unknown.
We characterized a Dynactin1a depletion model in the zebrafish embryo and combined in vivo molecular analysis of primary motor neuron development with live in vivo axonal transport assays in single cells to investigate ALS-related defects. To probe neuromuscular junction (NMJ) function and organization we performed paired motor neuron-muscle electrophysiological recordings and GCaMP calcium imaging in live, intact larvae, and the synapse structure was investigated by electron microscopy.
Here we show that Dynactin1a depletion is sufficient to induce defects in the development of spinal cord motor neurons and in the function of the NMJ. We observe synapse instability, impaired growth of primary motor neurons, and higher failure rates of action potentials at the NMJ. In addition, the embryos display locomotion defects consistent with NMJ dysfunction. Rescue of the observed phenotype by overexpression of wild-type human DCTN1-GFP indicates a cell-autonomous mechanism. Synaptic accumulation of DCTN1-GFP, as well as ultrastructural analysis of NMJ synapses exhibiting wider synaptic clefts, support a local role for Dynactin1a in synaptic function. Furthermore, live in vivo analysis of axonal transport and cytoskeleton dynamics in primary motor neurons show that the phenotype reported here is independent of modulation of these processes.
Our study reveals a novel role for Dynactin1 in ALS pathogenesis, where it acts cell-autonomously to promote motor neuron synapse stability independently of dynein-mediated axonal transport
Asymptotic theory for a moving droplet driven by a wettability gradient
An asymptotic theory is developed for a moving drop driven by a wettability
gradient. We distinguish the mesoscale where an exact solution is known for the
properly simplified problem. This solution is matched at both -- the advancing
and the receding side -- to respective solutions of the problem on the
microscale. On the microscale the velocity of movement is used as the small
parameter of an asymptotic expansion. Matching gives the droplet shape,
velocity of movement as a function of the imposed wettability gradient and
droplet volume.Comment: 8 fig
Interfacial layering in a three-component polymer system
We study theoretically the temporal evolution and the spatial structure of
the interface between two polymer melts involving three different species (A,
A* and B). The first melt is composed of two different polymer species A and A*
which are fairly indifferent to one another (Flory parameter chi_AA* ~ 0). The
second melt is made of a pure polymer B which is strongly attracted to species
A (chi_AB 0). We then show
that, due to these contradictory tendencies, interesting properties arise
during the evolution of the interface after the melts are put into contact: as
diffusion proceeds, the interface structures into several adjacent
"compartments", or layers, of differing chemical compositions, and in addition,
the central mixing layer grows in a very asymmetric fashion. Such unusual
behaviour might lead to interesting mechanical properties, and demonstrates on
a specific case the potential richness of multi-component polymer interfaces
(as compared to conventional two-component interfaces) for various
applications.Comment: Revised version, to appear in Macromolecule
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