23,832 research outputs found
Collective effects in cellular structure formation mediated by compliant environments: a Monte Carlo study
Compliant environments can mediate interactions between mechanically active
cells like fibroblasts. Starting with a phenomenological model for the
behaviour of single cells, we use extensive Monte Carlo simulations to predict
non-trivial structure formation for cell communities on soft elastic substrates
as a function of elastic moduli, cell density, noise and cell position
geometry. In general, we find a disordered structure as well as ordered
string-like and ring-like structures. The transition between ordered and
disordered structures is controlled both by cell density and noise level, while
the transition between string- and ring-like ordered structures is controlled
by the Poisson ratio. Similar effects are observed in three dimensions. Our
results suggest that in regard to elastic effects, healthy connective tissue
usually is in a macroscopically disordered state, but can be switched to a
macroscopically ordered state by appropriate parameter variations, in a way
that is reminiscent of wound contraction or diseased states like contracture.Comment: 45 pages, 7 postscript figures included, revised version accepted for
publication in Acta Biomateriali
Effect of adhesion geometry and rigidity on cellular force distributions
The behaviour and fate of tissue cells is controlled by the rigidity and
geometry of their adhesive environment, possibly through forces localized to
sites of adhesion. We introduce a mechanical model that predicts cellular force
distributions for cells adhering to adhesive patterns with different geometries
and rigidities. For continuous adhesion along a closed contour, forces are
predicted to be localized to the corners. For discrete sites of adhesion, the
model predicts the forces to be mainly determined by the lateral pull of the
cell contour. With increasing distance between two neighboring sites of
adhesion, the adhesion force increases because cell shape results in steeper
pulling directions. Softer substrates result in smaller forces. Our predictions
agree well with experimental force patterns measured on pillar assays.Comment: 4 pages, Revtex with 4 figure
Role of anisotropy for protein-protein encounter
Protein-protein interactions comprise both transport and reaction steps.
During the transport step, anisotropy of proteins and their complexes is
important both for hydrodynamic diffusion and accessibility of the binding
site. Using a Brownian dynamics approach and extensive computer simulations, we
quantify the effect of anisotropy on the encounter rate of ellipsoidal
particles covered with spherical encounter patches. We show that the encounter
rate depends on the aspect ratios mainly through steric effects,
while anisotropic diffusion has only a little effect. Calculating analytically
the crossover times from anisotropic to isotropic diffusion in three
dimensions, we find that they are much smaller than typical protein encounter
times, in agreement with our numerical results.Comment: 4 pages, Revtex with 3 figures, to appear as a Rapid Communication in
Physical Review
Focal adhesions as mechanosensors: the two-spring model
Adhesion-dependent cells actively sense the mechanical properties of their
environment through mechanotransductory processes at focal adhesions, which are
integrin-based contacts connecting the extracellular matrix to the
cytoskeleton. Here we present first steps towards a quantitative understanding
of focal adhesions as mechanosensors. It has been shown experimentally that
high levels of force are related to growth of and signaling at focal adhesions.
In particular, activation of the small GTPase Rho through focal adhesions leads
to the formation of stress fibers. Here we discuss one way in which force might
regulate the internal state of focal adhesions, namely by modulating the
internal rupture dynamics of focal adhesions. A simple two-spring model shows
that the stiffer the environment, the more efficient cellular force is built up
at focal adhesions by molecular motors interacting with the actin filaments.Comment: Latex, 17 pages, 5 postscript figures include
Focal adhesions as mechanosensors: the two-spring model
Adhesion-dependent cells actively sense the mechanical properties of their
environment through mechanotransductory processes at focal adhesions, which are
integrin-based contacts connecting the extracellular matrix to the
cytoskeleton. Here we present first steps towards a quantitative understanding
of focal adhesions as mechanosensors. It has been shown experimentally that
high levels of force are related to growth of and signaling at focal adhesions.
In particular, activation of the small GTPase Rho through focal adhesions leads
to the formation of stress fibers. Here we discuss one way in which force might
regulate the internal state of focal adhesions, namely by modulating the
internal rupture dynamics of focal adhesions. A simple two-spring model shows
that the stiffer the environment, the more efficient cellular force is built up
at focal adhesions by molecular motors interacting with the actin filaments.Comment: Latex, 17 pages, 5 postscript figures include
Planar sandwich antennas for submillimeter applications
A planar receiving antenna with a predictable pattern at submillimeter wavelength is demonstrated experimentally for the first time. It is single lobed and efficient, with a gain of approximately 8 dB at a wavelength of 119 µm
Mean encounter times for cell adhesion in hydrodynamic flow: analytical progress by dimensional reduction
For a cell moving in hydrodynamic flow above a wall, translational and
rotational degrees of freedom are coupled by the Stokes equation. In addition,
there is a close coupling of convection and diffusion due to the
position-dependent mobility. These couplings render calculation of the mean
encounter time between cell surface receptors and ligands on the substrate very
difficult. Here we show for a two-dimensional model system how analytical
progress can be achieved by treating motion in the vertical direction by an
effective reaction term in the mean first passage time equation for the
rotational degree of freedom. The strength of this reaction term can either be
estimated from equilibrium considerations or used as a fit parameter. Our
analytical results are confirmed by computer simulations and allow to assess
the relative roles of convection and diffusion for different scaling regimes of
interest.Comment: Reftex, postscript figures include
Effect of Poisson ratio on cellular structure formation
Mechanically active cells in soft media act as force dipoles. The resulting
elastic interactions are long-ranged and favor the formation of strings. We
show analytically that due to screening, the effective interaction between
strings decays exponentially, with a decay length determined only by geometry.
Both for disordered and ordered arrangements of cells, we predict novel phase
transitions from paraelastic to ferroelastic and anti-ferroelastic phases as a
function of Poisson ratio.Comment: 4 pages, Revtex, 4 Postscript figures include
Stochastic simulations of cargo transport by processive molecular motors
We use stochastic computer simulations to study the transport of a spherical
cargo particle along a microtubule-like track on a planar substrate by several
kinesin-like processive motors. Our newly developed adhesive motor dynamics
algorithm combines the numerical integration of a Langevin equation for the
motion of a sphere with kinetic rules for the molecular motors. The Langevin
part includes diffusive motion, the action of the pulling motors, and
hydrodynamic interactions between sphere and wall. The kinetic rules for the
motors include binding to and unbinding from the filament as well as active
motor steps. We find that the simulated mean transport length increases
exponentially with the number of bound motors, in good agreement with earlier
results. The number of motors in binding range to the motor track fluctuates in
time with a Poissonian distribution, both for springs and cables being used as
models for the linker mechanics. Cooperativity in the sense of equal load
sharing only occurs for high values for viscosity and attachment time.Comment: 40 pages, Revtex with 13 figures, to appear in Journal of Chemical
Physic
Cell organization in soft media due to active mechanosensing
Adhering cells actively probe the mechanical properties of their environment
and use the resulting information to position and orient themselves. We show
that a large body of experimental observations can be consistently explained
from one unifying principle, namely that cells strengthen contacts and
cytoskeleton in the direction of large effective stiffness. Using linear
elasticity theory to model the extracellular environment, we calculate optimal
cell organization for several situations of interest and find excellent
agreement with experiments for fibroblasts, both on elastic substrates and in
collagen gels: cells orient in the direction of external tensile strain, they
orient parallel and normal to free and clamped surfaces, respectively, and they
interact elastically to form strings. Our method can be applied for rational
design of tissue equivalents. Moreover our results indicate that the concept of
contact guidance has to be reevaluated. We also suggest that cell-matrix
contacts are upregulated by large effective stiffness in the environment
because in this way, build-up of force is more efficient.Comment: Revtex, 7 pages, 4 Postscript files include
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