34 research outputs found
Mechanics of motility initiation and motility arrest in crawling cells
Motility initiation in crawling cells requires transformation of a symmetric
state into a polarized state. In contrast, motility arrest is associated with
re-symmetrization of the internal configuration of a cell. Experiments on
keratocytes suggest that polarization is triggered by the increased
contractility of motor proteins but the conditions of re-symmetrization remain
unknown. In this paper we show that if adhesion with the extra-cellular
substrate is sufficiently low, the progressive intensification of motor-induced
contraction may be responsible for both transitions: from static (symmetric) to
motile (polarized) at a lower contractility threshold and from motile
(polarized) back to static (symmetric) at a higher contractility threshold. Our
model of lamellipodial cell motility is based on a 1D projection of the complex
intra-cellular dynamics on the direction of locomotion. In the interest of
analytical transparency we also neglect active protrusion and view adhesion as
passive. Despite the unavoidable oversimplifications associated with these
assumptions, the model reproduces quantitatively the motility initiation
pattern in fish keratocytes and reveals a crucial role played in cell motility
by the nonlocal feedback between the mechanics and the transport of active
agents. A prediction of the model that a crawling cell can stop and
re-symmetrize when contractility increases sufficiently far beyond the motility
initiation threshold still awaits experimental verification
On the quasi-static effective behaviour of poroelastic media containing elastic inclusions
The aim of the present study is to derive the effective quasi-static
behaviour of a composite medium, made of a poroelastic matrix containing
elastic impervious inclusions. For this purpose, the asymptotic homogenisation
method is used. On the local scale, the governing equations include Biot's
model of poroelasticity in the porous matrix and Navier equations in the
inclusions, with elastic properties of the same order of magnitude. Biot's
diphasic model of poroelasticity is obtained on the macroscopic scale, but with
effective parameters that are strongly impacted by the distribution of
inclusions, even at low volume fraction. The impact on fluid flow is strictly
geometrical, showing that the inclusions do not play the role of a porous
network
Theory of mechanochemical patterning in biphasic biological tissues.
The formation of self-organized patterns is key to the morphogenesis of multicellular organisms, although a comprehensive theory of biological pattern formation is still lacking. Here, we propose a minimal model combining tissue mechanics with morphogen turnover and transport to explore routes to patterning. Our active description couples morphogen reaction and diffusion, which impact cell differentiation and tissue mechanics, to a two-phase poroelastic rheology, where one tissue phase consists of a poroelastic cell network and the other one of a permeating extracellular fluid, which provides a feedback by actively transporting morphogens. While this model encompasses previous theories approximating tissues to inert monophasic media, such as Turing's reaction-diffusion model, it overcomes some of their key limitations permitting pattern formation via any two-species biochemical kinetics due to mechanically induced cross-diffusion flows. Moreover, we describe a qualitatively different advection-driven Keller-Segel instability which allows for the formation of patterns with a single morphogen and whose fundamental mode pattern robustly scales with tissue size. We discuss the potential relevance of these findings for tissue morphogenesis
Crawling in a fluid
There is increasing evidence that mammalian cells not only crawl on
substrates but can also swim in fluids. To elucidate the mechanisms of the
onset of motility of cells in suspension, a model which couples actin and
myosin kinetics to fluid flow is proposed and solved for a spherical shape. The
swimming speed is extracted in terms of key parameters. We analytically find
super- and subcritical bifurcations from a non-motile to a motile state and
also spontaneous polarity oscillations that arise from a Hopf bifurcation.
Relaxing the spherical assumption, the obtained shapes show appealing trends
Mechanical behavior of multi-cellular spheroids under osmotic compression
The internal and external mechanical environment plays an important role in
tumorogenesis. As a proxy of an avascular early state tumor, we use
multicellular spheroids, a composite material made of cells, extracellular
matrix and permeating fluid. We characterize its effective rheology at the
timescale of minutes to hours by compressing the aggregates with osmotic shocks
and modeling the experimental results with an active poroelastic material that
reproduces the stress and strain distributions in the aggregate. The model also
predicts how the emergent bulk modulus of the aggregate as well as the
hydraulic diffusion of the percolating interstitial fluid are modified by the
preexisting active stress within the aggregate. We further show that the value
of these two phenomenological parameters can be rationalized by considering
that, in our experimental context, the cells are effectively impermeable and
incompressible inclusions nested in a compressible and permeable matrix
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Theory of mechanochemical patterning in biphasic biological tissues.
The formation of self-organized patterns is key to the morphogenesis of multicellular organisms, although a comprehensive theory of biological pattern formation is still lacking. Here, we propose a minimal model combining tissue mechanics to morphogen turnover and transport in order to explore new routes to patterning. Our active description couples morphogen reaction-diffusion, which impact on cell differentiation and tissue mechanics, to a two-phase poroelastic rheology, where one tissue phase consists of a poroelastic cell network and the other of a permeating extracellular fluid, which provides a feedback by actively transporting morphogens. While this model encompasses previous theories approximating tissues to inert monophasic media, such as Turing’s reaction-diffusion model, it overcomes some of their key limitations permitting pattern formation via any two- species biochemical kinetics thanks to mechanically induced cross-diffusion flows. Moreover, we describe a qualitatively different advection-driven Keller-Segel instability which allows for the formation of patterns with a single morphogen, and whose fundamental mode pattern robustly scales with tissue size. We discuss the potential relevance of these findings for tissue morphogenesis