Collective behavior in active agent systems from experiments to models
November 16-17 2011
Université Paul Sabatier, Toulouse
The purpose of the workshop is to bring together people working in experimental and/or modeling studies of active agent systems. Active agent systems are systems of a large number of agent having autonomous capabilities of motion and decision making. Their interaction rules are most often fairly simple but they collectively produce coherent structures and large scale order. Examples of active agent systems are for instance suspensions of active particles such as algae, bacteria or spermatozoa, assemblies of social amoeba, cell assemblies in blastocysts or embryos, or, at a more macroscopic scale, animal groups such as fish schools, bird flocks, insect swarms, etc. Beyond the different scales involved in the previous examples, the workshop aims at finding similarities, both in structural properties and in observation and measurement methodologies. For this reason, the workshop will include talks about experiments, data processing, statistical treatment, and modeling.
The meeting dates are:
Wednesday 16 November 2011,
Thursday 17 November 2011.
The location of the meeting is at
Université Paul Sabatier
118 route de Narbonne
31062 Toulouse cedex
16/11/2011: Amphithéatre Concorde - building.U4
17/11/2011: Amphithéatre Laurent Schwartz, Institute of Mathematics, building R3
For directions, see below
The workshop is sponsored by the ANR project ‘MOTIMO’ , the CNRS-PEPII project ‘M2S’ and is held under the auspices of the ‘MIBS’ platform
MOTIMO is an acronym for ‘Seminal Motility Imaging and Modeling’
M2S is an acronym for ‘Sperm Motility Modelization’
Institut de Mathématiques de Toulouse
Université Pierre et Marie Curie, Paris
Title: Rheology and transport of active suspensions
Authors: Gaston Mino, Jérémie Gachelin, Anke Lindner, Annie Rousselet (PPMH)
Abstract: Assemblies of microscopic swimmers dispersed in a ﬂuid display emergent properties that differ strongly from those of passive suspensions. The active momentum sources distributed in the bulk as well as their ability to organize collectively, modify deeply the momentum and energy transfer balances as well the constitutive transport properties. Thus, on conceptual grounds, one may expect original constitutive properties such as active diffusivity, anomalous viscous response, enhanced mixing and also, the possibility to use the hydrodynamic ﬂuctuations to extract work. In this presentation, I will discuss these points and also present some effects have been observed experimentally. Finally, I will focus on recent experimental works done at the PMMH using wild-type E-Coli bacteria and artificial swimmers.
Title: Turbulent eddies and Motility conspire to generate small-scale accumulation of swimming micro-organisms
Authors: Eric Climent (IMFT), William M. Durham (MIT), Michael Barry (MIT) and Roman Stocker (MIT)
Abstract: Spatial distribution of the motile micro-organisms is observed to be heterogeneous at nearly all scales. These millimeter to centimeter scale aggregations are typically believed to result when turbulence stretches and folds larger mesoscale-sized patches. This communication demonstrates that this small-scale heterogeneity can also be generated ex novo at the smallest scales of turbulent motion via an active coupling between motility and turbulent flow. This new mechanism requires three simple ingredients : motility, asymmetric cell morphology, and turbulent fluid motion. To quantify this interaction, realistic flows are obtained by randomly forcing large-scale fluid motions within a fully periodic box and solving Navier-Stokes equations for the resultant cascade of turbulent motion using direct numerical simulation (DNS). This flow is seeded with hundreds of thousands of cells that swim in a direction prescribed by both their local flow environment and cell morphology. Statistical analyses reveal that this mechanism is capable of producing increased heterogeneity under routine conditions and different species are spatially segregated to disparate regions of the flow.
Gyrotaxis in a steady vortical flow. W.M. Durham, E. Climent and R. Stocker. (2011). Physical Review Letters – 106, 238102 (2011).
Turbulent unmixing: the sorting of motile phytoplankton by flow. W. Durham, E. Climent, M. Barry and R. Stocker (2010) – 63rd Meeting of the APS Division of Fluid Dynamics – Long Beach, USA.
Disruption of vertical motility by shear triggers formation of thin phytoplankton layers. W. M. Durham, J. O. Kessler, and R. Stocker, Science 323, 1067 (2009).
Title: Cell migration in the developing mouse embryo
Abstract: One of the most elusive facets of embryonic development is the coordination of cell movement, a process responsible for sculpting organs and shaping embryos. In the last decade, the emergence of live imaging techniques has allowed researchers to visualize migratory processes within developing embryos from fly, xenopus or zebrafish. From these analyses it became obvious that all cells move within the embryo and that long-distance migration during embryonic development is achieved by diverse locomotory behaviors. Analysis of cell movement in mammalian embryos is lagging behind other invertebrate or vertebrate model systems primarily because embryos are not accessible during gestation and are very difficult to maintain ex vivo. In my group we study a family of cell surface proteins that is involved in guiding migrating cells to their final destination during embryogenesis. I will present our current knowledge on the mechanisms by which these proteins control migratory processes during mouse development and I will conclude by discussing challenges facing this field of research.
Université Paul Sabatier, Toulouse
Title: Fast Image Registration
Abstract: An image registration method relying on brightness constancy assumption was developped in (Fehrenbach & Masmoudi), and tested on fluid images in (Auroux & Fehrenbach). The novelty of this approach is that the jacobian matrix required to estimate the motion field is assembled rapidly by reading only the the data. This allows a fast estimation of the motion field. Moreover, spatial regularization terms can be added. We will present this method, as well as its results on data issued from semen samples.
J. Fehrenbach, M. Masmoudi A fast algorithm for image registration , Comptes Rendus Mathématique, vol 346/9-10 (2008), pp. 593-598
D. Auroux, J. Fehrenbach Identification of velocity fields for geophysical fluids from a sequence of images, Experiments in Fluids, vol 50/2 (2011), pp. 313-
Title: Cilia and flagella on the move: structures and functions.
Abstract : Eukaryotic cilia and flagella are highly specialized cell appendage involved in different physiological and cellular processes. Both cilia and flagella share the same complex central structure, the axoneme, made by assembly of a microtubular system and mechano-chemical motors, dedicated to the generation of different types of movements. The talk will emphasize similarities and differences between cilia and flagella and focus on the generation of movement by the axonemal structure.
Title: Transition to collective motion in gliding bacterial colonies
Abstract: Collective motion of individual cells marks the onset of the transition to multicellularity in many microorganisms. This transition is often believed to be mediated by intercellular communication signals between cells. I will show that, in contrast to this picture, in an ensemble of gliding bacterial cells the transition from single cell to collective motion occurs in the absence of biochemical clues. Moreover, this transition is reminiscent of a dynamical self-assembly process of self-propelled rods.
Experiments were carried out with a mutant of the bacterium Myxococcus xanthus moving by means of the A-motility system only and without undergoing reversals. Cell movement is confined to a monolayer. At a critical cell packing fraction around 17%, cells start to organize into large moving clusters. The transition is characterized by a scale-free power-law cluster size distribution with an exponent 0.88 and giant number fluctuations. These findings suggest that the interplay of self-propulsion of bacteria and volume exclusion effects of the rod-shaped cell bodies lead to such phenomena.
I will argue that the observed change in the cluster statistics is a general property of self-propelled particle (SPP) systems. SPP systems exhibit two phases: i) a mono-disperse phase where the cluster size distribution (CSD) is dominated by an exponential tail, and ii) a collective phase characterized by the presence of non-monotonic CSD with a peak at large cluster sizes. At the transition between these two phases, the CSD is a power-law with a critical exponent. This transition can be used as a criterion to determine the onset of collective phenomena in SPP systems. Such criterion is particularly useful in SPPs system where global orientational order is often not observed, e.g., in the experiments with myxobacteria.
Title: Cytoskeletal pattern formation: Self organization of driven filaments
Abstract: Living cells rely on the self organization mechanisms of cytoskeleton to adapt to their requirements. Especially in processes such as cell division, intracellular transport or cellular motility the controlled self assembly to well defined structures, which still allow a dynamic reorganization on different time scales are of outstanding importance. Thereby, the intricate interplay of cytoskeletal filaments, crosslinking proteins and molecular motors plays a central role. One important and promising strategy to identify the underlying governing principles is to quantify the physical process in model systems mimicking the functional units of living cells. Here I will present in vitro minimal model systems consisting of actin filaments, crosslinking molecules and motor proteins exhibiting collective motion patterns and long range order. I will discuss how the balance of local force exertion and the influence of different crosslinking molecules affect the evolving dynamic structures.
Austrian Academy of Sciences Vienna
Title: How actin is organized to push in migrating cells
Authors: Marlene Vinzenz, Florian Schur, Maria Nemethova, Jan Mueller, Edit Urban Guenter Resch, Christoph Winkler, Christian Schmeiser, Akihiro Narita and J. Victor Small
Abstract: The Arp2/3 complex, an essential component of actin networks within cytoplasmic protrusions termed lamellipodia has been shown to induce the branching of actin filaments in vitro. Using electron tomography we now provide the first 3D images of lamellipodia actin networks and show that actin filaments are organized into subsets linked by branch junctions whereby the distances between branch junctions range up to 1µm. We developed mathematical tracking protocols for unbiased identification of actin filaments in order to elucidate their organization, distribution and length. Additionally we have been able to reveal the early stages of lamellipodial formation employing an intracellular wound healing model. Image averaging of branch structures enabled us to generate the first in situ 3D model of the Arp2/3 complex.
Austrian Academy of Sciences, Vienna
Title: Actin filament tracking and modeling of lamellipodia
Authors: Christian Schmeiser, Marlene Vinzenz, J. Victor Small, Dietmar Oelz, Jan Mueller
Abstract: The first objective is to introduce a method for the automatic tracking of actin filaments in electron tomograms of negatively stained lamellipodia. At the approach's heart is the use of a localized version of the Radon transform to derive filament directions. The approach allows to visualize the filament network structure in 3d and to compute statistical information such as directional distributions. Secondly this information is used to estimate parameters for a two dimensional stochastic simulation model of protrusion. Additionally the derivation of a model for symmetric lamellipodia with instantaneous cross-link turnover is presented.
Contact the conference secretariat
Registration is free but mandatory
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Coming from the train station Toulouse Matabiau
By cab : about 20 minutes.
By subway : about 30 minutes. Take the subway line A direction Basso-Cambo. At station Jean-Jaurès, change for subway line B direction Ramonville. Exit at station Université-Paul-Sabatier. Find the theatre using the campus map above.
Coming from Toulouse Blagnac airport
By cab : about 30 minutes.
By shuttle + métro : about 50 minutes. Take the shuttle « navette aéroport Tisséo » (special bus) and exit at the station Compans Caffarelli. Then, take the subway line B direction Ramonville. Exit at station Université-Paul-Sabatier. Find the theatre using the campus map above.