Spontaneous motility of actin lamellar fragments

We show that actin lamellar fragments driven solely by polymerization forces at the bounding membrane are generically motile when the circular symmetry is spontaneously broken, with no need of molecular motors or global polarization. We base our study on a nonlinear analysis of a recently introduced minimal model [Callan-Jones et al Phys. Rev. Lett. 100, 258106 (2008)]. We prove the nonlinear instability of the center of mass and find an exact and simple relation between shape and center-of-mass velocity. A complex subcritical bifurcation scenario into traveling solutions is unfolded, where finite velocities appear through a nonadiabatic mechanism. Examples of traveling solutions and their stability are studied numericall

Mechanical instabilities and dynamics of living matter. From single-cell motility to collective cell migration

[cat] Aquesta tesis s'emmarca dins del camp de la biofísica, en particular en l'estudi des de un punt de vista físic de processos biològics a l’escala cel·lular i multicel·lular que involucren fenòmens col·lectius d'auto-organització. Per tant, hem estudiat tres problemes qualitativament diferents: la locomoció sostinguda en fragments cel·lulars, l'expansió lliure d'una mono-capa de cèl·lules epitelials i l'evolució dinàmica de la forma de l'ala de la mosca Drosophila melanogaster. En el primer cas, mostrem com un fragment cel·lular es capaç de desplaçar-se de forma sostinguda, solament amb les forces de polimerització a la membrana cel·lular, si inicialment s'indueixi un trencament espontani de la simetria rotacional. Entre d’altres resultats, derivem una expressió exacte i fem palés que la velocitat de migració s'origina a través d'un mecanisme no-adiabatic. En el segon cas, estudiem la migració col·lectiva en un teixit cohesiu de cèl·lules epitelials que s'expandeix sobre d'un substrat elàstic. Per ajudar a entendre les propietats físiques i biofísiques d’aquest tipus de teixits, comparem directament observables físics amb els predits segons la nostre descripció continua del medi. D’aquesta forma, podem derivar l'evolució temporal de la força de tracció activa per cèl·lula, la correlació nemàtica del medi i la seva viscositat efectiva. Per altra banda, generalitzem el model anterior, incorporant nous efectes com: els esforços contràctils generats per l'activitat en la xarxa d'actomyosina. Hem observat que els materials actius permeten sostenir ones elàstiques, fins i tot si les propietats reològiques passives del medi són de tipus viscos. En el tercer cas, estudiem i classifiquem l'evolució temporal de les morfologies d'un teixit totalment polaritzat, que està sotmès a un creixement anisòtrop però espaialment homogeni. Estenem estudis teòrics previs, incorporant els efectes generats per la interacció entre els esforços actius produïts per la divisió cel·lular i tres tipus de forces passives: de tipus viscos, fricció amb el medi extern i capil·laritat. Demostrem que en general l' excentricitat d'un teixit allongat evoluciona de forma no-monòtona amb el temps, amb un màxim a temps finit, del qual hem derivat les seves lleis d'escala amb les paràmetres físics del model.[eng] The thesis belongs to the field of biophysicis, in particular we evaluate from a physicial perspective biological processes that occur at the celular and multicelular scales involving collective phenomena of self-organization. Our modelling approach is based on the formalism of the active gels theory. Similarly as living systems, an ideal active gel is intrinsically out of equilibrium, due to its capacity to consume chemical energy. Within a certain range of validity the cells, the cytoskeleton, the tissues or even schools of fishes are expected to satisfy the same material properties as an active gel. This approach, coarse grain the systems by assuming that the large-scale and long-time limits are well described by a limited number of continuum fields, like the density of cells or the velocity of actin monomers. We apply this formalism to three main topics: self-locomotion of lamellar fragments, free-expansion of an epithelial monolayers, and the morphodynamics of the wing disk of the Drosophila melanogaster. In the first topic, we show that actin lamellar fragments driven solely by polymerisation forces at the bounding membrane are generically motile when the circular symmetry is spontaneously broken, with no need of molecular motors or global polarisation. We base our study on a nonlinear analysis of a recently introduced minimal model for an actin lamellar fragment. We prove the nonlinear instability of the center of mass and find an exact and simple relation between shape and center-of-mass velocity. A complex subcritical bifurcation scenario into traveling solutions is unfolded, where finite velocities appear through a nonadiabatic mechanism. In the second topic, we study the collective cell migration occurring in expanding cohesive epithelial cell sheets. This process involves the coordination of single cell traction forces, which are mechanically transmitted to adjacent neighbours via cell-cell junctions. The maps of reactive intracellular forces display a complex and heterogeneous spatio-temporal distribution, and are directly compared with the analytical stress and velocity profiles, so that we are able to track the temporal variations of the active celular traction force, the nematic correlation length and the effective viscosity at ultra-slow time scales. Furthermore, we generalise the previous biophysical model by incorporating a more realistic description of the material properties of an active gel. In particular, we include active stresses originated in part from the interaction between myosin motors and the intertwined actin meshwork within epithelial cells. We unveil a transition into an oscillatory periodic pattern. Interestingly, the complex material properties of an active gel allows to sustain elastic waves, even if the passive rheology is viscous-like. We classify in a phase-diagram the nonlinear assymptotic steady profiles, showing a rich variety of phenomenology. In the third topic, we study and classify the time-dependent morphologies of polarised tissues subjected to anisotropic but spatially homogeneous growth. Extending previous studies, we model the tissue as a fluid, and discuss the interplay of the active stresses generated by the anisotropic cell division and three types of passive mechanical forces: viscous stresses, friction with the environment and tension at the tissue boundary. The morphology dynamics is formulated as a free-boundary problem, and conformal mapping techniques are used to solve the evolution numerically. We elucidate how the different passive forces compete with the active stresses to shape the tissue in different temporal regimes and derive the corresponding scaling laws. We show that in general the aspect ratio of elongated tissues is non-monotonic in time, eventually recovering isotropic shapes in the presence of friction forces, which are asymptotically dominant

El tràfic de creuers i ferris al port de Palamós

L’objectiu d’aquest PFC és estudiar la nova etapa que s’ha obert en la història del port de Palamós, amb l’arribada regular de creuers, des de fa 10 anys. El treball comença repassant la història del port que és, alhora, la de la vila, fins a arribar a la situació actual, centrant-nos principalment en el tràfic de mercaderies. A continuació s’analitza la indústria creuerística a nivell global i es fa una especial menció de les principals destinacions creuerístiques a part del Mediterrani, per veure quina és la situació actual i els problemes a què s’ha de fer front. En el tercer capítol hi ha l’anàlisi del destí creuerístic pel Mediterrani i passa, a continuació, a comentar quina ha estat l’evolució de Palamós dintre d’aquest mercat, amb estadístiques globals. En l’últim capítol, es comenta les principals destinacions turístiques que hi ha, des de la pròpia vila Palamós, fins a les que estan situades a més o menys distància respecte al seu port. També hi ha una anàlisi econòmica de la indústria dels creuers i del rendiment que en treu Palamós. S’acaba aquest últim capítol contrastant els principals reptes de futur del port al voltant del que seria el tràfic de creuers, fent especial referència als possibles problemes de saturació per la compaginació d’aquest tràfic amb el de mercaderies

Quantifying material properties of cell monolayers by analyzing integer topological defects

In developing organisms, internal cellular processes generate mechanical stresses at the tissue scale. The resulting deformations depend on the material properties of the tissue, which can exhibit long-ranged orientational order and topological defects. It remains a challenge to determine these properties on the time scales relevant for developmental processes. Here, we build on the physics of liquid crystals to determine material parameters of cell monolayers. Specifically, we use a hydrodynamic description to characterize the stationary states of compressible active polar fluids around defects. We illustrate our approach by analyzing monolayers of C2C12 cells in small circular confinements, where they form a single topological defect with integer charge. We find that such monolayers exert compressive stresses at the defect centers, where localized cell differentiation and formation of three-dimensional shapes is observed.Comment: 5 pages, 4 figure

Integer topological defects of cell monolayers -- mechanics and flows

Monolayers of anisotropic cells exhibit long-ranged orientational order and topological defects. During the development of organisms, orientational order often influences morphogenetic events. However, the linkage between the mechanics of cell monolayers and topological defects remains largely unexplored. This holds specifically at the time scales relevant for tissue morphogenesis. Here, we build on the physics of liquid crystals to determine material parameters of cell monolayers. In particular, we use a hydrodynamical description of an active polar fluid to study the steady-state mechanical patterns at integer topological defects. Our description includes three distinct sources of activity: traction forces accounting for cell-substrate interactions as well as anisotropic and isotropic active nematic stresses accounting for cell-cell interactions. We apply our approach to C2C12 cell monolayers in small circular confinements, which form isolated aster or spiral topological defects. By analyzing the velocity and orientational order fields in spirals as well as the forces and cell number density fields in asters, we determine mechanical parameters of C2C12 cell monolayers. Our work shows how topological defects can be used to fully characterize the mechanical properties of biological active matter.Comment: 41 pages, 11 figure

Valores en guardia

“Valores en guardia” es una unidad didáctica de diez sesiones de duración que utiliza un deporte de adversario, la esgrima, como medio para educar en valores al alumnado de 1.º de ESO. Las características intrínsecas de este deporte y, sobre todo, los valores que se desprenden de su práctica justifican la presencia de esta propuesta didáctica en la programación del área de educación física en los centros educativos. Para llevarla a cabo sólo será necesario adquirir un sable espuma para cada alumno (un material sencillo, económico y seguro) y adaptar la progresión metodológica de la esgrima que presentamos a continuación a las características del alumnado. La programación en educación física en unidades didácticas facilita la distribución de los contenidos del área a lo largo de la etapa. En este sentido, esta unidad de esgrima puede servir de orientación para elaborar otras, sobre todo por el papel relevante de los objetivos planteados, realistas y cuantificables, y su evaluación a partir de criterios observables

Adherence, satisfaction and preferences for treatment in patients with psoriasis in the European Union: a systematic review of the literature

Background and objective: Adherence to treatment in patients with psoriasis is often poor. An investigation of patient preferences and satisfaction with treatment may be important, based on the expected correlation with therapy compliance. This paper aims to examine and describe the current literature on patient preferences, satisfaction and adherence to treatment for psoriasis in the European Union (EU). Methods: Electronic searches were conducted using PubMed, ISI Web of Knowledge, Scopus, Spanish databases and Google Scholar. European studies published in English or Spanish between January 1, 2009 and December 31, 2014 regarding patient-reported outcomes in psoriatic patients were included. Studies conducted in non-EU countries, letters to the editor, editorials, experts’ opinions, case studies, congress proceedings, publications that did not differentiate between patients with psoriasis and psoriatic arthritis or studies related to specific treatment were excluded. Results: A total of 1,769 titles were identified, of which 1,636 were excluded as they were duplicates or did not provide any relevant information. After a full-text reading and application of the inclusion/exclusion criteria, 46 publications were included. This paper will describe publications on adherence (n=4), preferences (n=5) and satisfaction with treatment (n=7). Results related to health-related quality of life articles (n=30) have been published elsewhere. Adherence rates are generally low in psoriasis patients regardless of the type of treatment, severity of disease or methods used to measure adherence. Biologic therapy is associated with greater clinical improvement. There is a direct association between physician recommendations, patient preferences and several domains of treatment satisfaction. Conclusion: The results of this review support the conclusion that adherence rates in patients with psoriasis are suboptimal and highlight the need to improve patient compliance and satisfaction with treatment. Patients’ preferences should be taken into account in the treatment decision-making process in order to improve patients’ clinical outcomes by ensuring satisfaction and adherence

Unraveling the hidden complexity of quasideterministic ratchets: random walks, graphs, and circle maps

Brownian ratchets are shown to feature a nontrivial vanishing-noise limit where the dynamics is reduced to a stochastic alternation between two deterministic circle maps (quasideterministic ratchets). Motivated by cooperative dynamics of molecular motors, here we solve exactly the problem of two interacting quasideterministic ratchets. We show that the dynamics can be described as a random walk on a graph that is specific to each set of parameters. We compute point by point the exact velocity-force V ( f ) function as a summation over all paths in the specific graph for each f , revealing a complex structure that features self-similarity and nontrivial continuity properties. From a general perspective, we unveil that the alternation of two simple piecewise linear circle maps unfolds a very rich variety of dynamical complexity, in particular the phenomenon of piecewise chaos, where chaos emerges from the combination of nonchaotic maps. We show convergence of the finite-noise case to our exact solution