Laboratoire Matière et Systèmes Complexes

Deux séminaires consécutifs d’une demie-heure : Aurore van de Walle (laboratoire MSC) et Mathieu Rivière (laboratoire MSC).

Aurore van de Walle : "Magnetic nanoparticles in regenerative medicine" (Les nanoparticules magnétiques en médecine régénérative).

La réparation du cartilage articulaire suite à une maladie ou un traumatisme reste un défi clinique important. Une approche récemment explorée pour la fabrication de cartilage autologue est la différentiation de cellules souches en chondrocytes. Les contacts cellule-cellule étant connus comme nécessaires pour initier la chondrogenèse, notre laboratoire a mis au point une technique de condensation des cellules basée sur un ensemencement magnétique. Des cellules souches mésenchymateuses sont tout d’abord marquées avec des nanoparticules d’oxydes de fer, puis condensées magnétiquement au sein de matrices composées de polysaccharides. En associant l’ensemencement magnétique à une étape de maturation en bioréacteur, des chondrocytes fonctionnels qui synthétisent un niveau élevé de collagène II et agrécanes sont alors obtenus. Cependant, ces chondrocytes expriment également un niveau élevé de collagen X, marqueur spécifique du cartilage hypertrophique. Afin d’améliorer la cinétique de synthèse des composants de la matrice extracellulaire, l’influence de divers facteurs de croissance impliqués dans la différenciation (ex. TGF-β3, IGF-1) ainsi que du niveau d’oxygène (normoxie vs. hypoxie) a été étudié. Un contrôle précis du microenvironnement permet d’obtenir un tissu cartilagineux se rapprochant du tissu natif.

Repair of articular cartilage in case of disease or trauma remains a significant clinical challenge. An approach recently explored to engineer autologous cartilage replacements is the differentiation of stem cells into chondrocytes. As cell-cell contact is known to initiate chondrogenic differentiation, a method has been developed in the laboratory to condense cells based on magnetic seeding. Human mesenchymal stem cells are first labeled using iron oxide nanoparticles and seeded within porous polysaccharide scaffolds via magnetic condensation. By associating magnetic seeding and bioreactor maturation, functional chondrocytes that synthesize a high level of collagen II and aggrecans are then obtained. However, these chondrocytes also express a high level of collagen X, typically found in hypertrophic cartilage. With the goal of improving the synthesis kinetics of the extracellular matrix components, the influence of various growth factors involved in the differentiation (e.g. TGF-β3, IGF-1) as well as the oxygen level (normoxia vs. hypoxia) was assessed. A precise control of the microenvironment allows for the development of a cartilage tissue coming closer to the native tissue.

Mathieu Rivière : "Growth and motions of a compound leaf"

Contrary to popular belief plants move, often in spectacular ways. In particular, growth and posture regulation are accompanied by a variety of motions. Such growth motions are driven by the differential elongation of the organs’ tissues. The origin of differential elongation lies in mechanical and biochemical heterogeneities at the cell level. The study of plant growth motions thus allows to gain insight on growth and shape regulation at several scales.

We focus on Averrhoa carambola’s compound leaves which display typical and marked motions. The growing leaf is hooked and slowly unfurls throughout development. In the same time, it swings from left to right in a pendulum-like way (nutation). The mechanisms of nutation and the origin of such oscillations are still debated nowadays. Does nutation result from reversible (water content variations) or irreversible (growth) elongation ? In the latter case, can we connect the mechanical and biochemical properties of the cell wall to the motion ?

To tackle these questions, we first characterize motions and growth kinematics using several imaging techniques. Then, we probe the mechanical properties of the cell wall thanks to Atomic Force Microscopy. At last, we probe the biochemical status of the cell wall by immunohistochemistry. Doing so, we unravel the rich behaviour of growth motions and also confirm the possible involvement of reversible elongation in nutation.