Biophysics of multicellular self-organization 

Organoids and gastruloids

The first steps of organogenesis in mammals take place after the embryo's implantation in the uterus, making it challenging to study with live microscopy. Over the last years, multiple cell culture systems have been developed to understand better how pluripotent cells differentiate, self-organize in 3D and make organ-type structures, hence called organoids. 

Among these self-organizing in vitro multicellular systems, Gastruloids are 3D cellular aggregates of embryonic stem cells capable of differentiating and organizing in a manner reminiscent of the gastrulation process in the embryo. Cultured over several days in specific culture mediums, Gastruloids can develop advanced structures strikingly similar to organs such as (1) somites and neural-tube or (2) gut and heart.  The first days of self-organization of these organoids have identifiable commonalities among these different classes of chemical protocols leading to the formation of different organ-like structures. 

3D imaging of embyronic organoids

When studying the dynamics of cell aggregates in 3D, there are five significant challenges that need to be addressed. Firstly, to conduct long-term imaging, we need to minimize the amount of photo-damage done to the cells. Secondly, cell aggregates may move or rotate during the imaging process, so it is necessary to register the images to account for these movements. Thirdly, tracking individual cells can be complicated due to the presence of cell divisions, rearrangements, and large flows. Fourthly, as the optical properties of the sample change over time, aberrations can occur, and optical penetration can decrease, resulting in a decline in image quality. Finally, segmenting the images and differentiating the cells from the background can be challenging, particularly when the image changes over time.

We are developing and using multi-photon and light sheet microscopy to tackle these challenges. 

Gastruloids biophysical mechanisms of symmetry breaking

Understanding how a spherical aggregate of apparently homogeneous cells self-organize itself into a polarized structure with a recognizable axis is a fundamental question that has implications for the formation of the primary body axis during embryonic development, as well as the development of most, if not all, organoids. To address this fundamental question, we investigate the behavior of individual cells within the aggregate, such as cell movement, division, and death. We identify mechanisms of symmetry breaking including tissue flows and cell differentiation.

mechanical stresses and rheological properties of SELF-ORGANAZING MULTICELLULAR SYSTEMS

Cell-generated forces are responsible for driving the movement of both individual cells and larger tissue structures, and these movements are influenced by the material properties, such as viscosity, elasticity, and friction, and boundary conditions of the system. We investigate, on both the cell and tissue scales, the mechanical stresses and material properties shaping the gastruloids and how these affect cell fate and signaling. We develop and apply various biophysical tools to probe cell and tissue mechanics in situ to accomplish this.

Our approach involves combining three types of measurements: (i) kinematic measurements, where we determine the velocity field and strain rate in 2D and 3D using image-based methods such as Optic Flow; (ii) force measurements, where we will measure the forces generated by cells and the resulting mechanical stresses in the tissue; and (iii) rheological measurements, where we investigate the material properties of the aggregate, such as its elasticity and viscosity. Through this approach, we can identify zones of growth, contraction, and convergence-extension within the aggregate and gain a deeper understanding of the mechanical principles that govern cell and tissue movement.

Collaborations: Léo Guignard (LIS, CENTURI, Marseille), Simon Gsell (IRPHE, Marseille), Alfonso Martinez Arias (Universidad Pompeu Fabra, Barcelona), Matthias Merkel (CPT, CENTURI, Marseille), Philippe Roudot (I2M, CENTURI, Marseille), Vikas Trivedi (EMBL, Barcelona), Virgile Viasnoff (CINAM, Marseille)

Funding: Fondation de la Recherche Médicale (FRM), ANR, CENTURI, ERC