Control of organoid structure formation by the mechanical plasticity of collagen networks

Self-organization principles govern the formation of biological structures, from the molecular to the cellular and up to the tissue level. While the structure formation on a molecular and mesoscopic level has gained much of attention, very little is known about which physical principles govern the formation of tissues structures. The ability of growing organoids in vitro introduces the possibility to observe the dynamics of the structure formation processes on a higher organizational level. Despite its reported importance, very little is known about the role of the mechanical and structural properties of the surrounding matrix on organoid morphogenesis. The formation of single-cell derived human mammary organoids is one prime example, in which complex structure formation processes, such as branching morphogenesis and alveologenesis occur. Such shape transformations are based on the mechanical interaction of the epithelial cell layers and the complex mechanical properties of the surrounding collagen network. In preliminary experiments, we identified the mechanical nonlinear plastic response of the matrix as an essential factor for the observed structure formation and differentiation of the epithelial cell layers into ducts and alveoli.

The overarching goal of the present proposal is to identify the self-organization principles governing the morphogenesis of human mammary gland organoids. We aim to understand (i) the role of structural and mechanical plasticity of the ECM in organoid morphogenesis and (ii) the interplay of collective cell migration and differentiation in organoid morphogenesis.
To this end, the morphogenesis of primary human mammary epithelial cells embedded in collagen networks will be observed by live cell confocal imaging. We will determine the cell motility in the epithelial cell layers, the resulting elastic deformation field within the ECM and the structural changes of the surrounding collagen gel. Rheological characterization of the plastic behavior of the surrounding collagen will enable us to determine the feedback mechanism between branch formation, tension generation and collagen remodeling. We will apply precisely controlled laser ablation to infer the mechanical stress fields both in the extracellular matrix as well as within the organoid. The identified mechanical feedback mechanism will be ultimately used to design the local mechanical response of the surrounding matrix in order to manipulate and stir the structure formation process of the organoid. Single cell RNA sequencing of mammary organoids at different steps of morphogenesis will be used to further link different environmental cues to cell differentiation processes within the organoids. Hits will subsequently be verified via immunofluorescence to get a position-resolved expression pattern, and ultimately aimed to be traced by live cell imaging.