Multicellular rosette formation guides epithelial tissue assembly in pancreatic ductal adenocarcinoma cell organoids.

Mesenchymal-to-epithelial transitions are essential for epithelial tissue formation and thus the development of functional organs. Here, we demonstrate that the transition from a disordered mesenchymal state to a columnar epithelial structure in branched pancreatic ductal adenocarcinoma organoids is associated with rosette formation. We show that fluctuations in acto-myosin contractions on the emerging apical side of branches with high cell density create a tug-of-war mechanism, leading to regular spacing of rosettes. The distance between adjacent rosettes depends on the branch diameter, which we validate with a minimal theoretical model based on apical constriction. The resulting lumen formation occurs through the apoptosis of an inner cell mass, leaving an epithelial layer lining the cavity. In summary, our findings show that rosette architecture is set by geometrical confinement of the cell nuclei in combination with acto-myosin driven contractions. This underscores the critical role of mechanical processes in self-organized assembly of epithelial tissue.