Morphogenesis and chiral dynamics in epithelial organoids*

Collective cell dynamics plays a crucial role in many developmental and physiological contexts. While 2D cell migration has been widely studied, how 3D geometry and topology interplay with collective cell behavior to determine structure, dynamics and functions remain an open question. In this work, we elucidate the biophysical mechanism underlying rotation in spherical tissues. We find that epithelial spheres exhibit persistent rotation, rotational axis drift and rotation arrest. Using a 3D vertex model, we demonstrate how the interplay between traction force and polarity alignment can account for these distinct rotational dynamics. Surprisingly, our analysis shows that the sphere rotates as an active solid, and exhibit spontaneous chiral symmetry breaking in the cell shape orientation field. Using a continuum model, we demonstrate how the topological defects in the polarity field underlie this symmetry breaking process, which is revealed by asymmetries in the cell elongation pattern. Altogether, our work reveals the role of topological defects in determining collective cell behavior, with implications in morphogenesis of complex 3D organoids.