Emergent chirality in active solid rotation of multicellular systems

Symmetry breaking in 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 poorly understood. In this work, we elucidate the biophysical mechanism underlying rotation in spherical tissues. We find that epithelial spheres exhibit persistent rotation, drifting of the rotational axis, 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 exhibits spontaneous chiral symmetry breaking in the cell shape orientation field. Using a continuum model, we demonstrate that topological defects in the polarity field drive this symmetry breaking process, which is revealed by asymmetries in the cell elongation pattern. Altogether, our work shows that tissue chirality can arise via topological defects in the pattern of cell traction forces. This has implications for left-right symmetry breaking processes in diverse contexts, including the rotation of marine embryos and the morphogenesis of organoids.