Biological & Soft Matter Seminar: Chiral edge current in chaotic nematic cell monolayers

Abstarct:

Collective migration of cancer cells in the body is routinely observed close to confining structures such as muscle fibers or blood vessels. In vitro studies re-create such behavior by showing that fibrosarcoma cells collectively migrate at the border of their colony, even though within the monolayer cell flows obey turbulent chaotic dynamics characterized by an irregular array of vortices generated by self-propelled units. Even more surprising is that the edge currents always flow in the same direction—somehow cells collectively distinguish between their left and right near the edge. To understand this situation, we looked deeper at the organization of the cells within the monolayers.

Fibrosarcoma cells are elongated and align together, defining a patchwork of well-aligned domains between which orientational singularities (topological defects) position themselves. In the bulk of the monolayer, the position and orientation of these defects randomly change over time. However, close to the boundary, we find that comet-shaped “+½ defects” orient themself with an angle slightly smaller than 90° relative to the boundary, consistently tilting their tails to the right. Because of this left-right symmetry breaking, clockwise vortices are pushed closer to the border and generate the directed edge flow. Modeling the system as a chiral, active, nematic liquid crystal accounts well for our results and demonstrates that cell handedness is a critical ingredient for the emergence of the observed edge flows and not only for their direction.

Reference: Yashunsky V, Pearce D. J. G, Blanch-Mercader C, Ascione F, Silberzan P and Giomi L, Physical Review X (2022), 12(4):041017