Integer topological defects provide a new way to quantify and classify cell sheets

Abstract

Sheets of confluent cells are often considered as active nematics, with accumulation at +1/2 topological defects and escape from −1/2 defects being widely recognized. However, collective dynamics surrounding integer-charge defects remain poorly understood, despite its biological importance. By using microfabricated patterns, we induce diverse +1 topological defects (aster, spirals, and target) within monolayers of neural progenitor cells. Remarkably, cells are consistently attracted to the core of +1 defects regardless of their type, challenging existing theories and the conventional extensile/contractile dichotomy. We trace back the origin of this accumulation behavior to previously overlooked nonlinear active forces using a combination of experiments and a continuous theory derived from a cell level model. Our findings demonstrate that +1 topological defects can reveal key features of active nematic systems and offer a new way to characterize and classify cell layers.