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

Sheets of confluent cells are often considered as active nematics, with accumulation at topological defects and escape from 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.