Integer topological defects offer a methodology to quantify and classify active cell monolayers.

Area of Science:

• Cell biology
• Active matter physics
• Soft condensed matter

Background:

• Confluent monolayers of elongated cells exhibit active nematic behavior with topological defects.
• Existing models predict cell accumulation at +1 defects in extensile systems and escape from -1 defects.
• Collective cell dynamics around integer topological defects are not fully understood.

Purpose of the Study:

• To investigate the collective dynamics of neural progenitor cell monolayers around diverse +1 topological defects.
• To challenge conventional theories on cell behavior at topological defects.
• To identify underlying mechanisms driving cell migration toward defect cores.

Main Methods:

• Induction of +1 topological defects (asters, spirals, targets) in cell monolayers using microfabricated patterns.
• Experimental observation of cell migration patterns.
• Development and application of a continuum theory derived from a cell-level model.

Main Results:

• Cells consistently migrated toward the cores of all induced +1 topological defects, regardless of defect type.
• This behavior contradicts predictions of conventional extensile/contractile models for spiral and target defects.
• Nonlinear active forces, previously overlooked, were identified as driving the accumulation toward defect cores.

Conclusions:

• A unified framework explaining cell accumulation at various +1 defects was established.
• +1 topological defects serve as effective probes for uncovering nonlinear features in active nematics.
• The study provides a methodology for characterizing and classifying cell monolayers based on their nematic behavior.