Integer Topological Defects Reveal Anti-Symmetric Forces in Active Nematics

TL;DR

This paper investigates how integer topological defects in active nematic cell layers lead to cell accumulation due to anti-symmetric forces, challenging traditional classifications of cell layer activity.

Contribution

It introduces a particle-level model and a continuous theory that explain cell accumulation at +1 defects through effective active forces, providing new insights into cellular active nematics.

Findings

Both model and theory qualitatively match experimental cell accumulation at +1 defects.

Identification of two effective active forces responsible for cell accumulation.

Discussion of implications for cellular active nematics and existing theories.

Abstract

Cell layers are often categorized as contractile or extensile active nematics but recent experiments on neural progenitor cells with induced $+1$ topological defects challenge this classification. In a bottom-up approach, we first study a relevant particle-level model and then analyze a continuous theory derived from it. We show that both model and theory account qualitatively for the main experimental result, i.e. accumulation of cells at the core of any type of +1 defect. We argue that cell accumulation is essentially due to two generally ignored 'effective active forces'. We finally discuss the relevance and consequences of our findings in the context of other cellular active nematics experiments and previously proposed theories.