Active nematic defects and epithelial morphogenesis

TL;DR

This paper presents a minimal theoretical framework linking nematic defect dynamics, surface curvature, and cell growth, explaining morphogenetic phenomena in epithelial tissues and biological models like Hydra.

Contribution

It introduces a unified model connecting nematic defect behavior with tissue morphogenesis and validates it through experiments on neural progenitors and Hydra.

Findings

Defect locations correlate with curvature and growth patterns.

Cells accumulate at positive defects and deplete at negative defects.

Activity stabilizes specific defect configurations leading to tentacle formation.

Abstract

Inspired by recent experiments that highlight the role of nematic defects in the morphogenesis of epithelial tissues, we develop a minimal framework to study the dynamics of an active curved surface driven by its nematic texture. Allowing the surface to evolve via relaxational dynamics leads to a theory linking nematic defect dynamics, cellular division rates and Gaussian curvature. Regions of large positive (negative) curvature and positive (negative) growth are colocalized with the presence of positive (negative) defects. Applying this framework to the dynamics of cultured murine neural progenitor cells (NPCs) in an ex-vivo setting, we find that cells accumulate at positive defects and are depleted at negative defects. In contrast, applying this to the dynamics of a basal marine invertebrate \emph{Hydra} in an in-vivo setting, we show that activity stabilizes a bound $+1$ defect state by creating an incipient tentacle, while a bound $+1$ defect state surrounded by two $-1/2$ defects can create a stationary ring configuration of tentacles, consistent with observations.