Structure and Rheology in Vertex Models under Cell-Shape-Dependent Active Stresses

Area of Science:

• Soft Matter Physics
• Cellular Biophysics
• Materials Science

Background:

• Biological cells dynamically adjust their internal structure and shape.
• Cellular shape and mechanical properties are coupled, influencing tissue behavior.
• Active mechanical stress generation within cells is a key factor in tissue dynamics.

Purpose of the Study:

• To investigate the rheological implications of active cell shape tuning in a dense cellular material model.
• To identify critical transitions in material properties as a function of active stress.
• To explore the emergence of tissue flow and topological defects.

Main Methods:

• Utilized a minimal model of dense cellular material.
• Simulated active mechanical stress exerted by cells along their elongation axis.
• Analyzed transitions in material structure, order, and shear modulus with increasing active stress.

Main Results:

• Observed transitions from hexagonal crystal to anisotropic solid states.
• Identified critical activities where shear modulus vanishes, leading to distinct material phases.
• Discovered a re-entrant phase with hexatic order and spontaneous tissue flow with topological defects.

Conclusions:

• Active cell shape tuning significantly alters the rheology of cellular materials.
• The model predicts multiple phase transitions, including solid-to-fluid transitions and defect-driven flow.
• Findings align with experimental observations in epithelial tissues, supporting active nematic theory.