[Paper Review] Arrested coalescence of multicellular aggregates

David Oriola, Miquel Marín-Riera+7 more|arXiv (Cornell University)|Dec 2, 2020

Cellular Mechanics and Interactions4 citations

TL;DR

This study demonstrates arrested coalescence in stem cell aggregates, where viscoelastic properties prevent complete fusion, and develops a Kelvin-Voigt model to quantify effective viscosity, shear modulus, and surface tension. Agent-based simulations reveal that cell protrusion activity modulates a solid-to-fluid transition near the unjamming point, linking mechanical behavior to collective cell dynamics.

ABSTRACT

Multicellular aggregates are known to exhibit liquid-like properties. The fusion process of two cell aggregates is commonly studied as the coalescence of two viscous drops. However, tissues are complex materials and can exhibit viscoelastic behaviour. It is known that elastic effects can prevent the complete fusion of two drops, a phenomenon known as arrested coalescence. Here we report the presence of this phenomenon in stem cell aggregates and provide a theoretical framework which agrees with the experiments. In addition, agent-based simulations show that cell protrusion activity controls a solid-to-fluid phase transition, revealing that arrested coalescence can be found in the vicinity of an unjamming transition. By analysing the dynamics of the fusion process and combining it with nanoindentation measurements, we obtain the effective viscosity, shear modulus and surface tension of the aggregates. More generally, our work provides a simple, fast and inexpensive method to characterize the mechanical properties of viscoelastic materials.

Motivation & Objective

• To investigate whether arrested coalescence—a phenomenon seen in viscoelastic materials—occurs in multicellular stem cell aggregates.
• To develop a theoretical framework based on the Kelvin-Voigt model to describe the fusion dynamics of these aggregates.
• To quantify the effective mechanical properties (viscosity, shear modulus, surface tension) using fusion dynamics and nanoindentation.
• To explore the role of cell protrusion activity in modulating mechanical transitions via agent-based simulations.
• To establish a simple, non-contact method for characterizing the viscoelastic properties of biological tissues.

Proposed method

• The fusion dynamics of 3D multicellular aggregates were imaged over 10 hours at 10-minute intervals using high-content imaging on a PerkinElmer Opera Phenix system.
• End-to-end distance and fusion angle were extracted using ellipse fitting to segmented masks via the MOrgAna machine learning software.
• A non-linear least squares fitting procedure was applied to experimental data using the solution of a modified Kelvin-Voigt equation, numerically solved with scipy's odeint and curve_fit.
• Nanoindentation with a spherical probe (29.5 µm radius) was used to measure effective elastic modulus, from which shear modulus was derived assuming ν = 0.5.
• Agent-based simulations in CUDA C++ modeled cell protrusion activity using a modified overlapping spheres neighbor-finding method and Heun’s time integration scheme.
• The simulations were used to explore the relationship between cell activity and the emergence of arrested coalescence near the unjamming transition.

Experimental results

Research questions

• RQ1 Does arrested coalescence occur in multicellular stem cell aggregates, similar to what is observed in synthetic viscoelastic materials?
• RQ2 Can a Kelvin-Voigt model accurately describe the viscoelastic fusion dynamics of these biological aggregates?
• RQ3 How do cell protrusion activities influence the mechanical state and transition between fluid-like and solid-like behavior in the aggregates?
• RQ4 What are the effective mechanical parameters (viscosity, shear modulus, surface tension) of the aggregates, and how do they relate to fusion kinetics?
• RQ5 Can the fusion process serve as a non-contact, low-cost method to infer mechanical properties of viscoelastic tissues?

Key findings

• Arrested coalescence was experimentally observed in mouse embryonic stem cell aggregates, with stable non-spherical shapes persisting after partial fusion.
• The viscocapillary velocity $v_c = \gamma / \eta$ and shear elastocapillary length $\ell_e = \gamma / \mu$ were successfully extracted by fitting the Kelvin-Voigt model to fusion dynamics.
• Nanoindentation measurements yielded an average effective elastic modulus of $E_{\text{eff}} \approx 1.2$ kPa, from which a shear modulus $\mu \approx 0.8$ kPa was calculated assuming $\nu = 0.5$.
• The fusion process was found to be governed by a balance between capillary and viscoelastic forces, with the degree of coalescence dependent on the material's elastic response.
• Agent-based simulations revealed that cell protrusion activity controls a transition between fluid-like and solid-like behavior, with arrested coalescence occurring near the unjamming transition.
• The fusion-based method provides a fast, non-contact, and inexpensive alternative for measuring tissue mechanical properties without calibrated probes.