Jan Rozman: Tissues as Active Nematics: Flows, Sorting, and Morphogenesis

Jan Rozman: Tissues as Active Nematics: Flows, Sorting, and Morphogenesis

Active nematics are elongated, nematically aligning units that either actively generate flows inwards along their long-axis and outwards along their short-axis, or the reverse. While active nematic theories have been first developed to describe, e.g., the motion of bacterial colonies and kinesin-microtubule mixtures, there is now increasing evidence that tissues can also behave like active nematics. To better understand the role of nematic activity in tissues, we study an epithelium vertex model extended to include active nematic stresses 1. We first analyse its behaviour in channel confinement 2: Under the usual substrate friction dissipation dynamics of the vertex model, flows in the channel are chaotic. However, if dissipation is instead internal, channel-wide, sustained, unidirectional flows emerge because internal dissipation allows for the development of longer-range correlations in the model tissue.

We then turn to a mixture of extensile and contractile cells, finding that the two populations sort over time 3. Phase separation strengthens monotonically with an increasing magnitude of contractile activity, but depends non-monotonically on extensile activity, with sufficiently high values reducing the extent of sorting. We interpret this by showing that extensile activity renders the system motile, enabling cells to undergo neighbour exchanges. In turn, contractile activity acts as an effective adhesion between cells.

Lastly, we apply the active nematic vertex model to a specific biological problem: Understanding the physical underpinning gastrulation in avian embryos. Based on out finding, we propose an experiment that could distinguish between mechanical and chemical coordination of the gastrulation process.

References

1 S.-Z. Lin, M. Merkel, and J.-F. Rupprecht, Phys. Rev. Lett. 130, 058202 (2023).

2 J. Rozman, Chaithanya K. V. S., J. M. Yeomans, and R. Sknepnek, Nat. Commun. 16, 530 (2025).

3 J. Rozman and J. M. Yeomans, Phys. Rev. Lett. 133, 248401 (2024).