A lump of plasticine, when kneaded, behaves like an amorphous solid — not a crystal, but not a liquid either. Inside such a material, avalanches can occur: one tiny rearrangement triggers neighboring ones, and a wave sweeps through the entire sample. Living tissue, where cells constantly divide and rearrange, is also amorphous, but it has an energy source — divisions. Until now, it was unclear whether such an active medium exhibits avalanches similar to those in passive materials.
Researchers built a two-dimensional model where each cell is an elastic block, and cell division is an active event that injects energy. They found that below the yield threshold, divisions fluidize the tissue, allowing it to flow like very thick honey. But unlike passive counterparts, the active tissue does not exhibit giant rearrangement avalanches. Divisions act like many small shock absorbers: they dampen large fluctuations, preventing them from growing.
The reason for avalanche suppression lies in the energy balance. The energy brought by divisions allows the tissue to flow, but it is just enough to sustain motion, not to accelerate chain reactions. As a result, the tissue remains on the brink of stability, like a sandpile that only sheds small portions but never collapses in a full avalanche. This explains why growing organs do not break apart from internal rearrangements.
as written by the authors arXiv:2605.12109
Developing tissues often maintain mechanical coherence while continuously remodeling through cellular processes such as cell divisions and rearrangements. In this way, they are an example of amorphous solids. In passive amorphous solids, local rearrangements can trigger one another through long-ranged elastic interactions, leading to system-spanning avalanches near yielding. Whether similar collective dynamics should be expected in living tissues is unclear, because cell divisions generate stress and remodeling events independently of local mechanical stability. Here, we address this question using a two-dimensional elastoplastic model in which cell divisions are treated as active plastic events. We find that while cell divisions fluidize the tissue below the passive yield stress, but preserve the marginal stability in the quasistatic limit. However, they also strongly suppress the system-spanning avalanches of cell rearrangements, in constrast with the expected behavior in passive amorphous solids. Finally, we show that the avalanche supression originates from the energy balance in the system. Namely, the energy injected by cell divisions allows for shear flow below the yield stress, but also provides a finite budget for rearrangements. These results suggest that proliferating tissues display the structural hallmarks of marginal amorphous solids while exhibiting much shorter-ranged correlations in dynamics, compared to passive amorphous solids.
Amorphous solids, such as glass or a dense gel, can flow if you wait long enough. A stained-glass window in an old cathedral becomes thicker at the bottom over centuries.
In Peter Watts's novel 'Blindsight,' alien tissues are described that can self-organize without central control — somewhat reminiscent of active amorphous systems.