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.
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.