A colony of cells is like a city that grows from its outskirts: new houses appear where there's more space. Inside, it's crowded, and construction stalls. Similarly, cells divide more often at the edge, where neighbors squeeze them less. Scientists modeled this without any chemical commands — only mechanics.
In the model, each cell is a ball that presses on others. When pressure is high, division stops. A natural boundary emerges: outside, cells divide; inside, they sleep. The thickness of this active layer is set by a length the authors named χ (chi). It depends on cell stiffness and how fast they grow. This length governs the whole colony: shape, growth rate, stress distribution.
When the colony expands into empty space, the edge becomes wavy — finger-like protrusions appear. This resembles a finger poking through a water film. The protrusions speed up growth: the more protrusions, the faster the colony claims territory. The authors showed that such instabilities can be tuned by changing the geometry of the system relative to χ.
The model was linked to known growth models where cells divide depending on nutrients. It turned out that mechanics and nutrition give similar fronts. So, one can study colony growth without delving into chemical details. This simplifies calculations and helps predict the behavior of tumors and bacterial films.
Pressure in the center of a tumor can be so high that cells there stop dividing and even die, even though nutrients are still sufficient.
In Peter Watts's novel Blindsight, alien creatures use mechanical signals to coordinate growth, echoing this model.