A sheet of paper tears where the tension exceeds its strength. The cell sheet lining our organs behaves similarly. But until now, computer models could not show how tears appear in it because they always treated the tissue as continuous. Japanese scientists added a single operation that allows cells to detach from each other, and for the first time they saw how it happens.

Cells in a tissue are connected at points where several contacts meet. When tension at such a point becomes too high, the connection breaks. The scientists devised a rule: if tension exceeds a threshold, the point splits into three, and a gap forms between cells, which appears in the model as an empty polygon. This gap does not contribute to elasticity; it simply represents a hole.

The scientists stretched the cell sheet at different speeds. Under slow stretching, the tissue remained intact, while under fast stretching, holes and detachments appeared. If a crack was already present in the sheet, even slow stretching made it grow, and along a wavy line, just like in real tissue. The model can now be used to study wound healing and to predict the strength of cell sheets.