A piece of dough, when pulled from the sides, doesn't stretch evenly: in one spot it thins and tears. Living tissue behaves similarly, but instead of a tear, shear bands appear — narrow zones where cells begin to slide past each other, like cards in a deck when cut. These bands divide the tissue into regions that behave differently: some remain elastic, others become fluid.
Scientists built a computer model of tissue where each cell is a polygon, and its boundaries can change length and tension. They found that when the tissue is compressed or stretched, a pattern of solid and liquid regions spontaneously emerges. Shear bands don't appear randomly: their location depends on which is stronger — friction between cells or friction with the substrate on which the tissue rests.
This discovery helps explain how an embryo transforms from a clump of cells into an organism with organs and tissues. It also explains why cancer cells can leave a tumor: in places where the tissue becomes fluid, it's easier for them to detach and travel to other parts of the body.
Shear bands in tissue resemble slip lines in metal, which are visible on the surface of a bent aluminum spoon.
In Greg Egan's novel "Permutation City," people simulate entire biological worlds where body shape depends on mechanical stresses in tissues.