Cells are liquids – but behave like solids

romaricPRL_webScientists at the Institute for Bioengineering of Catalonia (IBEC) have revealed that, counter to previous understanding, the living cells in our bodies behave like solids rather than the liquids they are made of.

IBEC group leader and ICREA research professor Xavier Trepat, who led the research, describes the discovery as ‘truly counter-intuitive’. “It means we need brand new laws of physics to understand what ingredients a fluid needs to behave as a solid,” he says.

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Image: A monolayer of human mammary epithelial cells. The cell nucleus (DNA) is labelled in blue, the cytoskeleton is labelled in red (actin) and green (myosin).

The IBEC scientists, who focus their research on the ability of cells to migrate within living organisms – a crucial process that underlies a wide range of phenomena in development and disease – published their findings in Physics Review Letters today. The discovery stems from their development of a new technique called “self-rheology” that allows them to study the very slow behaviour of a material. This is usually very hard to do, as it requires applying a controlled force or deformation for a very long time; but using the technique, Trepat and his colleagues found out that cells are constantly applying forces on each other. “Thanks to the ability of cells to naturally pull on and squeeze each other, we realized we didn’t need to apply any force on the cells. We just let them do their thing, and then visualized the resulting forces and deformations,” explains Romaric Vincent, first author on the paper.

The results were truly astonishing. Living cells have long been thought to behave like fluids over periods of hours, but the experiments showed that they behave like solids rather than liquids. “No matter how long you wait, cells never flow,” say Trepat. “When a liquid is poured into a container, it adapts to its shape. This is the case of water and honey, and even silly putty if you wait long enough. When a force is applied to a liquid, like when you stir a cup of coffee, the liquid flows. In contrast, solid materials such as a rubber band or a spring, don’t flow; instead, they regain their original shape once an externally applied deformation is released.”

The key to the mystery lies in the so-called ‘active behaviour’ of cells. Traditional material science could never predict this behaviour, but cells are not traditional materials; they are ‘active materials’. This means that unlike common materials, cells are able to consume energy and transform it into a force, a movement, or a deformation. Thanks to their active nature, cells can circumvent established dogmas of materials science.

The finding changes the way we think about processes such as embryo development or wound healing. “When an embryo develops, or when a wound heals, cells undergo very large deformations to enable new shapes,” says Trepat, who believes the discovery will prompt the scientific community to develop the laws of active matter. “We thought they did so by behaving like fluids. Now we discover they do so as solids.”

Romaric Vincent, Elsa Bazellieres, Carlos Perez-Gonzalez, Marina Uroz, Xavier Serra-Picamal and Xavier Trepat (2015). Active tensile modulus of an epithelial monolayer. Physics Review Letters, 115, 248103