What do incipient organs, traffic jams and the frothy head of foam at the top of a beer glass have in common? Far more than expected, according to results published in Nature earlier this month. For the first time, using a series of clever, state-of-the-art techniques, scientists have uncovered the balance of physical forces that shapes tissues in developing embryos. And the process they’ve identified has turned out to be surprisingly familiar — not for its role in biology, but for its part in revolutionizing how physicists understand a slew of materials ranging from toothpaste to glass.
The team, led by Otger Campàs, a biophysicist at the University of California, Santa Barbara, studied the forces in a fish embryo that establish the long axis of its body. This crucial stage of development, they found, is governed by what’s known as a jamming transition: At the tip of what becomes the tail, cells can flow freely past one another like a fluid, but closer to the head, they get increasingly jammed together and eventually behave more like a solid. Those differences in turn determine how the body elongates and how various structures along that axis can be sculpted.
Physicists have been studying jamming theory in inanimate systems for the past 20 years. They found that materials such as glasses, colloids and foams get trapped in a state away from equilibrium, neither fully solid nor fluid. Consequently, even though their basic units remain exactly the same — whether soap bubbles, grains of sand or molecules in a polymer — they can sometimes still collectively flow like fluids or jam together like particles in a solid. (The same equations also describe the behavior of, say, cars in traffic.)
Soft matter physicists uncovered three criteria that characterized these systems: how much empty space exists between the particles, how densely the particles are packed and how much the particles can jiggle around.
Campàs and his colleagues measured all three properties in different regions of the developing zebra fish embryo, from head to tail, and showed that the way the cells get packed and change their behavior also fits the theory of jamming. Before, scientists had thought that cells essentially fine-tuned forces, applying more stress here, less there, to guide everything into place like a sculptor molding clay. “Instead,” Campàs said, “it’s more like blowing glass,” like liquefying a part that needs sculpting and then letting its new form set.
...
