The body marshals an astounding array of forces to heal wounds, Levine said. Many have to do with cell biology, the internal and external signals that tell a cell when to move, when to stop, when to split and when to die. His team's intent was to focus first on the cell's physical interactions with its neighbors and study what happened if all those complicating factors are eliminated from the simulations.
"We try to unravel what is physics and what is biology," Levine said. "We want to know which parts of the phenomenon don't require sophisticated signaling networks."
In the physics approach to cell motility, he said, "the first thing to do is see how far we can get if we assume that all the cells are following the same rules. Then the only thing that's creating the dynamics of the system is that they're interacting with each other. This is the type of problem that physicists have studied before, usually in nonbiological contexts."
In the Harvard experiment, he said, "They had taken a millimeter-sized tissue that was spreading and showed it wasn't just cells on the end that were pulling on the tissue while the others were spectators." But that work didn't explain how cells in the center of the tissue knew the direction of the edge.
Levine's team looked to the skies for inspiration. "Birds look around and decide which way all their neighbors are flying," Levine said. "The idea that they would move as independent birds but also coordinate is where the idea of flocking came from. This way of thinking hadn't been applied to epithelial tissue motility in wound healing."
What cells "see" are their sticky neighbors, which pull and tug them as they move on lamellipodia, thin sheets that serve as "feet" powered by actin filaments that act something like the treads on a tank. The overlapping lamellipodia of adjacent cells influence each other. "The
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