News Feature: The cells that flock together.

Flesh really does crawl. Crowds of cells can surge forward together, form gyrating swirls and waves, segregate into types, and send off meandering tendrils or mobile exploratory clusters. Collective cell motions are crucial for everything from embryo development to wound healing to cancer metastasis. These cell colonies seem to move in concert, almost like flocks of birds, shoals of fish, or herds of wildebeest. Some scientists are taking note of these parallels, hoping to find crucial clues about how cells coordinate. A burgeoning field of research on collective cell motion is drawing on mathematics originally developed to model animal flocks. The goal is to find order in the cellular melee and reveal how distant cells can coordinate their actions to move in unison. “We want to know if we can begin to find some general behavior in these systems,” says Cristina Marchetti of Syracuse University, New York. For all that’s known about cells, their coordinated movement largely remains a mystery, one that cell-motion models may help to solve. Marchetti once studied superconductivity, but then joined the growing band of physicists who have been drawn to the sticky problem of cell motion in recent years. “Cell biology is becoming more quantitative, so physicists are becoming more interested,” she says. “We bring mathematical tools and ways of thinking: Can we characterize this behavior in terms of a few parameters, such as density?” Several different models can already mimic cells’ complex collective motion, and suggest that it emerges from very simple behavior at the level of individual cells. Some models are even providing a few clues about the underlying molecular mechanisms. Researchers say that these models could eventually offer insights into wound healing and disease processes, telling us how cancers become invasive and perhaps even giving us new tools to stop them. “What I like about these approaches is that one can begin to think about how mechanisms within individual cells must be cooperating to give rise to these global tissue dynamics,” says Ryan Petrie of the National Institutes of Health in Bethesda, who works on individual cell motility.