The force of a cell's footsteps.

Using a set of tiny weighing stations etched onto a silicon chip, researchers at the Duke University Medical Center in Durham, N.C., have measured the traction forces exerted during the motion of connective tissue cells from chickens.

Since cells travel within the animal during embryonic development and wound healing, its important to understand those movements, says Catherine G. Galbraith. She and her colleague Michael P, Sheetz report their findings in the Aug. 19 Proceedings of the National Academy of Sciences.

The silicon device consists of 5,904 pads, each supported on the end of a beam. By videotaping cells slowly crawling over the device, the researchers could monitor the displacement of the pads and infer the forces exerted.

From their results, Galbraith and Sheetz suggest a model for how the cells move. To go forward, a cell's leading edge pushes backward on the surface like a swimmer at the start of a crawl stroke. The rear of the cell--behind the nucleus--is dragged forward, breaking its sticky contacts. Moreover, the measured forces fluctuate, indicating that the movement is not a "continuous and smooth treadmill," the authors note.