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Vol. 10, Issue 11, 3745-3769, November 1999

Keratocytes Generate Traction Forces in Two Phases

Kevin Burton,^* Jung H. Park, and D. Lansing Taylor

Center for Light Microscope Imaging and Biotechnology, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Forces generated by goldfish keratocytes and Swiss 3T3 fibroblasts have been measured with nanonewton precision and submicrometer spatial resolution. Differential interference contrast microscopy was used to visualize deformations produced by traction forces in elastic substrata, and interference reflection microscopy revealed sites of cell-substratum adhesions. Force ranged from a few nanonewtons at submicrometer spots under the lamellipodium to several hundred nanonewtons under the cell body. As cells moved forward, centripetal forces were applied by lamellipodia at sites that remained stationary on the substratum. Force increased and abruptly became lateral at the boundary of the lamellipodium and the cell body. When the cell retracted at its posterior margin, cell-substratum contact area decreased more rapidly than force, so that stress (force divided by area) increased as the cell pulled away. An increase in lateral force was associated with widening of the cell body. These mechanical data suggest an integrated, two-phase mechanism of cell motility: (1) low forces in the lamellipodium are applied in the direction of cortical flow and cause the cell body to be pulled forward; and (2) a component of force at the flanks pulls the rear margins forward toward the advancing cell body, whereas a large lateral component contributes to detachment of adhesions without greatly perturbing forward movement.

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   Online version of this article contains video material for Figures 2, 4, 5, 6, 7, 9, and 10. Online version available at www.molbiolcell.org.
^*   Corresponding author. E-mail address: KBurton{at}cmu.edu.
   Present address: 11400 NE 132nd Street, L202, Kirkland, WA 98034.

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Molecular Biology of the Cell
Vol. 10, 3745-3769, November 1999
Copyright © 1999 by The American Society for Cell Biology

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