Overview Cell Structures Cell Migration Cell Division  

Traction Forces of Normal and Ras-Transformed Cells

Munevar et al., Biophys. J. 80:1744-1757 (2001)

Traction forces are forces exerted by migrating cells on the underlying surface. Much about the mechanism of cell migration may be learned from analyzing these forces. We have developed flexible, collagen-coated polyacrylamide sheets as a powerful tool for detecting and quantifying traction forces.

Detection of Traction Forces by the Displacement of Beads Embedded in Flexible Substrates

The polyacrylamide sheet is embedded with fluorescent particles. Traction force causes displacement of the particles (left panel), which may be quantified with an optical flow algorithm to generate a map of displacement vectors (right panel). The displacement map, together with the elasticity modulus of the the material, is then used for the computation of traction stress (force per unit area). Recording time, 122 min.

Dynamics of Traction Forces Generated by NIH 3T3 Fibroblasts

The magnitude of traction stress may be rendered as a color image, with warmer color representing stronger forces. Strong, dynamic forces are found consistently at the leading edge. The tail region also shows sporadic strong forces, while the region underneath the nucleus is always inactive. Recording time, 130 min.

Discontinuities in Traction Forces as Revealed by Normalized Shear

Normalized shear is the spatial gradient of traction stress normalized by the magnitude of traction stress, i.e., percentage change per unit distance. High shear indicates possible discontinuities in structure. In normal 3T3 cells, a consistent high shear line is localized behind the leading edge. Recording time, 130 min.

Redistribution of Traction Forces Preceding Cell Reorientation

Adherent fibroblasts change the direction of migration by developing a persistent leading edge toward the new direction. Strong, persistent traction forces appear in this region before the cell shows the change in direction. Recording time, 135 min.

Transient Increase in Traction Forces upon Tail Detachment

While one might expect traction forces to build up as an adherent cell becomes elongated and stretched, experimental observations show the opposite. Traction forces increase transiently immediately upon retraction of the tail. Recording time, 122 min.

Disrupted Pattern of Traction Forces by H-Ras Transformed 3T3 Fibroblasts

With the magnitude of traction stress rendered in the same way as for normal 3T3 fibroblasts (left panel), H-Ras transformed 3T3 cells show substantially weaker traction forces than do NIH 3T3 cells. Moreover, the forces are highly disorganized, as indicated by the irregular and unstable pattern of normalized shear (right panel). Recording time, 133 min.