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J. Cell Biol.,
Volume 145, Number 3, May 3, 1999 589-604
* Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708-0300; Strong, actomyosin-dependent, pinching
tractions in steadily locomoting (gliding) fish keratocytes revealed by traction imaging present a paradox,
since only forces perpendicular to the direction of locomotion are apparent, leaving the actual propulsive
forces unresolved. When keratocytes become transiently "stuck" by their trailing edge and adopt a fibroblast-like morphology, the tractions opposing locomotion are concentrated into the tail, leaving the active
pinching and propulsive tractions clearly visible under the cell body. Stuck keratocytes can develop ~1 mdyn
(10,000 pN) total propulsive thrust, originating in the
wings of the cell. The leading lamella develops no detectable propulsive traction, even when the cell pulls on
its transient tail anchorage. The separation of propulsive and adhesive tractions in the stuck phenotype leads
to a mechanically consistent hypothesis that resolves
the traction paradox for gliding keratocytes: the propulsive tractions driving locomotion are normally canceled
by adhesive tractions resisting locomotion, leaving only
the pinching tractions as a resultant. The resolution of
the traction pattern into its components specifies conditions to be met for models of cytoskeletal force production, such as the dynamic network contraction model
(Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and
G.G. Borisy. 1997. J. Cell Biol. 139:397-415). The traction pattern associated with cells undergoing sharp
turns differs markedly from the normal pinching traction pattern, and can be accounted for by postulating an
asymmetry in contractile activity of the opposed lateral
wings of the cell.
Biomedical Engineering Department, Boston University, Boston, Massachusetts 02215; and § Cell Biology and Anatomy
Department and Lineberger Comprehensive Cancer Center, The University of North Carolina at Chapel Hill, Chapel Hill,
North Carolina 27514
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