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J. Cell Biol.,
Volume 145, Number 1, April 5, 1999 109-122


* Friedrich Miescher Institute, Basel CH-4002, Switzerland; and Cytoskeletal proteins tagged with green fluorescent protein were used to directly visualize the mechanical role of the cytoskeleton in determining cell
shape. Rat embryo (REF 52) fibroblasts were deformed
using glass needles either uncoated for purely physical manipulations, or coated with laminin to induce attachment to the cell surface. Cells responded to uncoated
probes in accordance with a three-layer model in which
a highly elastic nucleus is surrounded by cytoplasmic
microtubules that behave as a jelly-like viscoelastic
fluid. The third, outermost cortical layer is an elastic
shell under sustained tension. Adhesive, laminin-coated needles caused focal recruitment of actin filaments to
the contacted surface region and increased the cortical
layer stiffness. This direct visualization of actin recruitment confirms a widely postulated model for mechanical connections between extracellular matrix proteins
and the actin cytoskeleton. Cells tethered to laminin-treated needles strongly resisted elongation by actively
contracting. Whether using uncoated probes to apply
simple deformations or laminin-coated probes to induce surface-to-cytoskeleton interaction we observed
that experimentally applied forces produced exclusively local responses by both the actin and microtubule
cytoskeleton. This local accomodation and dissipation
of force is inconsistent with the proposal that cellular
tensegrity determines cell shape.
Department of Physiology, Michigan State University, East
Lansing, Michigan 48824-1101
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