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Regular fibroblast cells contain actin, as do most cells,
but they are incapable of mounting the degree of tension
or forming the kinds of intracellular and extracellular
bonds necessary to pull significantly on the ECM (Fig.
1.66A). Under mechanical stress, however, the fibroblast
will differentiate into a proto-MFB, which builds more
actin fibers and connects them to the focal adhesion
molecules near the cell surface (Fig. 1.66B). Further
mechanical and chemical stimulation can result in full
differentiation of the MFB, characterized by a complete
set of connections among the fibers and glycoproteins
of the ECM through the MFB membrane into the actin
fibers connected with the cytoskeleton (Fig 1.66C).
The contraction produced by these cells - which often
arrange themselves in linear syncytia as muscle cells
also do, like boxcars on a train - can generate stiffening
or shortening of large areas in the sheets of fascia where
they often reside (Fig. 1.67).
This discovery, though still in its early stages in terms
of research, promises myriad implications concerning
the body's ability to adjust the fascial webbing. This
form of 'pre-stress' - a middle ground between the
immediate contraction of pure muscle and the fiber-
creation remodeling shown by the pure fibroblast - can
prepare the body for greater loads or facilitate transfer
of loads from one fascia to another. In terms of the
responsiveness of fascia, we see a spectrum of contrac-
tile ability from the instant and linear pull of the skeletal
muscle through the more generalized spiral contraction
of the smooth muscle cell on into the varying degrees of
MFB expression to the more passive but still responsive
fibroblast at the other end of the connective tissue
spectrum.
Given how these MFBs can be stimulated by mechan-
ical (fibrous) loading or by fluid chemical agents, we can
also discern in this system the dance among the neural,
Fig. 1.66 MFBs are thought to differentiate in two stages. Though normal fibroblasts have actin in their cytoplasm and integrins
connecting them to the matrix, they do not form adhesion complexes or show stress fibers (A). In the proto-MFB stage, they do form
stress fibers and adhesion complexes through the cell's membrane (B). Mature MFBs show more permanent stress fibers formed by the
a-smooth muscle actin, as well as extensive focal adhesions that allow the pull from the actin through the membrane into the ECM (C).
(Redrawn from Tomasek J et al. Nature Reviews. Molecular Cell Biology; 2002.)
Fig 1.67 Stills from a video of a
melanoma cell migrating through a 3-D
collagen latticework over an hour's time.
Notice how the (green) collagen is
remodeled by the passage of the cell,
through an interaction with the integrins on
the cell's surface. (From Friedl 2004, with
kind permission from Springer-Science+
Business Media.)
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