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In many processes， cells assemble force-producing contrac-tile machines from myosin motor proteins， actin filaments，
and other structural and regulatory components. Examples
include the muscle myofibril whose contraction pumps the
heart or moves limbs， the contractile ring that pinches the
cell into two daughters during cytokinesis， and the stress
fiber (SF). SFs are tension-generating actomyosin bundles
terminating at one or both ends in transmembrane focal adhe-sions (FAs) anchored to the extracellular matrix (ECM) (see
Fig. 1). By coupling to the ECM and exerting force， they
enable cells to mechanically influence their environment
and sense its mechanical properties. SFs contribute to adhe-sion of vascular endothelial cells to the basal lamina (1)，
generate contraction in myofibroblasts which provokes
tissue reorganization during wound healing (2)， and may
assist cells in migration (3).
What are the working parts of SFs and how do they coor-dinate to generate force SFs are similar in some respects to
the thoroughly studied myofibrils of striated muscle (4). A
myofibril is built from many contractile repeat units (i.e.，
sarcomeres) arranged in series， each comprising an array of
parallel bipolar myosin aggregates (i.e.， thick filaments)
interdigitated with two oppositely oriented actin filament
arrays， one on either side. Sarcomeres contract when thick
filament myosins pull inward on the actin arrays whose
pointed ends lie in the central myosin zone. The actin barbed
ends and the actin cross-linkera-actinin reside at the sarco-mere boundaries (i.e.，Zdisks) which are connected to the
thick filament centers by the giant spring-like protein titin.
SFs in stationary cells appear to be organized in a sarco-meric myofibril-like fashion. Along the fiber axis， zones o