Formation, transport, contraction, and disassembly of stress fibers in fibroblasts

Swiss mouse 3T3 fibroblasts grown on a solid substrate in the presence of 10% serum exhibit cell movement, organelle transport, and cytokinesis. When the serum concentration in the culture medium is decreased to 0.2% for 48 h the serum-deprived cells virtually stop locomoting, spread, decreased organelle transport, and exhibit extensive arrays of stress fibers that are visible with video-enhanced differential interference contrast microscopy and that also incorporate fluorescent analogs of actin and conventional myosin (myosin II). The stress fibers form in a constitutive manner at the cytoplasm-membrane interface, transport toward the nucleus, and then disappear. The rate of transport of these fibers is quite heterogeneous with average rates in the range of 10-20 microns/h. When serum-deprived cells are stimulated with mitogens such as 10% serum or 10 nM thrombin, many of the stress fibers immediately begin to shorten, suggesting a contraction. The rate of shortening is approximately two orders of magnitude slower than that of unloaded smooth muscle cells. The fiber shortening is often accompanied by retraction of the edges of the cell and continues for at least the 1st hour post-stimulation.