FIGURE 3.

Cell spreading on collagen gels of FN^f/f and FN^−/− fibroblasts, and their ability to contract a collagen matrix. (A) In contrast to wildtype fibroblasts (FN^f/f, top left panel), fibronectin-null (FN^−/−) cells did not spread and tended to cluster on collagen gels after 24 h in FN-depleted conditions (top right). Spreading of FN^f/f fibroblasts was not influenced by adding 20 μg/ml purified FN to the medium (bottom left). A representative image shows partial rescue of cell spreading by FN^−/− cells in the presence of exogenous FN (bottom right) (Scale bar: 100 μm). (B) Mean percentage (±SD; significance was tested by a paired t-test) of “round” (black bar), “spikey” (light gray bar) and “spread” (dark gray bar) FN^−/− fibroblasts on collagen gels 24 h after seeding with or without exogenous FN (^∗∗∗p < 0.001). (C) Representative images of collagen gels 24 h after release; the initial area is given by the dashed red circle: After gel release, a “contracture ring” (pink dotted circle) defined the borders of the measured area with time (Scale bar: 2 mm). (D) The graph indicates the relative mean gel contracture of FN^f/f and FN^−/− fibroblasts in the absence or presence of 20 ug/ml FN added to the collagen gel (+FN), 0, 1, 5, and 24 h after gel detachment. Data are from at least three independent experiments done in quintuplicates and are expressed as contracture relative to the initial area in percent (±SD; significance after 24 h was tested by two-way ANOVA, followed by Tukey’s multiple comparisons test). FN^f/f fibroblasts exerted roughly a nine-fold higher collagen contracture ability after 24 h compared to FN^−/− cells (^§ p < 0.001). Addition of exogenous FN did not affect control fibroblasts. Collagen contracture exerted by FN^−/− cells was partially rescued by adding exogenous FN to the collagen gels, which caused a five-fold increase (^#p < 0.001).