Fig. 1. Actin cytoskeleton structure and podosome rosette appearance of spread 3T3 fibroblasts depend on substrate flexibility. 3T3 GFP-actin transfected fibroblasts observed by fluorescence microscopy. A 3T3 fibroblast spread on rigid substrate (glass) exhibits a larger number of actin stress fibres (A) than a fibroblast on very flexible substrate (0.08% bis-acrylamide) (B). The cell appears more spherical and loses most of its actin stress fibres on 0.08% gel. However 3T3 fibroblasts can exhibit podosome rosettes, i.e. large ring-shaped bands made of an actin cloud surrounding dense actin dots on both rigid (C) and flexible substrates (D) (0.2% gel), although the frequency of appearance of the rosettes is higher for rigid substrates than for flexible ones. Bars, 10 µm.