Figure 1.

Early steps of fibroblast spreading on fibronectin substrates. A fibroblast cell goes through several distinct phases when plated on a fibronectin surface. Attachment is followed by the formation of initial integrin clusters, best observed on supported lipid bilayer, using the mobile Arg-Gly-Asp (RGD) peptide as the integrin ligand (P0; figure modified from Yu et al. (27) (top panel) and Iskratsch et al. (28) (bottom panel). Cluster formation is accompanied by binding of the actin assembly protein FHOD1 and actin assembly (see kymograph). Myosin II then contracts the clusters (not shown), activating fast actin polymerization and cell spreading. As the cell spreads out, the membrane folds and blebbs flatten out and feed into the expanding membrane surface (P1, epifluorescence images of the membrane dye FM1-43; images modified from Gauthier et al. (38). Additionally, activation of exocytosis increases the availability of phospholipids. A decrease in the pool of available lipids leads to a sharp rise in membrane tension and a switch to the subsequent protrusion-retraction phase (P2). During the P2 phase, the cell tests its environment with local contractions (see the force map on PDMS pillar arrays from Ghassemi et al. (34)) during the periodic protrusion-retractions (kymograph of the spreading cell edge from Giannone et al. (3)). To see this figure in color, go online.