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Assembly of a fibronectin (FN) extracellular matrix is important during embryogenesis and during wound healing. The assembly process requires cell surface integrins, which form a bridge between FN and the intracellular actin cytoskeleton. Previous studies have monitored the dynamics of FN matrix assembly but been unable to image the early stages reliably or relate them to cytoskeletal rearrangements. Harold Erickson and co-workers have therefore performed dual-labelling with GFP spectral variants to visualize FN and actin dynamics simultaneously in living cells (see p. 1221). The authors show that matrix assembly begins at small spots at the ends of actin bundles in the cell periphery. FN fibrils appear to be anchored to the substrate at these points and elongate towards the cell centre. Since the unanchored end is mobile and fibrils contract in the presence of agents that disrupt the actin cytoskeleton, Erickson and co-workers propose that the fibrils grow by stretching towards the cell centre as new FN molecules are added probably at the mobile end and along the length of newer segments.
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