The actin-based nanomachine at the leading edge of migrating cells

Two fundamental parameters of the highly dynamic, ultrathin lamellipodia of migrating fibroblasts have been determined-its thickness in living cells ( 176 +/- 14 nm), by standing-wave fluorescence microscopy, and its F-actin d ensity (1580 +/- 613 mu m of F-actin/mu m(3)), via image-based photometry. In combination with data from previous studies, we have computed the densit y of growing actin filament ends at the lamellipodium margin (241 +/- 100/m u m) and the maximum force (1.86 +/- 0.83 nN/mu m) and pressure (10.5 +/- 4 .8 kPa) obtainable via actin assembly. We have used cell deformability meas urements (Erickson, 1980. J. Cell Sci. 44:187-200; Petersen et al., 1982. P roc. Natl. Acad. Sci. USA. 79:5327-5331) and an estimate of the force requi red to stall the polymerization of a single filament (Hill, 1981. Proc. Nat l, Acad. Sci. USA. 78:5613-5617; Peskin et al., 1993, Biophys, J. 65:316-32 4) to argue that actin assembly alone could drive lamellipodial extension d irectly.