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.