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 density (1580 ± 613 μm of F-actin/μm3), via image-based photometry. In combination with data from previous studies, we have computed the density of growing actin filament ends at the lamellipodium margin (241 ± 100/μm) and the maximum force (1.86 ± 0.83 nN/μm) and pressure (10.5 ± 4.8 kPa) obtainable via actin assembly. We have used cell deformability measurements (Erickson, 1980. J. Cell Sci. 44:187-200; Petersen et al., 1982. Proc. Natl. Acad. Sci. USA. 79:5327-5331) and an estimate of the force required to stall the polymerization of a single filament (Hill, 1981. Proc. Natl. Acad. Sci. USA. 78:5613-5617; Peskin et al., 1993. Biophys. J. 65:316-324) to argue that actin assembly alone could drive lamellipodial extension directly.
Abraham, V. C., Krishnamurthi, V., Lansing Taylor, D., & Lanni, F. (1999). The actin-based nanomachine at the leading edge of migrating cells. Biophysical Journal, 77(3), 1721–1732. https://doi.org/10.1016/S0006-3495(99)77018-9