The term “lamellipodia” was originally coined by Abercrombie to describe the thin cytoplasmic sheets extended at the front of moving cells and to distinguish them from the finger-like projections, termed “filopodia”. This section introduces the motile and structural features of lamellipodia.
The actin cytoskeleton of a living B16 melanoma cell. The cell was transfeced with a cDNA construct of actin tagged with green fluorescent protein (actin-GFP) allowing visualization of actin in the fluorescent channel. The lamellipodium is seen as a wide band of fluorescent label at the protruding cell front.
The actin cytoskeleton in a living fish fibroblast (CAR cell line) transfected as for the cell above. In this cell type the protruding cell front features segments of lamellipodia between filopodia.
The actin cytoskeleton in a rat 3Y1 fibroblast that was spread on polylysine, fixed and then stained with fluorescent phalloidin. A lamellipodium borders the entire cell periphery and is punctuated by actin bundles that protrude only marginally beyond the cell edge. We have termed these bundles microspikes, to distinguish them from filopodia.
The autonomy of the lamellipodium is also illustrated by experiments showing that mechanical manipulation of the keratocyte cell body has no influence on lamellipodium protrusion rate (Anderson et al., 1996):
A smooth microneedle is used to give the cell a gentle push. The rate of protrusion of the cell front is unaffected.
In the converse experiment the cell body is restrained: again the rate of protrusion of the cell front is not changed in the short term.
The sheet-like nature of the lamellipodium is best illustrated in electron micrographs of cross-sections of cells embedded in plastic, as shown here. In such images lamellipodia are found to be in the range of 0.1–0.3µm thick. (Left) cross-section of a fish keratocyte showing the thin lamellipodium extended from a very large cell body, bar 2.5µm. (Center) cross section of a rat 3Y1 fibroblast spread on polylysine, bar 10µm. (Right) enlarged view of the left part of the lamellipodium in the center image, bar 1µm.
Using correlative light microscopy and conventional electron microscopy (see Methods) the structure of lamellipodia in different states of motility may be analyzed (Figs 4.7−9), in this example during protrusion (Koestler et al., 2008):
Video frames of a cell expressing GFP-actin (green) and RFP-VASP (red; see section Nucleation Factors) that was arrested in a phase of protrusion with a mixture of glutaraldehyde and Triton X-100 at the time indicated (FIX).
Electron micrograph of the protruding zone of the cell shown in the video above, showing the diagonal network of actin filaments in lamellipodia and the bundles of actin filaments that make up the microspikes and filopodia. From Koestler et al., 2008.
Enlargement of region from top right in the figure above showing details of the lamellipodium actin meshwork and the bundled filaments in the microspikes.
- Anderson, KI., Wang, YL., Small, JV. (1996). Coordination of protrusion and translocation of the keratocyte involves rolling of the cell body. J Cell Biol. 134, 1209 – 1218.
- Euteneuer U., and Schliwa M. (1984). Persistent, directional motility of cells and cytoplasmic fragments in the absence of microtubules. Nature. 5–11; 310(5972): 58–61.
- Koestler, SA., Auinger, S., Vinzenz, M., Rottner K., Small, JV. (2008). Differentially oriented populations of actin filaments generated in lamellipodia collaborate in pushing and pausing at the cell front. Nat Cell Biol. 10, 306–313.
- Verkhovsky, A., B., Svitkina, T., M., Borisy, G., G. (1998). Self-polarization and directional motility of cytoplasm. Curr. Biol. 9, 11–20.