The actin cytoskeleton in Rat2 cells. Rat2 fibroblast cell stained for actin (white) and DNA (blue). The leftmost panel shows transverse actin stress fibers (S) and filopodia (F) as hair-like projections from the cell perimeter. The central panel shows counting of individual filopodia lengths (L1, L2, L3, L4) and distance separation (D1, D2, D3). The right panel shows a mitotic cell with retraction fibers (R) indicated. The red scale bar is 10 μm.