Fig. 4 Polarity of actin filaments in keratocyte cytoskeleton. Electron microscopy of detergent extracted cells after myosin S1 decoration. (a), leading edge; (b), middle portion of a lamellipodium; (c) transitional zone; (d), boundary bundle; (e), T-junctions between actin filaments at the leading edge; (f), retraction fiber at the cell rear. Directions of pointed ends of some filaments are shown by arrowheads located next to a filament. Filaments are oriented primarily with barbed end forward throughout the lamellipodia (a-c), while the boundary bundle has mixed filament polarity (d). At T junctions, filaments are oriented with pointed end toward the junction (e). The retraction fiber (f) contains uniformly oriented filaments with their barbed ends to the tip of the fiber. Unlabeled intermediate filaments can be seen in some panels. The cell's leading edge is oriented upward in all panels. Bars, 0.1 µm.