Figure 4.

Examples of cell orientation reversals during exposure due to temporally decreasing FMLP concentrations in a microscope stopped-flow chamber. Cells are shown before (column 1) and after (column 2) exposure to temporally decreasing concentrations of FMLP (10⁻⁸ to 10⁻¹⁰ M). Neutrophil orientation generally flips 180° relative to the initial direction of polarization after exposure to a temporally decreasing FMLP signal (i.e., cells detect that they are polarized in a wrong direction). (A and B) An example of a neutrophil reorientating after exposure to a temporally decreasing FMLP signal. In these experiments, cells reversed direction by disassembling their lamellipodia and uropods. (C and D) Occasionally, cells reverse direction by making a “U-turn.” In contrast to these orientational reversals, when neutrophils are exposed to a temporally increasing series of FMLP concentrations or HBSS pulses, reorientation is not observed. Nearby structures provide spatial confirmation of directional reversals. Quantitative data are shown in Fig. 3. (Magnification, ×1,333)