Fig. 6. Cells respond to increasing forces by an increased pseudopod frequency. D. discoideum cells adhering to glass were continuously monitored by phase-contrast microscopy. Cell response was triggered by application of a continuous 1.8 Pa shear stress (at t=0 seconds). Pseudopodia were detected by inspection over 50 cells as described under Methods and the pseudopodium emission frequency per cell was calculated at each time point. (A) Time course of cell pseudopodium emission frequency. The solid and dotted lines indicate the spontaneous and adapted pseudopodium emission frequencies, respectively. (B) A cell submitted to shear flow exhibiting nascent (empty triangles) and fully extended (filled triangles) pseudopodia, used in building up the curve shown in (A). The images are centered on the same cell and the indicated time is relative to the application of the flow. Different phases of cell response were selected: resting state (–2 seconds), onset of pseudopodium extension (+2 seconds), full immediate response (+6 seconds) and adapted phase (between +10 seconds and +30 seconds). Notice that most of the pseudopodia emitted after application of the flow protrude in the direction of the flow. Scale bar, 5 µm.