Fig. 3. PAK1 is required for lamellipodial stability during spreading. (A) WT and PAK1^–/– BMMs were plated onto glass-bottomed culture dishes in growth medium and cell spreading was visualised by time-lapse microscopy. Images at the indicated time points of the movies are shown. Scale bars: 1 µm. (B) The length and the perimeter of WT and PAK1^–/– lamellipodia was determined using movies as in A. Data shown are the mean ± s.e.m; n=145 and n=225 lamellipodia from WT and PAK1^–/– BMMs, respectively, from six separate experiments. **P<0.01 and ***P<0.001, Student's t-test. (C) Kymographs of extending lamellipodia. The final frame from the movie used (left panels) indicates the region of kymograph production. Right panels show kymographs; the inset shows the kymograph with the membrane edge highlighted (white line). (D) Quantification of the number of lamellipodia per cell observed at the specified times in movies. Mean ± s.e.m.; n=9 cells from six separate experiments for both WT and PAK1^–/– BMMs.