Fig. 4. MK cells display integrin 3ß1-dependent fan formation. (A) MK3 cells were plated sparsely onto LN-5 ECM and stained for F-actin and the 3 integrin subunit. Arrowheads indicate leading lamellipodium. Bar, 10 µm. (B) MK^–/– cells were plated sparsely onto LN-5 ECM and stained for F-actin. Arrows indicate filopodia. Corresponding phase-contrast images are shown in (A,B). (C) MK3 cells were plated sparsely in the presence of a control IgG, a function-blocking antibody against integrin 3ß1 (P1B5) or a function-blocking antibody against integrin 6ß4 (GoH3), as indicated, and cell morphology was observed by time-lapse video microscopy. (D) The proportion of MK cells that assumed a fan shape in either serum-free (open bars) or serum-containing (filled bars) conditions was determined. Data represent the average of three separate experiments, with 100 cells being scored per experiment. Error bars represent s.e.m.