PECAM-1^−/− MKs demonstrate defective cell migration toward a gradient of SDF1α. (A) Differential interference contrast (DIC) images of wild-type MKs exposed to an SDF1α gradient within the Dunn chamber. Scale bar = 10 μm. (B) The migration paths over 4 hours of 30 MKs from 8 mice in each graph were traced. The intersection of the x- and y-axis was taken to be the starting point of each cell path, whereas the source of the SDF1α was at the top. (i) PECAM-1^+/+ MKs exposed to SDF1α gradient. (ii) PECAM-1^+/+ MKs exposed to SDF1α but with no gradient. (iii) PECAM-1^+/+ MKs exposed to SDF1α gradient in the presence of AMD3100. (iv) PECAM-1^−/− MKs in the presence of SDF1α gradient. (C) Circular histograms showing the proportion of cells whose final position was located within each of 18 equal sectors (20°). The source of SDF1α was at the top. (D) The net translocation distance (displacement from the start to the end point) of each cell in the absence or presence of inhibitors AMD3100 or cytochalasin D (CytD). *P < .01; **P < .001. Error bars indicate standard error. DIC images were visualized using a Zeiss Axiovert 200 inverted high-end microscope (Zeiss, Welwyn Garden City, United Kingdom) equipped with a 20×/0.4 Plan-Neofluor objective lens. Image acquisition was otherwise performed as described for Figure 2C.