Migratory capacity of CD34+ tip cells and CD34- non-tip cells toward VEGF. (a) FACS-sorted CD34+ tip cells were labelled using violet dye and mixed with unstained CD34- non-tip cells and seeded in the observation area of the 2D µ-slide chemotaxis. Tracks of CD34- non-tip cells (b) and CD34+ tip cells (c) are plotted in the 2D µ-slide chemotaxis for a duration of 24 h. Tracks of single cells moving toward VEGF (left) are shown in red and single cells moving toward site without chemoattractant (right) are shown in black. (d) The percentage of tip cells migrating toward VEGF was higher as compared to that of non-tip cells. (e) Tip cells migrate more perpendicular to VEGF (x-axis) as compared to non-tip cells, whereas the migration parallel to VEGF was similar in tip cells and non-tip cells. (f) The center of mass on the x-axis show that tip cells migrate more perpendicular to VEGF as compared to non-tip cells. No differences were found in accumulated distance and euclidean distance (g), velocity (h) and the directness (i) between tip cells and non-tip cells. FMI: forward migration index. Results are shown as ± SEM of experiments with HUVECs of at least 3 donors. *P < 0.05 and **P < 0.01 as compared to non-tip cells (Unpaired Student’s t-test). P-values in fig. (b,c) were given by the statistical Rayleigh test.