VEGF-induced enhancement of the motility of Flk-1⁺ cells on OP9 feeder layer. Sorted Flk-1⁺ cells were cultured on OP9 feeder layer in the presence of VEGF or PlGF. After 3 days of culture, immunostaining with MoAb for Flk-1 was performed as described in Materials and Methods. Flk-1⁺ cells in the periphery of the sheets started to disperse at 3 ng/mL VEGF (A; arrows). At 50 ng/mL VEGF, Flk-1⁺ cells formed spiky clusters composed of spindle form cells (B; arrows). (C) In contrast to VEGF, 100 ng/mL PlGF had no effect on the motility of Flk-1⁺ cells. Scale bar = 100 μm.