OP9/ephrin-B2 promoted ephrin-B2⁺ cell proliferation and sprouting, whereas OP9/EphB4 caused inhibition.

(A) Whole-mount LacZ staining of the E9.0-9.5 yolk sac. The blue color in the vascular region indicates LacZ⁺ staining. The phenotype in each panel corresponds to panel B. The inset shows LacZ-negative staining in the wild-type yolk sac. (B) E9.5 P-Sp explants of mice heterozygous for ephrin-B2–targeted alleles, in which β-galactosidase (LacZ) expression is under the transcriptional control of ephrin-B2 promoter, were cocultured with OP9/ephrin-B2, OP9/EphB4, or OP9/vector stromal cells for 14 days. LacZ and PECAM-1 staining have shown that the vascular network was formed by LacZ/ephrin-B2 (blue) and/or PECAM-1 (red) double-positive ECs on OP9/ephrin-B2 stromal cells, and almost no LacZ/ephrin-B2⁺ECs were detected on OP9/EphB4 stromal cells. The vascular network was formed by LacZ/ephrin-B2 (blue) and/or PECAM-1 (red) double-positive ECs and PECAM-1⁺ cells on OP9/vector stromal cells. The right panels are a higher magnification of the area indicated by the box in the left panels. The bar indicates 1 mm (left panels) and 80 μm (right panels). The arrows on the OP9/EphB4 stromal cell plate indicate where some LacZ⁺ cells are available.