Effect of soluble NP-1 in P-Sp cultures.

P-Sp explants derived from E9.5 wild-type embryos (A, C, E, G) and mutant embryos (B, D, F, H) were cultured on OP9 stromal cells. NP-1–Flag (50 μg/mL; C,D,G,H), anti-Flag IgG (50 μg/mL; G-H), NP-1–Fc (50 μg/mL; E-F), or the same amount of control Flag (A-B) was added to this culture. In wild-type embryos, vb formation was suppressed when NP-1–Flag was added (C); however, on addition of NP-1–Fc (E) or NP-1–Flag plus anti-Flag IgG (G), which forms a dimer of NP-1, the vb was expanded compared with that on addition of control Flag (A). In homozygous mutants, vascular formation was slightly suppressed on addition of NP-1–Flag (D; arrow) compared with that on addition of control Flag (B); however, suppressed EC development was rescued by addition of NP-1–Fc (F) or NP-1–Flag plus anti-Flag IgG (H). Similar results were obtained in 4 independent experiments. The scale bar indicates 100 μm.