P-Sp culture system reflects vascular defects in

np-1 mutant embryos. Spinal cord (A-B) and heart region (C-D) of E12.0 np-1^+/+ (A, C) andnp-1^−/− (B, D) murine embryos were subjected to immunohistochemical staining with anti–PECAM-1 antibody to visualize all ECs. Capillaries showed little branching and were of large caliber (B, D; arrows). P-Sp explants derived from E9.5 wild-type embryos (E) and mutant embryos (F) were cultured on OP9 stromal cells. Vascular network (vn) formation was defective innp-1^−/− P-Sp explants (F), as observed in the CNS and pericardium. The scale bar indicates 100 μm (E-F); vb, vascular bed.