VE-cadherin–null allantois explants fail to undergo normal vascular morphogenesis in culture. (A) PECAM-immunolabeled allantois from a normal 8.5-dpc mouse embryo. (B) PECAM-immunolabeled allantois explant that was derived from an 8.5-dpc normal mouse embryo and cultured for 18 hours. (C-D) PECAM-immunolabeled allantois explants from 8.5-dpc VE-cadherin^+/+ (C) and VE-cadherin^–/– (D) embryos after 18 hours of culture. The interconnected networks of PECAM⁺ cells evident in wild-type explants (C) are absent in the VE-cadherin^–/– explants (D) and are replaced by isolated clusters of PECAM⁺ cells. Bars equal 100 μm. All panels are confocal images.