Explants of homozygous VE-PTP mutant allantois fail to undergo normal vascular morphogenesis in culture. (A-H) E8.5 allantoides of embryos of the following genotypes were explanted and cultured for 22 hours: wt (A,C,E,G), VE-PTP homozygous mutant (B), Tie-2^–/– (D), Flk-1^–/– (F), and wt (H); the latter was treated with blocking antibody against VE-cadherin. The vascular network was stained by indirect immunofluorescence either for VE-cadherin (A-F) or for PECAM-1 (G-H). Note that the VE-PTP mt/mt allantois displayed a reduction in the amount of avascular space and contained unusually large areas covered with endothelial cells. In contrast, wt and Tie-2^–/– allantoides contained normal filigran networks of endothelial cords. As expected, no cordlike vascular structures were detected in the Flk-1^–/– allantois. The allantois shown in panel H was cultured in the presence of an adhesion-blocking antibody against VE-cadherin, causing a complete disruption of vascular structures, while no such effect was seen in the control (G) treated with an isotype-matched control mAb. Bars represent 100 μm. (I) Percentage of surface area (±SD) stained for PECAM-1 in E8.5 wt/heterozygous and VE-PTP^mt/mt cultured allantoides explants, with the total area of each explant set at 100%. The fraction of PECAM-1–positive areas was 39% ± 8.7% larger in VE-PTP^mt/mt than in wt/heterozygous allantoides (P < .01). An HC Plan Apochromat 20 ×/0.70 NA objective was used to visualize all images in these panels.