Endothelial cells grow as cell layers in cultured VE-PTP homozygous mutant allantoides. Allantoides were isolated from E8.5 wt (A,C,E) and VE-PTP mt/mt embryos (B,D,F) and cultured as explants for 22 hours. In panels A-B, they were stained by indirect immunoperoxidase staining for PECAM-1; in panels C-F, by indirect immunofluorescence for VE-cadherin. In panel B, note that the additional area covered by PECAM-1–positive endothelial cells (marked by an arrowhead) is less strongly stained than the cordlike structures (marked by an arrow), appearing to be “thinner” than the cords. Such a “thin” area stained for VE-cadherin is shown in panel D and compared with cordlike endothelial structures in panel C. The z-scans shown in panels E and F are taken from the micrographs in panels C and D, respectively, taken at the position of the red line. Note that the endothelium in the z-scan of the wt allantois showed a multilayered organization (E), whereas the endothelium in the mt/mt allantois appeared to be a single cell layer (F). Single endothelial cell layers ranged from 5- to 6-μm thickness in 10 analyzed areas, whereas the thickness of endothelial cord structures in mt allantoides was 16 to 20 μm (5 cords determined) and in wt allantoides 22.1 ± 7 μm (19 cords determined). Bars represent 100 μm (A-B) and 25 μm (C-F). Objectives used: (A-B) Plan Apochromat 20 ×/0.60 NA; (C-F) HC Plan Apochromat 20 ×/0.70 NA.