Vascularization defects in VE-PTP mutant yolk sac. (A-D) Yolk sacs of E8.5 and E9.5 wt and mt/mt embryos (as indicated) were stained by indirect immunofluorescence for PECAM-1 and mounted on poly-l-lysine–coated glass slides. Whereas the vessel network in the mt/mt yolk sacs of E8.5 embryos (B) consisted of a rather normal honeycomb-like vascular plexus, the E9.5 mt/mt yolk sac (D) contained enormous continuous layers of endothelium, in contrast to the well-developed hierarchic organization of large and small vessels in the wt yolk sac (C). Arrows in panel D mark endothelium-free areas. (E-F) Paraffin sections of E9.0 wt and mt/mt yolk sacs still containing the embryo proper were immunostained for endomucin. Note the dramatically enlarged vessels in the mt/mt yolk sac (asterisk). The boxed areas in panels E and F are shown at higher magnifications in panels G and H. en indicates endoderm; me, mesothelium; and bv, blood vessel. Asterisks indicate blood cells. (I-J) Paraffin sections of the yolks sacs of E9.0 wt and mt/mt embryos were stained for the proliferation marker phosphohistone-3. Arrows depict nuclei in endothelial cells. Bars represent 50 μm (A-D,G-H) and 100 μm (I-J). Objectives used: (A-D) Fluar 20 ×/0.75 NA; (E-F) Plan Apochromat 10 ×/0.45 NA; (G-J) Plan Apochromat 20 ×/0.60 NA.