Collapsing vascular structure in the midbrain. Electron microscopic analysis of brain capillaries from Smad2^fl/fl;Smad3^+/−;Tie2-Cre (A-B) and EC-Smad2/3KO (C-D) embryos at E11.5. Panels A and C, 2000× magnification; panel B and D, 50 000× magnification. Samples were observed using an electron microscope (H7000; Hitachi) operated with 75 kV at 20°C. Each area surrounded by the circle reveals tight junction between ECs. (E) Transverse sections of midbrains from E10.5 embryos were stained with anti–claudin-5 antibody as well as with hematoxylin. EC indicates endothelial cell; P, pericyte; and RBC, red blood cell.