FIG. 9.

Electron microscopy of capillaries demonstrates unusual endothelial cell shapes in homozygous perinatal animals but not in adults. Ultrathin cross-sections of capillaries are shown for heterozygous (A) and homozygous (B to F) stage E18.5 embryos. (A) Representative heart capillary from a heterozygote with a single endothelial cell having a smooth inner surface to the capillary lumen (l) without cytoplasmic processes. (B) Heart capillary from a homozygote demonstrates multiple endothelial cytoplasmic processes. (C) Other heart capillaries show multilayered endothelial cells (arrows). (D) Endothelial cells (e) with vacuoles are seen together with their basement membranes (arrows) detached from an irregularly shaped cardiomyocyte (cm). (E) Enlarged renal capillaries show an irregular luminal surface with cytoplasmic processes (arrows), while a tubular epithelium cell (t) seems to be unaltered. (F) Multiple cytoplasmic processes (arrows) are recognizable in a brain capillary endothelial cell which is partially covered by a normal pericyte (p). (G and H) Heart capillaries from homozygous adults (H) do not show obvious alterations compared to heterozygotes (G). Bars: 4 μm (A), 1 μm (B, C, and E), 0.7 μm (D), 5 μm (F), 3 μm and (G and H).