Electron micrographs of E9.5 yolk sac cells. (A) Representative erythroid cell from EKLF/KLF2 double knockout. (B) Wild-type and (C) EKLF/KLF2 double-knockout sections indicating the cell types in the yolk sac. (D) Abnormal ingrowth of microvilli in epithelial cells in EKLF/KLF2 double knockout. The abbreviations used are erythroid cells (Ery), cytoplasmic projections (CP), mesothelial cells (Me), intercellular space (IC) between the mesothelial (Me L) and endothelial layers (En L), basal lamina (B Lm), microvilli (MV), and ingrowth of epithelial cells (IGEp). Scale is shown in each photograph. See “Analysis of EKLF^−/−KLF2^−/− embryos” for more image information.