Mutant homozygotes display an embryonic lethal phenotype, failing to develop past embryonic day 9.5. At embryonic day 7.5, homozygous embryos lack a primitive streak and mesodermal derivatives with a smaller epiblast surrounded by an expanded parietal endoderm. Mice homozygous for this targeted mutation exhibit a phenotype similar to homozygotes carrying the mesd deletion (Holdener et al., Development. 1994;120(5):1335-46. Hsieh, J-C et al Cell 2003; 112(3): 355-367). Wnt signaling is blocked in homozygous embryos (aged embryonic day 7.5) as assessed by crossing with B6.Cg-Tg(BAT-lacZ)3Picc/J (STOCK No. 5317) mice that express beta-galactosidase in the presence of activated beta-catenin. Expression of the pluripotency molecular markers, Oct4, Nanog, and Sox2, continues in the epiblast of homozygous embryos (day 8.5). LRP2, low density lipoprotein receptor-related protein 2, or Megalin, protein is not localized to the apical membrane of the visceral endoderm and is diffusely distributed in the cytoplasm. Ultrastructural analysis reveals a reduction in size and number of organelles in visceral endoderm cells. Lysosome number and size is also reduced and endocytosis function is defective in the visceral endoderm cells of mutant embryos. During backcrossing, the Y chromosome may not have been fixed to the C57BL/6J genetic background. Mice heterozygous for this targeted mutation are viable, fertile, normal in size.
