Visualising defective endoderm in E6.5 Smad4 mutant embryos. A. Collagen IV staining of the thickened basement membrane (green arrows) in Smad4 null embryos. Blue, DAPI nuclear staining. B. The Smad4 mutant epiblast strongly expresses Oct4 and is significantly reduced in size. C. Schematic of wild-type versus Smad4 null E6.0-E6.5 embryos. Blue corresponds to endoderm cells, grey represens the epiblast and green indicates the basement membrane layer.