Fig. 7. Mutant embryos exhibit a restricted differentiation potential. (A,B) Determination of ß-galactosidase expression in sections from E7.5 embryos that developed from aggregation chimeras between ROSA26 and mutant morulae revealed that mutant cells contributed mostly to extra-embryonic tissues and were only occasionally found in the embryonic ectoderm of the chimeras. In some cases the embryonic ectoderm cell layer was exclusively composed of ROSA26 cells (A,B). (C-E) Analysis of wild-type and mutant E6.5 embryos transplanted under the kidney capsule to induce teratomas. (C) Teratocarcinoma produced by transplantation of wild-type embryo. Several well differentiated tissues are visible, including bone, muscle, neural tissue and skin epithelium. (D) Adjacent section stained with antibody to ß-catenin. Higher magnification of area marked in panel C shows expression of ß-catenin in the membranes of the epithelial cells. (E) Histological section of tumor derived from transplanted mutant embryo. Cells similar to those present in extra-embryonic endoderm form small glandular structures (arrowhead). Hyalin substance corresponding to Reichert's membrane (arrow) fills extracellular spaces. Certain gland-like structures display substantial cellular and nuclear polymorphism (star). (F) Adjacent section stained with antibody to ß-catenin. There is intensive membrane and cytoplasmic staining. Cells exhibiting polymorphism (star) also show nuclear staining. Scale bar: 100 µm.