Fig. 1. Agenesis of the spleen in Pbx1^-/- embryos. (A,B) Comparative whole-mount preparation of upper abdominal organs at E15.5 shows the lack of a spleen, together with liver hypoplasia, in Pbx1^-/- embryos (B). The spleen forms lateral to the stomach, and appears as a reddish ribbon-shaped organ in wild-type embryos (black dashes in A). (C,D) Histology of Hematoxylin and Eosin-stained transverse sections at E15.5 show agenesis of the spleen in Pbx1^-/- embryos compared with wild-type littermates. (E,F) Histology of Hematoxylin and Eosin-stained transverse sections of E13.5 wild-type and Pbx1^-/- embryos. A visible spleen primordium forms as a mesenchymal condensation within the dorsal mesogastrium (Dm, arrows) in wild-type embryos (E). In Pbx1^-/- littermates (F), no mesenchymal condensation is detectable within the Dm (arrowheads). (G-J) Histology of Hematoxylin-stained transverse sections of wild-type and Pbx1^-/- embryos in the region of the stomach enlargement at E10-10.5. The splanchnic mesoderm lateral to the stomach enlargement (G,I) consists of a thick epithelial-like plate of cells (arrows) that encloses unorganized mesenchyme. The black boxes indicate the regions magnified (40x) in H,J, which highlight the organized epithelial-like cellular structure of the splanchnic mesoderm. C, coelomic cavity; Dm, dorsal mesogastrium; L, liver; Sp, spleen; St, stomach.