Fig. 2. Perturbed expression of Hox11 and Wt1, known regulators of spleen development, in Pbx1^-/- embryos. Transcripts for the spleen regulators Hox11, Wt1, Pod1 and Nkx3.2 were detected by in situ hybridization to frozen sagittal sections of wild-type and Pbx1^-/- upper abdominal organs at E11-11.5 and E12-12.5, as indicated. The splenic primordium is outlined by black dashes. (A-D) Both at E11-11.5 and E12-12.5 Hox11 expression is absent in the spleen primordia of Pbx1^-/- embryos, compared with wild-type littermates. (E-H) Wt1 expression is also absent in spleen anlage of Pbx1^-/- embryos compared with wild-type littermates both at E11-11.5 and E12-12.5. It is noteworthy that Wt1 expression is still present in the mesothelial lining that surrounds the mesenchyme of the splenic anlage (arrow). (I-L) Pod1 and (M-P) Nkx3.2 are normally expressed within the developing spleen primordia of Pbx1^-/- embryos compared with wild-type littermates, both at E11-11.5 and E12-12.5. L, liver; Sp, spleen; St, stomach.