Fig. 5. Similar reduction of spleen progenitor cell proliferation in Pbx1^-/- and Hox11^-/- embryos. (A,C) BrdU in vivo labeling of sagittal sections of upper abdominal organs shows a striking difference in the percentage of BrdU-positive nuclei (brown) within the spleen anlage of Pbx1^-/- and Hox11^-/- embryos, compared with wild-type littermates at E13.5. (B,D) Proliferation of spleen mesenchymal progenitors is reduced by 50% in Pbx1^-/- and Hox11^-/- embryos, respectively, when compared with wild-type littermates. The results are expressed as total BrdU-positive cells per mm² of spleen anlage. Data are mean±s.e.m. of four E 13.5 embryos analyzed for each genotype. Black bars, wild type; grey bars, Pbx1^-/- and Hox11^-/- embryos; Sp, spleen; St, stomach.