Fig. 6. Accelerated differentiation and diminished proliferation of chondrocytes in Pbx1^-/- embryos, demonstrated by cell-type specific gene expression and cellular proliferation assays. (A-H) Immunohistochemistry was performed on serial sections of wild-type (left panels) and Pbx1^-/- (right panels) cartilage at E15.5 using antibodies specific for markers of chondrocyte differentiation (A-F) and antibodies specific for cycling and dividing cells (G-J). Representative data are shown for embryos that were extensively sectioned and analyzed. (A,B) Collagen type II; (C,D) collagen type X; (E,F) stromolysin-1 (MMP3); (G,H) immunodetection of PCNA shows strong nuclear staining in numerous cells of wild-type but not Pbx1^-/- rib cartilage at E15.5; (I,J) BrdU in vivo labelling shows a marked difference in the percentage of BrdU-positive nuclei (dark brown) between wild-type and Pbx1^-/- rib cartilage at E13.5; (K) BrdU incorporation calculated as a percentage of BrdU-positive nuclei in rib cartilages at different developmental stages. Black bars, wild type; grey bars, Pbx1^-/-. Percentage PCNA staining of wild-type (black) and Pbx1^-/- (grey) at E15.5 is also shown. Bars represent means + s.d.