Fig. 7. (A) Safranin O stained longitudinal section of a rib of a 2-week-old wild-type mouse is shown. Rectangle indicates area of interest representative for B and C. (B) Rib of a Ext2^+/–. Arrow indicates the location of a chondrocyte nodule. Asterisk indicates location of the proliferative zone of the rib growth plate. The inset in the top right-hand corner shows a magnification of the nodule. (C) BrdU staining of adjacent serial section shown in B. BrdU-positive cells can be seen in the proliferative zone of the growth plate of the ribs (arrowhead). The chondrocyte nodule does not contain BrdU-positive cells (arrow). (D,E) Higher magnification of the area with the nodules shown in B and C. (D) In situ hybridization with Col10 antisense probe shows expression by chondrocytes located in the center of the rib cartilage. The nodule of displaced chondrocytes (encircled by the broken line) expresses Col10 and are separated from the centrally located hypertrophic cells by a layer of chondrocytes that do not express Col10. (E) Staining with the HS antibody 10E4 shows that the chondrocytes in the nodules (encircled by the broken line) produce HS. Inset in E shows staining with 10E4 after heparinase treatment. Scale bars: 200 µm in A-C; 50 µm in D,E.