Fig. 4. Exostosis formation in ribs of Ext2^+/– mice. (A) Dissected rib of a wild-type mouse and an Ext2^+/– mouse with an exostosis (indicated by the arrow). The boundaries between the costochondral cartilage and the bony part of the rib are indicated by the broken lines. (B) Higher magnification showing larger exostosis on a rib of an Ext2^+/– mouse. (C,D) Trichrome-stained histological section of the costochondral junction of rib from a wild-type mouse (C) and a rib from an Ext2^+/– mouse with an exostosis (D). The exostosis is composed of cortical and medullary bone with an overlying hyaline cartilage cap (indicated by arrows). The bone marrow cavity of the exostosis is continuous with that of the underlying bone. (E) Magnification of the boxed area in D. Within the cartilage cap, some chondrocytes appear to line up, resembling the organization of chondrocytes in a normal growth plate. (F) Staining of adjacent section to the one shown in E with the HS antibody 10E4 shows that chondrocytes produce HS. Inset in upper right-hand corner shows control staining with mouse IgG as primary antibody. B, bone; BM, bone marrow; C, cartilage. Scale bars: 3.5 mm in A,B; 300 µm in C,D; 50 µm in E,F.