Low and high power histology of adjacent sections (A, B and C, D) of OCst-KO (A, B, E, F) and WT (C D, G, H) 6-week-old mouse distal femur.

<p>Glycol methacrylate, 3 μm sections were stained histochemically for TRAP (<b>B, D, E, G</b>) and some were counterstained with toluidine blue (<b>A, C, F. H</b>). At low power, overall appearance was highly similar, with growth plates normal (purple, wavy band in <b>A</b> and <b>C,</b> asterisks in <b>B</b> and <b>D</b>), similar trabecular size and thickness in both the primary and secondary ossification centers, and open marrow space (<i>m</i>) in the diaphysis. TRAP stains (<b>B, D</b>) show overall similar distribution of osteoclasts, including along the chondroosseous junction, among the trabeculae of the primary ossification center, and notably, along the periosteal surface (arrows in B and D), where the bone is being removed to maintain the flared shape of the metaphysis. At higher power (<b>E, F, G, H</b>), only mononuclear osteoclasts are present in OCSt-KO mice. A row of such cells is indicated in <b>A</b> by arrows, attached to trabecular bone (<i>B</i>) in the primary spongiosa. In contrast, wild type mice had typical, multinucleated osteoclasts in this area, and individual nuclei (visible by lack of overlying TRAP label in <b>C</b>), are indicated by asterisks. Blinded observers consistently identified OCSt-KO and WT sections based solely on the presence or absence of multinucleated osteoclasts. Bar in A = 500 μm in A, B, C, D. Bar in E = 10 μm in E and G; 7.3 μm in F and H. <i>B</i> = bone, <i>C</i> = cartilage core.