Figure 4.

(A–C) Hematoxylin and eosin stained sections of the distal femur of (A) wild-type, (B) OPG^−/− mice, and (C) OPG^−/− mice bearing OPG transgene. OPG^−/− mice (B) show osteoporotic bone structure and lack of secondary spongiosa compared to wild-type (A). Osteopetrosis occurs in OPG^−/− mice bearing OPG transgene (C). (D–F) Polarized light microscopy of the cortical bone in mid-femoral diaphysis in wild-type mice (D), OPG^−/− mice (E), and OPG^−/− mice bearing OPG transgene (F). Note the woven bone in OPG^−/− mice (E), a characteristic of high bone turnover rate, compared to highly lamellar bone of OPG^−/− mice bearing OPG transgene (F). (G–I) TRAP-stained sections of wild-type mice(G), OPG^−/− mice (H), and OPG^−/− mice bearing OPG transgene (I). Note the significant increase in osteoclasts (black arrows) in OPG^−/− mice (H) compared with OPG^−/− mice bearing OPG transgene (I). The visible increase in osteoblasts (yellow arrowheads) indicates a compensatory increase in bone formation in OPG^−/− mice.