Osteoclast differentiation and characteristic trabecular bone formation during growth plate destruction in osteoprotegerin-deficient mice

Osteoprotegerin (OPG) is an osteoblast-derived secreted member of the tumour necrosis factor receptor superfamily that inhibits osteoclastogenesis. Mice that are OPG-deficient have severe bone loss, including growth plate cartilage destruction. Using OPG-deficient mice as a useful animal model, we attempted to clarify differentiation and ultrastructural features of osteoclasts located on destructed growth plate cartilage and trabecular bone matrix. In the humerus and femur of OPG homozygous (-/-) mice, adjacent to the growth plate cartilage, bone trabeculae without a calcified cartilage core were characteristically formed at the metaphyseal side of the medullary cavity, which resulted in an irregular chondrocyte distribution and arrangement in growth plate cartilage. During growth plate cartilage destruction, osteoclasts positive for tartrate-resistant acid phosphatase showed unusual localization on both type-II collagen-positive cartilage and type-I collagen-positive trabecular bone matrix at the ossification centre of the epiphyseal/metaphyseal border. Although multinucleated osteoclasts were distributed within open lacunar canals in the growth plate, those on uncalcified cartilage matrix lacked a ruffled border. Facing the calcified cartilage matrix within lacunar canals, osteoclasts showed irregularly formed ruffled borders. After growth plate destruction, a thin bone layer was deposited on the remaining cartilage surfaces by invading osteoblasts. Osteoclasts formed prominent ruffled border structures on bone matrix, deposited on the remaining growth plate cartilage. These results suggest that, in OPG (-/-) mice, terminal osteoclast differentiation requires the presence of newly produced bone matrix, as the coupled phenomenon of bone formation and resorption, as well as osteoblast-derived cytokines.