Bone Sialoprotein and Osteopontin Distribution at the Osteocartilaginous Interface

In situ hybridization and ultrastructural immunohistochemistry were used to study the synthesis and distribution of 2 bone matrix proteins, bone sialoprotein and osteopontin, in the proximal tibial epiphysis in 21-, 32-, and 84-day-old rats. Bone sialoprotein messenger ribonucleic acid expression was restricted to cells close to the osteocartilaginous interfaces, whereas osteopontin messenger ribonucleic acid was shown also in the central epiphysis. Moreover, bone sialoprotein synthesis decreased with age, while osteopontin synthesis remained unchanged. Bone sialoprotein and osteopontin immunolabeling were concentrated to a 1-micron thick zone at the border between mineralized articular cartilage and subchondral bone. Compared with the youngest animals, total bone sialoprotein immunolabeling increased in the 32-day-old group, but did not increase further in the oldest group. In contrast, osteopontin immunolabeling increased only in the oldest rats with a decelerating rate of growth. The accumulation and localization of osteopontin at osteocartilaginous interfaces differ from previous observations in metaphyseal bone where osteopontin has been found at the mineralization front of the osteoid, possibly reflecting different roles in the mineralization process in the 2 regions. The different pattern of synthesis of the 2 proteins in the epiphysis corroborates previous indications of different biological roles.