EXPRESSION OF BONE SIALOPROTEIN MESSENGER-RNA DURING BONE-FORMATION AND RESORPTION INDUCED BY COLCHICINE IN RAT TIBIAL BONE-MARROW CAVITY

In the rat tibial bone marrow cavity, following colchicine injection, there is a phase of osteogenesis in which bone trabeculae replace the necrotic bone marrow tissues and fill the marrow cavity, The newly for med bone is subsequently resorbed by osteoclasts and normal bone marro w is restored, In this study, we correlated these morphologic events w ith the pattern of gene expression of bone sialoprotein (BSP), an extr acellular matrix protein in mineralized tissues, to elucidate the poss ible functions of BSP in bone formation and resorption in vivo. The ex pressions of osteopontin (OPN) and type I collagen were also examined, Northern hybridization of the tibia demonstrated that OPN mRNA was gr adually increased and expressed at a maximal level 10 days after colch icine injection (during the bone resorption process), while BSP mRNA e xpression already reached a maximal level at day 6 (during the initial process of bone formation), Its expression was, thus, quite temporary at the beginning of bone formation and different from that of type I collagen, which was continually elevated from days 6 to 10, In situ hy bridization of the newly formed bone induced in the tibia revealed tha t BSP mRNA was evenly expressed in most osteoblasts and osteocytes, mo reover in interconnecting colonies of spindle-shaped cells, possibly p reosteoblasts, at day 6. At day 10, however, its expression became res tricted to some cells on the bone surfaces, some osteoblasts, and most osteoclasts. These observations suggest that BSP may play an importan t role mainly in the initiation of bone formation and is also associat ed with the functions of osteoclast in vivo.