ALTERATIONS IN THE EXPRESSION OF OSTEONECTIN, OSTEOPONTIN AND OSTEOCALCIN MESSENGER-RNAS DURING THE DEVELOPMENT OF SKELETAL TISSUES IN-VIVO

Heterogeneity in the expression of three members of non-collagenous ma trix proteins in osteogenic and chondrogenic development in vivo were investigated by in situ hybridization. Sections of several skeletal ti ssues from mice at various stages of development were hybridized with digoxigenin-labeled complementary RNA probes encoding osteonectin (Osn ), osteopontin (Osp) and osteocalcin (Osc). In calvariae and mandibula e, Osn messenger RNA (mRNA) was detected in cells in pre-osseous and o sseous tissues before mineralization. Osp mRNA was found in cells atta ched to the mineralized bone matrix together with Osn mRNA followed by the expression Osc mRNA. In long bones, mRNAs for Osn, Osp and Osc we re sequentially expressed with bone development from primary spongiosa to diaphyseal bone. In growth cartilage, Osn mRNA was observed in cho ndrocytes in non-mineralized cartilage, whereas Osp mRNA was detected in hypertrophic chondrocytes in mineralized cartilage matrix with a ch aracteristic switch in expression. Osc mRNA was not detected in any ch ondrocytes. These results indicate that osteogenic differentiation in bone development in vivo is characterized by the sequential expression of these three genes, and suggest that these genes are expressed diff erentially and specifically, in association with extra-cellular matrix mineralization.