EXPRESSION OF SELECTED OSTEOGENIC MARKERS IN THE FIBROBLAST-LIKE CELLS OF RAT MARROW STROMA

The fibroblast-like cells in the marrow stromal system were separated from endothelial cells and macrophages by negative selection of magnet ic beads. Immunocytochemistry confirmed that these fibroblast-like cel ls expressed fibronectin and collagen Type III, but not Factor VIII an d epithelial membrane antigen (endothelial cell markers) or Mac I (mac rophage marker). The fibroblast-like stromal cells (FSC) synthesized t he insulin-like growth factors (IGF)-I and -II in amounts equivalent t o that produced by unfractionated marrow stromal cells (UMSC); in both , the concentration of IGF-II was 10 times higher than that of IGF-I, Northern analysis revealed that FSC and UMSC expressed identical patte rns of mRNAs for IGF-I and transforming growth factor (TGF)-beta(2), f or osteopontin, and for procollagen Types I and III (Type I > Type III ). Type II procollagen mRNA was not expressed in both cell populations , The TGF-beta(2) gene mRNA was expressed at a lower level by the FSC than UMSC. The pattern of gene expression in these cells is consistent with an osteoprogenitor phenotype. Both FSCs and UMSCs express parath yroid hormone (PTH) and estrogen receptor genes (rtPCR technique). The study provides additional evidence that fibroblast-like marrow stroma l cells have an osteoblast signature, and that they are largely respon sible for the osteogenic performance of cells in unfractionated marrow .