Characterization of matrix-induced osteogenesis in rat calvarial bone defects: II. Origins of bone-forming cells

Two experimental models that separated demineralized bone matrix (DBM) impl ants from the host bone were utilized to identify the origins of bone-formi ng cells in the repair of calvarial defects in rats. Rat DBM, Guanadine HCl (Gdn-HCl) extracted insoluble residue of DBM, and Gdn-HCl extracted insolu ble DBM to which the dialyzed Gdn-HCl extract was added back, were implante d in the two models which prevented cells of the adjacent host bone from pa rticipating in the repair. in addition, cells in the dura and in the subcut aneous tissue overlying the calvarial defect were locally labeled with H-3- thymidine to identify the origins of those cells that were stimulated to di vide and differentiate to osteoblasts. Histological studies of the temporal events that occurred during the healing process in these defect models, co mbined with 3H-thymidine labeling demonstrated that the osteoblasts induced by DBM were initially derived from undifferentiated mesenchymal stem cells of the dura and later augmented by cells in the overlying connective tissu e covering the defect, and not from cells in the cranial bone surrounding t he circular defect. The cells of both dura and subcutaneous tissue were sti mulated to proliferate and differentiate principally to osteoblasts and to a very much lesser extent to chondroblasts by DBM and by reconstituted comp onents of DBM after Gdn-HCl extraction. Gdn-HCl-extracted insoluble DBM fai led to induce bone or cartilage. These results indicate that the cytokines or other factors present in DBM are required to induce bone-forming cells d erived from the dura and the overlying connective tissue for the repair of the calvarial defect.