Two different types of carriers combined with bone morphogetic protein (BMP) were implanted in the dorsal subcutaneous tissue of the rat in order to study and compare the osteogenesis-inducting activity and histological patterns. The carriers were insoluble bone matrix (IBM) and porous particles of hydroxyapatite (PPHAP). Only carriers were implanted in control studies. Each composite and its surrounding tissues were removed after 1, 2 and 3 weeks and were observed by routine histology. Our results demonstrated that BMP-IBM induced osteogenesis in s process resembling endochondral ossification and BMP-PPHAP induced osteogenesis in a process resembling intramembranous ossification. BMP-carrier composites differentiated immature mesenchymal cells to either osteogenetic or chondrogetic cells, depending on the physicochemical nature of the carrier that affected the tissue microenvironment for cell differentiation.
