Extraosseous bone formation obtained by association of mesenchymal stem cells with a periosteal flap in the rat

Background: Local bone cell therapy consists in grafting a large number of osteocompetent cells in the bone defect. Mesenchymal stem cells (MSC) have been demonstrated as an attractive cell source for tissue-engineering applications because of their ability to be easily isolated and expanded from adult bone marrow and their versatility for pluripotent differentiation into mesenchymal tissues. Methods: The purpose of our work was to evaluate in vitro the osteogenic potential (proliferation and differentiation) of rat MSC cultured in monolayer conditions and encapsulated in alginate beads and in vivo the osteogenic potential of encapsulated MSC implanted at an extraosseous site associated with a periosteal flap to obtain the equivalent of a vascularized bone autograft. Results: In vitro, the encapsulation of MSC in alginate beads maintains their degree of differentiation towards the osteoblastic lineage. In vivo, standard radiographs revealed "calcifications" adjacent to the area where alginate beads had been implanted in both groups (in the presence or the absence of MSC). In the group "beads alone," histologic analysis showed that calcifications reflected only a peripheral calcification with no bone formation. On the contrary, in the group "beads + MSC," a large mineralization process took place characterized by lamellar mature bone with osteocytes after 10 weeks.