Characterization of human cultured periosteal sheets expressing bone-forming potential: in vitro and in vivo animal studies.

Our recent clinical studies have demonstrated that autologous implantation of human cultured periosteal (hCP) sheets in combination with porous hydroxylapatite (HAp) particles at the site of periodontal bone defects strikingly facilitates tissue regeneration. To better understand how the hCP sheet functions at the implantation site, we have now examined its biochemical and morphological characteristics in vitro and its ectopic osteoinductivity in nude mice. Cultured human periosteal tissue segments produced periosteal cells that migrated out from the central region within 4-8 days and grew more rapidly with longer culture times. Alkaline phosphatase activity increased in parallel with actual osteoblastic induction. Cytokine array assays demonstrated that osteoblastic induction downregulated IL-6 and thrombopoietin, but upregulated IL-8, IL-13, IGF-I and IGFBP-2 in hCP sheets. When differentiated hCP sheets were implanted alone, areas of osteoid and mineralized tissue were formed within 2 weeks, but non-induced, immature hCP sheets did not produce much mineralization. These findings suggest that mature hCP sheets potentially function not only as seeds of ectopic bone formation without the need of synthetic tissue scaffolds, but also as living drug-delivery systems, to influence cells near implantation sites by producing several important cytokines. These two major characteristics indicate that a mature hCP sheet is a promising osteoinductive biomaterial, even without conventional scaffolds for periodontal regenerative therapy.