1. Division of Fixed Prosthodontics, Department of Restorative Dentistry, Tohoku University Graduate School of Dentistry, Seiryou 4-1, Aoba-ku, Sendai 980-8575, Japan
2. New Industry Creation Hatchery Center, Tohoku University, Aoba 6-6-10, Aramaki, Aoba-ku, Sendai 980-8579, Japan
3. Tissue Culture Laboratory, Showa University Fujigaoka Hospital, Fujigaoka 1-30, Aoba-ku, Yokohama 227-8501, Japan