1. Department of Oral Surgery, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
2. Department of Pathology, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
3. National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaragi 305, Japan
4. Kawase Veterinary Hospital, 1-64-11 Sakura, Setagaya-ku, Tokyo 156, Japan