1. Division of Microscopic Anatomy, Niigata University Graduate School of Medical and Dental Sciences, Niigata-shi, Japan
2. Office of Institutional Research, Hokkaido University, Kita-ku, Japan
3. Department of Pathology, Nihon University School of Dentistry, Tokyo, Japan
4. Electron Microscope Core Facility, Niigata University, Niigata-shi, Japan