1. Department of Veterinary Pathology, Faculty of Agriculture, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
2. The Institute of Environmental Toxicology, Uchimoriya-cho 4321, Mitsukaido-shi, Ibaraki 303-0043, Japan
3. Department of Biomedical Science, Faculty of Agriculture, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan