1. Laboratory of Anatomy, Department of Basic Veterinary Sciences, Faculty of Veterinary Medicine, Hokkaido University , Sapporo, Hokkaido 060-0818 , Japan
2. Laboratory of Agrobiomedical Science, Faculty of Agriculture, Hokkaido University , Sapporo, Hokkaido 060-8589 , Japan
3. Department of Laboratory Animal Medicine, Research Institute, National Center for Global Health and Medicine , Tokyo 162-8655 , Japan
4. Laboratory of Laboratory Animal Science and Medicine, Department of Applied Veterinary Sciences, Faculty of Veterinary Medicine, Hokkaido University , Sapporo, Hokkaido 060-0818 , Japan