1. Department of Veterinary Anatomy
2. Laboratory of Animal Genetics and Reproduction, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan
3. Unit of Anatomy and Cell Biology, Department of Animal Sciences, Kyoto University, Kyoto 606-8502, Japan
4. Laboratory of Experimental Animals, Faculty of Veterinary Medicine and Animal Sciences, University of Kitasato, Towada, Aomori 034-8628, Japan