1. Department of Veterinary Anatomy, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
2. Center for Experimental Animal, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113-8510, Japan
3. Department of Stem Cell Regulation, Medical Research Institute, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113-8510, Japan
4. Division of Germ Cell Biology, National Institute for Basic Biology and Department of Basic Biology, School of Life Science, Graduate University for Advanced Studies (SOKENDAI), Okazaki, Aichi 444-8585, Japan
5. Department of Anatomy, Kyorin University School of Medicine, Mitaka, Tokyo 181-8611, Japan
6. Department of Neurobiology and Anatomy, The University of Utah, Salt Lake City, UT 84132-3401, USA