1. Division of Germ Cell Biology, National Institute for Basic Biology, National Institute of Natural Sciences, Aichi 444-8787, Japan
2. Faculty of Agriculture, Shinshu University, Nagano 399-4598, Japan
3. Division of Bioimaging Sciences, Center for Molecular Medicine, Jichi Medical University, Tochigi 329-0498, Japan
4. Animal Breeding and Reproduction Research Division, NARO Institute of Livestock and Grassland Science, Ibaraki 305-0901, Japan