1. Division of Development and Differentiation, National Institute of Neuroscience, 4‐1‐1 Ogawahigashi‐machi, Kodaira, Tokyo 187‐8502, Japan
2. Department of Cell Biology and Morphology, Akita University Graduate School of Medicine, 1‐1‐1 Hondo, Akita City, Akita 010‐8543, Japan
3. Department of Paediatrics, Jichi Medical University, 3311‐1 Yakushiji, Shimotsukeshi, Tochigi 329‐0498, Japan