1. Nihon University Graduate School of Dentistry at Matsudo, Histology, Cytology and Developmental Anatomy, Matsudo, Chiba 271-8587, Japan
2. Department of Microbiology and Immunology, Nihon University School of Dentistry at Matsudo, Matsudo, Chiba 271-8587, Japan
3. Department of Histology, Nihon University School of Dentistry at Matsudo, Matsudo, Chiba 271-8587, Japan