1. Department of Laboratory Animal Science, Ohu University, School of Dentistry, Tomita-machi, Koriyama 963, Japan
2. Department of Oral Biochemistry, Ohu University, School of Dentistry, Tomita-machi, Koriyama 963, Japan
3. Laboratory Animal Research Center, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108, Japan