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Hypodontia of permanent teeth was evaluated from orthopantomograms of 2072 apparently healthy pediatric patients at The Hospital of Nihon University School of Dentistry at Matsudo. The prevalence of congenitally missing teeth (CMT) was 8.7% in boys and 10.8% in girls, and 9.4% for both sexes combined. Most cases (67.8%) involved either one or two missing teeth. There were in total 574 CMT, and on average 2.8 teeth were missing per child. The most commonly absent tooth was the mandibular second premolar. On the other hand, no first molars were missing in any case. A high frequency of CMT mandibular incisors (18.82%) was observed, and this seems to be a characteristic peculiar to individuals of Asian ethnicity. Oligodontia (6 or more CMT excluding the third molar) ranged from 6 to 14 teeth, with a prevalence of 1.4% in general: 1.8% for girls and 0.9% for boys. Symmetry of CMT was predominant: 214 pairs for bilateral symmetry and 107 pairs for symmetry between two antagonistic quadrants. The distribution of CMT between maxillary and mandibular hypodontia in the right and left quadrants for boys and girls no had significant association (P < 0.05).

We report an application of the interval mapping technique using DNA pools of inbred mice. The latest challenge in molecular genetics is to use microsatellite markers for gene mapping and cloning. Little is known about the interval mapping technique for detecting candidate linkages in inbred mice. We investigated the optimum interval length of microsatellite markers for gene mapping using DNA pools on mouse chromosomes, and found that between 25 and 35 centi Morgans (cM) was sufficient. We estimated that at least two to four microsatellite markers required for interval mapping should be present on each chromosome. The number required would depend entirely on the linkage map.

MRL/l strain mice, which possess a lymphoproliferative gene inducing swelling of systemic lymph nodes, develop a SLE (systemic lupus erythematosus)-like syndrome at around 8 w of age. MRL/n mice, which carry 99.6% of the genes of MRL/l mice, lack the gene for lymphoproliferation and exhibit only a slight degree of lymph node swelling late in life. This study investigated whether the salivary immune response caused by Streptococcus mutans(S. mutans) infection prevented dental caries in MRL/l and MRL/n mice after 8 w of age. A total of 10 MRL/l mice and 10 MRL/n mice were fed a commercial pellet diet without sucrose until 74 d of age, and then fed Diet 2000 containing 56% sucrose ad libitum from 75 to 130 d of age. On d 75, both strains of mice were inoculated with S. mutans JC-2 for 7 d. At 130 d of age, saliva samples were collected and caries scores were assessed. The results obtained suggested that the salivary immune response was one of the most important factors regulating caries occurrence.

Inbred mice are a suitable material for genetic studies, and mandibular shape in particular provides a highly quantitative hereditary trait. We investigated which genetic trait in F1, F2 and N2 hybrid mice was most strongly affected by the presence of a large or small mandible in the parents. Ten C57BL/6By strain mice as parents with a small mandible and 10 MRL/n strain mice as parents with a large mandible were employed. Twenty-five (C57BL/6By male X MRL/n female) F1 and 67 F2 hybrids, and 28 (F1 male X C57BL/6By female) N2 backcross hybrids were obtained by laboratory mating. The inter-landmarks of the right mandible were measured by an electronic digitizer. Each mean value of horizontal dimensions in F1 mice resembled that in MRL/l mice, and that in N2 mice was intermediate between C57BL/6By and MRL/n mice. On the other hand, the mean values of vertical dimensions in F1, F2 and N2 hybrids were intermediate between those of C57BL/6By and MRL/n mice. Hence we suggest that horizontal dimensions are predominantly inherited by mice with a large mandible, and that vertical dimensions show intermediate inheritance between mice with large and small mandibles in the C57BL/6By and MRL/n strains.