ハタネズミ (Microtus montebelli) 下顎臼歯の組織学的研究
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概要
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The morphology of lower molar teeth (rootless drycodont tooth type) of Japanese field voles was examined histologically. The results obtained were summerized as follows. 1. The dental formula of Japanese field vole was shown as follow : I 1/1 C 0/0 P 0/0 M 3/3=16 Of the molar teeth examined, the M_1 (1st molar) was the largest in size and molars gradually decreased their size as tooth positioned distally. 2. M_1, M_2 (2nd molar) and M_3 (3rd molar) were continuously growing teeth and their occulusal surfaces were completely flat, which were characteristics of drycodont type. 3. M_1 was composed of five lobes. M_2 and M_3 were composed of three lobes. 4. In M_1, the proximal portion of the 1st lobe and the lingual and buccal portions of the 5th lobe lacked the enemel layer. In M_2 and M_3, on the other hand, the proximal portions of the 1st lobes and the lingual and buccal portions of the 3rd lobes lacked the enemel layer. 5. Grooves between lobes were filled completely with filling cementum. 6. Formation of filling cementum began to start from each proximal portion of lobular grooves. 7. Filling cementum usually consisted of the cellular osseous cementum, but the cementum showing characteristics of the cartilage-like cementum or vasocementum was also observed occasionally. 8. Two types of pericoronal cementum, i. e., acellular pericoronal cementum covering the enamel surface and cellular pericoronal cementum covering directly the dentin at the enemel free areas, were observed. 9. Morphology of the cellular pericoronal cementum contained many cementum corpuscles and resembles that of secondary cementum of the rooted teeth. 10. It was suggested that cellular pericoronal cementum of Japanese field voles was highly adapted structures for masticatory pattern of herbivores.
- 1997-10-25