カイウサギ下顎臼歯の組織構造とその発生に関する研究

Abstract

The developmental process of mandibular molar teeth was studies histologically using rabbits at its varying developmental stages from 15 fetal days to 30 postnatal days and adulthood. Results were summarized as follows. Histology 1. Each molar tooth was characterized by the presence of a large indentation beginning from the buccal surface and dividing the tooth body into mesial and distal halves which were connected at the middle of the lingual surface. Both halves of the tooth were consisted of greater part of dentin which was filled with an osseous cementum substance which fused itself with the enamel layers of both halves of the tooth. 2. Of the molar teeth examined, the 1st premolar was the largest in size and most complex in structure with two indentations of small size, one incised on the buccal side of the mesial half of tooth and other on the middle of the mesial side of tooth. These indentations were also filled with an osseous cementum substance. 3. In the 1st premolar, the distal half of the crown lacked the enamel layer at distolingual side, but the rest of molars lacked the enamel layer even wider so that the lacked area extended from the middle of the lingual side toward the mesial side of the mesial half of the crown. The cementum substance directly covered the dentin layer in such regions. 4. The structure of the basal end of individual molars except the 1st premolar showed an incorporation of two tooth anlages. This means that, located in the middle of the basal end mesio-distally, there was present the compound enamel organ that is responsible for the formation of early phages of enamel in both distal side of the mesial half and mesial side of the distal half of the individual teeth. The tooth anlage of the mesial half of the 1st premolar was further divided into two parts, representing future cusps. The basal end was devoid of enamel and osseous cementum substances, and instead there was seen the cementblastic tissue, the dental sac. Genesis 1. By 15 fetal days the dental lamellae of deciduous molars (Pm_1, Pm_2) came into existence, and by 25 fetal days hard tissues were seen to be formed in these lamellae. Eruption of the newly formed teeth started by 3 postnatal days. On the other hand, there were seen signs of resorption in the roots of the teeth that were just being formed. 2. By 5 postnatal days, a greater part of the tooth crown came into eruption, but the root that had been well grown was largely resorbed, except its apical part which was yet to be fully completed. As mechanisms of the root resorption, two kinds of process were observed, of which the one acting through the osteoclasts that attacked from outside and the other from inside the root through spontaneous decalcification of the pulpal wall. 3. Over 20 days after birth the deciduous molars began to be shed and these were replaced by the rootless successors (PM_1, PM_2). 4. Order of these rootless teeth to erupt was as follows : M_1 (15 postnatal days), M_2 (20 postnatal days), PM_2, PM_1 and M_3. 5. Through entire process of development as above described, it was observed that no enamel was formed at every cuspal tips for both rooted and rootless teeth.