[in Japanese]

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[in Japanese]

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[in Japanese]

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The fact that the enameloid of shark teeth contains fluoride as high as about 3.3 per cent in the form of fluorapatite has been previously pointed out by the several investigators. In the present investigations, the distribution of fluoride in the developing and matured enameloid of the shark (Isurus glaucus, Blue shark, Mako) was investigated by means of the electron microprobe, in order to verify the mechanism of fluoride incorporation into the enameloid.
The undemineralized ground sections (about 70 micron thick) including a row of tooth germs (from the early stage to the late stage, of development) and matured tooth were made. They were, at first, microradiographed and, then, were subjected for the electron microprobe analysis. The condition of analysis are shown in Table 1.
The enameloid organic matrix begins to mineralize throughout the whole layer immediately after it was fully formed. However, the gradient of mineralization increase is different for the layers in the matrix (Fig.1, left, and Fig.5). It seems that the mineralization of enameloid has been completed before the dentin is formed on the inner wall of the enameloid (Fig.1, middlle and right). The mineralization degree in the fully matured enameloid is highest in the surface layer and decreases gradually towards the enameloid-dentin junction (Fig.6, c-d). The tubles containing the cytoplasmic processes of odntoblasts are observed as radiolucent spaces in the inner-half layer (Fig.6, a-d).
The fluoride concentration has been already relatively high in the enameloid (higher than 2 per cent in the middle layer) at the early stage (Fig.7, a). Then, as it approaches to the middle stage, the steep increase of fluoride concentration becomes observable in the outer-half layer (Fig.7, b). Finally, in the fully matured enameloid, the fluoride concentration is highest in the surface layer (higher than 3.5 per cent at the level of incisal one-third) and decreases gradually towards the enameloid-dentin junction (about 1.7 per cent)(Fig.7, c-f). However, in the dentin, the fluoride concentration is very low (lower than 0.5 per cent), even in the erupted tooth. Magnesium concentration is about 4 times higher in the dentin than in the enameloid.
In the fully matured and unerupted enameloid, the fluoride concentration in the surface layer is highest at the incisal tip (about 3.7 per cent) and decreases gradually towards the cervical region. On the other hand, the fluoride concentration in the inner-most layer shows almost the same degree throughout all level, except the incisal portion in which it is high as at the surface layer (Fig.8).
The results of present investigations seem to suggest that the enameloid forming cells have a peculiar ability to concentrate the fluoride in the enameloid.

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Human deciduous canines extracted for orthodontic reason were used in this investigation. The extracted teeth were fractured and immediately placed in 2% glutaraldehyde in phosphate buffer (pH 7.4) at 4° for 2 hours, and postfixed in barbital acetate buffered 2% osmic acid at 4° for 2 hours. After fixation, the specimens were washed in buffer solution and dehydrated in ethanol. They were dried by the critical-point technique after placed in iso-amylacetate. The dried specimens were coated with gold and carbon while rotating in a vacuum evaporator and examined in ASM-ST scanning electron microscope (SHIMAZU).
1. VonKorff fibrils of fibrous bundle penetrated from the pulp to the surface of the predentine where they spread out into fan-shaped arrangement and mingled with the matrix fibers of the predentine.
2. Von Korff fibrils were twisted and showed a marked spiralling because of passing through the odontoblast layer.
3. Von Korff bundles were coated with membranous structure and became hollow like guttershaped. Consequently, it was suggested that a cross section of Korff's fibrils made a present of crescend-shaped.
4. The bundles of fine fibers which were 3 microns in diameter and parallel to the odontoblastic process and extending from pulp across the mineralizing front of dentine without spreading out into fan-shaped arrangement, were observed. In many cases, Korff's fibrils appeared to spiral, but sometimes they were observed to be straight.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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Responses of trigeminal motoneurons were studied by intracellular recording in immobilized cats to repetitive stimulation of the orbital gyrus and the lateral amygdaloid nucleus which induced rhythmical masticatory movements before immobilization, with the following results:
1) There was a close parallelism between the pattern of masticatory movements before immobilization and that of intracellular potential changes after immobilization which were induced by either orbital cortical or lateral amygdaloid stimulation.
2) Orbital cortical stimulation induced an alternating masticatory rhythm of hyperpolarization and depolarization in jaw-closer motoneurons. Spike potentials were triggered and superimposed on the depolarizing potentials in some cases, but usually hyperpolarizing potentials were more prominent. Similarly, orbital cortical stimulation induced periodical depolarizing potentials superimposed by bursts of spikes in jaw-opener (anterior digastric) motoneurons. The rhythm of depolarizing potentials corresponded to that of mastication. No or little hyperpolarizing potential was seen between successive depolarizing potentials.
3) Repetitive stimulation of the orbital gyrus, which induced discharges in the anterior digastric nerve with masticatory rhythm after immobilization, evoked no response in supratrigeminal neurons.
These results were discussed in relation to their functional significance in the performance of mastication as well as to the neuronal mechanisms of cortically induced masticatory rhythm in trigeminal motoneurons.

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