Mineralization pattern and fluoride distribution of the developing and matured enameloid of the shark

Abstract

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.