Calcif Tissue Int (1990) 46:406-414

Calcified TissueInternational

9 1990Springer-VerlagNew YorkInc.

Changes in the Mode of Calcium and Phosphate Transport during Rat Incisal
Enamel Formation
Tadafumi Kawamoto and Masaharu Shimizu
Department of Biochemistry, School of Dental Medicine, Tsurumi University, Yokohama, JapanSummary. The distribution of 45Ca, 32po4, 22Na,
and calcein in the freeze-dried sections of rat lower
incisor was examined. Also, the ratio of 45Ca to32po 4 transported into the enamel at various developmental stages was studied after the simultaneous
injection of 45Ca and 32po 4. The distribution of calcein fluorescence indicated the presence of an extracellular route from capillary to enamel in the areas of both the secretory and smooth-ended ameloblasts. Autoradiograms showed that the 45Caincorporation into the enamel in the smooth-ended
ameloblast region was higher than that into thesecretory enamel, and a remarkably high incorporation was observed in the enamel of the apical twothirds of the ruffle-ended ameloblast region. Although the 32p incorporation into the enamel of thesmooth- and ruffle-ended ameloblast region washigher than in the secretory enamel, the differences
between these two regions were not so evident asthat observed in the case of 45Ca. The high labelingof 45Ca and 22Na was observed in the apical twothirds of the ruffle-ended ameloblasts. The 45Ca/
32po 4 ratio in the secretory enamel was significantly
lower than that in the blood, but in the enamel of thesmooth-ended ameloblast region the ratio was notsignificantly lower. Contrarily, the ratio in theenamel of the ruffle-ended ameloblast region wasmuch higher than that in blood. These results indicate that the mode of transport of these ions intoenamel is altered in relation to the morphologicalchanges of the ameloblasts.
Key words: Enamel formation-- Autoradiography
Calcium - - Phosphate - - Sodium.

Send reprint requests to T. Kawamoto, Department of Biochemistry, School of Dental Medicine, Tsurumi University, Yokohama, Japan.

It is well accepted that the mineralization of enamel
proceeds in two phases involving the matrixforming (secretory) and the maturation phase. In
the secretory phase, the mineralization begins
shortly after an organic matrix is secreted by theameloblasts, and further mineralization proceedsrather slowly. After the entire thickness of enamel
matrix is laid down, maturation phase takes place
wherein there is a sudden and large influx of calcium and phosphate, and a marked increase of mineralization occurs. It is very important for a thorough understanding of the mechanisms of enamelformation to clarify the control system of calcium
and phosphate transport by the enamel-forming
cells and the pathway of these ions into enamel.
Autoradiographic results [1, 2] showed that the calcium ion passed into developing enamel via theenamel organ, and the entry of the ion into enamelis controlled by the cells of enamel organ [3-5]. Inorder to define the control system and the pathwayof calcium transport from the blood to the mineralizing enamel, the location of calcium in the enamelorgan has been studied with histochemical methods[6-10], energy-dispersive X-ray spectrometry [9,11-14] and autoradiography [3, 4, 14-18]. Also, thepresence and localization of the Ca-ATPase activity
in ameloblasts and the possible role of the enzyme
in calcium transport has been intensively examined[19-24]. On the other hand, there is little information with regard to the phosphate transport [4,25-27], and the findings vary. The difference between the calcium and phosphate control systems
and their transport pathway through the enamelforming cell layers are still inconclusive.
The object in the present study was to investigate
the mode of ion transport and the pathway from theblood to the mineralizing enamel. In order to accomplish this, we examined (1) the possible existence of an extracellular route using calcein as atracer; (2) the distribution of radioactive calcium,