The progressive mineralization pattern of developing enamel of bovine permanent teeth was studied by means of microradiography, histochemistry and transmitted light microscopy. Undemineralized planoparallel longitudinal ground sections prepared by hand lapping after embed ding in polyester resin were used for the present study.
The mineralization pattern as revealed in microradiograms was investigated with the aid of two-dimensional densitometry (contour map representation by a colour television display system using a digital comparator method) and computer image processing (two-dimensional colour display and three-dimensional “birdseye image”).
During the stage of matrix formation, the enamel matrix, except for a very narrow layer adjacent to the dentin, was mineralized slightly. At the end of this stage, the inner two-thirds of fully formed matrix was a little more highly mineralized than the outer one-third. The very narrow inner-most layer adjacent to the dentin mineralized very heavily immediately after matrix formation had commeced (Figs. 1-5).
When the whole width of enaml matrix was formed, the maturation stage began and there was a secondary heavy increase in mineralization throughout the entire layer of enamel matrix (Figs. 1-5). However, the degree of mineralization and the gradient of increase were different for each layer. At the early stage of maturation, a very steep increase in mineralization appeared first in the surface layer, and then, in the inner layer except for the narrow inner-most layer which was already heavily mineralized. The highly mineralized inner layer showed a gradient of mineralization which increased towerds the middle layer (Figs. 3 and 4).
At the middle stage of maturation, the highest degree of mineralization and the steepest gradient of increasing mineralization became observable in the middle layer (Figs. 5 and 6). On the other hand, the narrow surface layer showed the least degree of mineralization and the least gradient of increasing mineralization for the entire layer of enamel.
As it entered into the late stage of maturation, the surface layer began to mineralize very quickly and finally it showed the highest degree of mineralization for the layers in contrast to the rest of the enamel in which the degree of mineralization did not increase further (Figs. 6 and 7).
In the fully matured enamel, the degree of mineralzation which was highest in the outer-most layer decreased gradually towards the inner layer except for the inner-most layer next to the dentin which showed a little higher mineralization than the inner layer (Fig. 8). Some times, the narrow surface layer was less mineralized than the adjacent middle layer even after the enamel surface was covered by cementum. This fact was probably due to the early involution of the ameloblasts (Figs. 9).
Transmitted light microphotographs of dried ground sections in air showed that almost the entire layer of enamel matrix which was transparent during the stage of matrix formation became suddenly very dark as it entered into the early stage of maturation soon after matrix formation was completed. The secondary increase of mineralization during maturation stage took place mainly within this dark area (Figs. 1-7). As the secondary increase in degree of mineralization was completed, the matrix became gradually transparent again from the inner layer towards the surface layer (Figs. 8). The dark area seemed to be related to the appearance o microspaces produced by removal of organic matrix, which facilitated a rapid growth of enamel crystals.
Serial ground sections were stained by various hisctochemical methods (Fig. 10). At the stage of matrix formation, the enamel matrix was stained very intensly by the coupled tetrazonium reaction for protein and the inner layer was stained by PAS reaction, however, the matrix except for the inner-most narrow layer was not stained by Alcian blue.