[Stratification of the experimental incipient caries (polarized light microscopy and microradiography)].

The enamel caries establishes when at least a part of enamel dissolves by the action of cariogenic bacteria which produce insoluble glucan and lactic acid. The experimental caries should be prepared at the condition which simulates the natural environment. The author made 1 mm thick sections from 29 teeth extracted for convenience of orthodontic reason. The surface was covered except the "window" which will be exposed to the acid milieu. The samples were immersed for 4 to 120 days in the gelatin added lactate buffer of pH 4.5, pH 5.0, and pH 5.5. Ground sections were made after the experiment. The structure of the experimental incipient caries was described after Darling (1963); surface layer, body of the lesion, dark zone, and translucent zone. When the section was immersed for shorter period in the buffer of pH 4.5, the lesion was made up only two layers, namely, the surface layer and the body of the lesion. This type of lesion will be called Type I, hereafter. When the action of acid was more intense, only one layer, the body of the lesion remained after the dissolution of the surface layer. This type of lesion will be called Type II. The lesion which provided with all 4 layers will be called as Type III, and the lesion which was made up of 3 layers which remained after dissolved surface layer, will be called as Type IV. Both Type III and Type IV were established after immersing for longer period in pH 5.5, although they could be observed in groups which immersed in lower pH. Of the samples examined, a few example showed a five-layered structure; the surface layer, the first body of lesion, the first dark zone, the second body of lesion, the second dark zone, and the translucent zone. This phenomenon suggests that the speed of caries development was not uniform throughout the experiment. The relationship between the depth of the lesion, and the pH and the duration of acid action was studied. In the pH 5.0 group, the development of caries lesion was temporarily arrested between 30 and 60 days. This phenomenon would be an explanation to the formation of five-layered lesion.