Carbonate Concentration, Crystal Centers, Core Dissolution, Caries, Cross Striations, Circadian Rhythms, and Compositional Contrast in the SEM

Enamel crystal centers, C03 and Mg. I propose that each enamel crystallite has a carbonate-rich center. The developmental data. Dr. Robinson's data show that the youngest enamel contains the most carbonate in relative proportion to the mineral. The youngest enamel is composed of enamel crystallite centers. Dr. Arend's results, concerned with synthetic apatite crystal growth behavior, indicate that the crystals grow long and thin like enamel crystals only if carbonate is present. Dr. Ingram showed that his calcium phosphate (hydroxyapatite) crystals would not grow in a heated system. Presumably, he had driven off carbon dioxide. Perhaps the presence of carbon dioxide (carbonate) near the growing enamel crystals permits or favors their growth. Dr. Ingram also found that bicarbonate inhibited crystal growth. The dissolution/destruction data. Generally speaking, carbonates are less stable than phosphates under electron bombardment. If you look at cross-cut enamel crystals in the TEM and cook them with the electron beam, it is the centers which "burn out" first. The size and shape of those burnt out centers more or less correspond with the first part of each crystal that was formed (D. B. Scott personal communication). Carbonates are generally more soluble than phosphates. In natural and artificial enamel caries, i.e., in mild acid dissolution conditions, central holes develop within the enamel crystals (Johnson, Archs. Oral Biol., 12:1505-1521, 1967). If the crystallite center were pure carbonate, it would be much more soluble in its own right. But even if lower amounts of carbonate were present they would account for the high strain, hollow core phenomenon described by Arends and Jongebloed (Caries Res., 11:186-189, 1977 and J. Biol. Buccale, 5:219-237, 1977). Arends also showed that only his carbonate-rich synthetic apatite crystals dissolved with a hole in the center, in the way that natural enamel crystals dissolve. The change in carbonate content during dissolution. Vogel showed with his infrared data that the relative carbonate content dropped during the first stages of dissolution of the enamel crystals corresponding to the time at which the crystal centers disappear. However, the most important data relating to these questions are those derived from wet chemical analyses (Hallsworth, et al., Caries Res., 6:156-1 58, 1972), which show that there is a striking reduction in both magnesium and carbonate in cariously involved enamel. Hallsworth, Weatherell and Robinson (Caries Res., 7:345-348, 1973) provided further documentation of the loss of carbonate during the first stages of enamel caries, including one example where the carbonate apparently dropped by 55% compared with the surrounding uninvolved tissue. Again, I point to the correlation between the loss of carbonate (and magnesium) and the loss of the crystal centers. Cross striations. During early caries and in acid dissolution of enamel, the cross striations of the enamel prisms also become very prominent (Fig. 1, Darling, Brit. Dent. J., 105: 1 19-135, 1958). I suggest that variations in the carbonate content of the enamel mineral correspond with the cross striations, and explain this acid etching effect. Cross striations can be seen in a section of perfectly normal enamel as differences in the optical density of the prisms along their long axis, but I suggest that they probably could be attributed to differences in the refractive index of the enamel mineral due to variations in its net composition, particularly with respect to carbonate. Artifactual cross striations result from the misidentification of transversely sectioned rows of prisms as longitudinally sectioned prisms (Boyde, in Tooth Enamel, Bristol: Wright, 1966). This kind of mistake