L'LTRASTRrCTURAL AND CHEMICAL STUDIES ON CARIES 205 



TABLE VI. Trace Elements of Sound and Soft Carious Human Dentin 

 (Ash weight; means of 12 samples. From Johansen, Steadman, and Steadman, 



to be pubHshed) 



1000 microns be\'ond the location of the l^acteiial plaque on the 

 tooth surface (Sognnaes, 1962). Such lesions frequently show an 

 intact and densely mineralized surface laver, while deeper regions 

 may be extensively but not uniformly demineralized (Gustafson, 

 1957 ) . The electron microscopic findings show that apparent regional 

 differences in degree of demineralization extend to the ultrastruc- 

 tural level. In enamel it was occasionally seen that crystallites at 

 the periphery of rods were better preserved than the majority of 

 crystallites within rods. Similar advanced demineralization of prism 

 cores has also been noted in microradiographs (Darling, chapter 6 

 in this volume). The electron microscopic observations on dentin 

 further showed more extensive demineralization in intercanalicular 

 areas than in pericanalicular zones. On the basis of the latter ob- 

 servations it was hypothesized that the persisting crystallites in these 

 regions were altered chemically and had become more resistant to 

 dissolution (Johansen and Parks, 1959, 1961). This hypothesis is 

 supported by the chemical data previously discussed (Johansen and 

 Nordback, 1962) and summarized in this report. 



It is known that the surface layer of enamel shows an increase in 

 fluoride and a decrease in carbonate with age (Brudevold et al., 

 1956; Little and Brudevold, 1958). This phenomenon may be part 

 of a slow "maturation process" of dental tissues, not necessarily 

 limited to the enamel surface. Deeper within the tooth, correspond- 

 ing changes may occur on a limited scale in areas of relatively 

 greater permeability. In enamel it is likely that rod sheath areas, 

 because of their higher organic content, represent pathways of dif- 

 fusion. In dentin, pericanalicular zones would appear to be more 

 directly exposed to tissue fluid than would intercanalicular matrix. 



