ULTRASTRUCTURAL AND CHEMICAL STUDIES ON CARIES 193 



pattern of distribution of mineral. The greatest concentration was 

 invariably found in areas lining the canals, often forming a hyper- 

 mineralized pericanalicular laver interposed between the lumen of 

 the canal and the extensively demineralized intercanalicular matrix 

 (Figs. 13 and 14). It is not known whether this layer represents a 

 persistence of the pericanalicular zone found in sound dentin ( Fig. 

 15), or whether its occurrence is related to the carious process. 



The most surprising finding in the studv of carious dentin was the 

 presence of distinct, relativel)' large crystallites in both pericanalicu- 

 lar and intercanalicular areas (Figs. 16 and 17). Stereoscopic ob- 

 servations (Johansen and Parks, 1960) showed that these structures 

 resembled the crystallites of sound dentin in size and in their plate- 

 like shape (Figs. 18 and 19). This unexpected finding led us to pro- 

 pose that these crystallites persisted in the lesion because they were 

 chemically different from the crystallites which had been dissolved. 

 It was considered that fluoride preferentially deposited in the lining 

 of dentinal canals and in persisting crystallites of intercanalicular 

 areas could be responsible for this phenomenon (Johansen and 

 Parks, 1959, 1961). 



Crystallites of carious dentin were also found to be closely as- 

 sociated with collagenous fibrils isolated from homogenate prepara- 

 tions. Usually the crystallites were oriented with their long axes 

 parallel to the long axis of the fibril (Fig. 20) in an arrangement 

 similar to that seen in the heavily mineralized fibrils of sound dentin 

 (Fig. 21) isolated by similar techniques. 



Fig. 1. Section of undecalcified carious enamel showing enamel rods (R) 

 outlined by wide rod sheath areas (RS). The sparse distribution of crystallites 

 indicates extensive demineralization. (X approx. 14,250.) 



Fig. 2. Section of undecalcified carious enamel. Crystallites (AC) at the 

 periphery of rods are generalK' more conspicuous than those within rods. ( X 

 approx. 11,400.) 



Fig. 3. Thin fragment of carious enamel exhibiting silver particles (Ag) 

 within spaces in the tissue. The carious lesion was exposed to silver nitrate 

 solution to test the permeability of the tissue. Dry preparation of specimen. ( X 

 approx. 66,500.) 



Fig. 4. Section of developing deciduous himian enamel, illustrating the gen- 

 eral organizational pattern of the tissue. Rods (R) delimited by rod sheaths 

 (RS) and separated by so-called interrod areas (IR) are seen. (X approx. 

 15,200.) 



