DENTAL HARD TISSUE DESTRUCTION 141 



teeth of the primate species, as reviewed elsewhere ( Sognnaes, 1955, 

 1957a, 1957Z?,1960Z7). 



Precise quantitative in vivo tracer studies in monkeys, utiHzing 

 high levels of various radioactive isotopes, have conclusively demon- 

 strated the capacity of the enamel to take up radioactive elements 

 from the salivary circulation and to a lesser degree from the internal 

 connective tissue fluid of the pulp, depending upon the stage of 

 dental calcification and eruption (Sognnaes et al., 1955). 



Of particular pertinence to the present discussion is the information 

 obtained by studies on phosphate deposition in enamel as illustrated 

 in Fig. 29. The arrows in this x-ray of the skull of a young rhesus 

 monkey point to the relative degree of radioactive phosphorus up- 

 take (8 days after injection of 5 millicuries of P^-) in the enamel of 

 two unerupted and two erupted teeth (the corner figures, per cent 

 of dosage X 10^ per mg of enamel, give three values, namely for 

 surface, subsurface, and internal enamel, reading from top to bot- 

 tom ) . It will be noted that of the two unerupted teeth, the second 

 permanent molar (upper right) is slightly farther advanced in cal- 

 cification (i.e. less "radioactive") than the first permanent premolar 

 (upper left). The slightly younger tooth shows a radioactive phos- 

 phorus uptake per milligram of enamel more than twofold higher 

 in the peripheral layer, more than threefold higher in the subsurface 

 and internal layers. The two erupted teeth, which were exposed to 

 the salivary environment, show that the central permanent incisor 

 (lower left arrow), which had been exposed to the salivary mouth 

 environment for a slightly shorter time than the first permanent 

 molar (lower right arrow), has twice as high a radioactive phos- 

 phorus uptake in the peripheral enamel layers as does the "older" 

 molar enamel. 



In view of the fact that the two unerupted teeth were surrounded 

 by the same internal environment (connective tissue fluid), it can 

 be concluded that their marked difference in radioactivity must be 

 due to considerable differences in the enamel structure itself. Pre- 

 sumably the permeability of the younger tooth enamel is greater, 

 and the crystals are still relatively smaller and hence exhibit a rela- 

 tively larger total surface for exchange as well as active growth of 

 the inorganic apatite crystals. 



