PREFACE Vll 



coral skeletons by boring sponges is countered by extensive rebuild- 

 ing through the calcifying powers of the living coral polyps. As 

 rock-boring mussels channel their way into mineralized structures, 

 the dissolved calcareous material is deposited on the walls of the 

 burrows. In vertebrate hard tissues, redeposition of new large in- 

 organic crystals takes place, at least at the ultrastructural level, 

 within superficially altered tooth substance both in erosion and in 

 caries. Such intermittent recrystallization may in part have a "re- 

 parative" significance. For example, the large crystals filling the den- 

 tinal tubules in dental erosion could possibly explain the failure of 

 oral microorganisms to invade the dentin substance; and, similarly, 

 in dental caries the large crystals noted in partially demineralized 

 areas have been found to contain an exceptionally high amount of 

 fluoride, which presumably would make such tooth substance less 

 soluble. Moreover, when the dental enamel is exposed to a de- 

 mineralizing solution, the "first order" diffusion-controlled reaction 

 can be inhibited bv deposition of protective reaction products in 

 equilibrium with acid solutions (dicalcium phosphate and calcium 

 fluoride on the surfaces of hydroxyapatite and fluorapatite respec- 

 tively). In brief, dental erosion and caries, destructive as they ar^«^ 

 can no longer be looked upon as entirely one-way processes, at^T^rRflk 

 not from the point of view of molecular biology. 



In terms of protective mechanisms there is limited knowledge 

 regarding certain organic coatings which appear to modify the de- 

 structive processes in teeth as well as in shells, and possibly in bone. 

 Thin salivary films which cover the tooth surfaces appear to have a 

 significant bearing on the relative protection of the thin external 

 layer of enamel in early caries. Furthermore, it has been suggested 

 that dental erosion may in part be due to the absence of the pro- 

 tective action of such a salivary film. The mussels, whose rock- 

 boring capacity is assisted by a calcium-dissolving secretion, have 

 an organic protection against decalcification of the mussels' own 

 shells, these being covered by a thick periostracal horny covering. 

 When snails and octopuses erode the shells of oysters and abalone, 

 it must be presumed also that a preferential shell destruction of 

 their prey can occur only if the gastropod's own radula is protected 

 from both mechanical and chemical action through the presence of 



