Teeth 



497 



done by the cell. The formative life span in 

 days may then be obtained by dividing the 

 length of the enamel rod by the daily rate of 

 formation. Thus, L (length of enamel rod) = 

 G. P. (growth potential) = (Daily) rate X 

 life span, = R X T = Time of cellular activ- 

 ity (functional life span). 



In the dentin the length of the dentinal 

 tubule represents the growth potential or 

 growth work of the odontoblasts. Such quan- 

 titative stvidies permit a detailed analysis of 

 the growth pattern and size and form of dif- 

 ferent classes and types of teeth. 



It is of interest to note that these rates 

 and gradients of growth are not readily al- 

 tered by environmental factors. Characteris- 

 tic gradients in different regions of the same 

 tooth and in different classes and types of 

 teeth appear to correspond with their par- 

 ticular form and contour. This conforms with 

 the statement of D'Arcy Thompson ('17): 

 "A very large part of the specific morphology 

 of the organism depends upon the fact that 

 there is not only an average rate of growth 

 common to the whole, but also a variation 

 of rate in different parts of the organism. 

 The smallest change in the relative magni- 

 tudes of these partial or localized velocities 

 of growth will soon be manifested in more 

 and more striking differences of form." 



The functional life span of the amelo- 

 blasts shows wide gradients, with a maximal 

 life span for the cells over the growth center 

 and a minimal one for those near the ce- 

 mento-enamel junction. While the odonto- 

 blasts, in contrast to the ameloblasts, persist 

 and function throughout life, that period 

 of their activity which is responsible for 

 the formation of the primary dentin tends 

 to be constant (in man, about 350 days 

 for deciduous and about 700 days for 

 the permanent teeth). Thus the presence 

 and morphology of the pulp can be ex- 

 plained on the basis of the limited and 

 decelerating rate of appositional activity of 

 the odontoblasts. 



CALCIFICATION 



Calcification consists of the deposition of 

 mineral salts and their crystallization. It is 

 a process which does not involve a change 

 in size, but a reorientation of molecular 

 structure and content of the deposited matrix 

 leading to increased polymerization of the 

 ground substance and the precipitation of 

 minerals. 



The enamel and dentin in most species 

 show a common basic incremental calcifica- 



tion rhythm which recurs at intervals of ap- 

 proximately 16 micra. The common basis of 

 this 16 micron calcification unit is possibly 

 associated with the physicochemical factors 

 concerned in calcospherite formation and the 

 Liesegang ring phenomenon. Striae of 

 Retzius in the enamel and Owen's lines of 

 contour in the dentin constitute physiologi- 

 cal or pathological accentuations of the nor- 

 mal incremental rings (Schour and Hoff- 

 man, '39). 



In the dentin, calcification follows apposi- 

 tion in close succession (one day interval in 

 the rat). In the enamel, the matrix first con- 

 sists of organic substance (and water) and 

 mineral salts in the ratio of two to one. 

 During the secondary or final calcification 

 the organic substance becomes increasingly 

 impregnated by mineral salts until the ma- 

 ture enamel consists of 96 to 98 per cent 

 inorganic material. 



ERUPTION 



Eruption is the process by which the tooth 

 migrates from its intraosseous position within 

 the jaw into the oral cavity in order to reach 

 and maintain articulation. The piercing of 

 the tooth through the oral mucosa is only 

 a momentary and transitory event. Eruption 

 continues throughout the life of the tooth. 



In the rat, the rate of eruption is about 

 2 mm. per week in the upper incisors, and 

 about 3 mm. in the lower. Studies in the 

 rabbit show that most of the eruption occurs 

 when the animal is at rest and the incisors 

 are out of occlusion (Rink, '29). Experi- 

 mental removal of the opposing incisor re- 

 leases the eruption potential and results in 

 the doubling of the rate of eruption. 



ATTRITION 



Attrition may be defined as the normal 

 wearing of the teeth due to functional ac- 

 tivity. Attrition is a degrowth process and 

 provides an exception to the rule that tooth 

 development proceeds without influence of 

 function (note the development of teeth in 

 dermoid cysts). The charting of the rate 

 of attrition results in a straight line, sug- 

 gesting that it is a mechanical process which 

 is independent of growth (Hoffman and 

 Schour, '40). The continuous process of at- 

 trition is compensated by erviption and serves 

 to regvilate articulation. In some species, as 

 in the herbivora and rodentia, the teeth are 

 made functional by attrition. 



In the rodent incisor, the rate of eruption 



