496 



Special Vertebrate Organogenesis 



Accentuations of these rings may occur at 

 any given time as a result of severe met- 

 abolic fluctuations or disturbances. They are 

 seen as the striae of Retzius in the enamel 

 and Owen's lines of contour in the dentin 

 and may be of physiological or pathological 

 origin. The neonatal ring, for example, re- 

 flects the physiological readjustments coin- 

 cident to birth and is the result of the brief 

 neonatal arrest in growth (Schour, '36). An 

 analysis of various experimental endocrine 

 and vitamin disturbances shows that each 

 particular dysfunction produces character- 

 istically disturbed incremental rings which 

 are superposed on the basic formative pat- 

 tern. The annual rings of the tree similarly 

 reflect the variations and vicissitudes in cli- 

 mate that the tree had experienced during 

 its growth period. 



3. The Permanence of the Tissues. Enamel 

 once completely formed and calcified can be 

 destroyed by oral environmental factors but 

 not by systemic alterations. Although com- 

 pleted, the dentin of deciduous teeth under- 

 goes physiological resorption. Enamel, in 

 fact, has lost its formative organ and has not 

 even the power of repair. All available evi- 

 dence shows that the dentin, as well as the 

 enamel, is not subject to calcium withdrawal. 

 These tissues serve, therefore, as permanent 

 records of physiological or pathological dis- 

 turbances of metabolism that may occur 

 within the individual during their formative 

 and calcifying stages. This permanence of 

 structure is not found in bone althovigh it 

 also grows in an appositional manner and 

 registers within its structure the effects of 

 disturbances in body metabolism. The rec- 

 ords in bone are erased by constant resorp- 

 tions and reconstructions. Bone thus possesses 

 two of these physiological characteristics of 

 the tooth but lacks the third — its perma- 

 nence. 



The Growth Centers. The dentino-enamel 

 junction is characterized by definite high 

 points which correspond to the number of 

 cusps (in the posterior teeth) or lobes (in 

 the anterior teeth). Amelogenesis and den- 

 tinogenesis, just as proliferation and histo- 

 differentiation, begin at these individual 

 points and proceed at a specific rate and 

 gradient of growth. Each summit on the 

 dentino-enamel junction thus acts as an in- 

 dividual growth center from which the 

 growth begins and radiates outward in a 

 definite growth plan. 



The Incremental Cones. Beginning at each 

 growth center, sviccessively adjacent amelo- 

 blasts begin their formation at successively 



later intervals, possibly a day apart. The 

 cellular activity spreads peripherally along 

 the dentino-enamel junction like a ripple 

 will spread from a pebble dropped into calm 

 water. Each ameloblast proceeds outwardly 

 away from the dentino-enamel junction at its 

 own characteristic rate and gradient of 

 growth until the required length of the 

 enamel rod is reached. Similarly each odon- 

 toblast recedes inwardly away from the 

 dentino-enamel junction. Any given in- 

 cremental layer of enamel or dentin assumes 

 in three dimensions a conical form. Its apex 

 is directed occlusally and its base rests upon 

 the dentino-enamel junction. These incre- 

 mental layers are apposed at each growth 

 center, one over the other in the enamel and 

 one within the other in the dentin. The 

 resultant incremental growth pattei'n con- 

 sists of a series of gnomonic curves whose 

 form is determined by the dentino-enamel 

 junction. 



When the incremental layers of adjacent 

 growth centers meet, as in the molar teeth, 

 the subsequent incremental layers are de- 

 posited as fusions of individual cones. 



Rates and Gradients of Appositional Growth. 

 Vital staining with sodium fluoride (Schour 

 and Poncher, '37) and alizarine red S 

 (Schour et al., '41) offer a ready method for 

 measuring the rate of apposition. Each injec- 

 tion produces a distinct ring in the enamel 

 or dentin formed at the time. The distance 

 between two successive experimental rings 

 represents the amount of deposition during 

 the time interval that elapsed between the in- 

 jections. 



In human teeth, the average rate of apposi- 

 tion of enamel and dentin is approximately 4 

 micra per day. This rate, however, decreases 

 as one proceeds from the incisal or cuspal 

 tip toward the gingival level {locus gradi- 

 ent) ; from the anterior incisors to the pos- 

 terior molar teeth {anteroposterior gradient) ; 

 and within the same cell, from the begin- 

 ning of functional activity toward its ter- 

 mination {age gradient). As Thompson ('17) 

 has pointed out, the method of appositional 

 growth combined with growth gradients may 

 result in a spiral form which is especially 

 evident in the rodent incisor. 



Appositional Growth Potential and Forma- 

 tive Life Span. In the tooth it is possible to 

 measvire the growth potential of the amelo- 

 blast and to assess its functional life span 

 (Massler and Schour, '46). Since only one 

 ameloblast is responsible for a given enamel 

 rod, its length may be taken as a measure 

 of the growth potential and the growth work 



