Section VII 



CHAPTER 7 



Teeth 



ISAAC SCHOUR 



INTRODUCTION 



The ontogenetic history of the tooth is an 

 interesting and fruitful chapter in develop- 

 mental histophysiology. The tooth does not 

 belong to the osseous system. It is a highly 

 specialized appendage and passes through 

 many developmental stages. These permit an 

 analysis of different types of growth and are 

 governed by definite principles the integra- 

 tion of which makes for orderliness in the 

 elaboration of the form, size and function of 

 the tooth. 



The continuously growing incisors are es- 

 pecially useful for the quantitative and 

 qualitative analysis of developmental proc- 

 esses. Thus the rodent incisor may be re- 

 garded as nature's gift to research in tooth 

 development (Schour and Massler, '49). 



The component dental structures when 

 growing are highly sensitive to physiological 

 and metabolic processes which become per- 

 manently recorded in the completed and 

 calcified enamel and dentin. The tooth is an 

 organ of mastication, but it can also be uti- 

 lized as a permanent biological kymograph 

 of the life history of the growing individual 

 (Massler, Schour and Poncher, '41). 



The tooth proper consists of the calcified 

 enamel, dentin and cementum and the soft 

 internal pulp. However, function calls for a 

 close organic integration with the svirround- 



ing gingiva, periodontal membrane and al- 

 veolar bone, and topographical correlation 

 with the adjacent and opposing teeth. This 

 section will be confined largely to a consid- 

 eration of the growth of enamel and dentin. 



The progressive development of the teeth 

 consists of the following stages (Noyes, 

 Schour and Noyes, '48) (Fig. 191): 

 I. Growth 



1. Initiation (chemodifferentiation of 

 Huxley) 



2. Proliferation 



3. Histodifferentiation 



4. Morphodifferentiation 



5. Apposition 

 II. Calcification 



III. Eruption 



IV. Attrition. 



Table 15 outlines the morphologic se- 

 quence of events and the corresponding phy- 

 siological processes. These overlap consider- 

 ably and many of them may occur at the 

 same time in different parts of the tooth. 

 Table 15 also summarizes the developmental 

 physiology of the tooth. 



GROWTH 



INITIATION 



Specific basal cells at definite sites on the 

 dental lamina of the oral epithelium become 



ABODE 

 Initiation Proliferation. Histodifferentiation Apposition v iintr 

 (Bud 3ta(^ ICap sta^e) (Bell sta(^e) . V_ 



GROWTH CALCIFICATION ERUPTION ATTRITION 



Fig. 191. Diagrammatic representation of life cycle of tooth (from Schour and Massler, '40). 



492 



