Teeth 



495 



ferentiation, the cells acquire their func- 

 tional assignment and their appositional 

 growth potential to form enamel or dentin. 

 This potentiality may be defined in terms of 

 the amount of work capacity of the cell and 

 is expressed in an orderly sequence of events 

 and processes which occur during the stage 

 of apposition. 



The growth energy released at the initia- 

 tion of development is unorganized and must 

 be distributed according to a definite growth 

 pattern. The process of differentiation is like 

 the direction of a stage play. It occurs in that 

 period in development at which "roles are 

 assigned, cues fixed, appearances timed, and 

 the stage set" (Weiss, '39). The orderliness 

 of the actual performance (apposition) 

 which follows is entirely dependent on the 

 proper differentiation of the cells and their 

 proper environmental condition. 



MORPHODIFFERENTIATION 



This stage marks the assumption of the 

 morphological pattern of the tooth. The cells 

 of the inner layer of enamel organ arrange 

 themselves to outline the dentino-enamel 

 junction, which serves as a blueprint pattern 

 of the future form and size of the tooth. This 

 junction must be established before any 

 enamel or dentin is deposited, since these 

 structures become calcified soon after they 

 are formed and cannot change thereafter. 

 Both histo- and morphodifferentiation first 

 occur at the tip of the tooth and then proceed 

 toward the apex. 



Hertwig's Epithelial Sheath. At the margins 

 of the bell-shaped enamel organ, the inner 

 and outer layers of the enamel epithelium 

 proliferate and give rise to Hertwig's epithe- 

 lial root sheath. This epithelial sheath out- 

 lines the dentino-cemental junction and acts 

 as the blueprint pattern for the shape, size 

 and length of the future root or roots, just 

 as the inner enamel epithelium outlines the 

 shape and size of the crown. In addition, 

 the epithelial sheath initiates the differentia- 

 tion of the radical odontoblasts just as the 

 ameloblasts initiate the differentiation of 

 the coronal odontoblasts. The cementoblasts 

 probably owe their differentiation to the 

 chemical stimulus of calcified dentin. As soon 

 as the formation of the dentin and cementum 

 of the root is begun, the sheath disintegrates 

 and vestiges can be found later as epithelial 

 rests in the periodontium. 



A disturbance in morphodifferentiation 

 and also in proliferation results in a dis- 



turbance in the form and size of the dentino- 

 enamel junction. This leads to abnormal 

 forms and sizes such as the peg tooth or 

 Hutchinson's incisor (screwdriver-shaped in- 

 cisor). 



The field concept of development has been 

 applied by Butler ('39) to phylogenetic prob- 

 lems of tooth morphology. He pointed out 

 that certain tooth characters are manifested 

 in groups of teeth to a maximum degree in 

 particular key teeth in each of the incisor, 

 canine, and molar groups. Dahlberg ('45) 

 suggested that tooth anomalies are related 

 specifically to the tooth districts rather than 

 to the dentitions as a whole. Thus, certain 

 points in the human dentition are more sus- 

 ceptible to change than are others. 



APPOSITION 



In contrast to the rapid, multiplicative, 

 mitotic and celkdar type of proliferative 

 growth, appositional growth is slow, additive, 

 incremental and extracellular (Huxley, '32). 

 Apposition of enamel constitutes the fulfill- 

 ment and full expression of the growth po- 

 tential acquired by the ameloblasts during 

 histodifferentiation. 



Appositional activity of the ameloblasts 

 begins at specific sites, the growth centers, 

 and proceeds at a definite time and chro- 

 nology, at definite rates and gradients and 

 for a definite number of days, the functional 

 life span of the formative cells. The end re- 

 sult is the incremental pattern (Schour and 

 Massler, '40). 



Physiological Characteristics of Enamel and 

 Dentin. An analysis of the incremental pat- 

 tern is facilitated by three unique physiolog- 

 ical characteristics of the enamel and dentin: 



1. The Rhythmic Manner of Appositional 

 Growth and Calcification. The hard struc- 

 tures of the tooth, like the trunks of trees, 

 grow by the regular and rhythmic formation 

 of concentric layers or rings. 



2. The High Sensitivity of the Growing 

 and Calcifying Tissues to Fluctuations of 

 Metabolic Processes (Particularly Mineral 

 Metabolism). The growing tooth depends for 

 its raw materials vipon the substances elab- 

 orated by the body. Fluctuations in the 

 metabolism of the individual are, therefore, 

 reflected in the layers or rings of the tooth 

 forming at that time. The normal incre- 

 mental rings are accurate records of the 

 ontogenetic development of the tooth and 

 the normal physiological fluctuations in the 

 metabolism of the individual. 



