DEVELOPMENT OF THE DIGESTIVE TUBE 611 



the dental papilla (fig. 288D, E). The enamel organ differentiates into three 

 layers (fig. 288E): 



( 1 ) an inner enamel layer, surrounding the dental papilla, 



(2) an outer enamel layer, and 



(3) between these two layers, a mass of epithelial cells, giving origin to 

 the enamel pulp. 



The cells of the enamel pulp eventually form a stellate reticulum. 



Development thus far serves to establish the basic mechanisms for tooth 

 development. Further development of the tooth may be divided into two 

 phases: 



(1) formation of the dentine and enamel and 



(2) development of the root of the tooth and its union with the alveolus or 

 socket of the jaw. 



The initial phase of tooth formation begins when the inner cells of the inner 

 enamel layer of the enamel organ become differentiated into columnar epi- 

 thelial cells. These cells form the ameloblasts (fig. 288E, F). Following this 

 change in the cells of the inner enamel layer, the mesenchymal cells, facing 

 the ameloblasts, become arranged into a layer of columnar odontoblasts (fig. 

 288F). The odontoblasts then begin to deposit the dentine of the tooth. The 

 initial phase of formation of dentine consists first in the elaboration of an 

 organic substance or matrix. The organic matrix then becomes impregnated 

 with inorganic calcareous materials to form the dentine, a hard, bone-like 

 substance. As the dentinal layer becomes thicker, the odontoblasts recede 

 toward the dental pulp of the papilla. However, the odontoblasts do not with- 

 draw entirely from the dentine already formed, as elongated, extremely fine 

 extensions from the odontoblasts continue to remain within the dentine to 

 form the dentinal fibers (fig. 286B). 



Dentine is deposited by the odontoblasts; the ameloblasts deposit the enamel 

 layer in the form of a cap, surrounding the dentine (fig. 286A, B). In doing so, 

 a slight amount of organic substance is first deposited, and then the ameloblast 

 constructs in some way a prismatic column of hard calcareous material at 

 right angles to the dentinal surface (fig. 286B). The columnar prisms thus 

 deposited around the dentine form an exceedingly hard cap for the dentine. 

 As in the formation ,of the dentine, the elaboration of enamel begins at the 

 crown or distal end of the tooth and proceeds rootward. 



The development of the root of the tooth and its union with the jaw socket 

 (alveolus) is a complicated procedure. This phase of tooth development is 

 accomplished as follows: The mesenchyme, with its contained blood vessels 

 and nerves of the dental papilla, lies within the developing dentinal layer of 

 the forming tooth. At the base of the tooth (i.e., the end of the tooth opposite 

 the crown), the mesenchyme of the dental papilla is continuous with 



