THE DEVELOPMENT OF THE TEETH 



l6 5 



The enamel cells about the future root of the tooth remain cuboidal or low col- 

 umnar in form, come into contact with the outer enamel cells and the two layers 

 constitute the epithelial sheath of the root which does not produce enamel prisms. 

 The Dental Papilla. — The outermost cells of the dental papilla at the end 

 of the fourth month arrange themselves as a definite layer of columnar epithelium. 

 Since they produce the dentine, or dental bone, these cells are known as odonto- 

 blasts. When the dentine layer is developed, the odontoblast cells remain in- 

 ternal to it and branched processes from them (the dental fibers of Tomes) extend 

 into the dentine and form the dental canaliculi. Internal to the odontoblast 

 layer the mesenchymal cells differentiate into the dental pulp, popularly known 



HH 



-\n(N 



Fig. 158. — Section through a portion of the crown of a developing tooth showing the various layers 

 (Tourneux in Heisler). 1, 2, cells of enamel pulp; 3, ameloblast layer of enamel-forming cells; 4, 5, 

 enamel prisms; 6, layer of dentine containing processes of 7, odontoblast cells; 8, cells of dental pulp. 



as the "nerve" of the tooth. This is composed of a framework of reticular tissue 

 in which are found blood-vessels, lymphatics and nerve fibers. The odonto- 

 blast layer persists throughout life and continues to secrete dentine so that 

 eventually the root canal may be obliterated. 



Dental Sac. — The mesenchymal tissue surrounding the anlage of the tooth 

 gives rise to a dense outer layer and a more open inner layer of fibrous connective 

 tissue. These layers form the dental sac (Fig. 159). Over the root of the tooth 

 a layer of osteoblasts or bone forming cells develops, and, the epithelial sheath 

 formed by the enamel layers having disintegrated, these osteoblasts deposit about 

 the dentine a layer of bone which is known as the substantia ossea or cement. 

 The cement layer contains typical bone cells but no Haversian canals. As the 



