246 



THE ORGANS 



b '■ 



The DENTINE is the first of the dental tissues to become hard. Both dentine 

 and pulp develop, as noted on p. 243, from the mesoderm of the dental papilla. 

 When the latter is first formed it is of the same structure as the surrounding 

 mesoderm with which it is continuous, except that it is somewhat more dense; 

 later it assumes more the character of embryonal connective tissue, blood-vessels 

 and nerves growing into it from the underlying connective tissue. The most 

 peripheral cells of the pulp, those lying nearest the enamel organ, differentiate 

 from the rest of the pulp to form a single layer of columnar or pyramidal cells — 



the odontoblasts. The outer end of each 

 cell is broad, while the inner end which 

 contains the nucleus narrows to a point 

 from which a delicate process extends 

 into the pulp and probably anastomoses 

 with other cell processes. These cells 

 are analogous to the osteoblasts of de- 

 veloping bone and like them appear to 

 determine the deposition of lime salts. 

 The lime salts are first laid down in a 

 membrane-like structure — the mem- 

 brana preformativa — which the odonto- 

 blasts apparently form between them- 

 selves and the enamel. From this 

 membrane the transformation of the 

 connective tissue into dentine progresses 

 inward toward the pulp, additional 

 dentine continuing to be laid down in 

 layers, each new layer internal to the 

 preceding. In this way the dental 

 papilla is reduced in size to form the 

 pulp cavity. In the outer part of the 

 dentine spaces remain in which no lime 

 salts are deposited. These are the in- 

 terglobular spaces. As calcification pro- 

 ceeds the odontoblasts do not become 



fe^£Si&>iS&isiai:^rsi* 



V a'.'Ji 







Fig. 150. — From Cross-section through 

 a Developing Tooth. X 720. (Bohm and 

 von Davidoif.) Note close relationship 

 between odontoblasts and tissue of dental 

 pulp, a, Dental pulp; b, odontoblasts; c, 

 dentine; d, inner enamel cells; e, enamel 

 pulp. 



enclosed within the dentine as do the 

 osteoblasts within bone. They leave merely long slender processes, the dentinal 

 fibres, lying in minute channels through the dentine, dentinal canals, while the 

 bodies of the cells form a single layer along the inner margin of the dentine. 

 There are thus no lacunae and no cells within the dentine. This relation of 

 odontoblasts to dentine, and probably the original odontoblasts, persist, not 

 only throughout embr>'onic but through adult life. 



While the tooth lies within the gum, the somewhat condensed connective 

 tissue which surrounds it constitutes the dental sac. 



As the germs of the milk teeth develop, the dental shelf broadens by extend- 

 ing inward toward the tongue. Along this inner margin appear the germs of 

 the permanent teeth, the development of the various teeth structures from the 

 germs being identical with the process described in the case of the milk teeth. 



