THE MOUTH 



formation of the papillae, these cells undergo a differentiation into three layers. Those which 

 are in immediate contact with the papilla become elongated, and form a layer of well-marked 

 columnar epithelium coating the papilla. They are the cells which form the enamel fibers, 

 and are therefore termed enamel cells or adamantoblasts. The cells of the outer layer of the 

 special dental germ, which are in contact with the inner surface of the dental sac, presently to 

 be described, are much shorter, cubical in form, and are named the external enamel epithelium. 

 All the intermediate round cells of the dental germ between these two layers undergo a peculiar 

 change. They become stellate in shape and develop processes, which unite to form a net-work 

 into which fluid is secreted; this has the appearance of a jelly, and to it the name of enamel pulp 

 is given. This transformed special dental germ is now known under the name of enamel organ 

 (Fig. 1011). 



While these changes are going on, a* sac is formed around each enamel organ from the sur- 

 rounding mesodermal tissue. This is known as the dental sac, and is a vascular membrane 

 of connective tissue. It grows up from below, and thus encloses the whole tooth germ; as it 

 grows it causes the neck of the enamel organ to atrophy and disappear; so that all communi- 

 cation between the enamel organ and the superficial epithelium is cut off. At this stage there 

 are vascular papillae surmounted by caps of epithelial cells, the whole being surrounded by 

 by membranous sacs. 



Formation of the Enamel. The enamel is formed exclusively from the enamel cells or adaman- 

 toblasts of the special dental germ, either by direct calcification of the columnar cells, which 

 become elongated into the hexagonal rods of the enamel; or, as is more generally believed, as 

 a secretion from the adamantoblasts, within which calcareous matter is subsequently deposited. 



The process begins at the apex of each cusp, at the ends of the enamel cells in contact with 

 the dental papilla. Here a fine globular deposit takes place, being apparently shed from the end 

 of the adamantoblasts. It is known by the name of the enamel droplet, and resembles keratin 

 in its resistance to the action of mineral acids. This droplet then becomes fibrous and calcifies 

 and forms the first layer of the enamel; a second droplet now appears and calcifies, and so on; 

 successive droplets of keratin-like material are shed from the adamantoblasts and form successive 

 layers of enamel, the adamantoblasts gradually receding as each layer is produced, until at the 

 termination of the process they have almost disappeared. The intermediate cells of the enamel 

 pulp atrophy and disappear, so that the newly formed calcified material and the external enamel 

 epithelium come into apposition. This latter layer, however, soon disappears on the emergence 

 of the tooth beyond the gum. After its disappearance the crown of the tooth is still covered 

 by a distinct membrane, which persists for some time. This is known as the cuticula dentis, or 

 Nasmyth's membrane, and is believed to be the last-formed layer of enamel derived from the 

 adamantoblasts, which has not become calcified. It forms a horny layer, which may be sepa- 

 rated from the subjacent calcified mass by the action of strong acids. It is marked by the hexagonal 

 impressions of the enamel prisms, and, when stained by nitrate of silver, shows the characteristic 

 appearance of epithelium. 



Formation of the Dentin. While these changes are taking place in the epithelium to form 

 the enamel, contemporaneous changes occurring in the differentiated mesoderm of the dental 

 papillae result in the formation of the dentin. As before stated, the first germs of the dentin are 

 the papillae, corresponding in number to the teeth, formed from the soft mesodermal tissue 

 which bounds the depressions containing the special enamel germs. The papillae grow upward 

 into the enamel germs and become covered by them, both being enclosed in a vascular connective 

 tissue, the dental sac, in the manner above described. Each papilla then constitutes the forma- 

 tive pulp from which the dentin and permanent pulp are developed; it consists of rounded cells 

 and is very vascular, and soon begins to assume the shape of the future tooth. The next step 

 is the appearance of the odontoblasts, which have a relation to the development of the teeth 

 similar to that of the osteoblasts to the formation of bone. They are formed from the cells 

 of the periphery of the papilla that is to say, from the cells in immediate contact with the 

 adamantoblasts of the special dental germ. These cells become elongated, one end of the 

 elongated cell resting against the epithelium of the special dental germs, the other being tapered 

 and oftened branched. By the direct transformation of the peripheral ends of these cells, or 

 by a secretion from them, a layer of uncalcified matrix (prodentin) is formed which caps the 

 cusp or cusps, if there are more than one, of the papillae. This matrix becomes fibrillated, and 

 in it islets of calcification make their appearance, and coalescing give rise to a continuous layer 

 of calcified material which covers each cusp and constitutes the first layer of dentin. The odon- 

 toblasts, having thus formed the first layer, retire toward the center of the papilla, and, as they 

 do so, produce successive layers of dentin from then: peripheral extremities that is to say, 

 they form the dentinal matrix in which calcification subsequently takes place. As they thus 

 recede from the periphery of the papilla, they leave behind them filamentous processes of cell 

 protoplasm, provided with finer side processes; these are surrounded by calcified material, and 

 thus form the dental canaliculi, and, by their side branches, the anastomosing canaliculi: the 

 processes of protoplasm contained within them constitute the dentinal fibers (Tomes' fibers). 

 In this way the entire thickness of the dentin is developed, each canaliculus being completed 





