140 



NORMAL HISTOLOGY. 



a transitional zone known as the intermediate layer. This is best marked 

 over the upper part of the crown, at the sides thinning out and entirely 

 disappearing at the margin of the enamel-organ. The inner layer of the 

 enamel-organ comprises a single row of closely set tall columnar elements, 

 the enamel-cells > also called adamantoblasts or ameloblasts, through whose 

 active agency the enamel is produced. The ameloblasts are best developed 

 over the top of the dental papilla, where they measure from 25-40 /* in 

 length and from 4-7 p. in breadth. They possess oval nuclei that usually 

 lie close to the outer ends of the cells. The ameloblasts are united by a 

 small amount of cement-substance and defined from the intermediate layer 

 by a distinct border. Over the sides of the dental papilla, corresponding 

 to the lateral limit of the future crown, the ameloblasts gradually diminish 

 in height until they are replaced by low cuboidal cells which, at the margin 

 of the enamel-organ, are continuous with the epithelium of the outer layer. 

 Preparatory to the formation of the dentine of the root of the tooth, this 

 margin of the enamel-organ grows towards the base of the elongating dental 

 papilla, which is in consequence embraced by the extension of the enamel- 

 organ. This investment is known as the epithelial sheath (Fig. 177, D), 

 a structure of importance in determining the form of the tooth, since it serves 

 as a mould in which the dentine is deposited; there is, however, insufficient 

 evidence to regard the epithelial sheath as an active or even necessary factor 

 in the production of the dentine. 



The formation of the enamel results from the activity of ectodermic 

 epithelium and may be regarded as a cuticular development carried on by the 

 ameloblasts. The initial phase in the production of the enamel is the appear- 

 ance of a delicate cuticular zone at the inner end of 

 each ameloblast; this fuses _ with the zones capping- 

 the adjoining cells to form a continuous homogene- 

 ous layer. Within the latter differentiate the 

 rod-like processes of Tomes, which soon give 

 rise to tbejt&rous layer, containing rudimentary 

 enamel prisms and interprismatic substance. The 

 latter gradually decreases in amount as the forma- 

 tion of the enamel-columns progresses, since the 

 greater part of this intercolumnar substance is 

 transformed into the cortical portion of the enamel- 

 prisms, while the remainder persists as the meagre 

 cement-substance between the mature prisms. 

 The fibrous productions are for a time uncalcified, 

 but later the calcareous material is deposited as 



granules and spherules, which appear first in the axis of the prisms. The 

 developing enamel increases in thickness by the addition of the increments 

 formed at the inner ends of the ameloblasts, the same cells sufficing for the 

 production of the entire tissue. The earliest formed enamel lies in apposition 

 with the oldest dentine, the youngest enamel immediately beneath the amelo- 

 blasts. The enamel is deposited, therefore, from within outwards, or in the 

 reverse of the direction followed by the growth of the dentine. The oldest 

 strata of both substances lie in contact; the youngest on the outer and 

 inner surfaces of the tooth. After the requisite amount of enamel has 

 been produced, differentiation into prisms ceases; consequently the last- 

 formed enamel remains as a continuous homogeneous layer which invests 

 the free surface of the crown and constitutes the enamel-cuticle or membrane 

 of Nasmyth. 



FIG. 180. Isolated ameloblasts 

 from new-born child, a, basal 

 plate ; b, cuticular border ; c, 

 processes of Tomes; d, homo- 

 geneous mass still capping proc- 

 ess. X 400. (Ebner.) 



