DEVELOPMENT OF THE TEETH. 



1559 



Fig. 1318. 



When engaged in the latter process the cells measure from .035-050 mm. in length 

 and from .005-. 010 mm. in breadth, but over the sides of the papilla they gradually 

 become lower until towards the base they blend with and become indistinguishable 

 from the deeper cells of the mesoblastic elevation. So long as the tooth grows, 

 division proceeds and odontoblasts are differentiated in the vicinity of the last-formed 

 parts of the root ; after, however, the odontoblasts are engaged in forming dentine, 

 mitosis is no longer to be observed in these elements. 



The formation of the dentine is accomplished through the agency of the 

 odontoblasts much in the same manner that the osteoblasts produce the matri.x of 

 bone. The earliest trace of the dentine appears as a thin homogeneous stratum, the 

 membrana pmformaliva, overlying the coincidently forming la}'er of odontoblasts. 

 Although separable by certain reagents as a cuticular structure, the membrane is only 

 a part of the general dentinal ground-substance with which it blends ; it is resolvable 

 into collagenous fibrils similar to those of bone-lamellae. The dentinal matrix 

 deposited through the influence of the odontoblasts, is for a time without fibrous 

 structure and uncalcified, the deposition of the lime salts occurring first near the apex 

 of the papilla and next the enamel, a zone of uncalcified matrix around the pulp- 

 cavity marking the youngest dentine. The calcareous material is first deposited in 

 the form of globules, the dentinal spheres, the calcification being completed by the 

 subsequent invasion of the interstices between the spherical masses. When for any 

 reason calcification is incomplete these clefts remain 

 lime free, a condition seen in the interglobular spaces 

 already described. The spherical form of the calca- 

 reous deposits is indicated by the uneven condition of 

 the inner surface of the dentine in macerated teeth, the 

 wall of the pulp-cavity presenting numerous minute 

 hemispherical projections which correspond to the 

 globular masses of lime salts. The scalloped border 

 and pitted outer surface of the dentine, together with 

 the extension of the dentinal tubules as far as or into 

 the enamel, point to the absorption of the primary 

 dentine constituting the preformed membrane, proba- 

 bly through the influence of the enamel. As empha- 

 sized by Ebner,' the formation of the fibrillae of the 

 ground-substance takes place independently of the 

 direct influence of the dentine-cells, since the general 

 disposition of the earliest fibrillae is at right angles to 

 that of the odontoblasts and their processes. The 

 dentinal matrix differs from that of bone in being the 

 production of a single set of cells, while the osseous 

 tissue is the collective work of difterent elements, 

 many of which, after contributing their increment, be- 

 come surrounded by the ground-substance to form 

 the bone-corpuscles within the lacunae. In human 

 dentine, on the contrary, the odontoblasts are only 

 rarely, under normal conditions, imprisoned within the 

 ground-substance which they have formed. The de- 

 mands made upon the odontoblasts during their active role as dentine producers are 

 met by the nutrition supplied by the rich vascular supply of the dentinal papilla, so 

 that for a time the cells are enabled not only to increase th^ dentinal matrix, but also 

 to extend their processes, which they send into the tubules of the dentine as the den- 

 tinal fibres, without diminution in size. With the completion of dentine production, 

 and the consequent decrease in the area upon which they rest, the odontoblasts 

 become narrower and smaller (Walkhof?) ; later they exhibit evidences of impaired 

 vitality and degeneration, their dentinal processes likewise growing thinner and less 

 flexible and assuming the characteristics of the fibres of Tomes of the adult tissue. 

 According to Walkhoff, the dentinal fibres suffer in size as the result of their activity 

 in the production of the sheath of the tubules. 



* In KoUiker's Gewebelehre des Menschen, 6te Auf., 1899. 



Isolated odontoblasts from incisor 

 tooth of new-born child, a; b, fioni 

 upper part of crown ; r, d, e, from lat- 

 eral region. X 400. {Ebner.) 



