262 R. M. Frank 



(Fig. 1). Similar structures have been described in the osteoblast by Dudley and 

 Spiro (1961). Around the odontoblasts near the predentine, terminal bar-like struc- 

 tures form a continuous belt-like attachment between adjacent cells. Rounded 

 spaces occupied by bundles of collagen fibrils can be seen at these levels permeating 

 the intercellular regions. These collagen bundles, visible all along the intercellular 

 spaces between lateral walls of the odontoblasts, are embedded in a ground substance 

 denser than the adjacent areas and containing granular and short filamentous struc- 

 tures. These bundles, also noticed by Nylen and Scott (1960), Johansen and Parks 

 (1962) and Frank (1965) correspond to the classical Korff's fibres. 



Compared with the fetal odontoblast during dentinogenesis, it would appear that 

 in the adult odontoblast there is a reduction in the importance of the Golgi apparatus 

 and the number of endoplasmic reticulum vesicles and mitochondria. However, all 

 these cytoplasmic organelles are well-developed and the cell exhibits all the ultra- 

 structural characteristics of secretory activity. 



4. Ultrastructure of predentine 



The predentine is essentially composed of odontoblastic processes separated by a 

 non-calcified collageneous matrix (Fig. 2 and 3). In transverse sections, it appears 

 clearly that the odontoblastic process is a cytoplasmic extension of the odontoblast, 

 limited by a cell membrane and containing dense granules with the same appearance 

 and size as the ribosomes, also numerous filaments 50 to 80 A in diameter and 

 some scarce endoplasmic reticulum vesicles. Large vacuoles, delineated by a dense 

 membrane, which is sometimes folded, and containing a loosely dispersed granular 

 and filamentous material, are located in the core of the process and have a similar 

 ultrastructure than the large odontoblastic vacuoles. 



The non-calcified collagen matrix is made up of numerous collagen fibrils with 

 640 A periodicity, interweaving obliquely and transversely around the odontoblastic 

 process. A minority of the collagen fibrils have a longitudinal orientation (Fig. 3). 

 All the collageneous material is located outside the cell membrane of the odonto- 

 blastic process, but in close association with it. The collagen fibrils are embedded in 

 a transparent ground substance in which is dispersed a granular and finely fila- 

 mentous material. 



5. Ultrastructure of dentine 



One of the main difference of dentine compared to predentine is that in dentine 

 the collageneous matrix embedded in a ground substance is highly calcified and the 

 odontoblastic processes are enclosed in tubules. 



In the inner dentine layer situated next to the predentine, these odontoblastic 

 processes have the same ultrastructure as previously described in the predentine. They 

 are limited by a cell membrane and contain a cytoplasmic mass with numerous small 

 filaments, ribosome-like granules and very occasionally some endoplasmic reticulum 

 vesicles (Fig. 5). Similar large vacuoles, previously described in the odontoblast 

 cytoplasm and the predentinal odontoblastic process, are observed in these dentinal 

 processes. Between the calcified wall of the tubule and the cell membrane of the 

 odontoblastic process a small space is visible (Fig. 5). This space contains non- 

 calcified collagen fibrils embedded in a ground substance containing some finely 

 granular and filamentous material (Fig. 5 and 8). Small lateral extensions of the 



