260 R. M. Frank 



3. Ultrastructure of the adult odontoblast 



An investigation of the ultrastructure of the adult odontoblast cannot be dis- 

 sociated from a study of dentine as this should also provide a better understanding 

 of the odontoblastic process. Accurate electron microscopical observations on the 

 adult odontoblast have not been reported and most authors have investigated the 

 characteristics of these cells during dentinogenesis (Watson and Avery, 1954; Nylen 

 and Scott, 1958, 1960; Lenz, 1959; Nalbandian and Frank, 1962; Noble et ai, 

 1962; Frank and Nalbandian, 1963 a). The similarity of the adult cell cytoplasm to 

 the contents of the odontoblastic process near the pulpo-dentinal border, has been 

 noticed by Bernick et al. (1952) and Frank (1957). 



In the present study, the mature human odontoblast appeared as an elongated 

 cell limited by a cell membrane. Glutaraldehyde fixation gave a typical appearance 

 to the nucleus in which the chromatin was condensed peripherally and in the cyto- 

 plasm more of the ultrastructural elements were retained (Fig. 1 and 2). 



The nucleus of the odontoblast is normally located near the pulpal end of the cell 

 and in its larger diameter measures 4 to 5 microns. With glutaraldehyde fixation it 

 was difficult to determine accurately the nucleoli but with osmic fixation alone, the 

 nucleolar material appeared as denser chromatin granules arranged in irregular rows 

 surrounding clear areas of pars amorpha. A double membrane surrounded the nucleus 

 (Fig. 1). In the vicinity of the latter, a pair of centrioles are located near a well- 

 developed Golgi apparatus situated between the nucleus and the predentine layer. 



In transverse sections, the Golgi apparatus was seen to occupy the core of the 

 odontoblast and consist of a smooth-surfaced system of lamellae, micro-vesicles and 

 granules (Fig. 2). Around the Golgi appartus, rough-surfaced endoplasmic reticulum 

 in close association with mitochondria could be seen. With aldehyde fixation, a finely 

 granular material masked the cristae mitochondrialis (Fig. 1 and 2). In the pulpal 

 cytoplasmic part of the odontoblast and around the nucleus the sacs of the endo- 

 plasmic reticulum were scarcer, but numerous mitochondria could be seen in these 

 regions. 



Numerous grains of ribonucleoprotein were disseminated throughout the cyto- 

 plasm as well as small filaments, 50 to 80 A in diameter, similar to the intracyto- 

 plasmic filaments found in young mesenchymal cells of the dental papilla (Frank, 

 1965). Clusters of glycogen granules were dispersed in the cytoplasm (Fig. 2). 



Near the pulpo-dentinal border, the odontoblast cytoplasm is filled mainly with 

 these filaments, with ribosomes and some endoplasmic reticulum and mitochondria. 

 In these regions some vacuoles 0.4 to 0.5 /i in diameter, composed of an amorphous 

 transparent material limited by a dense membrane, could be seen beside larger 

 vacuoles, 1.5// to 2.5// in diameter. These larger vacuoles contained in addition a 

 loose filamentous and granular material. The large vacuoles seemed to arise from 

 fusion of smaller vacuoles. 



A continuous cell membrane limits the odontoblast and its small lateral extensions 

 which, in transverse section, appear as small rounded or oval cytoplasmic islands in 

 the intercellular spaces (Fig. 1 and 2). When the plasmalemma is properly sectioned, 

 it appears to be composed of an Inner leaflet separated from an outer leaflet by a 

 small clear space. Two types of attachment devices were noticed between adjacent 

 odontoblasts. At the level of the nucleus, desmosome-like structures could be seen 



