264 R.M.Frank 



odontoblastic process can be seen in this space (Fig. 5). In fact they cross it and 

 penetrate in the lateral interconnecting micro-tubules. 



This small periodontoblastic organic space, limited on the inside by the cell 

 membrane of the odontoblastic process and on the outside by the calcified wall of 

 the tubule, seems to correspond to the "manchon juxta-cytoplasmique immature" of 

 Weill (1959), confirmed by Symons (1961). According to Weill (1959), this space 

 exhibits metachromasia with methylene blue and stains with alcian blue and with 

 fuchsin paraldehyde after potassium permanganate oxidation. This space is, therefore, 

 rich in acid mucopolysaccharides. 



The large vacuoles, located within the odontoblastic process, can approach the cell 

 membrane where their limiting membrane fuses with the plasmalemma. At the point 

 of fusion, there is rupture of the membrane so that the content of the vacuoles is 

 secreted in the periodontoblastic organic space without any discontinuity in the cell 

 membrane. A similar mechanism of secretion has been noticed in the ameloblast 

 (Frank and Nalbandian, 1963 b). Numerous collagen fibrils, with 640 A bandings, 

 appear in the space where the vacuoles discharge their secretion (Fig. 8). It is inter- 

 esting to note that Weill (1959) also mentions the secretion of globules from the 

 odontoblastic process as well as from the odontoblast. These globules stain with 

 alcian blue. 



The ultrastructure of the odontoblastic process changes in the more peripheral or 

 outer layer of dentine. In a transverse section of a tubule (Fig. 6), the odontoblastic 

 process contains a large vacuole centrally surrounded by an annular peripheral layer 

 of cytoplasm with a hyaline appearance close to the calcified wall of the tubule 

 (Fig. 6 and 7). That this annular layer is a peripheral cytoplasmic condensation can 

 be deduced from the presence at this level of typical lipo-pigment inclusions and 

 from intermediate stages ranging from typical cytoplasmic contents to the hyaline 

 appearance. This centrally located vacuole containing fine granular material (Fig. 7) 

 has an ovoid or elongated form, as judged from different planes of section. The long 

 axis of the oval type of vacuole occurs parallel to the long axis of the odontoblastic 

 process. This configuration has previously been described as the thin-walled or tube- 

 like odontoblastic process (Helmcke, 1955; Scott, 1955; Frank, 1957, 1959). 



It must be mentioned that in some tubules of the peripheral dentine, no structure 

 could be located except an amorphous epon content. This observation is either related 

 to a fixation defect or to the so-called dead tracts (Bradford, 1960). 



The calcified parts of the dentine can be divided into peritubular dentine and 

 intertubular dentine. In the inner dentine, near the predentinal layer, the tubule 

 lumen is immediately surrounded by intertubular dentine and no peritubular dentine 

 is present, this has been shown by microradiography (Frank, 1957; Blake, 1958; 

 Wyckoff and Croissant, 1963), and by electron microscopy (Frank, 1959). In the 

 outer layer of dentine, the peritubular zone is also absent around some tubules 

 (Frank, 1959) and in the interglobular dentine (Blake, 1958). Elsewhere, however, 

 a typical peritubular zone surrounds the lumen of the tubules as a densely calcified 

 wall, whose inner side is in contact with the odontoblastic process or the periodonto- 

 blastic organic space and whose outer surface is continuous with the intertubular 

 dentine (Fig. 6 and 7). 



This peritubular dentine has been described by Bradford (1955) using the optical 

 microscope and by Miller (1954), Baud and Held (1956), Blake (1958), Dreyfuss 



