270 R. M. Frank 



progressively more hyaline in character and is then applied directly along the 

 calcified walls of the tubule. Complete obliteration of the dentinal lumen through 

 apatite crystal growth sometimes occurs in these processes. In young erupted coronal 

 dentine, the presence of dentine sclerosis has, therefore, to be considered as a physio- 

 logical phenomenon. 



The study of the ultrastructure of human mature dentine suggests that this hard 

 tissue, containing in contrast to enamel, sensitive terminal nerve fibres and cyto- 

 plasmic cell extensions which can assume secretory activities, is able to perform a 

 certain number of complex metabolic activities. 



Summary 



Mature human coronal dentine was studied under the electron microscope in 

 young normal erupted permanent teeth, extracted for orthodontic purposes. The 

 odontoblast appears as an elongated cell with a large nucleus and a cytoplasm 

 containing a pair of centrioles, a well-developed Golgi apparatus, an endoplasmic 

 reticulum in close relationship with mitochondria, ribosomes, clusters of glycogen and 

 numerous fine intracytoplasmic filaments. Laterally the odontoblasts are connected 

 by desmosome and terminal bar-like structures. Collagen bundles are located in the 

 intercellular spaces. At the predentinal level, the odontoblastic process is limited by 

 a cell membrane and contains a cytoplasm rich in fine filaments with occasional endo- 

 plasmic reticulum vesicles as well as some secretion vacuoles which have a limiting 

 membrane and contain granular looking material. Interwoven non-calcified collagen 

 fibrils can be observed in the intertubular substance. In the calcified dentine, adjacent 

 to the predentine, large vacuoles can be seen approaching the cell membrane and 

 discharging their content into the periodontoblastic organic space between the 

 calcified wall of the dentinal tubule and the cell membrane through a membrane- 

 fusion phenomenon without any discontinuity in the latter. Non-calcified collagen 

 fibrils can be seen in these spaces. At a more peripheral level in transverse sections, 

 the odontoblastic process assumes an annular outline, related to a peripheral con- 

 densation of cytoplasm through a centrally located large and elongated vacuole 

 (over 1 micron In transverse diameter) and filled with a fine granular looking material. 

 The peripherally condensed cytoplasm has a hyaline appearance. Densely calcified 

 peritubular zones separated by Intertubular substance surround the odontoblastic 

 processes and in the peripheral layers progressive obliteration of some tubular lumens 

 has been observed thus indicating the possibility of dentine sclerosis in young calci- 

 fied dentine. Last but not least, typical amyellnated sensory nerve fibers could be 

 located for the first time under the electron microscope In the predentine as well as in 

 the calcified dentine, where they are located In the dentinal tubules in close relation- 

 ship with the odontoblastic processes. 



References 



Anderson, D. J.: Sensory Mechanisms in Dentine. Oxford: Pergamon Press 1963. 



Arwill, T.: Innervation of the teeth. Trans, roy. Schools Dent. Stockhohn and Umea 2, 1 



(1958). 

 AwAZAWA, Y.: Electron microscope investigation of the dentin with particular regard to 



the nature of the area surrounding the odontoblast process. J. Nihon Univ. School Dent. 



5, 31 (1962). 



