Ultrastructure of Human Dentine 269 



1963), Hattyasy (1961) and Stella and Fuentes (1963) have described amyelinated 

 beaded intratubular nerve fibres in the calcified dentine of erupted teeth, whereas 

 amyelinated nerve fibres could be located only in the predentine by Arwill (1958), 

 Zerosi (1959), Kerebel (1964) and Bernick (1964). With the electron microscope, 

 myelinated and unmyelinated nerve fibres have been described in the pulp tissue 

 (Arwill, 1958; Mathews et al., 1959; Uchinozo and Homma, 1959; Dhman and 

 Engstrom, 1960) but no nerve fibres could be located in the predentine and dentine 

 with the electron microscope (Fearnhead, 1957; Arwill, 1958). 



In the present study, typical unmyelinated nerve fibres have been identified for 

 the first time under the electron microscope in the human predentine and dentine of 

 erupted teeth (Fig. 4). These free sensory axons are in close contact with the odonto- 

 blastic process. Their limiting cell membranes are parallel to the cell membrane of 

 the odontoblastic process and separated by a small space about 150 A to 220 A wide 

 (Fig. 4). No specific relationship of the nerves to odontoblasts could be observed so 

 far. In cross section, at the predentinal level, the axoplasm of the nerve fibre, about 

 0.5 // to 0.7 // in diameter, contains numerous mitochondria and some synaptic 

 vesicle-like structures. Such sensory nerve fibres of a terminal type are also present in 

 transverse sections of calcified dentine, where they present the same ultrastructure 

 and relationship to the odontoblastic process. At the latter level they are, however, 

 smaller, about 0.5 to 0.3 // in diameter, and contain less mitochondria and synaptic 

 vesicle-like structures. 



This ultrastructural demonstration of non-myelinated nerve fibres in the pre- 

 dentine and dentine of erupted teeth confirms the light microscopical investigations 

 performed mainly with silver Impregnation methods and with polarizing microscopy. 

 As has been pointed out by Fearnhead (1963), the demonstration of nerve fibres in 

 human dentine is indicative of and provides anatomical basis for the sensitivity of 

 this tissue. 



6. Conclusion 



From this survey on the ultrastructure of dentine, it appears that this tissue is 

 permeated by numerous cytoplasmic extensions of the odontoblasts. In mature 

 erupted teeth, these cells and their processes assume a definite secreting activity. 

 Secretion vacuoles discharge their content through a membrane fusion phenomenon in 

 the periodontoblastic organic space, located between the cell membrane of the 

 odontoblastic process and the calcified wall of the tubule. From electron micro- 

 scopical evidence and histochemistry it seems that organic matrix precursors of the 

 peritubular dentine are secreted by the odontoblast through this mechanism. The 

 periodontoblastic organic space, surrounding the cell process, contains collagen fibrils 

 embedded in ground substance. Amyelinated nerve fibres are also located in this 

 space in close association with the odontoblastic process. This organic space can 

 undergo calcification and participate to the elaboration of the peritubular dentine. 

 A new periodontoblastic organic space is then elaborated under the control of new 

 odontoblastic secretions. 



In peripheral dentine, the odontoblastic process shows structural changes. Large 

 vacuoles collect in the center of the process. These elongated vacuoles are not dis- 

 charged Into the periodontoblastic space, on the contrary, they cause condensation of 

 the cytoplasm peripherally. This cytoplasmic extension of the odontoblast becomes 



