The dentinal tubules are designed to protect the odontoblast processes, which leave behind a cytoplasmic process during tooth development. Around the cytoplasmic process, a dentin matrix is deposited, which eventually mineralizes. However, the question of the role and structure of the cytoplasmic process of the odontoblast in each dentinal tubule after dentin has completely finished the formation process remains an unresolved issue. We investigated the dentinal tubules of 20 permanent teeth (premolars and molars) of people of different ages using a scanning electron microscope. The occlusal surfaces and longitudinal fracture planes of both decalcified and non-decalcified native tooth preparations were examined. In longitudinally split teeth at the periphery of the pulp chamber of a young human premolar, odontoblasts can be seen forming a continuous layer adjacent to the parapulpal dentin. Each of the odontoblasts has a separate process that penetrates into the dentinal tubule. More peripheral parts of the dentinal tubules were either empty or contained cylindrical structures that were better visualized after acid etching on longitudinally split samples. Etched split dentinal tubules in the outer dentin more often contained cylindroid tubular structures. In each individual dentinal tubule, the odontoblast processes are arranged in the form of thin-walled tubules, which, with the help of a supporting fibrillar framework, occupy a central position. On the occlusal surface of a mature human molar, the intertubular dentin has the appearance of a smooth-surfaced structure. Practically all dentinal tubules contain more or less preserved dentinal canaliculi, indicating their fairly pronounced resistance to adverse factors. Inside the lumen of the dentinal tubules, three different types of structures are observed: odontoblast processes, cylindrical structures, and collagen fibers. Odontoblast processes are visualized both along the entire length of the dentinal tubules and only in the parapulpal parts of the dentinal tubules. Their peripheral parts were either empty or contained cylindrical structures that likely correspond to the laminae limitantes. Collagen fibers often form a fibrillar network that on one side intertwines into the dentinal canaliculus, and on the other side is connected to the walls of the dentinal tubules due to perpendicular microtubules for anchoring fibrils. Collagen fibers are most numerous in the parapulpal parts of the dentinal tubules. This study only partially explained certain aspects of the dentin microstructure, but further in-depth studies of dentin ultrastructure are necessary to more fully understand the pathology of hard tooth tissues in people of different ages and the possibilities for its treatment and prevention.
