Dentin

DENTIN, dentinum, substantia eburnea (from Latin ebur—ivory), the tissue that constitutes the main mass of the tooth and determines the general shape of the latter. After enamel, dentin is the hardest tissue of the human body and is characterized by a low content of water (10%) and organic substances (26–28%). The elements of dentin, as with any connective tissue formation, are cells and an intermediate (or ground) substance. The ground substance of dentin is built from collagen fibers arranged in thin bundles (up to 2 μ) and joined together, as if embedded in a special cementing substance. The bundles of fibers of the ground substance in dentin are generally arranged in a direction parallel to the walls of the pulp cavity and root canals; i.e., in the region of the root they are almost vertical, in the region of the masticatory surface of the crown (in molars) they are almost horizontal, and in its side walls they occupy an intermediate position. The organic basis of dentin is impregnated with inorganic salts, predominantly calcium phosphate (calcium phosphate—66.7%, calcium carbonate—3.4%, magnesium phosphate—1.1%, and other salts 0.8%—according to Bibra); this compound is not a simple mechanical mixture but most likely has the character of adsorption. The cells participating in the formation of dentin, with which the metabolic processes in dentin are associated, are located not in the ground substance itself but border it at the periphery; they are called odontoblasts and are usually considered cells of the dental pulp [see separate table (vol. V, col. 27–28), Fig. 4]. However, the long protoplasmic processes of these cells—Tomes' fibers (which exist in addition to the short processes by which odontoblasts anastomose with each other and with deeper-lying pulp cells)—penetrate the entire thickness of the ground substance of dentin, lying in the dentinal tubules (see Figures 1 and 2); the latter are arranged (in general) radially, diverging from the pulp to the periphery of the dentin such that in the root region they are directed almost horizontally, in the side walls of the crown they take an ascending direction, and in the part of the crown forming the cutting edge or masticatory surface, they run almost vertically. Consequently, the directions of the dentinal tubules and the bundles of fibers of the ground substance intersect each other at almost a right angle. The presence of dentinal tubules in the ground substance, the complex intertwining of collagen fibrils, and the difference in the refractive indices of these and the cementing substance explain the peculiar silky luster of the dentin surface seen in incident light. The dentinal tubules have their own wall—Neumann's sheath, which is apparently a layer of ground substance that is distinct in its physicochemical properties; its characteristic property is significant resistance to acids and alkalis; when heated in a solution of caustic alkali, the entire ground substance dissolves except for the walls of the tubules, which can be isolated in this way. On their path from the pulp to the periphery of the dentin, Tomes' fibers give off numerous lateral branches by which they anastomose with each other; particularly abundant branching of Tomes' fibers is noted in the peripheral layer of dentin (at the boundary with enamel and cementum); corresponding to the branching of Tomes' fibers, Neumann's sheath also forms anastomosing lateral tubules (see Figure 1). Individual dentinal tubules also penetrate beyond the limits of the dentin into the enamel substance, ending in its interprismatic substance. Apparently, a connection also exists between the peripheral branches of the dentinal tubules and the cementum of the root.—The layer of dentin closest to the pulp, bordering directly on the odontoblasts, is called the dentinogenic layer; it represents a still uncalcified zone of the ground substance of dentin (corresponding to the osteoid zone during the apposition of bone tissue). The picture of initial calcification of dentin observed in this layer presents the following characteristic feature: calcification is not uniform—the union of organic matter with lime salts manifests as the precipitation of spherical masses of colloid (globuli), in which the lime is in a bound state; these round formations vary in size, stain somewhat more intensely (with hematoxylin), and therefore stand out clearly against the paler background of the uncalcified ground substance; over time, the latter also undergoes more uniform calcification, and thus the mass of calcified dentin in a stained preparation has a more or less uniform tint. In the peripheral areas of dentin (near the enamel and cementum), as a rule, only globular calcification occurs; the zone of ground substance located between the lime-containing spherical areas remains little or not at all calcified and is called interglobular dentin (see Figure 2). Thus, in chemical composition and histological structure, dentin is a variety of bone tissue; there is only an external difference between them, consisting in the different arrangement of cellular elements relative to the ground substance: while osteoblasts, in the process of forming the ground substance of bone, are themselves surrounded by this substance and remain embedded in it as osteocytes, odontoblasts are not enclosed in the forming ground substance of dentin but are located at its periphery and are considered pulp cells. But if one takes into account that osteocytes, being located in the mass of the ground substance, anastomose with each other by protoplasmic processes embedded in tubules, and that these tubules have their own walls isolated from the surrounding ground substance, then the indicated difference essentially turns into identity.—With age (as in bones), the content of inorganic salts in dentin increases, especially in the root region; in this case, salts are deposited both in the ground substance and in the dentinal tubules, which leads to narrowing and, in part, to complete filling of their lumen; the mass of dentin thus becomes more homogeneous, and the difference in the refractive indices of its individual structural elements is smoothed out, which leads to the loss of the silky luster characteristic of dentin; the dentin becomes as if translucent—it turns into so-called transparent dentin; the transparency of individual areas of dentin is also observed in certain pathological processes (see Caries). The pathways through which metabolism in dentin occurs—the manifestation of which, in particular, are fluctuations in the content of lime salts (see below—changes in dentin in rickets)—are the dentinal tubules or the Tomes' fibers embedded in them. This question has not yet been resolved: some believe that Tomes' fibers do not fill the lumen of the dentinal tubules, and thus a space remains between them and the walls of the tubules in which lymph circulates; through the smallest branches of the dentinal tubules, this fluid penetrates into the enamel and cementum, which has been proven experimentally. Others believe that during life, Tomes' fibers completely fill the lumen of the tubules, and thus they alone are the pathways of exchange. The processes of odontoblasts are also credited with the role of conductors of pain stimuli from the periphery to the pulp (Walkhoff); the special sensitivity of the peripheral layers of dentin is explained from this point of view by the presence of the terminal branches of Tomes' fibers in them. Some attempt to prove the presence of nerve fibers in dentin, described partly in the tubules themselves alongside Tomes' fibers, and partly even in the ground substance (Adrion). The described histological structure mainly characterizes the dentin formed during the embryonic development of the tooth. This dentin is given the name primary dentin, in contrast to secondary dentin, the deposition of which from the side of the dental pulp continues even after the tooth has erupted and begun to function (i.e., participate in the act of mastication). Compared to primary dentin, the structure of secondary dentin is distinguished by a certain irregularity: 1) in the number, direction of the path, and character of the branching of the dentinal tubules, which in some places are absent altogether, while in other places they are arranged very densely and most often, running an extremely tortuous path, give dense and complex networks of anastomoses; 2) in the character of calcification—sometimes strong, sometimes insufficient, sometimes with clearly expressed globularity, sometimes layered, without clear sphericity; 3) in the structure of the ground substance—uneven thickness and irregular path of the bundles of collagen fibrils. Due to these features, secondary dentin can generally be characterized as irregular dentin (see Figures 3 and 4). To designate irregular dentin, the term "dentinoid" has been proposed, which is essentially incorrect, as it corresponds only to the concept of the uncalcified ground substance of dentin (analogous to the term "osteoid").—It is customary to distinguish two groups of secondary dentin: 1) irregular dentin proper, deposited mainly on the walls of the root canals and on the floor of the pulp cavity, apparently as a response to physiological functional stimuli; it is noted as a rule in all teeth.

The deposition of irregular dentin is of great practical importance, as it leads to the narrowing of the canals and deformation of the pulp cavity (see Figures 3 and 4), which must be taken into account during the treatment of dental diseases; these changes are generally more pronounced the older the subject is; 2) reparative dentin, deposited primarily on the lateral walls and roof of the pulp cavity (also altering its configuration) in response to pathological irritation (caries, tooth attrition). In some cases of abundant secondary dentin deposition, individual cells or small areas of the pulp, sometimes containing capillaries, remain enclosed within the newly forming ground substance; such secondary dentin bears a great resemblance to bone (osteodentin) and sometimes remotely resembles vasodentin—a variety of dentin found almost exclusively in certain species of fish. The ground substance of vasodentin is permeated by a system of blood capillaries, due to which the teeth of some fish have a red tint during life. Vasodentin is deposited with the participation of odontoblasts but, as a rule, does not contain dentinal tubules. Between typical vasodentin and the dentin of mammals, there are a number of transitional forms. As a variety of bone tissue, dentin undergoes changes in all diseases that affect the structure of bone in general; thus, in rickets, insufficient calcification of the ground substance of the dentin is manifested 1) in the presence of a wider-than-usual dentinogenic layer (see above) and 2) in the fact that almost the entire thickness of the dentin has the appearance that, under normal conditions, is characteristic only of the zone of interglobular dentin (see above); in this case, the arrangement of the calcified globules in the uncalcified ground substance, their size, and degree of stainability present extreme diversity. These changes are observed almost exclusively in the dentin of tooth germs or teeth that have recently erupted in children; in the dentin of teeth in adults who suffered from rickets in childhood, these changes, if noted at all, are present to a very insignificant degree. This circumstance serves as an indication that the restoration of normal calcium metabolism with age is also manifested in the teeth: the degree of dentin calcification gradually increases and can reach the norm. In osteogenesis imperfecta, the dysfunction of the odontoblasts of the tooth germ, which apparently intensifies periodically, is manifested in the layered deposition of ground substance, which sometimes approaches normal and sometimes is irregularly constructed; in this case, the entrapment of individual odontoblasts in the forming ground substance (osteodentin) is noted. As shown by a single observation by Adloff, the irregularity of the dentin is preserved even in the teeth of an adult if a child born with symptoms of osteogenesis imperfecta survives. In scurvy, a sharply pronounced irregularity of the dentin and the formation of numerous denticles (see below) are also noted; the fiberization and dissolution of the ground substance, up to the formation of cavities in the dentin, have been observed in experimental scurvy in guinea pigs (Kotanyi). Denticle (Dentikel)—areas of tissue constructed like irregular dentin, having rounded-oval outlines and occurring a) isolated in the tissue of the dental pulp—free denticles, b) in connection with the dentin of the wall of the root canal or pulp cavity—parietal denticles, c) embedded in the mass of the dentin—interstitial denticles. According to histological structure, until recently, a distinction was made between: 1) Highly organized denticles, characterized by the presence of dentinal tubules; it must now be considered proven that these formations actually represent sections of irregular dentin in places of its abundant deposition on the walls of the root canals and pulp cavity (their connection with the wall can always be established when making serial sections). 2) Low-organized denticles, containing no dentinal tubules or cellular elements in the ground substance; these denticles often have a layered structure and are surrounded by a belt of elongated cells separated from the calcified substance by a light uncalcified zone; the presence of the latter indicates the continuing apposition of the ground substance with the participation of the pulp cells bordering the denticle. In the center of the denticle, a more calcified area is often visible; apparently, this is a degenerated pulp cell (or group of cells) that has undergone calcification and served as a kind of nucleus for the subsequent formation of dentin-like tissue around it. The process of denticle formation, which is the result of the active activity of pulp cells, should be distinguished from the so-called petrification of the pulp—calcification due to the unorganized deposition of lime salts into the pulp tissue during its atrophic states. The practical significance of denticles consists in the following: 1) they more or less sharply change the configuration of the pulp cavity; 2) being located at the orifices of the root canals, they hinder access to the canals for instruments and constitute an obstacle to the penetration of medications; 3) being located near nerve trunks and gradually increasing in size, they can, as is supposed, exert pressure on the nerve trunks and cause phenomena of trigeminal neuralgia (see Figure 5). However, there is no direct connection between these phenomena; the presence of denticles and petrification in teeth, often in immediate proximity to nerve trunks, is an almost everyday occurrence, whereas trigeminal neuralgia is relatively rare.