Structure of tooth dentin –
Dentin consists of calcified intercellular substance, which is penetrated by the so-called “dentinal tubules” (dentinal tubules), which ensure trophism and mineralization of dentin. Dentinal tubules run radially, in the coronal part of the tooth - in the direction from the wall of the pulp chamber to the enamel-dentin border, and in the root part of the tooth - from the wall of the root canal to the surface of the tooth root. However, if in the root part of the tooth, as well as in the area of the occlusal surface of the tooth crown, the dentinal tubules have an almost straight shape, then in the lateral parts of the crown they will already be S-shaped.
Dentinal tubules (electron microscopy) –
The diameter of the tubules ranges from 0.5 to 4 microns, and they will be wider in the internal parts of the dentin and gradually narrow outward (like a flattened cone). Inside the tubules there are processes of odontoblasts, unmyelinated nerve fibers, and tissue fluid also circulates. The odontoblasts themselves are located outside the dentin - their bodies are located in the superficial layer of the pulp, and their processes, passing along the entire length of the dentinal tubules, end in the area of the enamel-dentin border (Fig. 8).
The lateral canals extend from the dentinal tubules in a perpendicular direction. There are especially many of them in the predentin and inner layers of peripulpal dentin (100–200 µm from the border with the pulp). There are quite a few of them in the middle sections of dentin and again become numerous at the periphery - in the area of mantle dentin. In the lateral canals there are lateral branches of the processes of odontoblasts, which anastomose with each other. Let us recall that it is odontoblasts that participate in the formation of the organic dentin matrix and its further mineralization.
Odontoblasts, predentin and dentinal tubules (histology) –
Please note that on the histological specimen (Fig. 6) predentin looks like a light stripe, turning into a zone with spherical formations of a darker color (globules). The predentin layer has minimal mineralization and consists mainly of an organic matrix - intertwined collagen fibrils (Fig. 7). Spherical formations at the border of predentin and peripulpar dentin are nothing more than “calcospherites”, which are foci of mineralization. Low-mineralized dentin located around such globules is called “interglobular.”
Subsequently, the globules merge, forming homogeneous, highly mineralized dentin. It is also worth noting that when the dentinal tubules approach the enamel-dentin border or the cementum layer, their ends form large terminal branches (similar to tree branches). A number of terminal segments of dentinal tubules penetrate even through the enamel-dentin boundary, forming the so-called “enamel spindles”. According to most authors, these formations are involved in the mineralization of deep layers of enamel.
Architectonics of dentinal tubules –
The density of the tubules differs in different layers of dentin. Some of them are most densely located in the peripulpal dentin, where there are from 50,000 to 75,000 of them per 1 mm2 of dentin. For 1 mm2 of the mantle layer of dentin there will be only from 15,000 to 30,000. But let us draw your attention to the fact that in this case it decreases not the number of tubules as such, but precisely the density of their location in the surface layers of dentin (due to their radial direction and with a simultaneous increase in the area of the outer layers).
Other components in the enamel
In addition to the already mentioned main components of the enamel layer of the tooth, its chemical composition also contains a set of other components:
- Neonatal line - present exclusively on baby teeth, it looks like a dark-colored stripe (almost black). This line is located in the area of contact between two types of enamel, the first of which was formed before the baby was born, and the second after.
- Bundles and plates of dental enamel are special enamel formations containing prisms of a hypomineralized type, between which the interprismatic substance consists of the same material. It is noteworthy that the molecular structure of this material involves a large number of protein compounds. Many dentists are of the opinion that through the mentioned bundles and plates, various microorganisms penetrate into the enamel from the oral cavity, making their way to deeper dental tissues, causing caries, etc.
- Gunter-Schräger stripes are lines that stand out on the tooth enamel in a darker or lighter shade, the width of which does not exceed 100 microns. They are located perpendicular to the surface of the enamel layer and are formed as a result of opening its prisms.
- Retzius lines - in shape they resemble arches offset from the central one, located symmetrically in relation to each other. When cut across a tooth , these formations resemble rings inside a tree trunk. The formation of Retzius lines corresponds to different periods of mineralization of the enamel layer.
Structure of dentinal tubules –
Dentinal tubules pass in the so-called dentinal tubules. The wall of each dentinal tubule is formed by the so-called “peritubular dentin,” which is characterized by a very high degree of mineralization and the absence of collagen fibers. Between the dentinal tubules there is the so-called “intertubular dentin”, which is characterized by a lower degree of mineralization and a large number of collagen fibers (fibrils). Dentinal tubules are hollow inside, which is why they are often called dentinal tubules.
Each tube is internally coated with a thin layer of organic matter - this layer contains a high concentration of glycosaminoglycans and is commonly called the Neumann membrane. In the center of the dentinal tubules there are processes of odontoblasts (Toms fibers). The space between the odontoblast process and the wall of the dentinal tubule is filled with dentinal fluid, which is similar in composition to blood plasma. In addition to the processes of odontoblasts and tissue fluid, they also contain non-myelinated nerve fibers (however, they can only be found in peripulpar dentin).
Scheme of the structure of dentin –
Peritubular and intertubular dentin –
During the process of dentinogenesis, intertubular dentin, located between the dentinal tubules, is formed earlier than peritubular dentin (24stoma.ru). The deposition of peritubular dentin occurs from within the already formed dentinal tubules (the processes of odontoblasts take an active part in this). The thickness of peritubular dentin from the pulp end of the tube is from 40 to 50 nm, and from the enamel-dentin border from 500 to 700 nm.
As we said above, peritubular dentin has a significantly higher mineralization. For example, the content of hydroxyapatite in it will be 40% higher than in intertubular one, but there are practically no organic components in it. This leads to the fact that when moderate caries occurs, peritubular dentin will be destroyed much faster than intertubular dentin. And as a consequence, the ongoing processes of demineralization will lead to the expansion of dentinal tubules and an increase in dentin permeability, including for pathogenic bacteria.
In turn, intertubular dentin contains many organic components, for example, calcified collagen fibrils with a diameter of about 75 nm (hydroxyapatite crystals are located along the axis of the fibrils). Collagen fibrils form a so-called “framework”, which serves as a basis for the deposition of mineral salts. Below you can see what the collagen framework of dentin looks like in electron microscopy images taken after its forced demineralization.
What are dentinal tubules
They look like thin tubes that taper in the outer direction and penetrate the tissue from the pulp to the periphery. In the depths they branch, and inside they are filled with Toms fibers, which nourish dentin and saturate it with mineral salts. It is the dentinal tubules that ensure the high permeability of dentin, despite its dense structure. This allows the pulp to quickly respond to damage.
The dentinal tubules also contain dentinal fluid. Its movements cause pain and sensitivity in the teeth. It is believed that due to temperature, mechanical or other effects, the liquid begins to move, irritating the nerve endings in the pulp.
Organic dentin matrix –
The organic matrix of dentin is located between the dentin tubules (in intertubular dentin). It consists of collagen fibrils and an amorphous substance located between them. It is interesting that the direction of collagen fibrils and their structure will differ in the mantle and peripulpar layers of dentin. This is due to the fact that during primary dentinogenesis, the organic matrix of mantle dentin is first produced, and only then the matrix of peripulpar dentin.
In mantle dentin, fibers running in a radial direction, parallel to the course of the tubules (“Korff fibers”) will predominate. But the peculiarity of Korff fibers is not only their directionality, but also the fact that they will consist of fairly thick fibrils united into conical-tapering bundles. Moreover, it is worth clarifying that the radial-parallel direction of Korff fibers is more typical for that part of the mantle dentin that is closest to the occlusal surface of the tooth crown. And on the lateral surfaces of the crown and in the root area, the Korff fibers acquire an increasingly oblique direction.
As we said above, the matrix of peripulpal dentin is formed later than that of mantle dentin. Odontoblasts during this period synthesize much thinner fibrils, which intertwine with each other (see figure below). These fibers will be located tangentially, i.e. they extend from the dentinal tubules at almost a right angle (they are called “Ebner’s fibers”). But collagen fibers are not the only component of the organic dentin matrix, and we must not forget about the amorphous substance surrounding them.
Collagen fibrils in peripulpar dentin –
Amorphous substance –
Collagen fibers are surrounded on all sides by a basic amorphous substance, which consists mainly of glycosaminoglycans (chondroitin sulfates). In addition to them, the organic matrix includes a large amount of non-collagen proteins (their share is about 20% of the organic dentin matrix). The amorphous substance also contains proteoglycans, which are formed as a result of the combination of chondroitin sulfates and non-collagenous proteins.
Non-collagenous proteins play an important role in the processes of dentin mineralization. Below we have listed their main varieties -
- calcium binding proteins
- bone morphogenetic proteins (BMPs),
- glycoproteins (fibronectin, osteonectin),
- calcium ATPase and alkaline phosphatase,
- collagenases and collagen-digesting enzymes necessary for the restructuring of the organic matrix.
In the organic basis of dentin, lipids (glycolipids and phospholipids) have also been identified, probably participating in the mineralization of the matrix.
Nerve fibers -
Unmyelinated nerve fibers penetrate into dentin from the peripheral parts of the pulp, and they can only be detected in predentin (they penetrate into it to a depth of several micrometers, and only rare fibers - to a depth of 100 to 200 microns). In addition, nerve fibers are not found in all, but only in some dentinal tubules. In the more peripheral layers of dentin, there are no nerve endings at all, and in this case the main role in the formation of pain impulses is played by dentinal fluid (changes in hydrodynamic conditions).
Composition of mature dentin
- Inorganic substances – phosphate salts of magnesium and calcium (70%);
- organic substances – type 1 collagen (20%);
- water (10%).
Dentin looks like bundles of collagen fibers containing mineralized intercellular substance. The bundles are penetrated by dentinal tubules with processes of odontoblasts, or Toms' processes. The fibers are arranged differently in different areas. In the outer, or mantle, dentin, formations of the radial direction predominate, in the inner - tangential.
Features of the inorganic matrix –
Dentin consists primarily of calcium phosphate (in the form of hydroxyapatite crystals), as well as small amounts of calcium fluoride and carbonate.
Hydroxyapatite crystals in dentin are much smaller and thinner when compared with tooth enamel, and have the following dimensions - length 20 nm, width 18-20 nm, thickness 3.5 nm. Those. The crystals are quite small and needle-shaped. Electron microscopy made it possible to establish that these crystals are located not only outside the collagen fibers, but also inside them (located even between the collagen fibrils). Dentin is characterized by a special form of deposition of crystals of mineral salts. If, for example, in the main substance of bone tissue the deposition of mineral salts occurs evenly (in the form of tiny crystals), then in dentin the mineralization process occurs in several stages. At the first stage, the formation of spherical crystalline structures occurs (in the form of “globules” - calcospherites), between which areas with non-calcified or slightly calcified basic substance - interglobular dentin - are still preserved.
Globular and interglobular dentin (histology) –
Globular and interglobular dentin are best visible at the border of peripulpar dentin and predentin (in the coronal part of the tooth), because It is in this zone that mineralization of secondary-formed dentin actively occurs. In addition, it is in this zone that the largest calcospherites can be found. Gradually, the globules increase in size and merge, forming homogeneous, highly mineralized dentin.
What are Owen's lines and Abner's lines?
Primary (physiological) dentin is formed during the period of tooth formation and eruption. It is produced by odontoblasts at a rate of approximately 4-8 µm/day, and the activity of odontoblasts has periods of activity and rest. This periodic change in odontoblast activity leads to the presence of so-called growth and contour lines in dentin.
Owen's contour lines reflect the daily rhythm of dentin deposition by odontoblasts. They are located at right angles to the dentinal tubules and correspond to “rest periods” in the activity of odontoblasts. During these periods, much less intense mineralization of dentin will occur - with the formation of very small interglobular spaces. The number of Owen lines can increase in pathological conditions of the body that affect the processes of mineralization of hard dental tissues.
Ebner's growth lines correspond to a slower 5-day cycle of formation of the organic dentin matrix by odontoblasts, i.e. periods of less mineralization of the dentin base substance.
Dead pathways in dentin –
Also, so-called dead paths (on thin sections of teeth) can be found in dentin, which arise when some of the odontoblasts die. Some authors say that only the exposure of 1 mm of the child’s area leads to the death of up to 30,000 odontoblasts. At the same time, the processes containing in the dentinal tubules undergo decay; accordingly, the cavities of the dentinal tubules will be filled with decay products and gaseous substances.
Due to the presence of gases, such dentinal tubules (dead paths) appear black on polished sections of teeth. By the way, the term “dead paths” was coined by E. Fish. It is also worth noting that the sensitivity of dentin in such areas is reduced, and from the side of the dental pulp in these areas there will be increased production of tertiary dentin.
Text of the book “Fundamentals of clinical dental morphology: a textbook”
Chapter 5 Histology of teeth and surrounding tissues
5.1. Hard tooth tissues
5.1.1. Dentin Dentin (dentinum) is
the tissue that forms the bulk of the tooth and determines its shape. In humans, dentin in the crown area is covered with enamel, and in the root area - with cement. Thus, in a healthy tooth, dentin does not come into contact anywhere with the external environment and tissues surrounding the tooth. Like enamel, mature dentin is another ossified tissue of the tooth. In terms of its formation, structure and physiological characteristics, dentin is most likely closer to compact coarse-fibrous bone tissue, but differs from it in greater hardness and the absence of cells. Dentin and bone share some similar properties:
– growth by accession (apposition);
– the presence of a canalicular system containing cell processes, as well as specialized extracellular fluid and mineralizing matrix vesicles;
– structure of an organic matrix of collagens. However, between these two tissues there are obvious differences
differences.
Main features of dentin:
– dentin comes from ectomesenchyme (cells of the cranial part of the neural crest);
– dentin is restored and modified throughout the life of the individual;
– dentin is an acellular tissue (contains only processes of odontoblasts) that does not have blood vessels;
– odontoblasts participate in both the formation and mineralization of the organic basis of dentin.
Physical properties and chemical composition.
Dentin has a light yellow color, has some elasticity, it is stronger than bone and cement, but 4–5 times softer than enamel. The high hardness of dentin is explained by the presence in it of a large amount of mineral salts, the content of which reaches 70-80%, while the remaining 20-30% consists of organic substances (12-18%) and water (8-12%). In this regard, organic and inorganic components (matrices) are distinguished in mature dentin.
Organic matrix
Dentin is made up of collagen. Under an electron microscope, rather large fibers with an axial periodicity of 600–700 nm, characteristic of type 1 collagen, are visible. this is a genetically specific form of collagen found in the most ossified tissues (dentin, bone, cement).
In dentin, collagen fibers are oriented randomly, as in dense, unformed connective tissue. Only at the dentinoenamel border - in mantle dentin - are the fibers characterized by regularity, precise organization and orientation.
The collagen fibers of the dentin matrix are embedded in a basic amorphous substance consisting of glycosaminoglycans such as chondroitin sulfates. The latter can combine with non-collagen proteins, resulting in the formation of proteoglycans - the main components of the dentin matrix.
About 20% of the organic dentin matrix consists of non-collagen proteins, which play a role in the mineralization of dentin:
– proteins rich in gamma-carboxyglutamic acid;
– proteins that bind calcium;
– bone morphogenetic proteins;
– glycoproteins: fibronectin, osteonectin and, possibly, dentinonectin;
– proteins associated with membranes: calcium ATPase and alkaline phosphatase;
– collagenases, as well as collagen-digesting enzymes, necessary for the restructuring and change of the organic matrix.
In the organic basis of dentin, lipids (glycolipids and phospholipids) have also been identified, probably participating in the mineralization of the matrix.
Inorganic matrix
Dentin, like bone, cement and enamel, consists mainly of calcium phosphate in the form of hydroxyapatite crystals, which in dentin are small, thin, needle-shaped.
Transmission electron microscopy data made it possible to establish that these crystals are located both inside collagen fibers and between them. The inorganic dentin matrix contains small amounts of calcium fluoride (fluorapatite), calcium carbonate, magnesium and sodium.
5.1.1.1. Structure of dentin
Dentin consists of a ground substance and many thin dentinal tubules (canaliculi dentales) that penetrate the ground substance (Fig. 89).
Rice. 89. Scheme of the structure of dentin and periodontium (according to R. Krstic).
1 – Toms fibers; 2 – odontoblasts; 3 – predentin; 4 – dentin; 5 – cellular cement; 6 – acellular cement; 7 – fibers of the periodontal ligament; 8 – periodontal blood vessels; 9 – bone alveolus; 10—Thoms granular layer (interglobular dentin); 11 – alveologingival ligaments; 12 – gums; 13 – enamel; 14 – gingival epithelium.
Dentinal tubules provide trophism to dentin and are thin cone-shaped tubes with a diameter of 1 to 3–4 μm, running in the radial direction from the dental pulp to the enamel or cement. They are wider in the inner parts of the dentin and gradually narrow outward.
The number of dentinal tubules varies in different parts of dentin. Due to the radial direction in relation to the tooth cavity, the tubules in the internal parts of the dentin (near the pulp) lie more closely: there are 50,000-75,000 dentinal tubules per 1 mm2 of dentin, and from 15,000 to 30,000 tubules closer to the periphery. There are more of them in the crown of the tooth than in the root. In molars there are 1.5 times less of them per 1 mm2 of dentin surface than in incisors.
The dentinal tubules in the crown are S-shaped curved, and in the area of the tooth root they are almost straight and run perpendicular to the axis of the tooth.
In the thickness of the dentin, the tubules branch and give off lateral processes that anastomose with each other (Fig. 90).
Rice. 90. Topography of dentinal tubules in peripulpar dentin (scanogram)
1 – dentinal tubules; 2 – Toms fibers; 3 – collagen fibers of the organic dentin matrix; 4 – boundary plate (Neumann’s shell).
The branching of the tubules is especially clearly visible at the dentinoenamel and dentinocement border, where each of them is divided into several terminal branches. In some cases, the tubules can cross the dentinoenamel border, penetrating into the thickness of the enamel and forming enamel spindles.
Due to the presence of a huge number of tubes penetrating dentin, the latter has high permeability. This circumstance is of clinical significance, causing a rapid response of the pulp to damage to dentin.
The ground substance surrounding the tubules is more compacted (hypermineralized) and homogeneous than the substance in the spaces between them. In this regard, peritubular (around the tubular) and intertubular (intertubular) dentin are distinguished.
Peritubular dentin is the layer of dentin that immediately surrounds each dentinal tubule, forming its wall (Fig. 91).
Rice. 91. Electronic scanogram of a dentin area.
1 – openings of dentinal tubules; 2 – peritubular dentin; 3 – intertubular dentin.
The thickness of the layer of peritubular dentin at the pulp end of the tube is about 40–50 nm, and at the dentinoenamel border – 500–700 nm. This dentin has a higher (35–40%) mineral content than intertubular dentin. The amount of organic matter in peritubular dentin is minimal, since it almost completely disappears during decalcification. Therefore, during caries during dentin demineralization, peritubular dentin undergoes intense destruction, which leads to the expansion of the tubes and an increase in its permeability.
During tooth development, intertubular dentin is formed first in both mantle and peripulpal dentin. It consists mainly of calcified collagen fibrils with a diameter of 100–200 nm, with hydroxyapatite crystals located along the axis of the fibrils.
The contents of dentinal tubules are varied:
– processes of odontoblasts (Toms fibers);
– unmyelinated nerve fibers;
– tissue dentinal fluid;
– non-calcified collagen fibrils (intertubular fibrils);
– hydroxyapatite crystals.
Some of these substances are involved in the sensory function of dentin, others are involved in the restructuring of its organic matrix. From the inside, the wall of the dentinal tubule is lined with a thin film of organic matter - the boundary plate (Neumann's membrane, or membrane), running along the entire length of the dentinal tubule and containing high concentrations of glycosaminoglycans. The tubes can serve as conductors for various substances necessary for dentin restoration, cellular debris from degenerated odontoblasts, microorganisms and their metabolic products. The latter can reach the pulp, causing some damage to it.
The processes of odontoblasts, being a direct continuation of the apical sections of their cell bodies, as a rule, stretch along the entire length of the dentinal tubules, ending at the dentinoenamel border. They contain few organelles, but a significant number of cytoskeletal elements, as well as vesicles, lysosomes and polymorphic vacuoles. The lateral branches of the processes are numerous in the predentin and internal parts of the dentin (100–200 µm from the border with the pulp), there are few of them in its middle parts and again many at the periphery. The branches form contacts with branches of neighboring odontoblasts, which can play a significant role in the transfer of nutrients and ions, as well as contribute to the spread of microorganisms and acids during caries.
Nerve fibers directed into dentin from the peripheral parts of the pulp usually penetrate into it to a depth of several micrometers, individual fibers - to 100–200 microns. Some of the fibers already in the predentin are divided into numerous branches; others pass inside the dentinal tubules along the process of the odontoblast or have a spiral course, entwining it and occasionally forming branches running at right angles to the tubules. Nerve fibers are usually thinner than the processes of odontoblasts and in some places form connections with them such as tight junctions and gap junctions. Most researchers believe that nerve fibers in the dentinal tubules influence the efficiency of odontoblast activity, that is, they are efferent, and do not perceive changes in their environment.
The main substance of dentin, located between the dentinal tubules, has a fibrillar structure and consists of collagen fibers and a homogeneous cementing substance (Fig. 9 2).
Rice. 92. Electronic scanogram of a demineralized area of peripulpar dentin.
1 – collagen fibers of the organic matrix; 2 – openings of dentinal tubules.
The location of collagen fibers and their structure change in different parts of dentin. In this regard, two layers of dentin are distinguished: outer, or mantle (mantle) dentin and inner, or peripulpal dentin. In mantle dentin, the predominant fibers are those running in the radial direction (Korff fibers), parallel to the course of the tubules. this arrangement remains closer to the occlusal surface of the tooth crown, and on the lateral surfaces of the crown and in the root area the radial fibers acquire an increasingly oblique direction.
Mantle dentin smoothly transitions into peripulpal dentin, in which, along with radial fibers, there are a large number of fibers located parallel to the surface of the pulp. The mantle dentin matrix is less mineralized than the peripulpal dentin matrix and contains relatively fewer collagen fibers.
In peripulpar dentin, the fibers are located tangentially (Ebner fibers) and almost at right angles to the dentinal tubules.
The location of Ebner fibers in the peripulpal dentin coincides with the location of the dentinal plates - layers of dentin deposited from the inside, from the pulp side, during the development of the tooth. The expression of rhythmic growth and layered dentin deposition are Owen's contour lines and Ebner's growth lines.
Owen's lines usually run at right angles to the dentinal tubules and correspond to periods of rest in the activity of odontoblasts. During this period, less intense calcification of the dentin substance and the formation of very small interglobular spaces occurs (Fig. 93).
In baby teeth and first permanent molars, a contour (neonatal) line is often visible, separating the layer of dentin that formed during the prenatal period of life from the dentin that formed after birth. According to B. Orban [72, 73], this line corresponds to incompletely calcified dentin that arose in the first 2 weeks after birth due to metabolic disorders during the period of adaptation of the newborn to sudden changes in environment and nutrition.
The number of Owen lines can increase in pathological conditions of the body (long-term illnesses accompanied by increased body temperature, unstable nutrition, etc.).
Rice. 93. Microphotograph of a longitudinal section of a tooth (from L.I. Falin).
1 – Owen lines; 2 – dentinal tubules.
The thinner Ebner lines, directed almost perpendicular to the dentinal tubules, are located closer together than the Owen lines, with a periodicity in the crown of about 18–20 μm. Between Ebner's lines there are lines, the distance between which is usually constant (about 4–5 µm) depending on the location - at the apex or at the root of the tooth. It is assumed that the latter reflect the daily rhythm in the formation of dentin, and the Ebner lines correspond to the 5-day cycle of the formation of the organic dentin matrix, i.e., periods of less mineralization of the main substance of dentin.
Secondary and tertiary dentin.
Dentin deposited in the teeth of an adult during his life is called secondary (regular, or physiological) reparative dentin (Fig. 94).
In addition to the slower rate of formation, it differs from primary dentin (which arose during the embryonic development of the tooth) in having a less regular structure. This is expressed in a change in the course and number of dentinal tubules and collagen fibers with a lower degree of mineralization. The most active deposition of secondary dentin occurs in the side walls and in the roof of the pulp chamber, and in multi-rooted teeth - in its bottom. In this regard, with age, the shape of the pulp chamber changes: the pulp horns become less protruding, and its volume decreases. The intensity of secondary dentin deposition in men is higher than in women; it decreases with age. The thickness of the secondary dentin layer can be used as one of the indicators to estimate the dental age of an individual.
Rice. 94. Microphotograph of a vertical section of a mandibular molar.
1 – secondary (reparative) dentin; 2 – vestibular horn of the pulp; 3 – lingual horn of the pulp; 4 – dentin; 5 – enamel; 6 – tooth cavity.
The production of secondary dentin sharply increases with the destruction or abrasion of enamel and exposure of dentin (caries, increased tooth abrasion, exposure to chemicals, etc.). At the same time, in areas of the pulp corresponding to the area of tooth damage, there is a deposition of more or less significant masses of replacement dentin, which can protrude into the tooth cavity and change its configuration. Such dentin is called tertiary (irregular). Unlike secondary dentin, which completely lines the surface of the pulp facing the dentin, the formation of tertiary dentin occurs more or less locally, that is, only in places of the strongest influence of an unfavorable factor (Fig. 95).
The main function of tertiary dentin is to protect the dental pulp from the spread of bacteria, toxins, etc. It can form in any part of the wall of the tooth cavity, but most often appears in the area of the pulp horns. According to its structure (presence of osteoblasts, irregular course of dentinal tubules or even their absence, weak mineralization), it is closer to the bone and is therefore called osteodentin by some authors. In the dental cavity of elderly people, foci of osteodentin are also normally found, which can significantly change the configuration of the cavity, up to its complete obliteration.
Rice. 95. Topographical features of tertiary dentin (scheme).
1 – enamel; 2 – dentin; 3 – tooth cavity; 4 – defect in the area of the tooth neck; 5 – tertiary dentin; 6 – enamel defect (caries).
Denticles.
In the dental pulp, round or irregularly shaped bodies are sometimes observed, consisting of dentin or dentin-like tissue, which are called denticles, or pulp stones. According to their position in the pulp, they are divided into free, that is, lying directly in the pulp; parietal, maintaining connection with the tooth wall; interstitial, which arise when free or parietal denticles are overgrown with new layers of secondary dentin (Fig. 96).
As a result, the denticle becomes immured in the wall of the tooth (this process is more often observed at the root of the tooth, closer to its apex). Depending on the structure, it is customary to distinguish between highly organized (canalized) and low-organized (devoid of canaliculi) denticles. The sources of their formation are odontoblasts. As noted, denticles have the structure of dentin or dentin-like tissue, therefore they are called true denticles, in contrast to false denticles, which are foci of limited calcification in the pulp tissue.
The sizes of denticles are very variable - from barely noticeable grains to 2–3 mm in diameter. As a result of their growth, denticles can merge with each other, filling the entire tooth cavity or root canals, preventing endodontic manipulation. By squeezing the nerve fibers of the pulp, denticles can cause pain of a pulpitic nature. The reasons for their formation are not clear enough. Denticles are found both in the teeth of elderly people and in the teeth of young people, and even in tooth germs before their eruption [42, 61].
Rice. 96. Topography of denticles in the dental pulp (diagram).
1 – enamel; 2 – dentin; 3 – interstitial denticles; 4 – free denticles; 5 – parietal denticles; 6 – dental pulp.
5.1.1.2. Age-related changes in dentin
Dentin is a living tissue that is produced continuously throughout life. The formation of secondary and then tertiary dentin leads with age to a decrease in the size and volume of the tooth cavity - a process called pulp recession by clinicians. In addition, in the teeth of elderly people there are areas of dentin in which lime salts are deposited not only in the main substance, but also in the dentinal tubules and processes of degenerating odontoblasts. As a result, obliteration (“physiological” sclerosis) occurs, i.e., complete closure of the lumen of some groups of dentinal tubules, while the refractive indices of the tubules and the main substance are aligned and such areas look transparent. These areas are called sclerotic (transparent) dentin. Such dentin is also formed during caries or increased wear of teeth (“pathological” sclerosis), which can be considered as a protective reaction of the tooth to the action of destabilizing factors, which protects the pulp from irritation and infection (Fig. 97).
Rice. 97. Microphotograph of a vertical section of a mandibular incisor.
1 – enamel; 2 – dentin; 3 – dead paths in dentin; 4 – sclerotic dentin; 5 – reparative dentin; 6 – tooth cavity.
The formation of transparent dentin most often begins in the apical part of the root and progresses slowly towards the crown.
Typically, mineralization occurs first in the periodontoblastic space, and then captures the odontoblast process, less often vice versa.
Due to the fact that dentin sclerosis reduces its permeability, it can prolong the period of pulp viability. Obliteration of the dentinal tubules also leads to a decrease in tooth sensitivity.
With age, especially with increased tooth abrasion, the death of part of the odontoblasts and their processes and blockage of the inner ends of the corresponding dentinal tubules with tertiary dentin are also often observed. The contents of such tubules disintegrate, and the cavities of the tubules are filled with air or other gaseous substances. As a result, on thin sections of a tooth, groups of such tubules appear black in transmitted light (Fig. 98).
E. Fish [51] called groups of such tubules “dead paths”. The sensitivity of dentin in these areas is reduced.
In most cases, the dead spaces on the pulp side are, as noted, closed by the formation of irregular tertiary dentin.
Rice. 98. Microphotograph of a section of dentin.
1 – peripulpar dentin; 2 – pulp; 3 – tertiary dentin; 4 – “dead paths” of dentin.
5.1.1.3. Morphology of dentin in a clinical aspect
The “instant” reaction of the pulp to damage to dentin is due to the presence of a system of dentinal tubules, which can become pathways for the invasion of microorganisms, contributing to the rapid spread of the carious process. It has been shown that in cases where 1 mm2 of the dentin surface remains unprotected by enamel, about 30,000 odontoblasts are destroyed. Toxins, drugs and chemical reagents can penetrate through unprotected dentin, and the receptor apparatus of the pulp is also vulnerable to thermal influences. Dentin is most sensitive in the area of the dentinoenamel junction.
With demineralization of dentin, destruction of the dentinal processes of odontoblasts is observed. Under the action of enzymes secreted by microorganisms, the organic matter of demineralized dentin is dissolved. Along the periphery of the carious cavity towards the dental pulp, the dentinal tubules expand and become deformed. Closer to the tooth cavity there is a layer of compacted transparent dentin with significantly narrowed dentinal tubules.
The morphology of dentin must be taken into account when determining the treatment tactics for defects of hard dental tissues of both carious and non-carious origin.
One of the important clinical stages in the treatment of caries is the removal of softened and discolored (pigmented) dentin using various dental instruments (excavators, burs, etc.). An exception is the treatment of deep caries. In these cases, pigmented dentin is left at the bottom of the crown cavity and therapeutic pads containing calcium hydroxide (calmicin), zinc-eugenol paste and other materials are placed on it for several weeks or months. During this period, pulp cells become odontoblasts through cytodifferentiation, and the latter form reparative dentin, after which the dentist can remove pigmented dentin without fear of opening the tooth cavity.
Secondary and tertiary dentin –
Secondary dentin is deposited (from the pulp chamber side) throughout the life of the individual, which leads to a gradual reduction in the volume of the pulp chamber. Secondary dentin, like primary dentin, is produced by odontoblasts. Secondary dentin differs from primary dentin (formed during the formation and eruption of teeth) by a less regular structure, which is expressed in a change in the direction and number of dentinal tubules and collagen fibers, as well as a lower degree of mineralization. The size of the dentinal tubules itself also decreases.
Deposits of secondary dentin occur more actively in the area of the roof of the pulp chamber, as well as in its side walls. In addition, secondary dentin is actively deposited in the area of the bottom of the pulp chamber (in multi-rooted teeth). In connection with all this, the shape of the pulp chamber gradually changes - the pulp horns become flattened, its volume decreases. An interesting fact is that in men the intensity of secondary dentin deposition is higher. In addition, it is noted that with age, the intensity of secondary dentin deposition decreases.
Another type of dentin is the so-called “tertiary dentin” (dentinum tertiarium), which is also called reparative or irregular. Unlike secondary dentin, which is produced fairly evenly from the inner surface of the pulp chamber, the formation of tertiary dentin occurs locally. Local dentin formation is caused by the influence of strong irritating factors on the tooth, for example, this occurs when the enamel is destroyed or increased abrasion, or dentin is exposed.
With slowly developing caries, the production of tertiary dentin allows, for some time, to prevent the penetration of pathogenic bacteria and their toxins into the pulp. Thus, tertiary dentin performs a protective function. It is also worth noting that tertiary dentin is also characterized by the incorrect direction of the dentinal tubules, or they may be completely absent.
Tooth dentin: histology
Below you can see the histology of tooth tissue in stunning resolution, as well as an excellent lecture on dentin morphology in English (if you wish, you can turn on subtitles and select translation from English into Russian in the settings).
Strengthening baby teeth enamel
As was said earlier in relation to baby teeth, their enamel is more vulnerable. To protect it, saving the child from premature loss of dental units and problems in the future, doctors perform the following actions to provide temporary protection:
- Fluoridation involves treating teeth with special fluoride-based compounds; it is recommended to repeat this procedure 2-3 times a year.
- Fissure sealing - the dentist performs the procedure of filling the recesses and grooves of chewing teeth with temporary filling material, protecting the dental structures from the negative effects of harmful microorganisms and other unfavorable factors.
- Application gels and preventive mouth guards for teeth - the method is based on enriching the enamel layer with useful components (fluorine, calcium, vitamins) through the use of special products.
Age-related changes in dentin –
We have already said above that with age, the deposition of secondary and tertiary dentin occurs, which also leads to a decrease in the size of the pulp. But in the teeth of older people you can often notice areas of dentin in which mineral salts are deposited not only in the main substance, but also inside the dentinal tubules themselves (this occurs against the background of degeneration processes of odontoblast processes). As a result, complete obliteration of the tubule lumen occurs, i.e. their physiological sclerosis.
Obliteration of the lumen of the tubules leads to a decrease in tooth sensitivity. In addition, the refractive indices of light in the tubules and in the main substance are aligned in this case, and therefore such areas of dentin appear transparent. Accordingly, such dentin is often called “transparent” or “sclerotic.” The formation of transparent dentin most often occurs first in the apical part of the root, and then slowly spreads towards the coronal part of the tooth. We hope that our article was useful to you!
Sources:
1. Higher professional education of the author in dentistry, 2. The European Academy of Paediatric Dentistry (EU), 3. “Anatomy of human teeth” (Gayvoronsky, Petrova). 4. “Therapeutic dentistry” (Politun, Smolyar), 5. “Histology of the oral cavity” (Glinkina V.V.).
Cement for dental crowns and dentures
In dentistry, cement means not only the natural material that covers the root of the tooth, but also the artificial material with which dentures, crowns and veneers are attached. Artificial cement for crowns is used for their permanent fixation; it is constantly in the patient’s mouth and therefore should be as close to natural as possible. It should not affect the dental pulp or injure the soft tissues of the oral cavity, and should not dissolve in saliva or shrink during use.
Therefore, in modern dentistry, only those materials are used that best meet the requirements.
Denture cement is also called dental glue, because it is used to glue not only crowns and bridges to teeth, but also veneers, lumineers, and other dental structures, the main purpose of which is to give the patient a snow-white Hollywood smile.
Functions of tooth root cement –
1) Protective function – the content of inorganic components in cement reaches 70%, which makes it resistant to mechanical loads. Consequently, one of its functions will be to protect root dentin from damaging effects.
2) Participation in the formation of periodontium - the formation of periodontal fibers occurs simultaneously both from the cementum of the tooth root and from the bone plate of the alveoli. According to a number of authors, these collagen fibers are subsequently intertwined with each other through immature collagen (procollagen), turning them into a single whole. The depth of immersion of periodontal fibers into the cement of the tooth root is from 3 to 5 μ.t.
3) Fixing (retaining) - cement of the tooth root together with the compact plate of the alveolus and periodontal fibers - ensures fixation of the tooth in the alveolus.
4) Compensatory function - when the length of the tooth decreases as a result of physiological abrasion of the enamel, increased production of cement occurs in the area of the apex of the tooth root. As a result, the tooth is pushed out of the alveoli into the oral cavity, and thus the size of the clinical crown of the tooth increases. This becomes especially noticeable in elderly patients.
5) Participation in reparative processes - for example, when the cause of root resorption is eliminated, its partial restoration may occur. Or, if there is a crack in the tooth root, cement may form between the fragments, which can lead to the elimination of the defect.
Cementocytes and cementoblasts: functions and composition
These two types of cells make up the cellular cement. They have different compositions and perform different functions.
Cementocytes are located in lacunae and resemble osteocytes in structure. The short processes in their composition are directed towards the periodontium. In turn, functionally active cementoblast cells are located on the surface of the cement and are responsible for the regular appearance of new layers on it. They are the ones who take part in the restoration of damaged parts of the tooth, for example, in the event of a traumatic root fracture, they form a “coupling”.
