2.2.2 Tubules
Dentin is laid down by odontoblasts during tooth development. As the odontoblast retreats away from the secreted dentin, it leaves behind a process of the cell which remains into adult life, inside a fine tube or tubule. There is a greater density of tubules in cervical and root dentin than in coronal dentin (▶ Fig. 2.8 and ▶ Fig. 2.9).
The odontoblastic process continues to occupy most of the tubule into adult life. It extends the entire length of the dentinal tubule in the crown of the tooth but only about a third of the tubule in dentin around the neck of the tooth. There is less and less recognizable cell content in the tubule as the odontoblastic process gets further away from the cell body and nutrients. Furthest from the cell body the process has no organelles but some microcisterns and microfilaments. There is a space around the odontoblastic process which is occupied by fluid. This fluid is able to flow back and forth along the tubule and may transmit vibrations which convey information to sensors in the body of the odontoblast. Fluid movement within the tubule is thought to cause sensations of pain arising in the nerve endings in the dental pulp. The ease with which fluid is able to move through the dentinal tubules is referred to as dentin permeability.
Fig. 2.8 A SEM image (magnification × 300, window × 1,500) of dentin tubules viewed from the coronal pulp. The odontoblasts and their processes have been removed. Note the high density and large diameter of the tubules, which would make this area of the pulp–dentin highly permeability to fluid movement.
Fig. 2.9 A SEM image (magnification × 300, window × 1,500) of dentin tubules viewed from the root pulp. Note the higher density of tubules in comparison with coronal pulp in the previous image. This high density makes root dentin more permeable and therefore more sensitive to irritants than coronal dentin. The larger apertures are accessories to the apical root canal and may be responsible for spread of infection from the root pulp to the periodontium.
The odontoblastic process secretes dentin around the inner walls which is called peritubular dentin. This secretion continues throughout life but occurs more rapidly if there is mild irritation such as wear of the adjacent enamel or dental caries. However, peritubular dentin is also laid down in the absence of attrition, even in unerupted teeth. It is a feature of aging and is used by forensic dentists to determine the age of human remains.
Peritubular dentin reduces the tubule’s dimension, and may eventually block it completely (▶ Fig. 2.10). When this happens, the dentin is called sclerotic. The high mineral content of sclerotic dentin makes it translucent to light and is easily identified by holding up a ground section of a tooth up to the light.
Peritubular dentin restricts the fluid flow through the dentinal tubule and thus reduces tooth sensitivity from irritants such as wear of enamel and dentin, or loss of cementum covering root surfaces. When peritubular dentin completely blocks the tubule, it protects the pulp from the bacteria and bacterial products occupying more superficial layers of a carious lesion. As production of peritubular dentin occurs most rapidly in response to irritation, it is most pronounced in the dentin nearest the source of irritation at the tooth surface (caries or tooth wear).
Fig. 2.10 A SEM image of dentin which has been cracked in preparation so as to reveal dentin tubules. The dentin in this image was close to the amelodentinal junction. (Magnification × 300, window × 1,500). There are relatively few open tubules as most have been blocked with peritubular dentin. This zone of the pulp–dentin would have a very low fluid permeability.
2.2.3 Permeability
Dentin permeability is influenced by the diameter of the dentinal tubule. This diameter varies according to the distance of the tubule from the pulp tissue. The tubules are at their narrowest dimension near the dentin enamel junction (1 μm) and widest nearer the odontoblast, where the average tubule diameter is 2 to 3 μm. The permeability of dentin to fluids is thus greatest near the pulp. The pulp tissue is therefore particularly vulnerable to deep carious lesions or to tooth preparation near the dental pulp (▶ Fig. 2.11). Dentin permeability is also influenced by the amount of peritubular dentin which has been laid down inside the tubule.
After cavity preparation, the permeability of dentin is effectively increased due to the opening of many tubules, 20,000 for every square millimeter of cut dentin. This permeability is reduced within hours of the tooth preparation. This appears to be due to the obturation of the tubule with fibrinogen from the serum.
If dentin is exposed to the oral cavity before peritubular dentin has formed, the tooth becomes sensitive to sweet, hot, and acid foods. These sensations are thought to be due to fluid movement within the tubule which is sensed by the odontoblast or surrounding free nerve endings (see Chapter 10.4.3 Dentin Sensitivity). Young adults who have bulimia suffer from enamel demineralization due to the presence of strong stomach acids in the oral cavity. Sensitivity of dentin occurs as peritubular dentin has not formed. Dentin hypersensitivity is also a common complaint in patients with receding gingiva. This sensitivity is caused by the loss of cementum covering the root surface and the exposure of root dentin tubules.
2.2.4 Response to Irritation
When dentin is exposed to oral fluids because of enamel wear, or enamel caries, bacterial products may diffuse down the dentinal tubules and cause a hypersensitive response from the odontoblast to normal stimuli. Cutting or grinding procedures on exposed dentin to remove dental caries is painful and usually requires anesthesia. It has been suggested that a normally cooled preparation of a completely caries-free tooth would not be particularly painful as the dental pulp is not hypersensitive. For obvious reasons this is not routine. The most common clinical sign of dentin hypersensitivity is a painful response of the pulp–dentin to hot and cold foods.
When odontoblasts are subjected to chemical or physical irritants, they increase in size and start laying down new peritubular dentin. They also lay down dentin in the roof of the pulp chamber, called secondary dentin, and retreat from the irritation behind this layer.
2.2.5 Secondary Dentin
Secondary dentin may entirely fill the original pulp chamber in an old and worn tooth. It is a normal physiological process related to tooth wear and aging of the tooth. With age, secondary dentin reduces the diameter of the root canal, causing difficulty in performing endodontic therapy for elderly patients. When secondary dentin is gradually formed, the microscopic appearance differs little from primary dentin. The dentin tubules are less regularly arranged, but they are continuous with the primary dentin. If the progression of tooth wear or dental caries is more rapid, the secondary dentin tubules are quite irregular