Dentin forms bulk of the tooth tissues and occupies both the crown and the root. It is surrounded by the enamel in the crown and cementum in the root. Physically and chemically it resembles bone. Dentin consists of 65% inorganic and 35% organic material. The organic substance consists mainly of type I collagen fibers embedded in ground substance of mucopolysaccharides. The inorganic component consists of plate shaped hydroxyapatite crystals.
Structure of Dentin
- Dentin is made up of dentinal tubules. The dentinal tubules contain the protoplasmic process of the odontoblasts called odontoblastic process.
- The inner wall of the tubule is highly calcified and is termed as peritubular or intratubular dentin. Between the tubules is the intertubular dentin and it is calcified to a lesser degree than peritubular dentin.
- Between the odontoblastic processes and the peritubular dentin is the periodontoblastic space which contains the dentinal fluid.
- The tubules are broader at their point of origin, i.e. at their pulpal end (3–4 mm) and become gradually narrower at their point of termination (1 mm), i.e. at dentinoenamel or cementoenamel junction. Also the tubules are closer together at the
pulpal end (50–90,000 tubules/mm2) and further apart at their terminal end. Therefore the number of tubules per unit area is more at the pulpal end by 4 times than at their terminal end. Hence the intertubular dentin is well appreciated at the terminal end.
- The tubules generally have a doubly convex course, the primary curvature (often called ‘S’-shaped tubules), the first convexity of which is directed towards apex. This is well appreciated in the middle portion of the tooth because the convexity lessens and the tubules gradually become straighter near the incisal/cuspal regions and the apex.
- Apart from this primary curvature the tubules show smaller undulations called secondary curvatures all along their course. Also, all along the course the tubules show lateral branches and terminal branches at their terminal end. The odontoblastic process which contains microtubules, microfilaments, and mitochondria extends into these branches. Few of the odontoblastic process cross the DEJ and they are known as enamel spindles
The dentin which forms before root completion is called primary dentin. Primary dentin is of two types—the [[mantle dentin]] and the circumpulpal dentin. The mantle dentin found along the DEJ is about 20 mm thick, and it contains largerdiameter argyrophilic (silver stained) collagen fibers arranged perpendicularly to the DEJ. The circumpulpal dentin contains smaller diameter closely packed collagen fibers.
The dentin which forms after root completion is called secondary dentin. This forms at a slower rate (1 mm/day) than primary dentin (4 mm/day) and regularly but not uniformly—more formation is seen on the roof and floor of the pulp chamber. Hence this dentin is also known as regular secondary dentin.
Tertiary Dentin (Reparative Dentin)
If the dentin forms as a reaction to an irritant (trauma or dental caries) to protect the pulp, it is called reparative/reactive/tertiary/irregular secondary dentin. The tertiary dentin shows irregularity in the number, size, and arrangement of tubules—the irregularity increases with the rapidity of formation.
Formation of Dentin
- Whenever dentin forms, it forms in two phases—a distinct organic matrix known as predentin which calcifies subsequently but only after another layer of predentin is laid down. Hence a layer of predentin always exist adjacent to the odontoblast.
- Many factors are involved in differentiation of odontoblast, secretion of organic matrix, and in subsequent mineralization. Like other mineralized tissues dentin formation is not a continuous process, the periods of rest are denoted by incremental lines—the incremental lines of Von Ebner which are 4–8 mm apart run at right angles to the dentinal tubules.
- The accentuated incremental lines are known as contour lines of Owen, while those formed during the period of birth are referred to as neonatal lines.
- The mineralization of dentin occurs in relation to collagen fibers as linear deposits (linear mineralization) or by fusion of globules (globular mineralization). Incomplete fusion of globules leads to the formation of interglobular dentin. The interglobular dentin is found near DEJ, and are visible in ground sections as dark spaces under transmitted light.
- Similarly, the Tomes granular layer is visible near cementodentinal junction as minute dark spaces, and it is related to looping and coalescing of terminal portions of dentinal tubules in that region.
Hypersensitivity of Dentin
The nerves which enter dentin do not extend beyond the inner two thirds of dentin. The stimulation of dentin by any agent causes a pain like sensation called hypersensitivity. The theories that explain sensitivity are based on the direct stimulation of the nerves (direct neural stimulation theory) or due to inward or outward movement of dentinal fluid exciting the nerve endings (hydrodynamic theory) or due to stimulation of odontoblastic process (transduction theory).
Age Changes in Dentin
The age changes in dentin include the formation of sclerotic dentin and dead tracts. Sclerotic dentin occurs when hydroxyapatite crystals are laid down in the intertubular dentin and within dentinal tubules to block the tubules against the entry of bacteria, etc. The dentin then becomes transparent in transmitted light hence often called transparent dentin. Permeability of dentin becomes reduced while hardness increases in these regions. Breakage of apical thirds of roots during extraction of teeth of elderly is due to the brittle nature of sclerotic dentin. Dead tracts are areas containing degenerated dentinal tubules which appear dark in transmitted light. These are often seen in attrited or abraded teeth or below the carious lesions.