Histology of Dentin (Notes & Video)
The dentin is a hard tissue that forms the bulk of the tooth.
The dentin is similar to bone but is slightly harder, although softer than enamel.
Dentin is elastic, and provides protection in that it prevents fracture of the overlying enamel (enamel is hard and brittle).
The dentin has numerous dentinal tubules that run across its length. Each dentinal tubule houses the cytoplasmic process of an odontoblast (odontoblastic process).
NOTE
- An odontoblastic process is also called Tomes fiber.
- Also, do not get confused with Tomes process and Tomes fiber.
Tomes process is the distal extension of an ameloblast that is responsible for secretion of rod and inter rod enamel.
Whereas, Tome’s fiber is the cytoplasmic process of the odontoblast present within dentinal tubules in the fully formed dentin.Composition of Dentin
The organic matrix comprises mainly type I collagen with fractions of type III and type V collagen. This comprises 90% of the organic material.
The rest of the organic material is composed of Dentin phosphoprotein (DPP), dentin sialoprotein (DSP), dentin sialophosphoprotein(DSPP), osteonectin, osteopontin and Gla protein. These make up for non-collagenous proteins.
Together they make up around 20% of dentin and along with water comprise 30% of dentin.
The rest of the dentin (70%) is made of inorganic material composed of calcium hydroxyapatite crystals (inorganic component).
Histology of Dentin
Dentinal Tubules
The odontoblastic processes are housed in canaliculi called dentinal tubules.
These dentinal tubules run or traverse the entire thickness of the dentin, extending from the inner dentin (actually from the mineralization front just before the pulp) to the dentino-enamel junction (DEJ) (coronally) and cementum dentin junction (in the root portion).
A dentinal tubule is doubly curved and follows an “S” shaped or sigmoid course. These curves are called primary curvatures.
The first curve from the inner dentin (near the pulp) has its convexity rootward (towards the apex).
Dentinal tubules show numerous branches terminally towards the DEJ and cementum dentin junction. These are called terminal branches. Tubules also show lateral branches along their length.
Terminal branching of dentinal tubules is more prominent in the root dentin than in the coronal dentin.
The course of a dentinal tubule is not straight. Each tubule shows minute curvatures or minor kinks throughout its length. These curvatures or kinks are called secondary curvatures.
Dentinal tubules have a larger diameter towards the pulpal surface.
NOTE
- Terminal branching and looping of these branches give rise to the Tomes Granular layer, in the root dentin just below the cementum.
- Secondary curvatures are believed to coincide with one another to give rise to Contour lines of Owen.💡KNOW THY FACTS
The dentinal tubule housing the odontoblast process, is also known to have a thin periodontoblastic space.
This space is thought to be filled by dentinal fluid.
Peritubular dentin
As the name describes, it is the dentin immediately surrounding the dentinal tubule.
Ironically, the peritubular dentin does not actually surround the tubule but rather lines the inner wall of the dentinal tubule. Hence, it may be more appropriately termed “intra-tubular” dentin.
The peri-odontoblastic space in the dentinal tubule exists between the odontoblast process and the peritubular or intratubular dentin.
The peritubular dentin is more mineralized (by 5 – 12%) than the intertubular dentin.
Intertubular dentin
Forms the main portion of the dentin and is found between the dentinal tubules.
It mainly consists of type I collagen fibrils and is less mineralized than the peritubular (or intratubular) dentin.
It is hypothesized that the area or zone between the peritubular and intertubular dentin in hypomineralized and is called the Sheath of Neuman.
This is thought to mark the outer boundary of the dentinal tubule.
Primary dentin
Primary dentin is formed before root completion.
This forms the bulk of the dentin and is of two types – mantle and circumpulpal dentin.
Mantle Dentin
Mantle dentin is the first formed dentin which is 15 – 20 microns thick and is confined to the upper layers, near the enamel. The rest of the dentin is called circumpulpal dentin.
This layer is less mineralized than the circumpulpal dentin.
Mantle dentin matrix comprises of mainly type I collagen and partly von-Korff’s fibres comprising type III collagen.
Von Korff’s fibers are large diameter collagen fibers (larger than collagen fibers in the rest of the dentin).
Circumpulpal dentin
The circumpulpal dentin forms the bulk of the primary dentin.
Is more mineralized than the mantle dentin.
Predentin
There is also a layer of “predentin” that is unmineralized, situated below the circumpulpal dentin, just above the odontoblast cell bodies.
Secondary dentin
Secondary dentin is formed after root completion.
This is a narrow zone of dentin near the pulp especially near the roof and floor of the pulp chamber.
It appears like the primary dentin, though it contains fewer but regularly arranged tubules.
There is a characteristic bend in the zone where the primary and secondary dentin meet.
Deposition of secondary dentin near the pulp (especially near the floor and roof of the pulp chamber) could cause a reduction in the size of the pulp chamber. This is called pulp recession.
Interglobular dentin
Interglobular dentin is usually found in the circumpulpal dentin just below the mantle dentin.
Interglobular dentin are areas of hypomineralized dentin.
During globular mineralization, globules or concentric masses of calcification (calcospherites) enlarge and fuse with adjacent globules. When these globules fail to coalesce, there are hypomineralized zones in between these globules – hence called interglobular dentin.
Dentinal tubules pass through the interglobular dentin.
Interglobular dentin is usually found in teeth of people with Vitamin D deficiency or fluoride excess.
Tomes Granular layer
When root dentin is viewed under transmitted light, there is a zone of granularity in the dentin just below the cementum.
This granular layer is called Tomes granular layer.
The granularity of this layer increases from the CEJ to the apex of the tooth.
Tomes granular layer is believed to be due to the coalescing and looping of the terminal branches of the dentinal tubules.
This layer is hypomineralized when compared to the circumpulpal dentin.
Incremental lines of von Ebner
In ground sections of the tooth, there are fine lines that are visible running perpendicular to the tubules. These are called incremental lines of von Ebner.
These lines represent the boundary of incremental and rhythmic dentin deposition.
Contour lines of Owen
These lines also run perpendicular to the dentinal tubules and are known to be visible due to coinciding of secondary curvatures of adjacent dentinal tubules.
The other school of thought is that these zones result from hypomineralization.
Neonatal lines
Usually found in the deciduous teeth and permanent first molars, the neonatal line is an accentuated line separating the prenatal and postnatal dentin.HIGHLIGHTS - VIVA & ENTRANCE EXAM PERSPECTIVE
The dentin has numerous dentinal tubules that run across its length. Each dentinal tubule houses the cytoplasmic process of an odontoblast (odontoblastic process).
An odontoblastic process is also called Tomes fiber.
A dentinal tubule is doubly curved and follows an “S” shaped or sigmoid course. These curves are called primary curvatures.
Dentinal tubules show numerous branches terminally towards the DEJ. These are called terminal branches.
Terminal branching of dentinal tubules is more prominent in the root dentin than in the coronal dentin.
The course of a dentinal tubule is not straight. Each tubule shows minute curvatures or minor kinks throughout its length. These curvatures or kinks are called secondary curvatures.
Ironically, the peritubular dentin actually lines the inner wall of the dentinal tubule. Hence, it may be more appropriately termed “intra-tubular” dentin.
Primary dentin forms the bulk of the dentin and is of two types – mantle and circumpulpal dentin.
Mantle dentin is the first formed dentin which is 15 – 20 microns thick and is confined to the upper layers, near the enamel.
The circumpulpal dentin forms the bulk of the primary dentin.
During globular mineralization, globules or concentric masses of calcification (calcospherites) enlarge and fuse with adjacent globules. When these globules fail to coalesce, there are hypomineralized zones in between these globules – hence called interglobular dentin.
Interglobular dentin is usually found in the circumpulpal dentin just below the mantle dentin.
Tomes granular layer is believed to be due to the coalescing and looping of the terminal branches of the dentinal tubules.
REFERENCES AND FURTHER READING
Berkovitz BKB, Hollan GR, Moxham BJ. Oral Anatomy, Histology and Embryology. 4th ed. Mosby Elsevier; 2009.
Nanci A. Tencate’s Oral Histology. Development, Structure and Function. 8th ed. Elsevier; 2013.
Kumar GS. Orban’s Oral Histology and Embryology.13th ed. Elsevier; 2011.
Avery JK. Oral development and Histology. 3rd ed. Thieme Medical Publishers; 2002.