Dentin

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Dentin (BE: dentine) is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel or cementum and lays over the pulp. By weight, seventy percent of dentin consists of the mineral, hydroxylapatite, twenty percent is organic material, and ten percent is water.^[1] Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel.^[2]

Dentin consists of microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp to the exterior cementum or enamel border.^[3] These tubules contain fluid and cellular structures. As a result, dentin has a degree of permeability which can increase the sensation of pain and the rate of tooth decay.

The formation of dentin, known as dentinogenesis, begins prior to the formation of enamel and is initiated by the odontoblasts of the pulp. Unlike enamel, dentin continues to form throughout life and can be initiated in response to stimuli, such as tooth decay or attrition.

There are different types of dentin, differentiated by appearance and stage of development. Primary dentin forms most of the tooth. Secondary dentin develops after root formation is complete and forms much slower than primary dentin. Tertiary dentin forms as a biological response to stimuli.

Structure

Dentinal tubules are structures that span the entire thickness of dentin and form as a result of the mechanism of dentin formation. From the outer surface of the dentin to the area nearest the pulp, these tubules follow an S-shaped path. The diameter and density of the tubules are greatest near the pulp.^[4] Tapering from the inner to the outermost surface, they have a diameter of 2.5 μm near the pulp, 1.2 μm in the middle of the dentin, and 900 nm at the dentino-enamel junction. Their density is 59,000 to 76,000 per square millimeter near the pulp, whereas the density is only half as much near the enamel.

Within the tubules, there is an odontoblast process, which is an extension of an odontoblast, and dentinal fluid, which contains a mixture of albumin, transferrin, tenascin and proteoglycans.^[5] In addition, there are branching canalicular systems that connect to each other. These branches have been categorized by size, with major being 500-1000 μm in diameter, fine being 300-700 μm, and micro being less than 300 μm.^[6] The major branches are the terminal ends of the tubules. About every 1-2 μm, there are fine branches diverging from dentinal tubules at 45 degree angles. The microtubules diverge at 90 degree angles.

The porous, yellow-hued material is made up of 70% inorganic materials (mainly hydroxylapatite and some non-crystalline amorphous calcium phosphate), 20% organic materials (90% of which is collagen type 1 and the remaining 10% ground substance, which includes dentine-specific proteins), and 10% water (which is absorbed on the surface of the minerals or between the crystals). Because it is softer than enamel, it decays more rapidly and is subject to severe cavities if not properly treated, but dentin due to its elastic properties it is a good support for enamel. Its flexibility prevents the brittle enamel fracturing.

The three dimensional configuration of the dentinal tubules is under genetic control and is therefore a characteristic unique to the order, although in many mammalian species the tubules follow a gentle helical course through the solid matrix.

Types

There are three types of dentine, primary, secondary and tertiary. primary dentine is the most prominent dentine in the tooth, it outlines the pulp chamber. The outer layer is mantle dentine, it is formed by newly differentiated odontoblasts and is approximately 150 micrometer. It is different from the rest of primary dentine in that it lacks phosphoryn, has loosely packed collagen fibrils and is less mineralized.

Newly secreted dentine is unmineralised and is called predentine. It is easily identified in haematoxylin and eosin stained section since it stains less intensely then dentine. It is usually 10-47 micrometer and lines the innermost region of the dentine. It is unmineralized and consists of collagen, glycoproteins and proteoglycans. It is similar to osteoid in bone and is thickest when dentinogenesis is occurring.

Secondary dentine is dentine that is formed after root formation is complete and the tooth is functional. It continues at a slower rate in incremental growths. It has a similar structure to primary dentine. Deposition is not always even around pulp chamber. Deposition causes a decrease in pulp chamber size, this means cavity preparation in young patients greater risk of exposing pulp.

Tertiary dentine is dentine formed as a reaction to external insult such as caries. It is of two type, either reactionary, where dentine is formed from pre-existing odontoblast or is it reparative, where newly differented odontoblast like cells are formed. Tertiary dentine is only formed by odontoblast directly affected by stimulus, the architecture and structure depends on intensity and duration of the stimuli e.g. if the stimulus is a carious lesion, there would be extensive destruction of dentine and damage to the pulp. Thus tertiary dentine would be deposited rapidly, with a sparse and irregular tubular pattern with cellular inclusion know as osteodentine. However if the stimuli is less active, it would be laid down less rapidly with a more regular tubular pattern and hardly any in any cellular inclusions.

Elephant ivory is solid dentin. The structure of the dentinal tubules contributes both to its porosity (useful for piano keys) and its elasticity (useful for billiard balls.) Elephant tusks are formed with a thin cap of enamel, which soon wears away, leaving the dentin exposed. Exposed dentin in humans causes the symptom of sensitive teeth.

Because dentin is softer than enamel, it wears away more quickly than enamel. Some mammalian teeth exploit this phenomenon, especially herbivores such as horses, deer or elephants. In many herbivores, the occlusal (biting) surface of the tooth is composed of alternating areas of dentin and enamel. Differential wearing causes sharp ridges of enamel to be formed on the surface of the tooth (typically a molar), and to remain during the working life of the tooth. Herbivores grind their molars together as they chew (masticate), and the ridges help to shred tough plant material.

Dentin may be demineralized and stained for histological study, unlike enamel. Dentin rates approximately 3 on the Mohs scale of mineral hardness.

A material similar to dentin forms the hard material that makes up dermal denticles in sharks and other cartilaginous fish.

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