Dentin in Teeth Structure and Functions

Dentin, also known as dentine, is a calcified body tissue that is one of the four main components of teeth, cementum, together with enamel, and pulp. It covers the entire pulp and is normally protected by enamel on the peak and cementum on the base.

Dentine meaning states that it is composed of 45 percent mineral hydroxyapatite, 33 percent organic content, and 22 percent water by volume. Owing to the translucency of enamel, it has a significant impact on the colour of a tooth. Dentin, which is far less mineralized and fragile than enamel, is needed to keep the enamel in place. Upon Mohs scale of mineral hardness, dentin is a 3 on the index.

Dentin is differentiated from enamel by two important aspects: first, it shapes during life, and second, it is sensitive. The calcification of dentinal tubules characterises dentinal sclerosis/transparent dentin-sclerosis of primary dentin, a regressive alteration in the tooth. It may happen as a result of caries or abrasion to the dentin, or as a result of the natural ageing process.

Development

Dentin formation, also recognized as dentinogenesis, starts before enamel formation and is caused by the pulp's odontoblasts. Dentin is extracted from the tooth germ's dental papilla. The tooth germ is the primordial structures through which a tooth is developed, such as the enamel organ, the dental papilla, and the dental sac surrounding them. The cell bodies of the odontoblasts persist in the pulp within the tooth, across its exterior periphery, and project into tiny tubules in the dentin after predentin development and maturation into dentin. Dentin is formed during one's life and can be triggered by stimuli including tooth decay or attrition.

Structure

Dentin, with exception of enamel, can be demineralized and dyed for histological examination. Dentin is made up of microscopic channels termed dentinal tubules that radiate outward from the pulp to the cementum or enamel boundary on the outside. Dentinal tubules connect the dentinoenamel junction (DEJ) throughout the crown and the dentinocemental junction (DCJ) in the root to the pulp's outer wall. Such tubules take an S-shaped course from the dentin's outer surface to the pulp's vicinity. The tubules' diameter and density are highest near the pulp.

They do have a diameter of 2.5ηm at the pulp, 1.2ηm in the centre of the dentin, and 0.9ηm at the dentino-enamel junction, easing from the internal to the outermost surface. Close to the pulp, their density varies from 59,000 to 76,000 per square millimetre, while towards the enamel, the density is just half as high.

An odontoblast form, which is an extension of an odontoblast, and dentinal fluid, that includes a combination of albumin, tenascin, transferrin, and proteoglycans, are found inside the tubules. There are also branching canalicular networks that communicate with one another.

Significant divisions have a diameter of 500-1000 nm, small branches have a diameter of 300-700 nm, and micro branches have a length of less than 300 nm. The tubules' terminal points are indeed the main branches. There have been delicate branches diverging from dentinal tubules about 45-degree angles per 1-2ηm. At 90 degrees, the microtubules diverge.

The cytoplasmic extensions of odontoblasts that formerly developed and retain the dentin are found in the dentinal tubules. The odontoblast cell bodies were positioned across the inner aspect of dentin against a sheet of predentin, in which they often form the dental pulp's external boundary. 

Dentin does have a degree of permeability due to dentinal tubules, which can raise the sense of pain as well as the frequency of tooth decay. Dentinal hypersensitivity is thought to be caused by changes throughout the dentinal fluid caused by the processes, which is a form of hydrodynamic mechanism.

Types of Dentin

Main, secondary, and tertiary dentin are the three main types of dentin. Secondary dentin is a coating of dentin that forms only after the tooth's root has been fully developed. A stimulus, including a carious attack or wear, causes tertiary dentin to develop.

Primary Dentin: 

Between the enamel and the pulp chamber is primary dentin, the much more visible dentin in the tooth (near dentinoenamel junction). Mantle dentin is the outermost layer nearest to the enamel. This layer distinguishes itself from the rest of the primary dentin. Mantle dentin is produced by newly differentiated odontoblasts and has a consistent thickness of 15-20 micrometres (ηm).

Mantle dentin lacks phosphorylation, contains loosely packed collagen fibrils, and are far less mineralized than main dentin. The circumpulpal dentin, which is much more mineralized and forms up the majority of the dentin layer, is metabolised by the odontoblasts just after mantle dentin. Just before root formation is finished, the circumpulpal dentin is produced.

Secondary Dentin:

Once root formation is finished, usually, after the tooth has exploded and is functional, secondary dentin (adventitious dentin) is developed. It develops at a far slower rate than primary dentin, but it still has an incremental growth pattern. Even though its deposition may not always be aligned around the pulp chamber, it does have a similar structure to primary dentin. The shrinking of the pulp chamber with age is due to the expansion of this dentin. This is referred to as pulp recession in dentistry, and cavity preparation in young patients brings a higher chance of being exposed to the pulp. If this happens, various treatments, including direct pulp capping, may be used to cure the pulp.

The much more effective pulp capping is preceded by a stainless steel crown. Attempts have been made to not draw a pulpal exposure in order to save space in the primary dentition.

Tertiary Dentin: 

Dentin that forms as a result of external stimuli, including cavities or wears, is known as tertiary dentin. It is either reactionary, in which dentin is produced from a pre-existing odontoblast, or reparative, in which newly differentiated odontoblast-like cells are created from a pulpal progenitor cell as a result of the death of the original odontoblasts. Since tertiary dentin is only produced by an odontoblast that is directly affected by a stimulus, its architecture and composition are determined by the severity and length of the stimulus. For example, if the stimulus is a carious lesion, severe dentin degradation and pulp damage result from the separation of bacterial metabolites and toxins.

As a result, tertiary dentin forms quickly, with a sparse, variable tubular pattern and also some cellular inclusions; it's also termed as "osteodentin" in this case. Osteodentin can be found when there is a Vitamin A deficiency observed during development. When the stimulus is less present, nevertheless, it is laid down more slowly, with a normal tubular pattern and few cellular inclusions.

• Dentin dysplasia type I is an inherited condition in which the "dentin" of a person's teeth develops abnormally. Dentin is the bone-like substance beneath the enamel that makes up the majority of the tooth. Its function is to keep the pulp of the tooth contained. The pulp is a porous tissue with a lot of nerves and blood vessels in it. Since the poorly developed, irregular pulp tissue is mostly found in the roots of the teeth, this condition is termed radicular dentin dysplasia. In small or unusually shaped roots, the teeth lack pulp chambers or might have half-moon shaped pulp chambers.