Dentin: The Predominant Framework of the Tooth

Dentin takes precedence as the primary component in the tooth's structure, stretching nearly the entire length of the tooth. Its external surface is shielded by enamel, which wraps around the anatomic crown, while the anatomic root is safeguarded by cementum. Internally, dentin establishes the encircling walls of the pulp cavity, encompassing both the pulp chamber and pulp canals.

Figure 1: Tooth section view: Beneath the coronal enamel, dentin are found

 Physical and Chemical Composition: The Mosaic of Dentin:

In youth, dentin wears a vibrant yellow hue, aging gracefully to a deeper shade over time. Unlike resilient enamel, dentin boasts a unique viscoelasticity, allowing for subtle deformations. It comprises 35% organic matter and 65% inorganic materials, it showcases a delicate balance. The central region stands firm with a harder texture, while near the pulp, it gracefully transitions into a softer periphery.

Figure 2 :Table showing Organic and Inorganic components of Dentin

Formation of Dentin: The formation of Dentin is known as dentinogenesis which is accomplished cell called odontoblasts. Odontoblasts cell bodies are found in pulp cavity and its long slender cytoplasmic cell process (Tomes fibers) extend well (100-200µm) in the mineralized tubule of dentin. Due to the presence of odontoblastic cell process, the dentin is capable of reacting with physiological and pathological stimuli and is considered living. Dentin formation starts subjacent to the cusp tips and gradually spread towards the apex of the root.

Structure of Dentin

Dentinal tubules : Dentin is permeated by microscopic channels known as dentinal tubules. These tubules extend from the pulp to the outer surface of the dentin, allowing for communication between the pulp and the external environment.

In the crown, a gentle curve prevails, contrasting with the root's S-shaped pattern. Originating at right angles from the pulpal surface, the tubules curve towards the apex, terminating perpendicularly at dentinoenamel and dentinocementum junctions. Terminal branches, more abundant in root dentin, exhibit almost straight paths near the root tip and occlusal edges.

Throughout their lengths, tubules display sinusoidal secondary curvatures, maintaining a thickness of 3 to 10 mm. Dentin thickness varies across teeth, surfaces, and genders, with buccal surfaces showing maximum thickness. Notably, boys exhibit thicker dentin than girls, a difference accentuated during puberty. This intricate variation underscores the complexity of dentin's structural dynamics.

Figure 3: Images showing arrangement of dentinal tubules and odontobastic process

Peritubular Dentin : It is found immediately surrounding dentinal tubules, it is denser and highly mineralized. It forms the wall of the dentinal tubules and its inner dentin is twice thick than that of outer dentin. By its growth it contrict the diameter of the dentinal tubules by 1µm near dentinoenamel junction. When peritubular structure are visualized in a calcified tooth section there is the presence of organic lining known lamina limitants.

Intertubular Dentin: It is the structure that lies between dentinal tubules or between the zone of peritubular dentin. It is highly mineralized and remain intact even after decalcification.Comprising approximately half organic matrix, mainly collagen fibers, it retains a random orientation around tubules. These collagen fibrils, with diameters ranging from 0.5 to 0.2 µm, display characteristic cross banding at 64 µm intervals, a typical collagen feature observed after decalcification.

Figure 4: Scanning electron microscope image showing dentinal tubules, peritubular dentin and intertubular dentin (present5.com)

Types of Dentin

Figure 5:Image showing Types of Dentin

                                                                          (present5.com)  

Predentin: It is the dentin formed just after the formation of enamel and is located adjacent to the pulp tissue and is 2 µm-6µm wide depending upon the extend of odontoblast activity. It is the unmineralized zone of dentin found next to the cell bodies.

Primary Dentin: Dentin is formed before the root completion and is of two types; mantle dentin and circumpulpal dentin.

Figure 5:Table showing the difference between the features of Mantle and Circumpulpal Dentin

Secondary Dentin

It is like a slender band near the pulp and developed post-root completion, contains fewer tubules than primary dentin. Typically, there's a tubular bend at the interface of primary and secondary dentin. Many suggest that secondary dentin forms more slowly, resembling primary dentin but with a lower tubular count.

Its formation isn't uniform, with an increased presence in the coronal pulp chamber's roof and floor, safeguarding the pulp in aging teeth. Unlike primary dentin, secondary dentin isn't prompted by external stimuli, displaying a structure akin to primary dentin. Recognized for tubular regularity, it's termed "regular secondary dentin." Apical dentin, in both primary and secondary forms, features tubular irregularities.

Tertiary Dentin

This pulp dentin complex respond to tooth pathology through the pulpal inflammation defence system and dentin repair formation. Deep dentin formation occurs simultaneously with the pulpal inflammation response and results in the formation of Tertiary Dentin at the pulp-dentin interface. The net effect of this process is to increase the thickness of dentin as a protective barrier for pulpal tissue. There are two types of tertiary dentin i.e, reparative dentin and reactionary dentin.

Figure 6: Two types of tertiary dentin (a)Reactionary Dentin: Its formation occurs in response to moderate dentine injury if primary odontoblast are present to secrete dentin,(b) Reparative dentin: its formation occurs if odontoblast have been destroyed or damaged if the formation of tertiary dentine is performed after stem/progenitor cell recruitment by odontoblast.(onlinelibrary.wiley.com)