Dentinogenesis

Dentinogenesis is the formation of dentin, a substance that forms the majority of  teeth bulk. Dentinogenesis is performed by odontoblasts, which are a special type of ectomesenhymal cells on the outer wall of dental pulps, and it begins at the late bell stage of a tooth development. The different stages of dentin formation after differentiation of the cell result in different types of dentin: mantle dentin, primary dentin, secondary, tertiary.

Odontoblast differentiation Odontoblasts differentiate from cells of the dental papilla. This an expression of signaling molecules and growth factors of the inner enamel epithelium (IEE).

Formation of primary dentin: Primary dentin, the most prominent dentin in the tooth, lies between the enamel and the pulp chamber (near Dentinoenamel junction). The outer layer closest to enamel is known as mantle dentin. This layer is unique to the rest of primary dentin. Mantle dentin is formed by newly differentiated odontoblasts and forms a layer approximately 150 micrometers wide

Odontoblasts secreting an organic matrix around the area directly adjacent to the IEE, closest to the area of the future cusp of a tooth. The organic matrix contains collagen fibers with large diameters (0.1-0.2 μm in diameter). The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process.

Thus, dentin formation proceeds toward the inside of the tooth Thus, dentin formation proceeds toward the inside of the tooth. The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. Unlike primary dentin, mantle dentin lacks phosphorylation, has loosely packed collagen fibrils and is less mineralized. Below mantle dentin ; lies the circumpulpal dentin, a more mineralized dentin which makes up most of the dentin layer and is secreted after the mantle dentin by the odontoblasts. Circumpulpal dentin is formed before the root formation is completed.

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

Whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process. Odontoblasts increase in size, eliminating the availability of any extracellular resources to contribute to an organic matrix for mineralization. Additionally, the larger odontoblasts cause collagen to be secreted in smaller amounts, which results in more tightly arranged, heterogeneous nucleation that is used for mineralization.

Other materials (such as lipids, phosphoproteins, and phospholipids) are also secreted. The organic matrix of dentin is called predentin which is located directly near the odontoblast process. The dentin in the root of a tooth forms only after the presence of Hertwig epithelial root sheath (HERS), near the cervical loop of the enamel organ. Root dentin is considered different from dentin found in the crown of the tooth (known as coronal dentin) because of the different orientation of collagen fibers, as well as the possible decrease of phosphophoryn levels and less mineralization.

Maturation of dentin or mineralization of predentin occurs soon after its apposition, which takes place two phases: primary and secondary. Initially, the calcium hydroxyapatite crystals form as globules, or calcospherules, in the collagen fibers of the predentin, which allows for both the expansion and fusion during the primary mineralization phase. Later, new areas of mineralization occur

as globules form in the partially mineralized predentin during the secondary mineralization phase. These new areas of crystal formation are more or less regularly layered on the initial crystals, allowing them to expand, although they fuse incompletely.

In areas where both primary and secondary mineralization have occurred with complete crystalline fusion, these appear as lighter rounded areas on a stained section of dentin and are considered globular dentin. In contrast, the darker arclike areas in a stained section of dentin are considered interglobular dentin. In these areas, only primary mineralization has occurred within the predentin, and the globules of dentin do not fuse completely. Thus, interglobular dentin is slightly less mineralized than globular dentin. Interglobular dentin is especially evident in coronal dentin, near the DEJ, and in certain dental anomalies, such as in dentin dysplasia.

Formation of secondary dentin: Secondary dentin is formed after root formation is finished normally after the tooth has erupted and is functional. It grows much more slowly than primary dentin, but maintains its incremental aspect of growth. It has a similar structure to primary dentin, although its deposition is not always even around the pulp chamber.

It is the growth of this dentin that causes the decrease in the size of the pulp chamber with age. This is clinically known as pulp recession; cavity preparation in young patients therefore carries a greater risk of exposing the pulp. If this occurs, the pulp can be treated by different therapies such as direct pulp capping. Pulp capping is most successful if followed by a stainless steel crown. In order to maintain space in the primary dentition, attempts are made not to extract a pulpal exposure. and occurs at a much slower rate. It is not formed at a uniform rate along the tooth, but instead forms faster along sections closer to the crown of a tooth. This development continues throughout life and accounts for the smaller areas of pulp found in older individuals

Formation of tertiary dentin: Tertiary dentin is deposited at specific sites in response to injury by odontoblasts or replacement odontoblasts from the pulp depending on the severity of the injury. Tertiary dentin can be divided into reactionary or reparative dentin. Reactionary dentin is formed by odontoblasts when the injury does not damage the odontoblast layer.

Reparative dentin is formed by replacement odontoblasts when the injury is so severe that it damages a part of the primary odontoblast layer. Thus a type of tertiary dentin forms in reaction to stimuli, such as attrition or dental caries. Tertiary dentin is only formed by an odontoblast directly affected by a stimulus; therefore, the architecture and structure depend on the intensity and duration of the stimulus, e.g., if the stimulus is a carious lesion, there is extensive destruction of dentin and damage to the pulp, due to the differentiation of bacterial metabolites and toxins.

Thus, tertiary dentin is deposited rapidly, with a sparse and irregular tubular pattern and some cellular inclusions; in this case it is referred to as "osteodentin". However, if the stimulus is less active, it is laid down less rapidly with a more regular tubular pattern and hardly any cellular inclusions

Sclerotic dentin: That has become translucent due to calcific ation of the dentinal  tubules as a result  of injury or normal aging.  Alsocalled transparent dentin.dentin char acterized by calcificat- ion of the dentinal tubules