dentin

During odontogenesis, odontoblasts are critical for the formation of a primary dentin, until the tooth 

becomes functional. When contacts between antagonistic cusps are established, then the formation of secondary dentin starts immediately, and continues throughout life. Initially, odontoblasts constantly produce matrix molecules that result in the formation of a 10 micrometers thick layer, reduced afterward to a daily 4 micrometers deposit. However, there is not much difference between primary and secondary dentin. The only major difference is morphological, and the S-curve of the tubules is more accentuated in the secondary dentin, due to the gradual space restriction of odontoblasts, located at the periphery of a withdrawing pulp

The concomitant formation of inter- and peri-tubular dentin results from two different type of mineralization. Inter-tubular dentin results from the changes occurring between a dynamic non-mineralized predentin and the dentin located behind the mineralization front, a border that we now call metadentin . Polarized odontoblasts are formed by a cell body that is key for the synthesis of extracellular matrix molecules (ECM) components along with a long process where the secretion of ECM takes place both in predentin and dentin. Processes are also implicated in the re-internalization of some fragments after the degradation of some ECM molecules. Some ECM molecules, namely collagen and proteoglycans, are secreted in the predentin, whereas other ECM molecules are secreted more distally near the mineralization front, or even further within the lumen of tubules. In the proximal predentin, odontoblasts are responsible for the secretion of native type I collagen together with some proteoglycans (decorin, biglycan, lumican, fibromodulin) implicated in collagen fibrillation . Some non-collagenous proteins (NCP) are implicated in the nucleation and growth of the mineral phase, or in its inhibition. Most phosphorylated proteins are secreted in the metadentin, near the mineralization front. In the proximal predentin (near the cell bodies) the mean diameter of collagen fibrils is about 20nm, whereas in the central part the mean diameter reaches 40nm, and in the distal part, near the mineralization front, fibril diameter vary between 55–75nm . This suggests that the increased diameter is due to lateral aggregation of collagen subunits . In contrast, in the mineralized dentin, the diameter of the collagen fibrils is stable

The formation of inter-tubular dentin provides a unique three-layer model, very convenient to study matrix-derived mineralization. Anatomically, three are successive layers: 1- the cellular stratum (odontoblast cell bodies and Höehl’s cells , located at the periphery of the pulp), 2- the immature predentin layer, with a constant 15–20 micrometers thickness), and 3- the mineralized dentin, starting at the mineralization front up to the mantle dentino-enamel junction. This model is similar to the compartmentalized bone model, where three parts are also found: the osteoblast/bone lining cells layer, osteoid and bone. This observation may shed light on a processes shared by bone and dentin. However, while there may be some similarities between bone and tooth formation it is also clear there they also have unique properties. For example, bone formation is followed by a constant remodeling due to osteoclast-osteoblast interactions, hormonal influences and matrix metalloproteinase (MMP)s degradation of the existing matrix proteins, whereas after its formation, dentin is a quite stable structure.

Peritubular dentin does not result from such transformation of predentin into dentin, but rather from the adsorption along the lumen of the tubules of an amorphous matrix, may be secreted by the odontoblast processes within dentin, or taking origin from the serum (dentinal lymph). Proteoglycans, lipids and other ECM proteins are implicated in the formation of a thin amorphous network, giving rise to a dense hypermineralized peritubular dentin. In two species, the elephant and the opossum (Didelphis albiventris) the formation of peritubular dentin occurs prior to intertubular dentin, with prominent calcospheritic structures present at the mineralization front [28, 29]. In the other species so far studied, the formation of peritubular dentin occurs within the tubules, some distance away from the mineralization front, and is mostly developed in the two inner parts of the circumpulpal dentin.

After eruption, as a reaction to carious decay or to abrasion, beneath a calciotraumatic line, interpreted as an interruption of normal dentinogenesis, reactionary or tertiary dentin is formed. This dentin is relatively? unstained by the “stains all” method, hence is deficient in acidic proteins some of which are presumed to be phosphorylated. As iss the case for the mantle dentin, this is due either to a defective post-translational modification (ie phosphorylation), or to the absence of these proteins. Reactionary dentin appears either as a layer of the osteodentin type, or as a tubular or atubular orthodentin, depending the speed and severity of the carious attack, the progression of the reaction and the age of the patient. Such dentin may also be a physio-pathological response to the release of some components of dental material fillings, free-monomers of resins or silver amalgam containing mercury. Reactionary dentin is synthesized by odontoblasts, or if these cells are altered, this layer is produced by the subjacent cells of the Höehl‘s layer, issued for the last division of pre-odontoblasts, which are latent adult progenitors. Reactionary dentin is different from what is named reparative dentin. However, this last type of so-called “dentin” does not result from the activity of odontoblasts or their associated cells, but specifically from pulp progenitors, implicated in the formation of a bone-like or in structure-less mineralization (pulp diffuse mineralization or pulp stones). Such structures 

are closer to bone rather than to dentin.