Author: Sanketh DS, MDS
Dentinogenesis is the process of formation of dentin. It starts off with the deposition of predentin which is an unmineralized organic matrix comprising majorly of collagen and a tiny fraction of non-collagenous proteins. This is followed by mineralisation of the organic matrix/proteins thus forming dentin. The organic matrix/proteins comprise mainly type I collagen with fractions of type III and type V collagen. Dentin phosphoprotein (DPP), dentin sialoprotein (DSP), dentin sialophosphoprotein(DSPP), osteonectin, osteopontin and Gla protein make up for non-collagenous proteins. Together they make up around 20% of dentin and along with water comprise 30% of dentin. The rest of the dentin is made of inorganic material composed of hydroxyapatite.
Formation of dentin begins in the bell stage, when the undifferentiated dental papilla cells near the inner enamel epithelium(IEE) start to differentiate into odontoblasts due to the influence of the overlying epithelium. The inner enamel epithelium, in the early bell stage, before differentiating into ameloblasts secrete growth factors and signalling molecules like transforming growth factor (TGF) and bone morphogenetic proteins (BMP) which help in differentiation of dental papilla cells to pre-odontoblasts. Pre-odontoblasts, then gradually become bigger in size, elongate to become odontoblasts and develop numerous organelles like rough endoplasmic reticulum (RER), ribosomes and golgi complex necessary for protein secretion. The nucleus become polarized and shifts to a position away from the inner enamel epithelium.
SECRETION OF ORGANIC MATRIX
Before we get to how dentin secretion happens lets understand that dentin has two compartments – the mantle dentin and the circumpulpal dentin. Mantle dentin is the first formed dentin which is 15 – 20 microns thick and is confined to the upper layers near the enamel. The rest of the dentin is called circumpulpal dentin.
Odontoblasts once fully differentiated start to secrete collagen molecules and non-collagenous proteins in the extracellular region towards the IEE. The initial mantle dentin matrix comprises of mainly type I collagen and partly von-Korff’s fibres comprising type III collagen. After the odontoblast secretes this initial mantle dentin matrix, it develops a process called the odontoblastic process which extends into the extracellular matrix. This odontoblastic process otherwise called Tome’s fiber keeps elongating as more organic matrix is deposited and odontoblast moves towards the pulp. Odontoblastic processes are finally embedded in dentinal tubules in the fully formed dentin. Hence the mantle dentin does not have dentinal tubules.
Tome’s process is the distal extension of an ameloblast that is responsible for secretion of rod and inter rod enamel. Whereas, Tome’s fiber is the odontoblastic process of the odontoblast present within dentinal tubules in the fully formed dentin.
Once organic matrix is secreted, the matrix is mineralized, but only upto a point where there is a small layer of unmineralized organic matrix between the odontoblast cell bodies and the mineralized portion.
This unmineralized matrix layer is called “predentin” and is always present throughout dentinogenesis between the odontoblast cell body and the mineralized portion of dentin. Predentin is around 15-20 microns in thickness.
Once the mantle dentin matrix is secreted and mineralized, the circumpulpal dentin matrix starts to be deposited. Circumpulpal dentin matrix consists mainly of type I collagen, non-collagenous proteins (NCP) and proteoglycans (PG). The matrix deposition is hypothesized to happen from two points. One is immediately near the cell bodies and this layer of predentin consists of collagen and PG. The NCP, however, are transported via the odontoblastic processes and deposited in the mineralization front i.e in the predentin immediately below mineralized dentin. Thus, the circumpulpal dentin has two compartments – the predentin and mineralized dentin. The predentin is 15-20 microns in thickness, with one part of the predentin immediately adjacent to the odontoblast cell bodies having only collagen and PG. The other portion of predentin is immediately below the mineralized dentin called the mineralization front and comprises of NCP in addition to collagen and PG.
Minerals in dentin are composed of hydroxyapatite. Mineralization starts from the mantle dentin matrix and proceeds to the circumpulpal dentin. When the odontoblast secretes the mantle dentin matrix, it buds off many membrane bound vesicles called matrix vesicles that come to lie in the matrix below the IEE. These vesicles contain enzymes like alkaline phosphatase, metalloproteinase and other proteins that can bind calcium and inorganic phosphate and initiate hydroxyapatite crystal growth. The seed of growth is initiated within the vesicles, then rupture from the vesicles and are deposited in the matrix. These growing crystals further fuse with adjacent crystals and mineralize the matrix.
As mineralization proceeds to the circumpulpal dentinal matrix, there are no matrix vesicles found. What rather happens is calcium is transported from the sub-odontoblastic area to the mineralization front either through the odontoblasts or, in between odontoblasts. Alkaline phosphatase activity and Ca ATPase activity have been reported within the odontoblasts, which may facilitate the transport of mineral ions to the mineralization front. Once in the mineralization front, the NCPs deposited here help in mineralization of dentin. For example, dentin phosphoprotein and osteopontin control and promote growth of apatite crystals, osteonectin promotes binding of apatite crystals to collagen. Gla proteins act as nucleators for the growth of crystals, where they attract and concentrate mineral ions at one point which then grow and fuse with adjacent crystals. Hence NCPs are responsible for the induction and regulation of mineralization.
Mineralization may be globular or linear. In the mantle dentin matrix, collagen may be arranged in whorls or a concentric manner. This may result in globules or concentric masses of calcification which may enlarge and fuse with adjacent globules. Hence globular calcification is mostly found in the mantle dentin and circumpulpal dentin may have both globular and linear calcification. Linear calcification is when the calcification or mineralization front appears uniform.
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