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Dentin. Dentin – Composition , Formation , and Structures. Dentinogenesis. O rigin of the dentin.
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Origin of the dentin Dentin is formed by cells called odontoblasts that defferentiated from ectomesenchymal cells of the dental papilla following an organizing influence that emanents from the inner dental epithelium. Thus the dental papilla is the formative organ of dentin and eventually becomes the pulp of the tooth.
For dentinogenesis and amelogenesis to take place normally, the differentiating odontoblasts and ameloblasts will receive signals form each other – “reciprocal induction”.
Stages of Apposition • 1. Elongation of inner dental epithelium; • 2. Differentiation of odontoblasts; • 3. Formation of dentin; • 4. Formation of enamel.
Preparation for the formation of tooth structures • Continued growth of the tooth germ leads to bell stage, where the enamel organ resembles a bell with deepening of the epithelium over the dental papilla; • Continuation of histodifferentiation (ameloblasts and odontoblasts are defined) and beginning of morphodifferentiation(tooth crown assumes its final shape).
Start of dentinogenesis • Begins after the final differentiation of the odontoblats and subodontoblasts; • Odontoblasts extended theire processes; • Released space under the basement membrane; • The first free space is in the growth center.
Enamel organ and dental papilla • The cells of inner dental epithelium exert an organizing influence on the underlying mesenchymalcells in the dental papilla, which later differentiate into odontoblasts; • Outer dental epithelium - cuboidalcells that have a function to organize a network of capillaries that will bring nutrition to the ameloblasts.
Dental Papilla and preodontoblasts • Before the inner dental epithelium begins to produce enamel, the peripheral cells of the mesenchymal dental papilla differentiate into odontoblasts under the organizing influence of the epithelium; • First, they assume a cuboidal shape; • The basement membrane that separates the enamel organ and the dental papilla just prior to dentin formation is called the “membranapreformativa”. preodontoblasts Membrana preformativa
Odontoblastsdifferentiation Preodontoblast; Undifferentiated cell; Basement membrane; 4 and 5 - subodontoblastic cells. It is brought about by the expression of signaling molecules and growth factors in the cells of the inner epithelium; a.The dental papilla cells are small and undifferentiated and exhibit a central nucleus and few organelles; At this time they are separated from inner epithelium by an acellular zone that contains some fine collagen fibrils; b. Committed dental ectomessenchymal cells that are in a state of mitosis or cell division; c.Daughter cells that are competent to become odontoblasts remain in the peripheral zone; d.Differentiatedodontoblasts with a polarized nucleus and citoplasmicextentions;
Preodontoblasts • A -preodontoblasts; • В – preameloblasts; • С –basement membrane; • With the begining of the production of dentin it becomes in dentinoenamel junction; • Differentiation of odontoblasts is mediated by expression of signaling molecules and growth factors in the inner dental epithelial cells.
Acellular zone • This zone gradually is eliminated as the odontoblasts differentiate and increase in size and occupy this zone; • They are characterized by being highly polarized, with their nuclei positioned away from the inner dental epithelium Acellular zone
Differentiation of ectomesenchymalcells of dentalpapillatopreodontoblasts Almost immediately after cells of the inner dental epithelium reverse polarity, changes also occur in the adjacent dental papilla; Ectomesenchymal cells rapidly enlarge and elongate to become preodontoblasts first and then odontoblasts as their cytoplasm increases in volume to contain increasing amount of protein-synthesizing organelles.
Short columnar cells bordering the dental papilla – inner dental epithelium will eventually become ameloblasts ; 5. The odontoblasts as they differentiate will start elaborating organic matrix of dentin, which will mineralize. 6. As the organic matrix of dentin is deposited, the odontoblasts move towards the center of the dental papilla, leaving behind cytoplasmic extensions which will soon be surrounded by dentin; 7. Therefore, a tubular structure of dentin is formed.
Differentiation of odontoblasts The odontoblasts appear like protein-producing cells;
Odontoblasts and subodontoblasts odontoblasts Dentin formation proceeds toward the inside of the tooth; Sub odontoblasts
Odontoblasts • А – nucleus; • В -secretory end; • С - secreted matrix. • They are the dentin-forming cells, differentiate from cells of the dental papilla; • They begin secreting an organic matrix around the area directly adjacent to the inner enamel epithelium, closest to the area of the future cusp of a tooth;
Odontoblast process or Tomes`fiber • The odontoblast develops a cell process, the odontoblast process, which is left behind in the forming dentin matrix; • 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;
Odontoblasts - large pear-shaped cells • Thus, dentin formation proceeds toward the inside of the tooth; • The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix.
Dentinal tubules formation • As the odontoblastic process elongates, a dentinal tubule is maintained in the dentin, and the matrix is formed around this tubule;
Odontoblasts processes • The plasma membrane of odontoblasts adjacent to inner epithelium extend stubby processes into the forming extracellular matrix; • On occasion one of these processes may penetrate the basal lamina and interpose itself between the cells of inner epithelium to form what later becomes an enamel spindles;
Matrix vesicles • As the odontoblasts form these processes, it also buds off a number of small, mambrane-bound vesicles known as matrix vesicles, which come to lie superficially near the basement membrane.
This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.
Odontoblast branching growths on the periphery of the dentin
Stages of deninogenesis • Formation of organic matrix: • Fibrogenesis; • Maturation of the organic matrix; • Mineralization of the matrix.
2 steps of dentinogenesis • Odontoblasts with cytoplasmic processes forming dentinal tubules; • 1. Formation of collagen matrix; • 2. Deposition of calcium and phosphate • (hydroxyapatite) crystals in the matrix.
Synthesis of collagen and its arangement in fibrils and fibers • The collagens in dentin are primery type I with trace amounts of type V collagen and some type I collagen trimer; • The type I collagen is a key structural component of dentin matrix.
Some of the fifteen known types of collagen Some types (of 15) of known collagen
Korff'sfibers (corkscrew fibers) passing between odontoblasts and reach predentin. The question of the origin of these fibers is controversial.
Korff's fibers • The first sign of dentin formation is the appearance of distinct, large-diameter collagen fibrils; • They are 0,1 to 0,2 µm in diameter and called von Korff's fibers; • They originate deep among the odontoblasts, extend toward the inner epithelium, and fan out in the structurless ground substance immediately bellow the epithelium.
Formation of thefirstloyer of dentin – mantledentin The next step in the production of dentin is formation of its organic matrix; Odontoblasts differentiate in the preexisting ground substance of the dental papilla; The first dentin collagen syntesized by them is deposited inthis groun substance. Layer of polarized odontoblasts with Tomes' fibers.
Von Korff`s fibers appear as convoluted, threadlike structures that originated deep between odontoblasts • Following differentiation of odontoblasts, first layer of dentin is produced, characterized by appearance of large-diameter type III collagen fibrils (0.1 to 0.2 μm in dia) called von Korff’s fibers, followed by type I collagen fibers.
Odontoblasts and corkscrew fibers (Korff's fibers) Odontoblastsandnerveendsbetweenthem.
Noncollagenous proteins • The other group of proteins is the noncollagenous proteins; • They are grouped into five categories; • The first and likely the most important group – two proteins, originally classified as dentin-specific: • DPP – dentin phosphoprotein; • DSP – dentin sialoprotein; • After type I collagen, DPP is most abundant of dentin matrix proteins and represent almost 50% of the dentin ECM.
DPP • DPP has a high affinity for type I collagen as well as calcium and protein for initiation of dentin mineralization and is therefore considered a key protein for the initiation of mineralization; • DPP may also affect the shape and size of apatite crystals.
DSP • DSP accounts for 5% to 8% of the dentin matrix; • DSP played a role in cell attachment;
A second category of noncollagenous proteins • Osteocalcin; • Bone sialoprotein; • They are classified as mineralized tissue-spacific, because they are found in all the calciffied connective tissies.
A third group of noncollagenous proteins • They are sinthesized by odontoblasts; • They are: • Osteopontin; • Osteonectin.
Predentin • Newly secreted dentin is unmineralized and is called predentin; • It is easily identified in haematoxylin and eosin stained section since it stains less intensely then dentin; • It is usually 10-47 micrometers 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.
Mantle predentin • As the odontoblasts continue to increase in size, they also produce smaller collagen type I fibrils that orient themselves parallel to the future dentinoenamel junction; • In this way a layer of mantel predentin appears.
The first stage is formation of mantle dentin: • Its means the deposition of matrix which is composed of collagen fibers in a ground substance rich in glycosaminoglycans. • The fibers known as Von Korff`s fibers are argyrophilic. • Subodontoblasts may be responsible for a proportion of the very first matrix including the von Korff`s fibers. • Odontoblasts form the major part of mantle dentin matrix.
Prymary dentin • 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.
