All About Dentine

• Dentine is the hard tissue that forms the bulk of the tooth covered by enamel in the crown or cementum in the root.
• It is composed of a large number of parallel tubules in a mineralized collagen matrix, with tubules containing odontoblast processes.
• Dentine is a sensitive tissue with a nervatic supply and nerve terminals in the tubules.
• It is formed throughout life at the expense of pulp.
• It is regenerative, unlike enamel.

Physical Properties

• Fresh dentine is pale yellow, darkening with age, while enamel is whiter.
• Dentine is harder than bone and cementum but softer than enamel.
• Enamel is harder than dentine, which is harder than bone and cementum.
• Tubular nature and organic matrix provide strength to withstand compressive, tensile, and flexural forces.
• Dentine's strengths are higher than enamel's.
• Dentine's permeability depends on the patency and size of its tubules.
• As people age, tubule occlusion occurs, and permeability decreases.

Chemical Composition

• Dentine consists of approximately:
• 70% inorganic components (calcium hydroxyapatite)
• 20% organic components (mainly collagen type 1)
• 10% water.
• The percentage of inorganic material in dentine (70%) is less than in enamel (96%), which is what makes enamel harder than dentine.

Inorganic Composition (70%)

• Made up of calcium hydroxyapatite crystals (Ca10(PO4)6(OH)2).
• Smaller than enamel hydroxyapatite because they don't stack or form mineralized structures like rods.
• Crystals are calcium-poor and carbonate-rich, increasing susceptibility to caries.
• Inorganic material is found on and between collagen fibrils.

Organic Composition (20%)

• Collagen: Over 90% of the organic material are collagen fibrils, mainly collagen type 1
• Non-collagenous proteins: Phosphophoryn is the main phosphoprotein implicated in mineralization.
• Proteoglycans: Mainly biglycan and decorin, have a role in collagen assembly, cell adhesion, migration, differentiation, proliferation, and mineralization.
• Major glycosaminoglycans: chondroitin 4-sulphate and chondroitin 6 sulphate
• Gla proteins: Small proteins present in low amounts, that bind strongly to hydroxyapatite crystals and may participate in mineralization
• Acidic proteins: such as osteonectin, osteopontin, used in histology or histopathology to identify odontoblasts and their products
• Growth factors: (insulin growth factor 2) IGF-2, (bone morphogenetic protein 2) BMP2, (tissue growth factor beta TGF-B, are absorbed from circulating tissue fluid.
• Lipids: Comprise 2% of the organic content, phospholipids may be involved in the formation and growth of apatite crystals.

Regions of Dentine

• Predentine: first layer formed.
• Primary dentine: Forms before birth and occlusion
• Secondary dentine: Forms after tooth eruption and occlusion
• Tertiary dentine: Forms due to pathological problems.
• Mantle dentine: Found near the enamel.

Dentinal Tubules

• Dentine consists of numerous tubules sitting on a mineralized collagen matrix called inter-tubular dentine.
• Dentinal tubules extend from the pulp surface to the DEJ & the CDJ.
• Collagen matrix lies between tubules.
• Matrix between tubules is highly mineralized
• Odontoblasts go inside secreting dentin until it sets in the pulp
• Longitudinal sections of tubules follow a curved sigmoid course in the crown, referred to as primary curvatures.
• The tubules are horizontal in the root, under cusps and incisal edges and vertical.
• Cross-sections near the pulp reveal the tubules as round shapes.
• Cross-sections near the DEJ show the tubules as circles but smaller in size.
• In areas where the S-shape goes up, the shape appears as slots (B section).
• Tubules are 2.5 um in diameter at the pulpal end, and 1um or less at DEJ
• As odontoblasts retreat inwards, they occupy a smaller area, causing the tubules to become closer.
• Near the pulp, 22% of the cross-sectional area is occupied by dentinal tubules, while at the DEJ, it's 2.5%.
• The number of tubules near the pulp is higher than near the surface because the surface area becomes smaller near the pulp
• The number is different from pulp to DEJ because at the pulpal end, the surface area becomes smaller than the surface area at the surface
• The tubules are gathered together in small spaces and it appears to be more numerous.
• Ground, partially demineralized section:
• Dentine between tubules (inter-tubular dentine) is hypomineralized.
• Dentine in the tubules (peritubular dentine / intra-tubular dentine) is hypermineralized.
• Odontoblastic processes seen inside tubules.
• Demineralized section:
• Dentine inside the tubules (pertubular dentine) disappears due to high mineral content (inorganic).
• Collagen matrix between tubules (inter-tubules dentine) remains because it is hypomineralized (organic)

Secondary Curvatures

• Crown tubules are not straight, instead form a curved S shape, known as primary curvature.
• Finer curvatures are called secondary curvatures.
• Secondary curvatures may coincide in adjacent tubules.
• Low magnification reveals a line crossing the dentine, named the contour line of Owen.
• Contour line of Owen are not commonly seen in most of the dentine.
• It is present at the junction of primary and secondary dentine.
• Odontoblasts change direction simultaneously.
• Contour lines of Owen can be seen:
• Many tubules form the same secondary curvature inside the primary curvature.
• Between the primary and secondary dentine.

Dentinal Tubule Branches

• Have lateral branches in-between tubules.
• The most profuse branching is in the peripheries/terminal branches.
• A) near enamel and mantle dentine.
• B) Near the cementum in the root, terminal branches loop to form the granular layer of Tomes

Contents of Dentinal Tubules

• Odontoblastic Processes
• Afferent & Sensory Nerve Terminals
• Antigen Presenting Cells Processes
• Extracellular Dentinal Fluid
• Peritubular or intratubular dentine
• (that covers the tubule from inside)*

Odontoblastic Processes

• Odontoblasts send processes inside the tubules, responsible for forming peritubular dentine.
• Variable structure
• More organelles in the predentine area.
• Microtubules & intermediate filaments along microfilaments and the process.
• Processes vary in length, may reach DEJ, or remain only in predentine.
• In predentine near the pulp, the process occupies almost the full width but thins towards DEJ, creating a periodontoblastic space.
• Length varies, sometimes reaching the terminals of the tubule.
• Three hypotheses explain the location of components at the end or periphery of tubules near the enamel:
  • When odontoblasts retreat toward the pulp, processes stay in place.
  • Predetermined length causes cells and processes to retreat together.
  • Processes degenerate for some reason (degradation at the peripheral end).

Afferent Nerve Terminals

• Mainly present in the inner layers.
  • Intimate relation with odontoblastic processes. Sensory Terminals:
• Extent in tubules is not certain.
• Mostly in coronal dentine beneath cusps (80% of tubules), sparse in cervical and root dentine.

Antigen Presenting Cells Processes

They appear as small processes in the tubules near the pulp. immunocompetent antigen presenting cells Within and beneath odontoblasts. Processes limited to the predentine. Extend to the circumpulpal dentin in tubules under carious dentine if there is a carious region.

Extracellular Dentinal Fluid

• Fluid coming from dental pulp into the tubules.
• The odontoblasts is responsible for the composition of this fluid.
• Higher potassium and lower sodium ions (higher osmolality helps get toxins and pathogens outside tubules during caries).
• This balance affects membrane properties of cells.
• Positive force from pulpal tissue pressure (defense criteria).

Peritubular or Intratubular Dentine

• Covers the tubule from the inside.
• Found at walls of newly formed tubules at the pulp surface.
• Made of highly mineralized type I collagen (5-12% more mineralized than inter-tubular dentine).
• Maturation reduces the lumen and may lead to complete obliteration.
• Lacks collagen matrix. Lost in demineralisation.
• Proteins inside differ from those located in the intertubular dentine.
• The peritubular proteins looks like fibers, but inside the tubules it's amorphous. which facilitates mineralizing and making it highly mineralized.
• Main protein in intratubular dentine differs from phosphophoryn (main protein in intertubular dentin).
• Matrix in demineralized sections looks amorphous in the electron microscope rather than fibrous (collagen appearance).
• Has a different crystalline form
• Has some hypocrite areas In outer dentine, peritubular dentine make up two-thirds of cross-sectional area and approximately 3% near the predentine.
• At DEJ, there is more peritubular dentine, smaller odontoblastic processes.
• At the pulpal end, there is less peritubular dentine, wider odontoblastic processes.

Translucent Dentine

• Physiologic aging causes complete obliteration of tubules with peritubular dentine.
• Most often in root dentine.
• Refractive index becomes similar.
• When a ground section of a root is placed in water (different refractive index), regions blocked by peritubular dentine will appear translucent.
• Shape: Butterfly
• Is due to convergence of the tubules (smaller surface area near the pulp).
• Increases with age.
• Used for age estimation in forensic dentistry.

Intertubular Dentine

• Between the tubules.
• Collagenous fibrous matrix like woven wool.
• Decreased radiographic and electron density.
• Less mineralized than peritubules.

Regions of the Dentine

• Properties and composition vary from predentine to DEJ.
• Mineral content decreases and mineral crystal thickness increases towards DEJ.
• Hardness and elastic modulus decrease towards the junction.

Crown

• Four regions:
  • Mantle dentine
  • Interglobular dentine
  • Circumpulpal dentine
  • Predentine

Root

• Hyaline layer
• Granular layer of Tomes
• Interglobular dentine
• Circumpulpal dentine
• Predentine

Regions Classified Differently

• Primary
• Secondary
• Tertiary
• Sclerotic
• Translucent

Mantle Dentine

• The outermost first-formed layer.
• Has a width of 20-150 micrometers.
• Differs from circumpulpal dentine
  • 5% less mineralised.
• Collagen fibrils lie perpendicular on DEJ to prevent small cracks in enamel near DEJ.
• Branching of tubules

Interglobular Dentine

• Underneath the mantle dentine is a layer of interglobular dentine (area between globules).
• Dentin minerals deposit as globules (calcospheres) that fuse to form tissue
• Some areas experience failure of fusion beneath the mantle layer in the crown.
• Incomplete fusion: If globules fail to calcify or areas between them don't calcify, it will form areas of interglobular dentin.
• Contain hypocalcified areas under the mantle dentine.
• Tubules pass through these areas without deviation, peritubular dentine
• In both crown and root

Circumpulpal Dentine

• Forms the bulk of the dentine, there is nothing special inside of it, tubules in mineralised matrix.
• Uniform in structure except at peripheries, interglobular dentine, and predentin.

Predentine

• The innermost layer.
• Collagen matrix before it calcifies.
• Pale-staining appearance in demineralized H&E sections.
• Mineralization shows a globular/linear appearance (dentinogenesis).
• Is 10-40um in width, is thicker in young teeth

Structural Lines in Dentine

1.) Lines associated with the primary curvatures of dentinal tubules. 2.) Lines associated with the secondary curvatures of dentinal tubules. 3.) Incremental lines: von Ebner's lines & Andresen lines. -All of them are approximately perpendicular to dentinal tubules.

Schreger Lines (Primary Curvatures Lines)

• S-shaped tubules from the pulp to DEJ, the line that shows this convexity and concavity together for all tubules creates these lines.
• Found in longitudinal sections.
• Few are seen in one section
• Difficult to see in cross sections.

Contour Lines of Owen (Secondary Curvatures Lines)

• Caused by: 1.) Small secondary curvatures inside the primary curvature. 2.) Neonatal line: A contour on the line when composition, minerlisation, and curvature of tubules change. 3.) A line between primary and secondary Dentin that can be considered a contour line.
  • Coincidence of secondary curvatures.
  • Rare in primary dentin.
  • Interrupted , doesn't follow the whole lane of dentine

Neonatal Line

• Change in composition of matrix and minerlisation of dentin before and after birth.
• Near the dentinoenamel junction.
• Lines are small and likely found with primary/secondary lines

Incremental Lines

• Short term striations (von Ebner's lines) & Long term striations (Andresen lines).
• Visible in ground sections, demineralized sections under polarized light or in microradiographs
• Cause: Fluctuations in acid-base balance effects the mineral content, causing refractive index change

1. Von Ebner's Lines:

• Are like short term striations(cross striation), -Almost daily deposition of dentine.
  • needs high magnification
  • Cuspal dentine: ~4um separate every 2 lines.
  • Root dentine near the granular layer: ~2um separate every 2 lines.

2. Andersen Lines:
   • Are long term lines, weekly in dentine secretion

Regions of the Dentine

• Mineral content decreases and mineral crystal thickness increases towards DEJ.
• Properties and composition vary from predentine to DEJ.
• Hardness and elastic modulus both decrease towards the junction.
• Primary
• Secondary
• Tertiary
• Sclerotic
• Translucent

Age Related and Post Eruptive Changes

• Physiological age changes affect
• Secondary dentine
• Translucent dentine
• Changes associated with dentinal responses to stimuli (pathological):
• Tertiary dentine
• Sclerotic dentine
• Dead tracts of Fish

Secondary Dentine

• Begins to form as soon as the root is completed and the tooth erupts in the oral cavity.
• Still forms in unerupted, impacted teeth.
• Very similar to primary dentine.
• Sudden change in tubule direction as line of Owen.
• Slower deposition, closer incremental lines
• Accelerated production on pulp floor, leading to narrowing of pulp chamber and canals w/ age In physiological ageing, especially in root dentine, the tubules can become completely occluded with peritubular dentine to form translucent dentine.
• Particularly pronounced at the root apex

Tertiary Dentine (Pathological)

• Dentine secreted in response to pathological stimuli like caries, leakage, or cavity preparation.
• Pathological stimuli induce the pulp to increase calcified material.
• Names: Irregular secondary, reparative, reactionary, response dentine, osteodentine.
• Each stimulus evokes different odontoblast reaction, so tertiary has many forms:
  • Alterable appearance and composition
  • May be tubular, contain irregular tubule, or be atubular. Continuity of dentine-dentinal tubules will be lost in many instances.

Sclerotic Dentine (Pathological)

• Slow stimuli like attrition or caries induce material deposition the in tubules to occlude all tubules under the caries.
• Area lacking structure and appearing transparent.
• Similar in appearance to translucent dentine with the exception that this only translucent is physiological
• Different composition from intratubular dentine.
• Easier to fracture.

Dead Tracts of Fish

If strong primary cells/stimulus could be killed the odontoblasts could retreat before intratubular dentine is formed

• Empty Tubules result, and after they get sealed from a pulp, that area will be filled with air, resulting in translucent spots.