Structure and Function of Enamel

• describe Enamel

  • outer layer of the dental crown

  • hardest tissue in our bodies.

  • it is extremely mineralised

  
• Mature enamel is acellular and avascular.

  TRUE/FALSE

  TRUE

  
  

  acellular= containing no cells

  avascular= few or no blood vessels.
• what are the components of mature mineralised enamel?

  - 96% inorganic material (hydroxyapatite)

  - 4% organic material

  - water
• what does enamel look like in radiographs compared to dentin?

  In radiographs, enamel is more radiopaque (lighter) than dentine and bone due to its high mineral content and high density

  
• what is the primary function of enamel?

  - to mitigate (to make less severe or painful) tooth wear

  - to protect the underlying dentine
• Healthy enamel is translucent, grey-white to light yellow in colour.

  TRUE/FALSE

  TRUE
• what is the typical thickness of enamel?

  2.5mm thick over cusps of permanent teeth (1.3mm in deciduous teeth), tapers to a thin layer along the cervical edge
• how is enamel studied?

  Due to its high mineral content, you need ground thin sections to study its structure.

  (tooth is embedded in resin, cut, and grinded down)

  
  

  -> Demineralised sections won't work because enamel is 96% mineral so there is no underlying protein scaffold that will be left behind so it cannot be visualised.
• how is mature enamel organised?

  Mature enamel is hierarchically organised

  (multiple levels of crystal groupings).

  
  

  - primarily made of Hydroxyapatite crystallites
• describe the Hydroxyapatite crystallites in enamel

  Hydroxyapatite crystallites= Ca10(PO4)6(OH)2

  -> can include Mg, F, CO3 2-

  
  

  Sometimes called hydroxyapatite nanorods (because of their elongate structure). -> we are unsure how long these are, but could be as long as the whole tissue!

  
• what are the Hydroxyapatite crystallites in enamel grouped into?

  two major units:

  1. enamel prism

  2. interprismatic enamel
• describe the enamel prisms

  prisms (sometimes called "rods"):

  clusters of millions of Hydroxyapatite crystallites fused into a ~5-7μm wide "thread", spanning the entire thickness of enamel.

  
  

  • Prisms are large enough to see under a light microscope, especially using crosspolarised light.

  • The directions/orientations of the enamel prisms will appear as darker or brighter regions under cross-polarised light
• what is the growth of each prism is regulated by?

  a single ameloblast (enamel producing cell).
• how are prisms orientated in the outer third of enamel?

  the prisms are roughly parallel to each other and are at high angles relative to the outer enamel surface.

  (This is called radial enamel)
• how are prisms orientated in the inner two thirds of enamel?

  the prisms are arranged into groups (10-13 layers of prisms) that extend at different angles (so the prisms in each group are differently oriented) to neighbouring groups, and even cross each other.
• what is the crossing of neighbouring groups of prisms called?

  prism decussation.

  
• in the inner two thirds of enamel, the bundles of differently oriented prisms occur in bands. what are the bands called?

  Hunter-Schreger bands

  
  

  Within the Hunter-Schreger bands, prisms that were cut longitudinally in section form parazones (dark bands), prisms cut transversely are called diazones (light bands)

  
• what are Hunter-Schreger bands?

  structural changes in the orientations of the neighboring groups of prisms
• why do we have Hunter-Schreger bands and radially oriented prisms within our enamel?

  • A mix of resistance to mechanical failure and the deflection of cracks.

  • Radially oriented prisms maximise the strength of each prism relative to occlusal load.

  • Hunter-Schreger bands mitigate (reduce) and deflect the paths of expanding cracks.
• describe the interprismatic enamel

  it is the enamel around each prism (rod) made of crystallites of a different orientation

  
  

  - labelled as interrod enamel in the picture

  
• what separates the prisms from the surrounding interprismatic enamel?


  the prism sheath

  
• do enamel prisms reach the outer surface of enamel?

  Enamel prisms in relatively unworn teeth do not extend all the way to the surface of the enamel.
• what is the aprismatic layer in enamel?

  - the outermost layer of enamel with a sheet like pattern of Hydroxyapatite crystallites

  (more highly mineralised than the enamel subsurface)

  
  

  (The outer 20-100μm of deciduous enamel and outer 20-70μm of permanent enamel)
• Mature enamel contains several other distinctive features, the development of which can be clinically and even forensically important.

  
  

  give examples.

  • Enamel cross-striations

  • Striae of Retzius

  • Perikymata

  • Neonatal line*

  • Enamel-Dentine Junction

  • Enamel spindles

  • Enamel tufts

  • Enamel lamellae
• describe Enamel cross-striations


  Incremental variations in prism thickness that occur from daily variations in enamel production.

  
  

  • Give the prisms a ladder-like appearance under the light microscope.

  • Under an electron microscope, these are the result of changes in the widths of the prisms.

  
• describe Striae of Retzius

  Sometimes called "Retzius lines",

  these mark the successive positions of the layer of ameloblasts as they form enamel.

  
  

  • Striae result from variations in prism structure that affect the whole crown during enamel formation.

  • There are 6-12 (usually 7-9) enamel cross-striations between each stria of Retzius, suggesting that there is an almost weekly variation in enamel formation.

  • Causes: internal physiological rhythm? Recruitment of new cohorts of ameloblasts as the developing crown enlarges?

  
• how are Striae of Retzius seen under a microscope?

  ------------> In longitudinal sections, the striae of Retzius extend obliquely around the enamel.

  - they are easy to see them under a light microscope, ground thin section because they are so widley spaced.

  
  

  • The lines are interrupted where they meet the outer enamel surface and the enameldentine junction

  
  

  ------------> In transverse sections, the striae of Retzius are concentric rings within the enamel. This is because of the three dimensionality of these incremental lines

  
  

  
  

  ( it is like cutting an onion in 2 different ways)

  
• describe Perikymata

  A groove along the surface of the enamel where a single stria of Retzius has reached the surface.

  
  

  • Perikymata appear as horizontal lines along the surface of a newly erupted tooth crown (these wear away over time).

  
• what is the Neonatal line?

  A particular type of stria of Retzius that corresponds to the physiological stress of being born

  - Results from a subtle change in thickness and direction of enamel prisms that occurs at birth

  
  

  
  

  -------> all the enamel interior to this line is the enamel produced before birth and all the enamel external to the line was produced after birth

  
• where is the Neonatal line found?


  found within the enamel (and dentine) of all teeth that were at the appropriate stage of development when a person is born.

  
  

  • These include: all deciduous teeth and the permanent first molars
• why is the Neonatal line Forensically important?

  as a marker of a date of birth, it can be used determine age (in days) when an infant passed away, or to determine if a body belongs to a still-born infant.

  
  

  
  

  ----> it can also be used in archaeological and anthropological studies of growth rates.
• what is the Enamel-Dentine Junction (EDJ)?


  The interface between the enamel (a hard, brittle tissue) and dentine (a softer, more elastic tissue)

  
• how does the EDJ increase the surface area for attachment of the enamel to the dentine?

  The EDJ is usually not a straight boundary, but a scalloped one in thin section, with the convex surfaces of the scallops facing the dentine.
• Along the EDJ, there are occasions where the dentine tubules appear to cross into the enamel.

  what are these extensions from the dentine into the enamel called?

  enamel spindles.

  
• How do dentine tubules end up crossing into the enamel?

  dentine forms first, followed by enamel.

  
  

  Odontoblast processes become "stuck" in the developing enamel, and may even be pulled some distance through the enamel as the ameloblasts begin to pile up enamel matrix.
• what are Enamel tufts?


  the short, branched spaces within the innermost layers of enamel along the EDJ

  
  

  These are developmental features that may arise from the sharp divergences of neighbouring prisms along the scalloped EDJ. -> so when the prisms go in opposite directions of eachother, they leave behind a pore region with a pooling of larger amounts of protein and water.

  
• what do enamel tufts contain?

  proportionately higher protein within them than the surrounding enamel.

  
  

  • One of the proteins found inside is unique to this region of the enamel à tuftelin
• describe Enamel lamellae

  • Unlike enamel tufts, lamellae are spaces in the enamel that start from the outer enamel surface and extend for different depths into the enamel.

  
  

  • Unlike cracks, enamel lamellae contain organic material (proteins and water) and are formed within the enamel prior to tooth eruption (they are not cracks!)