Enamel Histology Flashcards
Enamel ‘s physical properties: Strengths? Weaknesses? Variations?
Strengths:
- hardest tissue
- withstand shearing and impact force and has high resistance to abrasion
- high modulus of elasticity
Weaknesses:
- can’t be replaced or repaired
- brittle
- requires support of dentine
- low tensile strength
Variations:
- 2.5mm width over cusps and feather edge at cervical margins
- surface enamel harder and denser
- subsurface enamel porous
- hardness and density reduces from cusp tips
- young enamel appears whiter (increased translucency)
Enamel - Chemical composition?
96% inorganic, 2% organic and 2% water
Inorganic:
- Calcium hydroxyapatite (crystallite form)
Organic:
- free aas, peptides and large protein complexes (non/amelogenins)
Hydroxyapatite crystals - dimensions? shape? composition? structure? substitutions?
Dimensions: - 70nm width and 25nm thickness Shape: - hexagonal cross section Composition: - rich in magnesium and carbonate (compared to peripheries) Structure: - hydroxyl group surrounded by 3 calcium ions, that are surrounded by 3 phosphate ions - 6 calcium ions in a hexagon enclose the phosphate ions Substitutions: - HPO4 and CO3 for PO4 - Sr, Ba, Pb, Na, K and Mg for Ca - F, Cl, Br, I for OH
Fluoride incorporation inhibits caries
Carbonate incorporation promotes caries
Enamel - Water?
2% weight and 5-10% vol
- related to porosity
- present between crystals surrounding the organic component
- trapped within crystalline defects forming a hydration layer
- fluoride ions travel through the water component
Enamel’s organic matrix - small molecules? protein complexes? highest conc? lipid content? peptides? developing enamel? amelogenins? non-amelogenins?
- Mature enamel contains 1-2% (varies 0.05-3%
- 50-90%: small molecules; peptides and free aas (glycine and glutamic acid)
- Large proteins: non/amelogenins
- Highest conc of prot in enamel tufts (at dentine-enamel junction)
- Lipid content 1% by weight of enamel (remnants of cell mem)
- Proteins and peps account for less than 2% of enamel and 25-30% ealy enamel
- Developing enamel is mainly amelogenin produced by ameloblasts
Amolegenins: - hydrophobic and aggregate
- spread throughout developing enamel forming gel matrix and spread
- helps formation of large crystals
Non-amelogenins: tuftelin - derived from plasma albumin
- secreted by ameloblasts
- role in mineralisation
Enamel - Histology?
- Enamel totally lost in demineralised sections
- Enamel mainly studied on ground sections
- Immature enamel can be studied in demineralised sections due to its high protein content
Enamel - basic structure? composition? pattern?
Basic structure:
- prisms (rods)
Composition:
- consists of hydroxyapatite crystals packed into thin rods
Pattern:
- prisms separated by inter-rods (crystals with different orientation)
- slightly undulating course that reflects ameloblast path during secretion
Enamel prisms - cross section? section cuts?
Cross-section:
- pattern III predominates
- prisms have head and tail regions (the tail lies between heads of 2 adjacent prisms)
- an abrupt change of crystal orientation at prism boundary responsible for optical appearance of the boundary
Section cuts:
- variation of prism shape is dependent on the angle of cut
Enamel prisms - Hunter-Schraeger bands? Cusps?
Hunter Schraeger bands:
- every 10 to 13 layers of prisms follow the same direction, but blocks above and below follow a different directions (creating bands)
- seen due to light reflections
- outer enamel, prisms run in the same direction
Cusps:
- prisms appear twisted around each other called gnarled enamel
Aprismatic enamel - overview? pattern? differences?
Overview:
- outer surface layer is aprismatic
Pattern:
- crystallites aligned at right angles to surface and parallel to each other
Differences:
- surface more mineralised than the rest, this is attributed to absence of prism boundaries (organic material located)
Incremental lines - Formation?
Enamel formed in increments, of periods of activity alternating with periods of inactivity
This results in incremental lines:
- cross striations (short)
- enamel striae (long)
Cross striations - overview? formation? variations?
Overview:
- appear as lines crossing the enamel prisms at right angles to their long axes
Formation:
- reflect diurnal rhythm (incremental growth)
Variations:
- in organic matrix
Enamel striae - overview? cross section? number? internal differences? on the surface?
Overview: - run obliquely across the prism - represent incremental lines - known as striae of retzius - do not reach surface Cross section: - run circumferentially like tree rings Number: - 7 to 10 cross striations between the enamel adjacent stira - suggesting weekly intervals Internal differences: - striae further away in middle portion, while closer cervically On the surface: - stria appear as fine grooves - termed perikymata
Surface enamel - differences? pits? caps? focal holes? brochs?
Differences: - surface is harder, less porous, less soluble and more radiopaque - richer in trace elements - less carbonate and aprismatic (highly mineralised) Pits: - can be found on protected areas - within perikymata - mark ends of ameloblasts Caps: - small elevations - result from enamel deposition on top of debris late during tooth development Focal holes: - depressions on the surface - loss of enamel caps - happens through abrasion Brochs: - elevations - radiating groups of crystals - in premolars
Dentine junction - pattern? lateral surfaces?
Pattern:
- scalloped, where shearing forces would be high (beneath cusps and incisal edges)
Lateral:
- smooth surface
