I° Definition
ENAMEL is the calcified tissue of epithelial origin covering the dental crowns. It results from the mineralization of the organic substrate synthesized and secreted by the ameloblasts.
2°Physical Properties of Enamel
- It is the hardest of dental tissues.
- Brittle especially when not supported by the underlying dentin.
- Vulnerable to acid attack.
- Its color generally ranges from yellow to gray.
- It is semi-translucent, the color of dentin or any other restorative materials under the enamel greatly affects the appearance of the tooth.
- Healthy enamel is smooth and shiny.
- Enamel is less fluorescent than dentin
- Its density varies according to the regions, higher in the external zone of the enamel than in the internal zone.
- Its thickness is maximum in the occlusal zone (more than 2.5 mm), and gradually decreases as one approaches the dental neck.
- More radiopaque than other dental tissues.
3° Formation of enamel or amelogenesis :
Amelogenesis is the result of secretion of secretory ameloblasts whose origin is the cell of the internal adamantine epithelium (pre-secretory ameloblast) which evolves over time into ameloblasts, of which we will study the important stages:
- Histo-differentiation of the pre-secretory ameloblast:
The pre-Ameloblast will elongate and its nucleus migrates towards the proximal pole of the cell (near the stratum Intermedium); the majority of cytoplasmic organelles accumulate at the distal pole in contact with the basement membrane.
The cells are united between them by the desmosomes at the two poles and by the intermediate fibers fixed on the first ones radiating into the cytoplasm to form the terminal webs; thus the pre-secretory Ameloblast gradually acquires the characteristics of a secretory cell.
- Disappearance of the basement membrane and formation of the dentinal mantle:
The basement membrane is degraded by odontoblastic metalloproteases and the residual fragments are phagocytosed by pre-secreting ameloblasts, resulting in the formation of the dental mantle. This dentinal mantle can induce amelogenesis.
- The secretory ameloblast without extension of TOMES and formation of the internal Aprismatic enamel:
The cell will accentuate its polarization and deposit enamel proteins directly on the dentinal mantle and we will have the formation of the internal Aprismatic enamel, this layer of enamel generally measures 10 µm thick.
- Formation of the papillary layer:
The cells of the stellate reticulum (ER) disappear by apoptosis; there will be an adhesion called “collapse” between the EAE and the SI to form the papillary layer, which allows the approach of vessels from the dental follicle towards the secretory ameloblasts to ensure their nutrition.
- Ameloblast with extension of TOMES and secretion of immature Prismatic enamel:
The ameloblasts will form at their distal pole a short conical extension called the TOMES extension, and the enamel secretory granules are transported to two sites of this extension: a distal site and a proximal site.
1-proximal secretion site (secretion of the inter-prismatic substance): this inter-prismatic substance is secreted by several neighboring ameloblasts which will serve as a mold containing the extension of TOMES giving the enamel-ameloblast junction a sawtooth image.
2-distal secretion site (prisms):
-at the distal secretion site, at the bottom of this mold, each Ameloblast secretes a prism.
-each prism is thus secreted by a single Ameloblast, from the internal Aprismatic enamel (at the enamel-dentine junction) to the surface of the enamel. So each prism crosses the entire thickness of the enamel.
This enamel has a significant protein fraction (amelogenins)
- Maturation ameloblast and secretion of mature enamel:
Ameloblasts reduce their size and the number of their synthesis organelles and enlarge.
They cyclically create a pleated border then a smooth border at their distal pole (80% in the pleated state and 20% in the smooth state); this modulation or alternation allows:
-the balance between acidification and neutralization of the pH of the enamel.
-elimination of protein fragments.
-transport of calcium to allow crystal growth.
-in the end we obtain an enamel composed of 96% crystals and only 3.2% water and 0.8% organic matter.
EXAMPLE: An initial crystal thickness of 3.1nm becomes 29nm after maturation and an initial crystal width of 25nm becomes 65nm after maturation.
- The protective ameloblast:
The ameloblast becomes cubic, it has a significant decrease in these cellular organelles.
-It secretes a basal lamina on the surface of the enamel to which it adheres by the hemi-desmosomes.
-the protective ameloblasts then merge with the papillary layer (EAE+SI) and thus form the reduced adamantine epithelium (EAR).
The role of the EAR is to isolate the enamel from the dental follicle until the tooth has arrived in the mouth.
4° CHEMICAL COMPOSITION:
Enamel is composed of 92 to 96% mineral matter, 3% plasma (water) and 1% organic matter.
A- the phase or organic matrix 1%:
its percentage relatively decreases with the maturation of the enamel; it is made up of proteins, lipids and protein-polysaccharide complexes.
Aa-proteins:
a-1-Amelogenins:
They constitute 90% of the proteins in the enamel being formed, they are rich in proline (25 to 30%), glutamine, leucine and histidine).
Role: They have a strong affinity for hydroxyapatite, they control the orientation of the crystals and keep the crystals at a uniform distance from each other and give them a regular arrangement in the forming enamel; they are very important in immature enamel which is less resistant to chewing forces compared to mature enamel.
a-2-Proteases:
At the enamel maturation stage, these proteases degrade the amelogenin nanospheres, thus allowing the growth in thickness and length of the crystals, which explains the significant resistance of mature enamel to masticatory forces because it is highly mineralized.
a-3-non-amelogenic proteins (ameloblastin, enamelin and tuftelin):
They represent 10% of enamel proteins during amelogenesis.
They are promoters and guides of crystal formation, they initiate the nucleation of crystals and they serve as a guide allowing the crystals to have their hexagonal shape.
Ab- lipids:
are mainly phospholipids and phospholipoproteins.
Ac- protein-polysaccharide complexes:
are present in very small quantities 0.4 to 0.5%.
B-water:
Free water is present in enamel heated up to 200°C, located mainly in the intercrystalline spaces.
The bound water disappears after heating between 200 and 400°C, it contributes to the formation of a protein shell around the crystallites.
C-the mineral phase
It represents 92 to 96% of the composition of enamel, the major element is formed of hydroxyapatite whose formula is close to Ca 10 (P04)6 (OH)2.
according to Goldberg (2008) it is formed from the following elements:
| The element | The Percentage % |
| calcium | 33.6 – 37.4 |
| phosphate | 16 – 17.4 |
| carbonates | 1.95 – 3.66 |
| sodium | 0.25 – 0.90 |
| magnesium | 0.25 – 0.56 |
| chlorine | 0.19 – 0.30 |
| potassium | 0.05 – 0.30 |
other elements in trace form: fluorine, iron, zinc, bromine, copper, manganese, chromium and cobalt.
5°Structure of the enamel:
A) the basic unit = the crystallite:
-the smallest entity in enamel is the single crystal of hydroxyapatite, measured in nanometers, whose chemical composition is close to Ca10 (PO4)6 (OH)2.
-The crystallite: a cut perpendicular to its axis reveals a hexagonal or rhombohedral section depending on the authors (Nanci, 1983), composed of around 2100 crystals.
its height exceeds (2000 to 10000 A°), exceeds 1 to 2 mm in length or more, therefore can completely cross the enamel.
B) Aprismatic (non-prismatic) enamel:
internal in contact with the dentin or external located in the outermost layer of the enamel, it is made up of crystallites parallel to each other and perpendicular to the enamel-dentin junction, oriented like those of the inter-prismatic enamel, this enamel is more often present in the cervical regions than at the level of the cusps (Nanci, 1989).
C) prismatic enamel :
It is made up of two different structures, the prisms and the inter-prismatic substance, it forms the major part of the enamel.
C1: prisms: enamel is formed by the juxtaposition of elementary structures called enamel cords or prisms of 4 to 8 µm, united between them by an inter-prismatic substance.
Each mineralized prism passes through the enamel from the enamel-dentin junction to the tooth surface.
The prisms are hydroxyapatite crystals surrounded by an organic sheath nested within each other.
The prism matrix is secreted by the distal pole of the Tomes extension.
C2: the inter-prismatic substance: it is a mold containing the extension of Tomes whose composition is the same as that of the prisms but which differs only by the orientation of the crystallites which they contain; it is secreted by the proximal pole of the extension of Tomes.
Note: within the prisms, several thousand crystallites are deposited in a fan shape, while in the inter-prismatic substance a hundred crystallites are organized in a network.
C3: the enamel sheath:
It is a thin, non-mineralized border, enriched with organic matrix, located at the interface between prism and inter-prism.
D) Hunter-Schreger bands:
They were first described by John-Hunter in 1971 and later by Christian Heinrich Theodore Schreger.
Whatever the cutting plane, after coloring with cationic dyes such as Alcian blue, we see colored bands appear in the internal zone of the enamel which appear dark called “Diazonies” alternating with other bands which remain light called “Parazonies” which take less dyes.
Diazonies are formed of transversely sectioned prisms, while in Parazonies the prisms are oriented longitudinally without there being any difference in the composition of the prisms.
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E) Retzius striae:
on sections through wear of non-demineralized enamel we can observe lines like brownish bands, these are the striae of Retzius.
in longitudinal section, they form the perikymatia; series of furrows parallel to the neck of the tooth spaced 35 to 50 µm apart; but in places this scalloped interval can reach 100 µm.
In cross-section, these striations appear as concentric circles comparable to the growth lines of trees.
These Retzius striae represent a less calcified region probably resulting from a slowdown in enamel formation.
F) Knotty enamel
The prisms become straight again near the dentine, but in the superficial region their orientation is regular, thus obtaining a tortuous appearance in the depth reminiscent of wood knots, hence the name knotty enamel.
G) Enamel bushes
In places there are poorly mineralized aspects which will overlap to give images of enamel bushes.
The latter have their base at the level of the enamel-dentine junction and spread in a superficial direction.
These enamel bushes were found to be filled with organic substances stainable by histochemical reagents .
H) The enamel lamellae
These are tissue-like structures that extend deep from the superficial surface of the enamel towards the dentine; they are not mineralized but contain organic constituents; they correspond to the accumulation of cytokeratin-type proteins (tuft proteins); they were considered for some time as gateways to microbial invasion; it turns out that this is not the case.
I) The spindles
These are images frequently encountered in the vicinity of dentin and which represent the terminal portions of the tome fibers of odontoblasts.
J) NASMYTH cuticles
Shortly after tooth eruption, the enamel is covered with a thin, translucent amorphous layer or film called the NASMITH film, which represents the remnant of adamantoblasts which, after the formation of the enamel, degenerate leaving a hard keratinized layer on the surface of the crown.
During dental eruption, the remains of the gingival epithelium covering the tooth will form a secondary keratinized cuticle.
Normally after eruption the cuticles will disappear, however the third cuticle called acquired pellicle appears on the entire tooth in contact with saliva; it constitutes the first stage of the formation of bacterial plaque.
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