OE L9 Enamel Structure

Question 1

How mineralised is enamel?

Answer 1

96% inorganic material

Question 2

Is enamel a vital tissue?

Answer 2

No, enamel is a non-vital tissue.
Ameloblasts are lost as the tooth erupts.
It cannot be replaced or regenerated if damaged.

Question 3

How does enamel compensate for the fact it is non-vital?

Answer 3

Enamel has developed compensatory mechanisms:

• High mineralisation
• Complex organisation

Question 4

Describe the physical characteristics of enamel.

Answer 4

• Translucent
• Varies in thickness e.g. 2.5mm over working surface, much thinner at cervical margins
• 96% mineral, 4% enamel proteins and water
• No collagen
• Hard but brittle (dentine prevents enamel from fracturing)

Question 5

Describe the structure of enamel.

Answer 5

• Enamel rods (prisms) run from ADJ to surface of enamel (1 ameloblast secretes 1 rod)
• Interrod enamel between rods
• At cervical margins the rods meet perpendicular to the surface, at the cusps the rods meet the surafce at a 60-degree angle

Question 6

Are the crystals in enamel rods and interrod enamel the same?

Answer 6

The crystals have the same structure but have different orientation.

Question 7

Describe the secretion of enamel rods vs interrod enamel.

Answer 7

Enamel rod:
- Secreted from secretory surface of Tomes’ process

Interrod enamel:

• Secreted from non-secretory surface of Tomes’ process
• Very little and at much slower rate

Question 8

What is secreted from the edges of the ameloblast?

Answer 8

Residual cell material may be deposited from here along with some enamel proteins (this is where most of the organic material of enamel is likely to come from).

Question 9

What is the rod sheath?

Answer 9

• Narrow boundary between rod and interrod enamel
• Majority of organic material is found in rod sheath
• Formed from cell margins
• Gives a keyhole appearance- allows rods to align in organised arrangement and not slide over eachother

Question 10

What is the fundamental unit of enamel?

Answer 10

The rod and its associated interrod enamel.

Question 11

What are crystallites?

Answer 11

Each rod is made of many bundles of crystallites. Crystallites are narrow and long. Calcium hydroxyapatite is present in the form of these crystallites.

Question 12

What type of information does enamel composition hold?

Answer 12

Enamel composition holds info which can be used forensically.
- Various ions present during enamel formation can be incorporated into HA e.g. strontium, lead, fluoride. This developmental history is preserved in the enamel.
Comparison of enamel with hair/nails provides info on recent vs developmental history.

Question 13

What type of histological sections are used when studying enamel?

Answer 13

Ground sections. Little chemical treatment, tooth is ground down and polished until very thin.
- Can be damaging and possibly introduce artefacts

Question 14

Describe the appearance of the ADJ.

Answer 14

Scalloped. More attachment with dentine.

More scalloping where mastication forces are highest, less close to cervical margins.

Question 15

Is all enamel in prismatic form?

Answer 15

No, the outer 20-70um of enamel and the most inner layer next to the ADJ is non-prismatic due to no Tomes’ processes during its development.

• Called structureless/non-prismatic enamel.
• No organic material, more highly mineralised.

Question 16

What are 3 characteristic features found in enamel?

Answer 16

• Enamel tufts
• Enamel spindles
• Enamel lamellae

Question 17

What size are the 3 enamel features in comparison to eachother?

Answer 17

• Smallest: enamel spindles
• Medium: enamel tufts
• Largest: enamel lamellae

Question 18

What are enamel tufts?

Answer 18

• Project from ADJ up to 1/4 of the way into enamel
• Tufted/branched appearance
• Organised throughout enamel, found at 100um intervals
• Constitute faults in amelogenesis- thought to occur due to abrupt changes in rod direction
• Contain more organic material than rest of enamel, seems to provide organic gaps where small degree of compression may be permitted

Question 19

What are enamel spindles?

Answer 19

• Narrow tubules extending from ADJ to 25um into enamel
• Formed from trapped odontoblast processes in enamel layer (spindles are a continuation of process)
• Seen near cusps
• May inrease adherence between dentine and enamel, not completely certain
• Potential role in sensitivity, may serve as pain ‘receptors’ and transmit info down into dentine

Question 20

What are enamel lamellae?

Answer 20

• Run the entrie length of the enamel
• Difficult to differentiate from cracks
• Made of organic material
• Occur during development, likely due to big group of ameloblasts failing to differentiate so did not produce rods or enamel
• Clinical relevance: big pore running through enamel, risk of caries and sensitivity

Question 21

What are cross striations?

Answer 21

• Lines running perpendicular to rods and lamellae
• Represents changes in enamel secretory rhythm
• Cross striations at 4um intervals
• Enamel is deposited at rate of 2.5-6um per day, these striations represent this daily rhythm
• Appear as contrictions to the rods where less enamel has been deposited so rod appears more narrow

Question 22

What are Striae of Retzius?

Answer 22

• Weekly lines
• Reflect successive enamel-forming fronts
• Seen as concentric rings in transverse sections
• Formed by temporary constrictions of Tomes processes associated with corresponding changes in crystal orientation and degree of mineralisation
• Reduction in number of crystals form these striae

Question 23

What are Perikymata grooves?

Answer 23

Striae of Retzius reach enamel surface in a series of fine grooves running circumferentially around the crown.
Gradually worn down and disappear.
Not seen at cusps.

Question 24

What are neonatal lines?

Answer 24

Striae of Retzia can be accentuted due to systemic disturbances during amelogenesis e.g. fever.

• Neonatal lines are an example of enlarged Striae of Retzius found in all decidious teeth and the first permanent molars.
• All enamel before line is secreted before birth, all enamel after is secreted after birth.

Question 25

What are Hunter-Schreger bands?

Answer 25

• An optical phenomeon
• Ameloblasts activated in groups of 10-12, 1 group begins secreting enamel, move back, next group begins secreting enamel and moves back
• Don’t move outwards completely straight, have a sinusoidal direction
• If you shine a light at these orientations of rods there is a different transmition of light- light and dark bands

Question 26

Where are Hunter-Schreger bands most prominent?

Answer 26

Most prominent in inner third of enamel closest to ADJ, and in lower 2/3rds of the crown.

Question 27

Are Hunter-Schreger bands beneficial?

Answer 27

Yes the complex pattern of rods makes enamel more resistant to fracture and adds to the overall strength of the enamel.

Question 28

What are parazones and diazones?

Answer 28

Parazones: prisms cut longitudinally

Diazones: prisms cut transversely

Question 29

What is gnarled enamel and where is it found?

Answer 29

• Enamel with a more complex twisting of rods

- Found at cusps

Question 30

Describe changes to enamel with age.

Answer 30

• Enamel wears slowly with age depending on diet and masticatory habits
• Reduced translucency due to secondary dentine formation making teeth appear more yellow or due to enamel thinning
• Surface coating and/or staining
• Physical wear: attrition/abrasion
• Chemical wear: erosion