Structure and Function of Enamel

Question 1

What is the main function of enamel?

Answer 1

Withstand these forces

Question 2

What is the hardest known biological material? Why?

Answer 2

Enamel

Structure is fossilised and the mineral content is what keeps its shape.

Question 3

What is the building block shape of enamel?

Answer 3

Prisms

Question 4

Explain caries

Answer 4

• Demineralisation due to plaque acids
• Loss of mineral = radiolucency

Whiter = more mineral
Darker = less mineral

Question 5

Give information on enamel and dentine

Answer 5

Enamel is a transluscent material.
The dentine is opaque (where the yellow comes from).
Dentine becomes more prominent over time so teeth become more yellow.

Question 6

What is cusp size up to?

Answer 6

Up to 2.5mm thick.

Question 7

What is a fissure?

Answer 7

Where cusps come together.

Question 8

Where does tooth decay first occur?

Answer 8

Teeth decay first in grooves (fissures).

Question 9

What does an enamel fracture lead to?

Answer 9

Whole of the tooth structure of the tooth breaks down as the dentine is exposed and begins to demineralise.

Question 10

What is the name of the bumps on the insical edge of the tooth?

Answer 10

Mamillions

Question 11

What is the waves of the tooth called?

Answer 11

Perikymata

Question 12

What is the name of the pits in the enamel?

Answer 12

Tomes process pits (prism free enamel).

Question 13

Explain the structures visible with the naked eye

Answer 13

• Fractured tooth cleaves alone plains of weakness

- Following enamel prism direction

Question 14

When you fracture a tooth, where does it break?

Answer 14

Fractures across the planes of weaknesses within the material - along the prisms.

Question 15

Give information of enamel prism structure

What happens if all the crystallites line up?

Answer 15

Hydoxapatite Crystallites - 10,000 prism (hexagonal shape and line up in the long axis of the prisms). These crystallites do not line up so the prism boundary is visible so gives a point of weaknesses.

Enamel prism: 4-7um diameter (the prisms get bigger and smaller which helps to lock them together producing incremental lines)

Line up = we do not get any enamel prisms. Prism enamel acts as a glaze coat (aprismatical prism free enamel). The speed of movement of the enamel slows right down.

Question 16

What leads to incremental lines? Explain this

Answer 16

Changes in size of the prism.

Every 7-9 days there is change in secretion of matrix (its slows down). This means all the ameleoblasts slow down and change direction so a prominent incremental line every 7-9 days.

Question 17

What are Retzius Lines?

Answer 17

• Most visible in cervical 1/3rd
• Seen clearly in light and transmission microscopes
• Brown in transmitted light
• Neonatal line very important forensically
• Related to change in prism direction every 7-9 days

Question 18

Where do Retzius lines appear?

Answer 18

The gingival half of the tooth.

Form from ameoloblasts changing production rate every 6-9 days.

Question 19

Perikymata are —- of Straie of —- at surface of tooth.

Answer 19

Manifestation

Retzius

Question 20

Explain the mineralisaiton process which occurs in each stage

Answer 20

Cap stage:
Following epithelium-mesenchymal tissue interactions, dentine is laid down.
Dentine is formed from mesenchymal tissues (neural crest derived). Epithelial tissues line the mouth.
Condensation of cells to form the dental follicle which then go on to form the dentine (epithelial cells lay down above them).

Bell stage: One dentine is laid down, mesenchymal cells above this differentiate to ameleoblasts. A lamina forms from the epithelial cells. Mesenchymal cells lines a knot under the lamina. Dentine lines first and enamel follows

Question 21

What are the 4 tissues present in the tooth?

Answer 21

1. Internal enamel epithelium
2. Stratum intermedium - between ameloblasts and stellate reticulum
3. Stellate reticulum - cushion over surface of cells
4. External enamel epithelium

Question 22

What are the two types of matrix which is secreted by ameloblasts?

What does this form?

What comes from this?

Answer 22

Amelogenin (scaffolding) and non-anemlogenins (a glue to hold the cells together).
Forms Tomes Process

Proteins come from the pointy end of the ameoloblasts (the Tomes Process)

Question 23

What happens to ameloblasts in maturation?

Answer 23

They become shorter (differentiate). Matrix protein replaces by water replaced by mineral. The protein needs to be removes so the tooth is less porous.

Question 24

Explain how eruption occurs

Answer 24

Cell layers coalesce to make REE (reduced enamel epithelium). Forms a layer on crown of the tooth while the tooth erupts). The epithelium forms between the crown and the gingival surface which seals the tooth to the gum.

REE fuses with oral epithelium to make junctional epithelium.

Increased mineralisation and maturation of enamel in saliva.

Question 25

What substance causes the finishing of mineralisation?

Answer 25

Saliva

Question 26

What happens during the early stages of amelogeneisis?

Answer 26

tall (50 mm) columnar cells. – Developed from inner enamel epithelium. – Nucleus migrates away from basement membrane. – Massive RER for protein secretion. – Mitochondria next to stratum intermedium.

Question 27

How do cells get ready for matrix secretion? Explain what happens after this

Answer 27

- Stellate reticulum collapses at commencement of amelogenesis – Brings external enamel epithelium (complete with blood – vessels nutrients etc.) – close to ameloblasts (needed for mass production of proteins) Matrix secretion commences • Intracellular packets of matrix protein at secretory pole: the Tomes’ process. – End terminal bars give close intercellular attachments: to stop secretions going between the cells.

Question 28

Where do ameloblasts move and why? How long does each ameloblasts secrete matrix for? Does enamel or dentine form quicker?

Answer 28

Ameloblasts move away from the dentine surface as the matrix is secreted. Each ameloblast secretes matrix for about a year. Matrix secretion for a permanent molar will take 4-7 years. Enamel forms after dentine and forms slower.

Question 29

Are matrix proteins collagenous? | Explain this

Answer 29

No – majority: amelogenins unstable - molecular weight 25 kilodaltons. Hydrophobic end – minority: non amelogenins (enamelins) more stable – mw 55 kd. – ameloblastin, sulphated proteins, amelotin

Question 30

Explain the control of prismatic enamel

Answer 30

• Ameloblastin: 5% of non amelogenins – Possible role in communication between ameloblast and enamel extracellular matrix. – May be a residue of the Tomes process. – May act as an inhibitor to lateral growth of crystallites ~ so leading to prisms. – Maintain channels for escape of enamel proteins from the deeper layers. Prisms are found on the periphery of enamel prism.

Question 31

Are crystals secreted by cells? | What happens to the crystals?

Answer 31

No – over the period of secretion of the full thickness of the enamel – and for a long time afterwards They form in a horse shoe shape.

Question 32

Give information on adhesion of composite tooth coloured restorations

Answer 32

Acid etch technique: removal of smear layer, high energy surface – Enamel: differential dissolution of prism boundaries after 20s etch with H3PO4

Question 33

What are Hunter-Schreger bands?

Answer 33

* Sinuous course of prisms from EDJ to surface | * Sheets of prisms change angles relative to one another

Question 34

What are tufts?

Answer 34

Ameoloblasts cross over each other. Each ameoloblast takes a slightly different course. It goes curly in the inner third and then straightens out. Spaces between sheets of decussating prisms - high protein content - tuftelins quite widespread

Question 35

What mechanism is a Hunter-Schreger band?

Answer 35

Stress relieving mechanism. - Straight bit of the prism hits the curly bit which compresses the curly bit. If it was all straight, the enamel would break away with pressure.

Question 36

Diazones and parazones - explain this

Answer 36

Enamel prisms at cavity surface sectioned at different angles. Important for adhesive materials - if you cut a cavity, some prisms are end on and some are side on to the cavity.

Question 37

What are lamellae in teeth?

Answer 37

Have cracks that can go out the surface of the tooth called lamellae.

Question 38

What does enamel rely on for support?

Answer 38

Dentine

Question 39

Features of the EDJ (enamel-dentine junction):

Answer 39

Scalloped complex junction Enamel spindles Ameloblast spaces / odontoblast ends

Question 40

Give some details on cutting hard tissue

Answer 40

- Confocal microscope - Imaging interaction between burs and enamel - Tooth kept wet - Digitised images, recording load and displacement All burs/hand pieces cause cracking.

Question 41

Does enamel have collagen?

Answer 41

No - ONLY IN DENTINE

Question 42

Give details on the functioning of Amelogenins

Answer 42

They are a scaffold for mineralisation (thixotrophic gels). They form nanospheres that act as organised microstructure for initiation and oriented growth of crystal. They are secreted within tissue fluid and mineralisation then occurs in this fluid. The amelogenins stop the crystals from growing laterally. Mainly: glutamic acid, proline and histidine

Question 43

How does the hollow spiral inside the tooth form?

Answer 43

Dentine mineralises which induces the enamel to form. Globules secreted from the Tomes Process (from the ameloblasts). These globules from together into a spiral with a central hole in the middle where the mineralisation occurs (C-axis).

Question 44

What do amelogenin nanospheres encourage? What do amelogenin monomers cluster to form?

Answer 44

Linear crystal growth Form 20nm nanospheres

Question 45

What is the role of Non Amelogenins? Examples?

Answer 45

Initial nucleators of crystal growth at the EDJ. Remain in mature enamel as a glue for the crystallites, bond to cells due to acidic nature. These are stronger than the amelogenins. Examples: aspartic acid, glutamic acid, serine and glycine.

Question 46

Explain Amelogenin gene on the sex chromosomes

Answer 46

It has been mapped to both X and Y chromosomes. | Slightly different proteins can be used to determine individuals sex from small tooth fragments.

Question 47

What are crystals not secreted by? | How do they grow?

Answer 47

Not secreted by cells. They grow with their C axis at 90 degrees to the secreting surface of the Tomes process

Question 48

Maturation of ameloblasts after secretion is complete: | Briefly describe this

Answer 48

1. Mitochondria move to the enamel pole of the cell. Plasma membrane becomes deeply infolded (known as ruffle ended ameloblasts) 2. The ameloblasts have a smooth end and inter ameloblast spaces develop between then, known as smooth ended ameloblasts. pH changes during crustal growth (neutral to mildly acidic) 3.This produces a reduction in the matrix protein and water content of the enamel as mineralisation proceeds. MMPs break down proteins which stops the tooth being porous. 4. Fluoride and tetracycline particularly accessible to enamel at the maturation stage, especially SA. – Fluorotic porosties (white spots) have a striking resemblance to immature enamel. 5. Quality and hardness of enamel increase as maturation continues. – This will also continue once the tooth has erupted. Internal enamel epithelium becomes the reduced enamel epithelium (REE) 6. Hemi desmosome : internal basal lamina forms between maturation ameloblasts and enamel – eventually make up primary epithelial attachment. 7. Reduced enamel epithelium fuses with oral epithelial lining as the tooth erupts.