OE L8 Amelogenesis

Question 1

When does enamel formation begin?

Answer 1

Late bell stage.

Once the crown pattern has already been established and once dentinogenesis has proceeded sufficiently.

Question 2

What are the 3 stages of amelogenesis?

Answer 2

1. Induction
2. Enamel secretion
3. Enamel maturation

Question 3

Describe induction of amelogenesis.

Answer 3

Reciprocal induction

• When predentine is secreted the BM breaks down and IEE cells are exposed
• This signals to the IEE cells to differentiate into highly active secretory ameloblasts
• In turn, dentinogenesis is dependent on signals from IEE cells (reciprocal)

Question 4

Describe enamel secretion briefly.

Answer 4

• Secretory ameloblasts 1 unit of enamel (prism)
• Ameloblasts secrete an initial structureless layer of enamel (8um)
• Then Tomes processes form and structured enamel is secreted- prismatic enamel

Question 5

Describe the structure of an ameloblast.

Answer 5

• Large columnar cell
• Rich in ER
• Proximally positioned nucleus
• Tome’s process
• Terminal bar apparatus seperating distal end of ameloblast from Tome’s process

Question 6

In what order are the following structures found; dentine, enamel, ameloblasts, predentine, stratum intermedium, stellate reticulum, OEE.

Answer 6

1. OEE
2. Stellate reticulum
3. Stratum intermedium
4. Ameloblasts
5. Enamel
6. Dentine
7. Predentine

Question 7

What is the role of the stratum intermedium in amelogenesis?

Answer 7

• Cells of the SI are in direct contact with ameloblasts (proximal end)
• Transport organic and inorganic material to the ameloblasts
  E.g. SI cells produce high levels of alkaline phosphate (relevant to mineralisation process)

Question 8

What is the role of the stellate reticulum in amelogenesis?

Answer 8

• Nutritional support for ameloblasts

- Involved in glycogen accumulation

Question 9

What is the terminal bar apparatus?

Answer 9

• Linking process formed from desmosomes
• Links ameloblasts together
• Prevents ion exchange between cells, important in mineralisation

Question 10

What are the Tomes processes?

Answer 10

• A specialised organ allowing directional secretion of enamel
• Formed when ameloblasts migrate away from the ADJ
• Prior to their formation, enamel is structureless, afterwards prismatic enamel is deposited
• Tissue secreted perpendicular to the cells
• ‘Saw toothed’ appearance as processes jut into enamel matrix

Question 11

What is the degree of mineralisation at secretory stage compared to maturation stage?

Answer 11

At secretory stage: 30-40%

At maturation stage: 96%

Question 12

Briefly describe enamel maturation.

Answer 12

• Once the full template of enamel is laid down, the tissue can undergo mineralisation
• Involves addition of mineral and removal of most of the organic matrix
• Begins at cuspal tips and progresses

Question 13

What changes do ameloblasts undergo during the maturation stage.

Answer 13

• Shorten and lose Tomes processes

- Undergo further differentiation to either become a ruffled border or smooth border ameloblast

Question 14

What is a ruffled border ameloblast?

Answer 14

• Role: introduce enzymes for degradation of matrix template, and introduction of ions
• Have full vesicles at apical end (reflective of their role in secretion)

Question 15

What is a smooth border ameloblast?

Answer 15

• Role: removal of proteins and water

- Smooth ended (flat surface) indicating tole in uptake of water

Question 16

Maturation stage ameloblasts are involved in a cyclical process, what does this mean?

Answer 16

• Water and organic material selectively removed
• Additional inorganic material introduced
• Alternative bursts of activity to promote extensive mineralisation
  5-7 cycles in total

Question 17

What events occur post-maturation?

Answer 17

• Ameloblasts reduced in number (up to 50%) through apoptosis
• Basement membrane is re-synthesised
• Remaining ameloblasts differentiate into flattened epithelial cells forming the reduced enamel epithelium
• Reduced EE overlies the developing tooth and protects the enamel surface during eruption

Question 18

What stages does mineralisation occur in?

Answer 18

4 stages:

1. Formation of partially mineralised enamel matrix (first 8um secreted)
2. Increase in mineralisation, beginning at surface of enamel until it reaches initial 8um layer
3. Mineralisation front rebounds from innermost layer and heads back towards the enamel surface
4. Surface layer is mineralised more slowly, eventually reaches complete mineralisation and is the most highly mineralised at this surface

Question 19

What are the 6 key functions of the inner enamel epithelium?

Answer 19

• Determining crown pattern
• Induction of odotoblast differentiation
• Reciprocal induction: ameloblast differentiation
• Secretion of enamel matrix
• Mineralisation of enamel
• Production of newly formed enamel surface

Question 20

What is the enamel knot?

Answer 20

• When the tooth reaches late bud stage, a signalling centre forms at the tip of the bud
• This signalling centre (enamel knot) marks the initiation of crown development
• Enamel knot cells don’t proliferate themselves, they stimulate proliferation of cells in the flanking epithelium

Question 21

What theory explains how ultimate tooth formation is achieved?

Answer 21

Theory of a stem cell niche in the cervical loop area.

The epithelial stem cell niche gives rise to ameloblasts.

Question 22

Which 2 experiments support the notion of a stem cell niche in the cervical loops?

Answer 22

1. Dissection of mice teeth: cervical loop removed arrests tooth development. Cervical loop alone and other parts of tooth crown removed leads to regeneration of epithelium.
2. Stain dividing cells, can see abundant labelling of cells next to cervical loop. Most cells end up in region close to where hard tissue formation taking place. Cells of cervical loop differentiated into ameloblasts.

Question 23

How is the stem cell niche regulated? Which markers are involved?

Answer 23

• FGF-10
  Expressed in dental mesenchyme near cervical loop. Required for maintenace of stem or transit amplfying cells.
• Notch1
  Found in cervical loops.
• Lunatic fringe
  Found in cells of IEE. These cells produce a transient amplifying pool of cells (of ameloblast lineage).
• Serrate1
  Found in cells which have undergone differentiation to become preameloblasts.

Question 24

How does mesenchymal FGF-10 influence other signalling factors?

Answer 24

• FGF-10 stimulates lunatic fringe expression in the epithelium
• This causes notch1 signalling to be downregulated and cells expressing Notch1 in the stellate reticulum and stratum intermedium now become committed to the ameloblast lineage
• Ameloblasts express the Notch receptor ligand Serrate1 and thereby interact with stratum intermedium cells expressing Notch1

Question 25

What is HERS?

Answer 25

Hertwig’s epithelial root sheeth:
- Stellate reticulum is lost after crown formation, leaving a double layer of epithelium (HERS) that directs root formation
OEE + IEE = HERS

Question 26

What feature is key to root formation?

Answer 26

The loss of the epithelial stem cell niche.
FGF-10 expression stopped.
“The disappearance of Fgf10 signalling leads to the transition from crown to root formation, owing to the loss of a dental epithelial stem cell compartment.”