Tooth development (II) Amelogenesis and Dentinogenesis

Question 0

What does the amelodentinal junction signify?

Answer 0

The starting point for both amelogenesis and dentinogenesis.

Question 1

What are the sequence of early events leading to the formation of enamel and dentine?

Answer 1

• Inner enamel epithelium -> pre-ameloblasts
• Pre-ameloblasts stimulate dental papilla to pre-odontoblasts -> odontoblasts
• Basal lamina between them breaks down
• Odontoblasts produce the first layer of dentine matrix (dentinogenesis)
• Dentine stimulates pre-ameloblasts -> ameloblasts
• Ameloblasts produce enamel (amelogenesis)
• Amelodentinal junction is thus defined

Question 2

What are the five stages of amelogenesis?

Answer 2

1. Pre-secretory stage
2. Secretory stage
3. Transition stage
4. Maturation stage
5. Post-maturation stage

Question 3

Describe the pre-secretory (I) stage of amelogenesis.

Answer 3

• IEE cells cease to divide and become known as pre-ameloblasts
• Differentiation to polarised columnar cells
• Proximal terminal web forms
• Induction of odontoblast formation
• Resorption of the basal lamina of the IEE
• Inductive messages pass between ameloblasts and odontoblasts and matrix formation begins
• Distal cell membrane becomes irregular with many projections and pits

Question 4

In the pre-secretory stage of amelogenesis next to which layer of cells does the proximal terminal web form?

Answer 4

Nearest to the stratum intermedium.

Question 5

Describe the secretory (II) stage of amelogenesis.

Answer 5

• Inital layer of aprismatic enamel formed
• Ameloblasts develop Tomes processes and prismatic structure develops
• No formation of “pre-enamel” as mineralisation is immediate
• Ameloblasts move away from dentine surface
• Distal terminal web forms
• Prisms have a core secreted by the tip of the Tomes process and a boundary secreted from the region that joins ameloblasts to each other

Question 6

Where and when does the distal terminal web form?

Answer 6

Nearest to the Tomes process in the secretory (II) stage of amelogenesis.

Question 7

How is the shape of the mineralising front described?

Answer 7

“picket fence” arrangement

Question 8

Where are the “core” and the “boundary” found in the secretory stage ameloblasts?

Answer 8

Prisms have a core secreted by the tip of the Tomes process and a boundary secreted from the region that joins ameloblasts to each other

Question 9

What are enamel proteins? What percentage of developing and mature enamel do they make up? Mutations in their genes are associated with..?

Answer 9

Amelogenins and non-amelogenins likely to be important in mineralisation processes, cell signalling and control of secretion.
25% of weight in early developing enamel; <1% of mature enamel.
Mutations in their genes are associated with disturbances of enamel structure.

Question 10

Give examples of non-amelogenins.

Answer 10

Enamelins, ameloblastin, teftelin, amelotin.

Question 11

How many ameloblasts form one prism core?

Answer 11

1

Question 12

How many ameloblasts contribute to each interprismatic region? What is this region also known as?

Answer 12

4 and AKA prism boundary

Question 13

What do the Striae of Retzius denote? What is a special one?

Answer 13

Incremental elongation of the prisms; the neonatal line is present in both enamel and dentine.

Question 14

What is the structure of the final enamel surface? Why?

Answer 14

Aprismatic because the Tomes process retracts.

Question 15

Describe the transition (III) stage of amelogenesis.

Answer 15

• Conversion of young immature enamel to fully mineralised enamel
• Enamel secretion stops
• Ameloblasts shorten and 50% die by apoptosis
• Much of the matrix is removed including the majority of amelogenins
• Reformation of the ameloblast basal lamina

Question 16

Describe the maturation (IV) stage of amelogenesis?

Answer 16

• Enamel crystallites increase in width and thickness thus reducing intercrystallite space
• Ameloblasts move calcium, phosphate and carbonate ions into matrix then remove and degraded proteins from the matrix
• “Ruffle-ended” border forms on ameloblasts that alternates with a “smooth-ended” form
• Removal of more proteins and increase in relative mineral content and density

Question 17

Do ameloblasts in the maturation stage have Tomes processes?

Answer 17

No

Question 18

In which stage do shortened ameloblasts alternate between ruffle-ended and smooth-ended form?

Answer 18

Maturation (IV) stage

Question 19

What do the ruffle-ended ameloblasts do?

Answer 19

By active transport and diffusion they introduce minerals (Ca2+).