7 - Histogenesis of the enamel - Amelogenesis

Question 1

Enamel

Answer 1

• Epithelilal - derived protective covering for teeth
• It cannot renew itself - due to loss of ameloblasts during tooth eruption
• This means enamel acquires a complex structural organisation + a higher degree of mineralisation

Question 2

Ameloblasts

Answer 2

• Ectodermally derived cells ⇢ secrete matrix proteins
• They create extracellular environments ⇢ Favourable to mineral deposition
• The secreted matrix proteins ⇢ Enamelin + Amelogenin ⇢ Later mineralise to form enamel
• Secretory end of ameloblasts ⇢ ends in a 6-side pyramid like projection = tomes process

Question 3

Life cycle of ameloblasts

Answer 3

Morphogenic stage

Organising stage

Secretory stage

Maturative stage

Protective stage

Desmolytic stage

Question 4

Amelogenesis

Answer 4

Process of enamel formation

2 step process:

1. Produced partially mineralised enamel
2. Influx of additional mineral + removal of organic material + H2O

Stages:

1. Pre-secretory
2. Secretory
3. Maturation

Question 5

Pre-Secretory stage

Answer 5

Morphogenic stage:

• During bell stage ⇢ crown’s shape determined
• Cells of inner epithelium ⇢ separated from dental papilla by basement membrane
• Cells are cuboidal w/central nucleus

Differentiation stage:

• Cells of inner epithelium ⇢ differentiate into ameloblasts via elongation
• Nuclei shifts into stratum intermedium
• Basement membrane fragmented by cytoplasmic projections
• Golgi + ER ⇢ increase + migrate distally
• Ameloblast divides into ⇢ Body + tomes process

Question 6

Secretory stage - part 1

Answer 6

After ameloblasts differentiate:

• Matrix is synthesised in RER ⇢ migrates to golgi apparatus
• Matrix condensed + packaged in membrane-bound granules
• Vesicles migrate to apical end of cell + release content along DEJ
• Disintegration of basement membrane ⇢ allows pre-ameloblasts to come into contact w/pre-dentin
• This induces pre-ameloblasts to differentiate into ⇢ ameloblasts + start deposition of enamel matrix

Question 7

Secretory stage - part 2

Answer 7

Secretion of enamel matric occurs in 2 sites:

1. Proximal part of Tomes process
   * Results in formation of enamel portions that delimit a pit along enamel layer ⇢ inter-rod enamel
2. Distal portion of Tomes process

• Later fills the pit w/matrix that regulates enamel rod formation
• Creates narrow space between the 2 enamels which fill w/organic matrix ⇢ forms rod sheath

Question 8

Maturation stage

Answer 8

Mineralisation - 1° + 2° mineralisation

Maturation

Question 9

Mineralisation

Answer 9

When enamel matrix is completed + amelogenin is deposited ⇢ small mineral crystals begin to grow in length + diameter

• Small crystals = 25% of total enamel
• Crystal growth = 70%
• H2O = 5%

1° mineralisation

• Needle-shaped crystals appear
• Increase + become hexagonal

2° mineralisation

• Occur at amelo-dentinal junction
• inorganic material deposited into matrix
• Enamel turns from soft ⇢ hard substance

Question 10

Mineralisation part 2

Answer 10

1st matrix deposited = 1st enamel mineralised on DEJ

• Proceeds peripherally to cusp tips
• then laterally on crown sides
• then in cervical region

Question 11

Maturation

Answer 11

Ameloblasts undergo morphological changes

• Reduce in height + volume of organelles
• Lose tomes process
• Undergo cell death
• Ameloblasts remove H2O + organic material from enamel ⇢ additional inorganic material introduced
• Prevents reverse demineralisation of growing crystallites
• Ameloblasts contact stratum intermedium + outer epithelium ⇢ fuses to form reduced enamel epithelium