Amelogenesis

• it takes up to how many years to complete the crown of a permanent tooth?

  up to 5 years
• What is amelogenesis?

  formation of enamel
• what cell is amelogenesis accomplished by?

  ameloblasts

  
  

  - these epithelial cells do not produce mature enamel, but instead produce a special protein scaffold within which hydroxyapatite crystals will grow.
• how long does it take for enamel to mature?

  Up to two thirds of the time it takes to produce a full crown of enamel can be taken up by the maturation stage:

  ameloblasts remove water, proteins, and other organic materials to increase the mineral content of enamel.
• during what stage of tooth development does amelogenesis occur?

  at the advanced (later) bell stage
• where does amelogenesis occur?

  inner enamel epithelium of enamel oragn
• what are the stages of amelogenesis?

  -------> presecretory stage

  1. morphogenetic phase

  2. histodifferentiation phase

  
  

  -------> secretory stage

  3. initial secretory phase

  4. main secretory phase

  
  

  -------> maturation stage

  5. maturation phase-early

  6. maturation phase-late

  7. protective phase
• presecretory stage -

  1. morphogenetic phase

  
  

  what happens during the morphogenetic phase?

  • The cells forming the innermost lining of the inner enamel epithelium are low, cube-shaped cells.

  • Nuclei are centrally placed in the cell, organelles are not very numerous.

  • Cells of the inner enamel epithelium help shape the future crown

  
• presecretory stage -

  2. histodifferentiation phase

  
  

  what happens during the histodifferentiation phase?


  • Inner enamel epithelium cells differentiate into ameloblasts.

  • Nuclei shift to proximal sides of cells; cell bodies elongate into columnar ameloblasts.

  • The Golgi complex increases in volume and migrates distally, amount of rough endoplasmic reticulum increases (this is for protein synthesis and secretion)

  • Neighbouring ameloblasts are tightly joined by junctional complexes to prevent any leakage across the inner enamel epithelium.

  
• secretory stage -

  3. initial secretory phase

  
  

  what happens during the initial secretory phase?

  • The large Golgi complex and rough endoplasmic reticula synthesize and package enamel matrix proteins into secretory granules, which are then secreted on the distal end of the ameloblasts.

  • The secretory surfaces of the ameloblasts are flat

  -> produce aprismatic enamel matrix protein scaffold at this point.

  
• secretory stage -

  4. main secretory phase

  
  

  what happens during the main secretory phase?

  • Ameloblasts migrate away from dentine surface and develop their Tomes' process.

  ----> This outgrowth of the ameloblast makes the protein secretion staggered into two parts: one forms the prisms, the other forms the interprismatic regions

  
  

  • This phase is when most of the enamel is formed.

  • Calcium and phosphate ions are transported into the protein matrix to form the initial hydroxyapatite crystals.

  • This hardens enamel to about 30% of its final hardness.

  
• secretory stage -

  4. main secretory phase

  
  

  what is the tomes' process?

  CONE SHAPED outgrowth from the ameloblast cell THAT POINTS TOWARDS THE FORMING ENAMEL

  
  

  - the pointy part forms the enamel prisms (rods)

  - the sides form the interprismatic enamel (interrods)

  
• secretory stage -

  4. main secretory phase

  
  

  how is hydroxyapatite crystals formed?

  - Ameloblasts migrate away from dentine surface tomes process forms the enamel prisms and the interprismatic enamel

  (protein matrix)

  - Calcium and phosphate ions are transported into the protein matrix to form the initial hydroxyapatite crystals.

  - This hardens enamel to about 30% of its final hardness.
• secretory stage -

  4. main secretory phase

  
  

  what are the enamel matrix proteins (EMPs)?

  noncollagenous proteins that are helping initiate and guide the formation of hydroxyapatite crystals outside of the cells.

  e.g. amelogenin, ameloblastin, enamelin, enamelysin, enamel matrix serine proteases, and tuftelin
• secretory stage -

  4. main secretory phase

  
  

  what is the main enamel matrix protein (EMP)?

  Amelogenin.
• secretory stage -

  4. main secretory phase

  
  

  describe Amelogenin

  • Secreted in matrix vesicles by the ameloblasts

  • Self-aggregating, gel -like matrix

  • Creates nanospheres that aggregate into chains and ribbons

  • These prevent lateral growth of HA crystals.

  • The shape of the amelogenin scaffold dictates the shape of the hydroxyapatite crystals.

  
• secretory stage -

  4. main secretory phase

  
  

  describe ameloblastin

  • ~10% of the enamel matrix.

  • Has calcium-binding properties.

  • May assist ameloblasts in adhering to the forming enamel surface during the secretory stage
• secretory stage -

  4. main secretory phase

  
  

  describe elamelin


  • ~5% of the enamel matrix.

  • Appears to bind strongly to mineral and inhibit crystal growth. • May help promote crystal elongation in the earliest stages of enamel formation
• secretory stage -

  4. main secretory phase

  
  

  what does enamelysin (MMP20) do?


  cleaves amelogenin and ameloblastin to make active form
• secretory stage -

  4. main secretory phase

  
  

  what do enamel matrix serine proteases do?

  degrades residual EMPs during enamel maturation
• secretory stage -

  4. main secretory phase

  
  

  how do the ameloblasts get their nutrients?


  • The stellate reticulum collapses, bringing the inner and outer enamel epithelia closer together.

  • This brings surrounding vascular tissue closer to the ameloblasts, which nourishes them during this phase.

  • Ameloblasts are also supported by cells from the stratum intermedium

  
• secretory stage -

  4. main secretory phase

  
  

  what does tuftelin do?

  residual protein found in enamel tufts
• maturation stage

  5. maturation phase-early

  
  

  what happens during the maturation phase-early?

  • Ameloblasts shorten and lose their Tomes' processes.

  • This leads to the production of the outer layer of aprismatic enamel.

  • Calcium and phosphate ions are actively pumped into the enamel to increase the sizes of the hydroxyapatite crystals

  
• maturation stage

  6. maturation phase-late

  
  

  what happens during the maturation phase-late?

  • Ameloblasts secrete enamel matrix protein digesting enzymes to degrade and remove the protein scaffold from the immature enamel. -> creates more space for crystal growth

  • Ameloblasts also actively remove water from the developing enamel, increasing the mineral content of the maturing tissue.

  • Ameloblasts seem to cycle between the early and late maturation stages repeatedly

  
• maturation stage

  6. maturation phase-late

  
  

  during which stage is enamel most susceptible to fluoride and tetracycline?

  maturation stage

  6. maturation phase-late

  
• maturation stage

  6. maturation phase-late

  
  

  which phase is the longest lasting phase of amelogenesis?

  maturation stage

  6. maturation phase-late

  
• maturation stage

  7. protective phase

  
  

  what happens during the protective phase?


  •After the completion and full maturation of the enamel, the enamel organ collapses into a dense covering of epithelium.

  • This is called the reduced enamel epithelium, which then protects and covers the outer surface of the enamel until tooth eruption occurs -> enamel STOPS growing and it is incapable of repair if damaged (because cells that make it disappear)

  
• what happens to the reduced enamel epithelium as the tooth erupts?

  it eventually merges with the oral epithelium

  - This leads to the destruction of most of the reduced enamel epithelium, except for the cervical portions

  
  

  The cervical portions of the reduced enamel epithelium become the junctional epithelium, which acts as a seal from the oral environment