amelogenesis

• Amelogenesis

  The process of enamel formation and maturation

  
• Enamel matrix formation: Organic matrix is laid down and mineralises only up to 30%
• Enamel maturation: Organic matrix is broken down and mineralises up to 96%

  
• The lifecycle of ameloblasts


  1. Presecretory

  2. Secretory

  3. Maturation
• Presecretory stage

  • Differentiation
  • Change in polarity
  • Change in cellular organelles
• Secretory stage

  • Formation of the organic matrix
• Maturation stage

  • Ion modification and other processes for mineralisation
• Functional stages in the lifecycle of ameloblasts

  1. Morphogenetic stage

  2. Histodifferentiation stage

  3. Initial secretory stage (no Tome's process)

  4. Secretory stage (Tome's process)

  5. Ruffle-ended ameloblast for maturative stage

  6. Smooth-ended ameloblast of the maturative stage

  7. Protective stage
• Presecretory stage - Morphogenetic phase

  1. Tooth shape is determined during the bell stage

  2. A basal lamina exists between IEE and DP

  3. Dentine is not mineralised (see vesicles)

  4. IEE still cuboidal with poorly developed or scattered cytoplasmic organelles necessary for enamel secretion

  
• Presecretory stage - Differentiation phase


  1. IEE becomes columnar (preameloblasts)

  2. Nuclei become shift proximally (towards the stratum intermedium)

  3. Golgi complex increases in volume and migrates distally (towards differentiating odontoblasts)

  4. Rough endoplasmic reticulum increase significantly
• Presecretory stage - Initial secretory stage

  1. Ribosomes on the surface of the rough endoplastmic reticulum rER translate mRNA coding for enamel proteins

  2. Proteins translocate to the cisternae

  3. Proteins progress to the Golgi complex

  4. Packaged into granules and migrate into Tome's process

  5. The granule contents are released against the newly formed mantle dentine

  6. This results in the formation of a layer of rodless (aprismatic) enamel

  7. This layer almost immediately becomes mineralised and the hydroxyapatite cyrstals interdigitate with those of dentine
• Secretory stage


  1. Enamel protein secretion sites: First site proximal part of Tome's process (form enamel partitions that delimit a pit which houses the distal part of Tome's process, form interrod enamel)

  2. Second site: one face of the distal part of Tome's process (provide enamel matrix which forms an enamel rod that fills the pits)

  3. A narrow space (rod sheath) between the enamel rod and interrod enamel fills with organic material
• Towards the end of the secretory stage, ameloblasts become shorter and lose the distal part of their Tome's processes (similar in their shape during the initial secretory stage)
• Final portions of enamel forming are also rodless (aprismatic)

  
• Amelogenin (AMELX; AMELY)

  Main protein present in enamel; expression stops when enamel reaches its full thickness
• Ameloblastic (AMBN)


  ~10% of the matrix, found in newly formed (secretory stage) enamel, more in the outer surface
• Enamelin (ENAM)


  Least abundant (~5%)
• Loss of function or mutant protein of Enamelin: No defined enamel layer
• Maturative stage


  1. Hardening of enamel is due to the growth of crystals seeded during the formation of enamel matrix

  2. Amelogenins: only in early maturation stage

  3. Ameloblastin: continues till later stages

  
• Maturative stage - Transitional phase


  1. Reduction in height and volume of ameloblasts – including their organelles

  2. 25% of ameloblasts die in this transition

  3. Another 25% die during enamel maturation
• Maturative stage - Modulation


  1. Cyclic creation, loss, and recreation of invaginated ruffle-ended apical surface

  2. Basal lamina formed with hemidesmosomal attachment

  3. Every 8 hours

  4. Smooth-ended cells: Removal of water and organic material from enamel

  5. Ruffle-ended cells: Introduction of inorganic material
• Amelotin (AMTN)


  Secreted during and shortly after transition to the maturation stage
• Odontogenic ameloblast-associated (ODAM)


  Secreted during and shortly after transition to the maturation stage; disruption of periodontal integrity induces expression of the protein by epithelial rests
• Loss of function of Odontogenic ameloblast-associated (ODAM): Tooth phenotype to be defined; junctional epithelium defects