OE L11 Chemistry of Hydroxyapatite Flashcards
What are the origins of enamel defects?
Enamel defects may be of genetic or environmental origin.
What 2 types of defects lead to enamel malformation?
- Distrubances in initial matrix deposition = hypoplasia
- Disturbances in enamel maturation = hypomineralisation
How does hypoplastic amelogenesis imperfecta present?
Pits and grooves on enamel surface.
Secretory phase defect.
What are the 2 types defects have a hypomineralised phenotype?
Hypomineralised phenotype:
- Hypocalcified or
- Hypomature
Describe hypocalcified amelogenesis imperfecta.
Fully mature tissue, but mineral deposition in end stages of amelogenesis not complete. Normal shaped crowns.
Describe hypomature amelogenesis imperfecta.
Patchy appearance, enamel opaque instead of translucent.
What are 3 type of defects which cause AI and what do they affect?
- ADJ defects: results in enamel layer that shears easily
- Secretory stage defects: results in insufficient crystal elongation, leaves enamel layer thin and disorganised
- Maturation stage defects: deficient matrix degradation, produces thick but soft enamel
Give examples of components that may be mutated in hypoplastic AI.
- Amelogenin
- Enamelin
- Ameloblastin
- Acid phosphatase
- Laminin a-3 and b-3 chains
- Collagen 17a1-chain
Give examples of components that may be mutated in hypomaturation AI.
- Kallikrein 4
- Enamelysin
- Amelotin
- WDR72
- SLC24A4
What protein mutation causes the most prevalent form of AI in the US?
Mutations in FAM83H, which controls epithelial maintenance through keratin filament regulation.
Can AI exist in syndromic form?
Yes, AI usually exists in synromic form as the genes involved are not specific to hard tissue formation.
E.g. Jalili synrome (disease of retina) - CNNM4
E.g. Renal disease - Claudin 16 and 19
Which protein was first linked to AI, describe the mutation.
Amelogein
- Tri-tyrosine motif normally mediates interaction of nanospheres with enamelin
- Mutations prevents cleavage by MMP-20 thus preventing formation of TRAP fragment
- Retention of protein in tissue
- Causes hypomaturation
How does hypomaturation AI differ between males and females?
Hypomaturation AI related to mutation inherited on X chromosome.
- Men only have 1 X chromosome so more severely effected.
- Women carry 2 but only 1 chromosome is expressed in given group of cells, inactivation of 1 X chromosome is spontaneous, leads to banding pattern- alternative stripes of normal and hypomature tissue.
Name 5 defects with environmental causes.
- Infectious disease e.g. measles during tooth development
- Fluorosis, mottled appearance
- Incorporation of ingested supplements into mineralised matrix e.g. tetracycline causes extreme brown pigmentation
- Dietary defincies
- Trauma
How does pH differ during maturation stage?
- Ruffled border ameloblasts introduce enzymes and ions into the matrix for degradation lower pH (5.5)
- Smooth border ameloblasts resorb protein fragments raise pH (7-7.5)
pH cycling is essential to forming mature, mineralised crown.
How are ions transported from ameloblasts to the enamel space during matrix degradation?
Ions initially accumulate in stellate reticulum, then actively transported across ameloblast layer to reach enamel space.
Terminal bar apparatus prevents ions travelling back up across ameloblasts.
Why is an acidic environment needed for mineralisation?
For every unit of HAP, 8 protons are produced- creates acidic environment.
Low pH leads to dissolution of less stable mineral.
EMSP1 works optimally at lower pH.
How is the acidic environment created during mineralisation neutralised?
Carbonic anyhrase II is expressed by smooth ended ameloblasts at end of secretory stage and during maturation.
It produces bicarbonate.
Describe surface enamel.
Non-prismatic, harder and less porous. More mineralised.
Describe the molecular structure of hydroxyapatite.
Crystal lattice:
- Central OH
- Inner ring of 3 Ca ions
- Middle ring of 3 P ions
- Outer ring of 6 Ca ions
What is the stoichiometric formula of HAP?
Ca10(PO4)6(OH)2 or Ca5(PO4)3OH
What are the 2 forms of apatite growth?
- Rapid growth along C-axis: elongated hexagonal crystals
- Thickening on the side prism places: slow rate
Name 3 types of substituted apatite.
- Fluorapatite
- Carbonate
- Magnesium
Describe fluorapatite.
- Fluoride replaces central hydroxyl ion
- Stabilising effect on crystal lattice: symmetrical, higher charge density so creates a flatter plane
- Prevents caries
Describe carbonate substituted apatite.
- Destablising effect
- Replaces either hydroxyl or phosphate ion
Describe magnesium substituted apatite.
- Destabilising effect
- Replaces calcium
- Other ions such as sodium and rubidium can also substitute calcium
Do we want enamel to have a high or low solubility product constant?
Low.
Smaller solubility product constant = more stable salt.
Most stable salt is a mixture of hydroxyapatite and fluorapatite.
Describe the mineral composition changes during development.
- Hydroxyl increases
- Carbonate and acid phosphotase decrease
Net effect: mineral more stable and less prone to dissolution
How does acid affect hydroxyapatite?
- Acid generated in oral plaque biofilms demineralises the tooth’s calcified tissues.
- If the ionic activity product (IP) is less than solubility product it causes ion dissolution (caries)
What is acid etching?
Removes the smear layer and causes small amount of mineral dissolution.
This creates surface roughness so materials bond better to the tooth.
