chemistry of hydroxyapatite Flashcards
what will disturbance in initial enamel matrix deposition lead to?
hypoplasia
- defects occuring during secretory stage
- manifest as pits and grooves on enamel surface
what will defects during enamel maturation lead to?
hypomineralisation / hypomaturation
- intact surface but opaque appearance and soft
- discoloured
- hypomature
- not fully converted into the final tissue
- hypomineralised
- insufficient mineral deposition
what does hypocalcified mean?
- Tissue is fully mature but
- Mineral deposition hasn’t gone to completion
- Full thickness of crown has been deposited
- Normal shape
what is amelogenesis imperfecta
Inherited defect of dental enamel, affecting quality and/or quantity of enamel being laid down
give examples of inherited enamel defects
what is the result of them?
- DEJ defects results in an enamel layer that shears easily
- Secretory stage defects result in insufficient crystal elongation and leave the enamel layer very thin and disorganized
- Layer can be highly mineralised
- Maturation stage defects, i.e. deficient matrix degradation, produce thick but soft enamel
- Rapidly degrades
what is amelogenesis imperfecta caused by?
mutations in the human genome
at several different loci
what mutations can lead to hypoplastic amelogenesis imperfecta?
what mutations can lead to hypomaturation amelogenesis imperfecta?
mutation at 4q13.3
dominant
- affecting amelotin
- Overexpression & loss of amelotin leads to amelogenesis imperfecta
- involved in transition and maturation stage
- involved in reformation of the basement membrane
what does this show?
- brown-mottled hypomature enamel
- male with X-linked AI
- enamel is undermineralised
- contains non-degraded protein
- Mutation in cleavage site Leads to retention of excessive amount of protein in the tissue when it undergoes maturation
- contains non-degraded protein
what does this show?
- alternating areas of normal and hypomature enamel
- affected female member of family with X-linked AI
- only half of the tissue is compromised
what are common causes of environmental defects of enamel?
-
infectious disease in childhood
- temporary interruption of normal enamel development
-
fluorosis
- prolonged ingestion of fluoride excess 5ppm
- fluoride incorporated in mineral at great excess
- premature hypermineralisation
- opacity is irregular
-
ingested supplements are incorporated into matrix
- pigmentation
- irreversible
-
dietary deficiencies
- affect outer layer of tissue
-
trauma
- during development
describe the changes in physiology during the maturation stage of enamel
Ruffle ended
- Engaged in secretion of enzymes that degrade the matrix
- Active ion transport
- Transport ions required for mineralisation process
- Lower pH
Resorption of protein fragments and water
- pH is raised back to physiological levels
oscillating pH between low and neutral allows you to accumulate more stable mineral and dissolve more soluble mineral
More perfect formation of hydroxyapatite
why does the pH drop and rise during maturation stage?
- acidification is necessary for degradation
- EMSP1 works best at lower pH
- allows dissolution of less stable pH
- ameloblasts express carbonic anhydrase II during maturation stage
- CAII produces HCO3-
- neutralises acid
how is pH regulated during the secretory stage of amelogenesis?
Ameloblasts express basal membrane NBCe1 - ion transport protein
- Allows them to transport bicarbonate ions from the enamel organ into the ameloblast
- HCO3 - is then secreted apically via AE2 in exchange for Cl- ions
- therefore pH is stable
how is pH kept by smooth ended ameloblasts during the maturation stage of amelogenesis?
pH oscillates in maturation stage
- Smooth-ended ameloblasts may secrete HCO3 - to raise pH
- Transport across the cells of bicarbonate is insufficient to neutralise pH
- Due to lots of mineralisation taking place
- Carbonic anhydrase 2 - produces bicarbonate ions
- Both aid regulation of pH back to physiological levels
how is pH kept by ruffled ended ameloblasts during the maturation stage of amelogenesis?
pH is not stable but oscillates in maturation stage
- Ruffle-ended ameloblasts produce an extracellular pH of 5-6
- No NBCe1
- No longer pumps ions across membrane
- AE2 in lateral membrane instead of apical
- Role in pumping biocarbonate ions out of the cell instead of enamel space
- ATPase in apical membrane
- Actively transports protons across the membrane
- Acidificates the environment
- Active transport
what is the pattern of mineralisation?
Starts at cusp
Progresses down tooth crown
how does surface enamel differ from the underlying core enamel?
Harder
less porous
less soluble
how does apatite growth on the crystal surfaces occur?
two ways :
- rapid growth in epitaxial direction (above each hexagon) forms elongated hexagonal crystals
- thickening on the side prism planes at slow rate
what substitutions can take place in hydroxyapatite?
what effects do they have?
F- for OH- :
- stabilizing effect
- caries prevention
- fluoride taken up passively from circulation
- only incorporated if in saturation
CO3 2- for OH- or PO4 3- :
- destabilizing effect
- much higher solubility product of enamel
- developmental substitution
Mg2+ for Ca2+:
- destabilizing effect
- Cation substitutions
- occurs in small quantities
why does a fluoride substitution have a stabilising effect?
Fluoride ion fits more closely within calcium triangle
- High charge density and symmetry of fluoride ion reduces lattice energy
- more stable salt
- a flatter plane
what is the most stable salt?
Most stable is mixture of fluorapatite and hydroxyapatite
- 50:50 mixture
- 100x more stable
- dispersion of hydroxy ions along the c acid allows hydrogen bonding between fluoride
how does the mineral composition of enamel change while aging?
Older enamel -
- hydroxyl increases
- Relates to remineralisation
- Decrease in carbonate content
- Decrease acid phosphate content
- Incorporated during initial formation
- Adopting a more perfect hydroxyapatite as maturation occurs
The youngest secretory enamel is the most soluble and the solubility decreases with advancing developmental stages
- More stable as development advances
