dentine matrix proteins

Question 1

how is predentine mineralised into dentine?

Answer 1

• mineral is deposited
• formration of calcospherites
  
  • fusion
  • forms intertubular dentine
• ECM interactions and protein modifications
  
  • there are proteins in predentine which prevent mineralisation
    
    • must be removed and replaced

Question 2

stages of mineral crystal formation

Answer 2

1. Nucleation

• Formation of small embryo crystals
• Clusters of calcium and phosphates form those crystals

2. Crystal growth

• After stabilisation
• Deposition of Ca2+ and HPO42- onto a crystal surface

= formation of crystal

Question 3

two mechanisms of initiating nucleation of ionic crystals

Answer 3

homogenous nucleation

heterogeneous nucleation

Question 4

mechanism of homogeneous nucleation

Answer 4

• Formation of a crystal within a supersaturated solution without the presence of a pre-existing solid phase.
• Serum is supersaturated with respect to Ca2+ and HPO42-
• Requires an ionic concentration of 35 (mg Ca & P dl-1)2
• Unlikely to occur in vivo because:-
  
  • there is some difficulty in forming stable embryo crystal to allow for crystal growth.
  • other calcium phosphate salts form in preference to hydroxyapatite.
  • The concentration is not high enough - therefore no mineral deposition

Question 5

mechanism of heterogeneous nucleation

Answer 5

• Crystal growth induced by provision of a second solid phase on which a crystal lattice may be formed.
  
  • E.g. acidic protein
    
    • Has negatively charged carboxyl groups - will bind calcium
• Crystal growth is better promoted on crystalline material having similar lattice spacing
  
  • epitaxy .
• Likely to occur in vivo.
• But if calcium are bound too strongly to structure
  
  • Will act as inhibitors of mineralisation
• May involve organic constituents of bone, dentine, cementum and enamel.

Question 6

structure and formation of collagen

Answer 6

• triple helix of 2a-1 and 1a-2 chains
• intracellular formation of procollagen
  
  • peptides removed in processing of procollagen -> tropocollagen
    
    • tropocollagen molecules align to form collagen molecules
      
      • (fibril formation)
    • guided by non-collagenous proteins
• pyridiloline crosslink formation between lysine and hydroxylysine residues
  
  • involved in bone and dentine formation
  • increases cross linkes as predentine becomes dentine
• is scaffold for calcium phosphate and mineral crystals

Question 7

which type of collagen is abundant in connective tissues

Answer 7

type I

Question 8

where mineral crystals deposited within collagen? and how?

Answer 8

the gap zones within collagen fibrils

other proteins guide mineral to the gap zone

Question 9

predentine characteristics and structure

Answer 9

• collagenous matrix
  
  • provides a scaffold for dentine formation
• non-collagenous components
  
  • direct matrix organisation
  • prevents premature mineralisation
  • includes
    
    • macromolecules
    • growth factors
    • cytokines
    • enzymes

Question 10

composition of dentine

Answer 10

• Organic matrix (20%)
  
  • collagen 90% of organic matrix
  • non collagenous components proteoglycans, phosphoproteins and glycoproteins
    
    • direct mineral deposition
• Inorganic matrix (70%)
  
  • Hydroxyapatite Ca10(PO4)6OH2
  • Impurities F-, CO32-, Mg2+
  • Much less mineral than enamel (96%)
  • Smaller crystal size (20x4mm) compared to enamel
• Water

Question 11

which proteoglycans promote and inhibit mineral deposition and how?

Answer 11

• decorin
  
  • leucine and GAG chain
  • binds to specific loci on collagen fibres
  • in dentine, decorin is bound to chondroitn sulfate
    
    • chondroitn can weakly bind to calcium to guide it to gap zones
• versican
  
  • has up to 20 GAG chains
    
    • calcium will bind to each chain, moving between them
    • sequesters calcium and phosphate
    • releases calcium
  • inhibitor of mineralisation
    
    • found in pre-dentine

Question 12

how do GAGs in dentine contribute to mineralisation ?

Answer 12

• contain sulphate groups
  
  • these can bind calcium
• dermatan sulphate
  
  • binds calcium very strongly
    
    • does not release to form crystals
      
      • inhibitor of mineralisation

Question 13

which proteoglycans are synthesised in predentine and what are their roles

Answer 13

• decorin
• biglycan
• versican
• bind to collagen fibres at gap zone
• inhibit movement of calcium into gap zones
• removed/remodelled as predentine matures to allow for dentine formation

Question 14

which proteoglycans are synthesised in mineralising dentine

Answer 14

• CS decorin
• biglycan
• transported intracellularly up the odontoblast process
• lower levels than those found in predentine
• could be associated with guiding mineral deposition
• CS may be involved in transport of Ca2+ and HPO42- to gap zone.