OB D5 : Biological mineralisation

Question 1

what is the composition of enamel by weight?

Answer 1

Mineral - 96%
Matrix - 1-2%
Water - 2%

Question 2

what is the composition of dentine by weight?

Answer 2

Mineral - 70%
Matrix - 20%
Water - 10%

Question 3

what is the composition of cementum by weight?

Answer 3

Mineral - 65%
Matrix- 23%
Water - 12%

Question 4

what is the composition of bone by weight?

Answer 4

Mineral - 60%
Matrix- 25%
Water- 15%

Question 5

what is the structure of hydroxyapatite?

Answer 5

-Ca10(PO4)6(OH)2
-Crystallites:
70nm wide, 25nm thick and very long
Hexagonal cross-section
-OH- ion surrounded by 3 Ca2+ ions
-Surrounded by 3 PO43- ions
-Enclosed by 6 Ca2+ ions

Question 6

how are hydroxyapatite molecules arranged?

Answer 6

• repeats in chains to make sheets in one plane in 2D

- in 3D - latice

Question 7

Describe hydroxyapatite ionic replacement.

Answer 7

• any ion can be substituted
• F- may substitute OH-
  stabilises the lattice
  more acid resistant
  F- and prevention

Question 8

where should hydroxyapatite by deposited? (mineralisation)

Answer 8

• only in bones and teeth

- not to be deposited where not required i.e don’t want crystals in bloodstream

Question 9

what are the theories of mineralisation?

Answer 9

-alkaline phosphate hypothesis
-Nucleation theories :
homogenous nucleation
heterogeneous nucleation
-matrix vesicles

Question 10

what does the alkaline phosphatase enzymes do?

Answer 10

– hydrolyses organic phosphates

– releases inorganic phosphates (pushes reaction to the right to produce apatite)

Question 11

what are the problems with alkaline phosphatase hypothesis?

Answer 11

– [organic phosphates] too low to be an effective
source
– other sites contain alkaline phosphatase e.g. kidney
– normal serum [Phosphate] is sufficient

Question 12

what is nucleation?

Answer 12

The formation of a 1st solid from a solution of ions in which no solid previously existed

Question 13

How are copper sulphate crystals formed?

Answer 13

-e.g. growing CuSO4 crystals from a supersaturated solution of CuSO4

Question 14

Can crystals grow if serum is supersaturated with Ca2+ and PO43-?

Answer 14

• Hydroxyapatite crystallites do not form

- But can sustain crystal growth

Question 15

Discuss the failure of homogeneous nucleation.

Answer 15

• Homogeneous nucleation of hydroxyapatite is very difficult to achieve
• Importantly hydroxyapatite crystals do not spontaneously form in the blood stream
• Ions in serum can sustain crystal growth

Question 16

Describe the heterogeneous nucleation (epitaxy).

Answer 16

-a foreign solid will assist nucleation of a crystal
-especially if it has a similar lattice structure
-Epitaxy (Greek = “arranged upon”)
organic matrix may act as an epitactic agent

Question 17

How does collagen act as a nucleator?

Answer 17

collagen:
– 64nm spacing will allow nucleation (crystals form in spaces)
– collagen fibrils in bone and dentine different to collagens elsewhere

Question 18

Name other possible nucleators.

Answer 18

-Proteoglycans
-Lipids
-Phosphoproteins
– e.g. Dentine Phosphoprotein

Question 19

what are matrix vesicles produced by?

Answer 19

cells

Question 20

what does matrix vesicles contain?

Answer 20

high concentrations of Ca2+ and PO43- ions

Question 21

what is matrix vesicles role?

Answer 21

-Provide ideal microenvironment for crystal
formation and growth
-Released into extracellular tissue
-Crystals grow and rupture vesicle
-help initiate mineralisation in a tissue

Question 22

what is the sequence of mineralisation for a tooth?

Answer 22

1. Dentine
2. enamel
3. cementum

Question 23

what mineralisation are Matrix vesicles involved in?

Answer 23

Only in dentine, not enamel and cementum

Question 24

Is MV’s involved in bone mineralisation?

Answer 24

Yes