Theories of Mineralisation

Question 1

What are the theories of mineralisation?

Answer 1

• alkaline phosphatase hypothesis
• nucleation theories - homogeneous nucleation, heterogeneous nucleation
• matrix vesicles

Question 2

Give an overview of the alkaline phosphatase theory:

Any problems?

Answer 2

• alkaline phosphatase enzyme
• hydrolyses organic phosphates
• releases inorganic phosphates (hydrogen phosphates)

Precipitation into apatite

Problems:

• concentration of organic phosphates too low to be an effective source
• pther sites contain alkaline phosphatase e.g. kidney
• normal serum phosphate concentration is sufficient
• this hypothesis has been discounted, however has been involved

Question 3

What is homogeneous nucleation?

Answer 3

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

• serum contains Ca and PO4 –> can hydroxyapatite crystals form?
• hydroxyapaitie crystallites do not form but can sustain crystal growth
• do not spontaneously form in the blood stream
• this theory can be disgarded

Question 4

What is heterogeneous nucleation?

What materials may be possible nucleators?

Answer 4

Epitaxy:

• a foreign solid will assist nucleation of a crystal, especially if it has a similar lattice structure
• organic matrix may act as an apitactic agent (template)

Possible nucleators:

• collagen - spacing will allow nucleation, colagen fibrils i bone and dentine different to collagen elsewhere
• proteoglycans, lipids, phosphoproteins e.g. dentine phosphoproteins
• this theory works

Question 5

What is the matrix vesicle theory?

Answer 5

Matrix vesicles:

• produced by cells
• contain high concentrations of Ca and PO4 ions
• provide ideal microenvironment for crystal formation and growth
• released into extracellular tissue
• crystals grow and rupture vesicle membrane
• help initiate mineralisation in a tissue
• involved in mantle dentine formation and bone mineralisation