2. Mineralization
Process of deposition of insoluble calcium salts in a tissue.
Mechanism of calcification:
Normally, the concentration of Ca & Phosphate ions are
insufficient to form crystals. When certain local factors raises the
concentration of these ions, then initiation of crystals starts.
3. The mineral component of all hard tissues of our body are cheifly
Hydroxyapatite crystals.
Mineralization was thought to be based on precipitation
dynamics, with initially the only ions needed being calcium and
phosphate.
Though the ECF is supersaturated with calcium and phosphate,
yet the mineralization is not a common phenomenon. It may be
due to:
1. Presence of inhibitors of mineralization
2. Formation of unstable or insufficient amount of crystals to
proceed
4. Thus for precipitation to occur, Interplay of various factors in
the osteoid matrix may contribute to conditions that may favor
mineralization. They are:
1. Presence of increased concentration of Extracellular Ca &
P ions which facilitate mineralization – Homogenous
nucleation
2. Presence of nucleating substance – Nucleating substances
like collagen which will act as a template over which
mineralization occurs even with Ca & P ion concentration of
ECF is not elevated – Heterogenous nucleation
3. Absence of Inhibitors of Mineralization.
5. Theories of Mineralization
There are 3 main theories of mineralization. They are:
Alkaline phosphatase theory
Nucleation theory and
Matrix Vesicle theory
6. Alkaline phosphatase theory
Proposed by Robinson in 1923
This enzyme present in the organic matrix of calcifying
tissues
↓
Hrdrolyses organic phosphates → Inorganic phosphate
↓
↑ P ion concentration – boosting effect & combines with
Ca ions
↓
Amorphous Calcium phosphate
↓
HA crystal formation
It acts as a marker for active tissue mineralization.
7. Supporting
When slices of cartilage
from animals affected by
rickets are placed in Alk.
Phosphatase, there is
evidence of mineralization
More Alkaline Phosphatase
in calcifying cartilage than
non calcifying cartilage
Contrasting
Alkaline Phosphatase also
seen in other tissues that
do not calcify.
Inhibitors of some
enzymes other than Alk.
Phosphatase can inhibit
mineralization
The organic phosphates
present in tissue fluid
alone are insufficient to
produce sufficient number
of inorganic p ions.
8. Nucleation theory
Neumann and Neumann in 1953 put forward this
theory.
Also called as seeding theory or collagen template
theory
Some nucleating substances act as a template upon
which crystals can be laid down.
Collagen acts as most important seeding substance.
Lysine and Hydroxylysine – Phosphate binding site.
Carboxyl ions – Calcium binding site
9.
10.
11. Gaps between the collagen fibril
↓
First Proteoglycans attaches to the collagen
↓
Enzymatic degradation of proteoglycans releases Ca
which attaches to collagen fibril
↓
Later phosphoproteins will come and attaches to the
collagen and by the action of Alkaline phosphatase,
phosphate is released and attaches to Ca to form
apatite crystal in the gap zone of collagen.
12.
13. Supporting
Electron microscopic
studies supported parallel
arrangement of
Hydroxyapatite crystals
and collagen fibers
Contrasting
Enamel is a highly
mineralized tissue – No
collagen at all. So, some
other mechanism is
playing a role.
14. So, all the tissues which contains collagen should
mineralize. But this is not happening in soft tissues.
WHY?
In soft tissues, the charged site may be covered by
some ground substance material which prevent the
attachment of ions to initiate mineralization. This
substance is called - Crystal poison –
Pyrophosphate.
15. Collagen shows intrafibrillar pores through which
Ca & P ions should pass to reach nucleating sites
located inside the fibril.
Diameter of P ion – 0.4 nm
Gap between tropocollagen molecules in
Calcifiable tissues – 0.6 nm
Gap between tropocollagen molecules in Non-
Calcifiable tissues – 0.3 nm
16. Matrix Vesicle theory
Rich in phospholipids - Phosphatidyl serine - high
affinity for Ca ions.
Vesicles also contain annexins. Annexins in the
vesicles form a calcium channel, thus incorporating the
ion within the matrix vesicles.
Their membranes furnish binding sites for the
nucleation of hydroxyapatite crystals.
17. Matrix Vesicle theory
Matrix vesicles are small membrane bound structures
25 to 250 nm in diameter, lying free in the matrix,
where calcification is going to occur.
Are rounded outgrowths of cell membrane that bud
from osteoblasts, chondrocytes and odontoblasts.
There are 2 types of matrix vesicles
18. Matrix Vesicle theory
Rich in phospholipids - Phosphatidyl serine - high
affinity for Ca ions.
Vesicles also contain annexins. Annexins in the
vesicles form a calcium channel, thus incorporating the
ion within the matrix vesicles.
Their membranes furnish binding sites for the
nucleation of hydroxyapatite crystals.
19.
20. Once the HA crystal is formed
↓
After attaining certain thickness, they will be released into
ECM by action of Phospholipases which disintegrates
membrane of Matrix vesicles
↓
Progression of mineralization occurs as spheres or
globular masses
21. Inhibitors of Calcification
Pyrophosphate
Diphosphonates
Adenosine triphosphate
Citrate, magnesium, and proteins like albumin also acts as
inhibitors
22. Summary
All the 3 mechanisms are involved in mineralization.
Alkaline phosphatase helps in providing more phosphate
ions and in removing crystal poisons or inhibitors of
calcification.
Collagen acts as a seed – helps in intrafibrillar calcification
or linear calcification
Matrix vesicles – helps in extrafibrillar calcification or
globular calcification and
