Biomedical Engineering Reference
In-Depth Information
order width
thickness. In other words, the degree of interaction of
amelogenin with the OCP crystal faces was in the order (010)
<
length
<
(100),
meaning that the interaction with the (010) face was the strongest. Thus, the aspect
ratio was increased by preferential interaction of the amelogenin with the side faces
of the OCP crystals. This was observed regardless of the presence of the hydrophilic
C-terminus and the type of amelogenin used. Therefore, the interaction between
OCP and amelogenin was ascribed to the hydrophobic core part of amelogenin.
Fluoride (1 ppm) changed the mineral phase from OCP to HAP, yielding oriented
prism-like HAP crystals with a large aspect ratio [
139
]. It is possible to incorporate
F
into the lattice to enhance the interaction between the amelogenin and crystal
[
137
].
>
(001)
>
5.4.11
Cooperative Regulation of HAP Growth by Amelogenin,
Ca
2
C
,andMg
2
C
Analysis of the adsorption of amelogenin, Ca
2C
,andMg
2C
on growing enamel
crystals [
137
] revealed that amelogenins adsorb on enamel crystals more pref-
erentially than Mg
2C
. In the maturation stage, as the proteolytic cleaving of
amelogenin proceeds, amelogenin fragments are removed from the crystals while
Mg
2C
adsorption increases. Since Mg
2C
adsorption is suppressed by amelogenins,
the removal of amelogenin enables Mg
2C
to adsorb. The adsorption ability of Mg
2C
is about one-third that of Ca
2C
, and the adsorption of Mg
2C
decreases with the
activity ratio (
a
Mg
)/(
a
Ca
). This means that Ca
2C
reduces the inhibitory effect of
Mg
2C
.TheMg
2C
adsorption rate is determined by the (
a
Mg
)/(
a
Ca
) ratio. An increase
in a
Ca
facilitates the growth of HAP crystals with less disturbance of the amelogenin
and Mg.
Enamel fluid contains about 0.8 mM Mg
2C
and10mMCO
3
2
.TheMg
2C
inhibits the growth of HAP [
79
,
83
], and the degree of inhibition is synergistically
increased by CO
3
2
[
83
,
135
]. Nevertheless, enamel crystal has good crystallinity.
This can be ascribed in part to the cooperative regulation by amelogenin, Ca
2C
,and
Mg
2C
, maintaining an
a
Ca
level suitable for growing HAP with good quality.
5.5
Evolution and Adopted Mineral Phase in Hard Tissue
In the evolution of animals,
Lingula unguis
(Brachiopoda, Inarticulata) appeared
some 500 million years ago in the Ordovician period as an ancestor of animals with
hard tissue [
141
].
Lingula
still exists and is known as a living fossil. It has a pair of
shells that cover its soft body like a bivalve. The shell is composed of apatite and
-
chitin [
142
] instead of the typical CaCO
3
composition of Articulata (Brachiopoda)
and Mollusca shells. Not only did species with an apatite shell appear prior to those
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