Biomedical Engineering Reference
In-Depth Information
that amelogenin nanospheres preferentially bind to positively charged domains of
enamel crystal. Thus, functional control of crystal size, morphology, orientation,
and growth rate has been ascribed to amelogenin.
Since enamelin has a strong affinity for enamel crystal (2.2), it is considered
to induce nucleation and to regulate the growth rate through interaction with the
crystal. The 32 kDa fragment, one of the series of cleavage products, is the most
stable one. It has been shown to promote nucleation of apatite [
100
] and to change its
conformation from
-sheet, which interacts with Ca
2C
in HAP, depending
on the Ca
2C
concentration [
101
].
Cooperative regulation of enamelin and amelogenin through their interaction has
been suggested [
102
]. The hypothesis that amelogenin and enamelin jointly control
the elongated crystal growth of OCP was examined [
103
] using a dual membrane
experimental device in which a cation-selective membrane and a dialysis membrane
form a tiny reaction chamber, and Ca
2C
and PO
4
3
ions flow into the chamber
in a controlled manner. OCP was chosen as a model crystal for the post-secretory
growth on the basis of the evidence described in 4.6 and 4.7. A mixture of 10w/v%
recombinant porcine amelogenin (rP148) and 32 kDa porcine enamelin fragments
wasusedasacrystalgrowthmediumat37
ı
C and a pH of 6.5. The aspect ratio of the
OCP crystals was substantially higher at a certain ratio of amelogenin to enamelin.
Thus, the co-assembly of amelogenin and 32 kDa enamelin was suggested as a
mechanism regulating the morphology of OCP crystal.
'
-helix to
“
5.4.2
ACP Formation and Stability in Enamel Fluid
As described in Chap. 3.2, there is no general agreement yet on the solubility
product of ACP, which is required to evaluate the driving force (
) needed to
precipitate ACP. Several solubility products, ranging from 24.8 to 28.3, have been
proposed for ACP and amorphous tricalcium phosphate (ATCP). ACP slurry (for
which a large solid/solution ratio is the best experimental approach) has a nearly
constant
I
AP
of 1.6
10
25
in the pH range 7.4-9.25 when the formula is given
as Ca
3
(PO
4
)
1.87
(HPO
4
)
0.2
[
104
]. Accordingly, “the minimum
I
AP
needed to form
ACP de novo in physiological solution at pH7.4 is considerably greater than that
calculated for serum” [
105
]. In general, the spontaneous formation of ACP is
attained by mixing Ca
2C
and PO
4
3
solutions with rather high concentrations of
Ca
2C
and PO
4
3
.TheCa
2C
and PO
4
3
immediately co-aggregate into clusters
large enough to be separated from the solution. However, the Ca
2C
and PO
4
3
concentrations of the enamel fluid are too low to form ACP spontaneously (2.5, 3.1).
Therefore, “it would appear unlikely that ACP could form in vivo” [
105
] without a
mechanism to increase the local Ca
2C
and PO
4
3
concentrations or a reasonable
process to form nuclei. As described in 4.4, the mechanism could be the stepwise
construction of growth units by amelogenin and pre-nucleated CaP clusters.
Although ACP is not very stable and transforms into crystalline phase in pure
calcium phosphate solution, in vitro studies have shown that CO
3
2
,Mg
2C
,F
,