Biomedical Engineering Reference
In-Depth Information
5.4.6
Possibility of OCP Involvement in Enamel HAP
Formation
In the mechanisms of extremely long and well-crystallized enamel crystal for-
mation, Brown
et al.
recognized the importance of OCP and theorized that OCP
is a precursor of enamel HAP [
45
,
119
]. They considered it to be a precursor
because (1) OCP has a thin and platy morphology, while HAP has a hexagonal
one; (2) OCP grows more rapidly than HAP; (3) OCP is a metastable phase of
HAP; (4) unstable OCP readily transforms into stable-phase HAP, and (5) OCP
is structurally similar to HAP. HAP formation via OCP occurs in three stages:
(1) nucleus formation, (2) two-dimensional growth of OCP into a single unit-cell
thickness of OCP, and (3) precipitation of a unit-cell thickness of OCP on the
two-dimensional crystal and its hydrolysis. Since the transformation from OCP to
HAP is topotaxial, the crystallographic axes of the OCP are preserved after the
transformation. The phase transformation from OCP to HAP has been proposed as
the mechanism incorporating impurity ions, water, and vacancies into HAP [
119
].
Although many HRTEM studies of enamel in the early formation stage caught
images of OCP in the first-formed enamel crystals [
52
,
120
-
122
], Brown
et al.
's
OCP precursor theory has not been generally accepted due to the difficulty of
detecting a transient mineral phase.
HPO
4
2
is another indicator that supports the involvement of the acidic precursor
phase [
79
,
123
]. As described in 3.1, HPO
4
2
is a substantial lattice component of
developing enamel crystals. Fourier transform infrared spectroscopy (FTIR) studies
have shown that the HPO
4
2
group is a lattice component of the enamel crystals
[
74
]. The amount of HPO
4
2
decreases with the progress of crystal maturation [
76
].
The higher content of HPO
4
2
in the early secretory enamel is consistent with the
formation of an OCP-like phase.
Micro-Raman spectroscopy, which is very sensitive to the atomic order of the
phase, revealed the presence of OCP in living murine calvarial tissue [
124
]. This
was the first direct evidence without the artifact of dehydration. Moreover, there
was an indication that ACP may form prior to OCP. Thus, in the case of murine
calvarial tissue, bone HAP formation could include the transition of ACP to OCP.
5.4.7
Epitaxial Overgrowth of Apatite on OCP Template
CO
3
-HAp crystals with central planar defects were synthesized for the first time by
Nelson
et al
.(Fig.
5.4
a1) [
125
]. The cross-sectional HRTEM images of the crystals
resemble those of enamel HAP crystals with a central dark line (CDL) [
50
,
126
,
127
]. Fourier filtering analysis of lattice images of the CO
3
-HAp crystals showed
that the defects were a one- or two-unit cell thickness of two-dimensional OCP
(Fig.
5.4
a2) [
128
]. The two-dimensional OCP was parallel to the (100) plane of the
CO
3
-HAp. A growth model, epitaxial overgrowth of apatite on the OCP template,
