Biomedical Engineering Reference
In-Depth Information
(0,0,0.062). Because the OH
is slightly displaced from the mirror plane (
z
0.25
and 0.75), the stability of H-Ap is slightly lower than that of F-Ap [
87
]. The crystal
structure of human enamel was determined using the powdered enamel fraction with
a density higher than 2.95 g cm
3
and the Rietveld method [
88
]. Enamel HAP has
almost the same structure as H-Ap and structural water, and Cl
(about 0.3 wt%) is
located on the hexagonal axis between
z
D
0.11 and 0.14. Structural water is defined
as water in the lattice that is removed by heating at 400
ı
C. The lattice dimensions are
a
D
0.6878(1) nm. The loss of water from enamel HAP decreases
the a-axial dimension by 0.02 nm [
89
]. Structural aspects of HAP are also described
in Chap. 3.2.
As described in Chap. 3.2, H-Ap has two possible symmetries, hexagonal and
monoclinic. It has been noted that a small number of impurity ions, e.g., F
,
stabilizes the hexagonal structure [
90
]. Therefore, strictly speaking, any HAP,
including enamel HAP, that contains impurity ions is not genuinely hexagonal but
pseudohexagonal with its hexagonal symmetry stabilized by the ions.
D
0.9441(2) and
c
D
5.4
Mechanisms of Enamel Crystal Formation
5.4.1
Regulatory Roles of Amelogenin and Enamelin
How does enamel crystal formation start? Dentin at the DEJ was once thought
to be the nucleation site on the basis of transmission electron microscopy (TEM)
observations of the dentin and enamel crystallites at the DEJ [
91
]. However, there
is little other evidence supporting the nucleation of enamel crystal on dentin crystal
at the DEJ. Direct epitaxial growth of enamel crystals onto dentin crystallites has
not been observed [
42
,
92
]. Along with ultrastructural studies of developing enamel
crystals, in vitro HAP formation studies have shown that amelogenin accelerates the
nucleation kinetics and controls the crystal morphology, indicating that the crystal
formation mechanism involves amelogenin [
93
-
96
]. Nuclear magnetic resonance
(NMR) studies showed that the C-terminus of amelogenin, with four -COO
groups
and three -NH
C
groups, directly binds with the HAP crystal surface [
97
]. In vitro
nucleation and growth of OCP are promoted by recombinant amelogenin (rP179)
with the C-terminus [
98
]. Promotion of OCP nucleation is ascribed to the -NH
C
and -COO
groups in the charged C-terminal region exposed on the surface, which
could reduce the nucleation energy. A role of rP179 in the OCP formation is also
discussed in Chap. 4.4.
The adsorption behavior varies with the molecular weight [
24
]. Intact amelo-
genin (25 kDa for porcine) adsorbs strongly on HAP while the 20 kDa fragments
adsorb less strongly [
23
]. The 13 kDa fragments adsorb less than the nascent
25 kDa amelogenin molecules. Ultrastructural studies of developing enamel crystals
have shown that amelogenin nanospheres are on and between the developing
enamel crystals [
61
]. Chemical force microscopy observations [
99
] demonstrated
