Biomedical Engineering Reference
In-Depth Information
and P
2
O
7
4
stabilize ACP [
83
,
106
,
107
]. The potential role of a PO
4
3
group of
amelogenin in ACP stabilization was demonstrated: P148 with a PO
4
3
group on
serine-16 stabilized ACP for more than one day while non-phosphorylated rP172
did not [
108
]. ACP, once formed, could be stabilized in the enamel matrix for some
period.
5.4.3
Organization of ACP into Crystals
Supramolecules composed of amelogenin and ACP have been formed in vitro [
51
].
The unanswered question is, “what controls the arrangement and fusion of the
supramolecules and transformation into crystallines with their c-axes parallel to
each other?” Since there are charged domains on developing enamel crystal surfaces
[
99
], the domain structure of the enamel crystals is thought to be formed by the
organized fusion of the crystallites in the c-axis direction. HAP crystal formation by
stepwise fusion of the protein/mineral nanoparticles into long crystals was proposed
[
109
]: initially formed fine granules align in rows and form chains by fusion; the
chains fuse into plates, and the plates fuse along the c-axis direction, forming long
crystals.
In the absence of a structural framework or functional materials, it would be
hard to control the assembly of ACP and the final morphology of the crystalline
phase derived from ACP. ACP particles formed in solution frequently aggregate into
irregularly shaped branching clusters, and de novo crystallization yields crystals
with a rectilinear morphology [
105
]. Since a spherical-shaped ACP particle has
no long-range order in its structure, there must be mechanisms that organize
the assembly of ACP particles and their successive growth into crystals with
morphology specific to the hard tissue. Beniash
et al
. noted that partially ordered
structure formed in ACP particle could dictate the direction of assembly [
44
].
5.4.4
CaP-Cluster Models
The formation models postulated by Wang
et al
. and Yang
et al
. ascribe this function
to amelogenin and explain the process specifically [
95
,
96
]. Both of them studied the
effect of amelogenin (rP172) on nucleation and growth of HAP under strict control
of the degree of supersaturation with respect to the precipitating phases using a
constant composition technique [
110
].
Wa n g
et al.
used solutions supersaturated with respect to OCP (
1.45 and
2.32) with pH 6.8 and 37
ı
C to examine whether enamel HAP forms with OCP
as an intermediate phase [
95
]. Figure
5.3
illustrates the step-by-step mechanism
of the assembly of CaP clusters and amelogenin nanospheres into HAP crystal.
In the presence of amelogenin, the induction time is less. Nucleation clusters
OCP
D
