Biomedical Engineering Reference
In-Depth Information
with a CaCO
3
shell, the
c
-axis of the HAP and the fiber axis of the
-chitin are
parallel to each other [
143
,
144
]. This orientation relationship between the HAP
and organic matrix component indicates that a mechanism that organizes mineral
crystal growth is inherent in animals with hard tissues.
HAP in a dense fraction (2.5-2.7 g cm
3
)of
Lingula
shell contains CO
3
2
(2.5
“
0.01 wt%) with Cl
as a minor component and structural water. Hydroxyl was not detected by FTIR
analysis.
Lingula
shell HAP was identified as CO
3
,F,Cl-Ap[
145
]. TEM [
146
]and
SAXS [
147
] studies of the shell demonstrated that mineral granules (about 50 nm
in diameter) were distributed in the marginal part of the shell, i.e., the newly formed
part. After calcination at 1,000
ı
C,
Lingula
shell HAP produced
0.2 wt%), F
0.04 wt%), and Mg
2C
˙
(2.0
˙
(0.98
˙
-TCP (about 30%)
along with HAP with improved crystallinity. Since ACP transforms into
“
-TCP after
being calcined [
148
], the existence of ACP in
Lingula
shell HAP is highly probable.
Chimaera phantasma
(Holocephali), a relative of the shark, has a unique dental
tissue “tooth plate” instead of sharp, marginal teeth like a shark. The tooth plate
is composed of pleromin and osteodentin. The mineral phase of pleromin is
whitlockite [
149
], and that of osteodentin is HAP. The whitlockite crystallizes in
an oval shape, has high crystallinity, and deposits without orientation.
Chimaera
is
a living fossil that appeared some 400 million years ago in the Devonian period.
The lungfish (
Protopterus
) also has tooth plates; however, the mineral phase is HAP
[
150
]. Thus, only
Chimaera phantasma
has dental tissue composed of whitlockite.
Recently, Bentov et al. [
151
] reported coexistence of calcium carbonates and
calcium phosphate in the mandibles of a freshwater crayfish (the arthropod
Cherax
quadricarinatu
s): Amorphous calcium carbonate (ACC) and ACP located in the
base of the mandible, F-Ap crystals covered the amorphose minerals, forming an
enamel-like coating, and calcite located in the incisor. The use of both carbonates
and phosphates as component of a functioning hard tissue was found, probably, for
the first time in an evolution of biominerals.
“What determines the phase and organization of mineral in hard tissues” is an
unanswered question. The higher level organizing system is thought to control
the cells so that they provide a medium in which appropriate mineral can form.
Recent advances in gene analysis and cloning techniques have made it possible
to investigate the evolution of enamel proteins. The amino acid sequences of
amelogenin have a striking level of homology among species [
152
,
153
]. Further
progress in this field could help answer the question above.
“
5.6
Concluding Remarks
Nucleation and growth of enamel HAP crystal proceed in an enamel matrix, which
acts as a functional medium that changes the composition continuously, thereby
providing an optimal environment for crystal formation. The formation process is
determined by the local environment, which is determined by the tissue-specific
cells. Analyses of developing enamel HAP crystal and of the components and
