Biomineralization: Tooth Enamel Formation - Bioinspiration: From Nano to Micro Scales - page 174

was postulated to explain the central planar inclusion. Briefly, a primordial thin plate
of OCP crystal works as a template for successive epitaxial growth of HAP on the
(100) face, with the template embedded at the center of the HAP crystal. This model
well explains the formation mechanism of the CDL, which is observed in the cross-
section of enamel HAP.
A series of Ap/OCP/Ap lamella mixed crystals ([ 129a ]) (Fig. 5.4 b), developing
porcine enamel crystals [ 120 ], puffer enamel HAP [ 130 ](Fig. 5.4 c1), and carp
enameloid HAP [ 131 ](Fig. 5.4 c2) ensure HAP overgrowth on an OCP template
in vivo and in vitro. In the Ap/OCP/Ap mixed crystals, the thickness of the OCP at
the center decreased while that of the apatite overgrowth increased with an increase
in the concentration of F from 0.1 to 1 ppm. In the presence of 1 ppm F , HAP
with a central planar defect was obtained; cross-sectional HRTEM images of these
Ap/OCP/Ap lamella mixed crystals (Fig. 5.4 b) were similar to those of Nelson's
CO 3 -HAp crystal, carp enameloid crystals [ 131 ] and of porcine enamel crystals
[ 120 ]. TEM observation has shown that the OCP template grew to its critical size,
and then HAP overgrowth on the template crystal commenced [ 129b ]. The critical
size of the template depended on the solution conditions (pH, ion concentration,
etc.). Interestingly, crystals with the Ap/OCP/Ap structure have been found in
human dental calculus [ 132 ].
On the basis of HRTEM observation of developing fish enameloid, porcine
enamel HAP, and synthesized CO 3 -HAp crystals, the epitaxial overgrowth mecha-
nism was modeled at the atomic level [ 120 ](Fig. 5.4 d): (1) two-dimensional growth
of an OCP-like precursor with a one- or two-unit cell thickness and a ribbon-like
morphology; (2) epitaxial overgrowth of apatite on the thin ribbon in which small,
regular triangles and trapezoid crystals form as building units and are deposited on
screw dislocations of the precursor crystal; (3) lateral growth of the small triangles
on the template and fusion of the neighboring units; and (4) thickening of the
template crystal by two-dimensional growth on the prism plane. Screw dislocations,
which initiate HAP overgrowth, have been observed in both synthetic CO 3 -HAp
[ 125 ] and human fetal enamel crystals [ 127 ].

5.4.8 Control of Morphology and Orientation

by One-Directional Ca 2 C Diffusion

The ameloblasts secrete enamel precursor materials in the direction of their long
axis. Osborn suggested that the crystal orientation is controlled by the secretory
force generated by the ameloblasts [ 133 ]. This suggestion led to the supposition
that one-directional Ca 2C ion supply from the ameloblasts causes the lengthwise
and oriented growth during the initial crystalline phase, which was assumed to be
OCP [ 134 ]. A model system constructed for this uses a cation-selective membrane
to create one-directional diffusion of Ca 2C ions, thereby simulating the flow of Ca 2C
ions in the enamel matrix. OCP crystals elongated in the c-axis direction grow on
the membrane with the c-axes parallel to each other. Details are reviewed by Iijima
[ 135 ].