Biomineralization: Tooth Enamel Formation - Bioinspiration: From Nano to Micro Scales - page 175

The formation of OCP usually starts with deposition of granules about 100 nm in
size on a membrane [ 136 ]. These granules do not show any distinctive XRD reflec-
tion, suggesting that they are ACP-like precipitates. They deposit on the membrane
and assemble into irregularly shaped branching clusters, which subsequently grow
into bank-like structures and then flake-like crystals. The flake-like crystals increase
in height perpendicular to the membrane and eventually grow into OCP crystals
elongated in the c -axis direction. The transition from amorphous to crystalline phase
is speculated to be a de novo rearrangement of the internal structures.

Correlated Interaction of F , Amelogenin, and Ca 2 C
in HAP Growth

5.4.9

The effects of F on the formation and properties of HAP crystals are well
known. F (1) increases the growth rate, (2) accelerates the hydrolysis of OCP to
HAP, (3) improves the crystallinity, (4) increases the crystal size, and (5) reduces
the solubility. In the combination of F and amelogenin, the F induces and
precedes F-HAp formation, and the amelogenin disturbs this reaction. Amelogenin
adsorption on the F-HAp crystals increases with the F content. F-HAp adsorbs
more amelogenin than H-Ap. Since the F in F-HAp interferes with protease
activity, the amelogenin on crystals is less degradable, so it is retained on the F-
HAp longer [ 137 ].
In addition, Ca 2C is related to the adsorption of amelogenin on the crystals. The
higher the Ca 2C concentration, the more rapidly the amelogenin is degraded and
removed from the crystal. A higher Ca 2C concentration also promotes F-mediated
conversion of OCP to HAP in vitro [ 137 ]. In the secretory stage, when Ca 2C
transport is limited, degradation of amelogenin progresses slowly, while early in
the maturation stage, when Ca 2C transport increases dramatically, the transition
from OCP to HAP, the degradation of amelogenin, and the removal of the degraded
fractions are accelerated. The removal of the fractions enables the F to adsorb
on the crystal, leading to formation of F-HAp. Since F-HAp enhances amelogenin
adsorption, there is an amelogenin reaction that disturbs crystal growth. Thus, F ,
Ca 2C , and amelogenin interact with enamel HAP in a coordinated manner.

5.4.10 Regulation of OCP and HAP Formation by Amelogenin

and F

Considering OCP to be another transient phase for enamel crystals, Iijima
et al . investigated the growth mechanism of elongated OCP crystals using a dual
membrane experimental device [ 138 - 140 ]. OCP crystals preferentially grew in
the c-axis direction on the membrane regardless of the presence of amelogenin.
Amelogenin decreased the OCP crystal size. The degree of decrease was in the