Biomedical Engineering Reference
In-Depth Information
The first detectable crystals in enamel formation are flat thin
ribbons [691-693], that were reported to be OCP [569, 701-703],
β
[702], DCPD [624, 627], or ACP [704]. The
formation process of enamel is different from that for bone or
dentine: amelogenin being hydrophobic self-assembles into nano-
sized spheres that guide the growth of the ribbon-like dental enamel
crystals. During maturation of enamel, the mineral content increases
from initially ~45 wt.% up to ~98-99 wt.% [624, 676, 677]. The
enamel crystal rods widen and thicken by additional growth [624,
627, 705] with a simultaneous increase of the Ca/P molar ratio [705]
and a decrease in carbonate content [706-708], finally resulting
in the most highly mineralized and hardest substance produced
by vertebrates. It is interesting to note that in the radular teeth of
chitons, ACP was found to be the first-formed calcium orthophosphate
mineral, which over a period of weeks was transformed to dahllite
[709].
The crystal faces expressed in enamel are always (100) face and
at the ends presumably (001) [710, 711], which are the ones usually
found in HA. The centers of enamel crystals contain a linear structure
known as the “central dark line” (this line was also observed in bone
and dentine), which consists of OCP [192-196, 635]. As described
above for bones, X-ray diffraction shows that the crystals of younger
dentine are less crystalline than those of more mature dentine [662].
Therefore, maturation of dentine also means a slow transformation
(re-crystallization?) of biological calcium orthophosphates from
ion-substituted ACP to a better-crystallized ion-substituted CDHA.
The development of individual enamel and dentine crystals
was studied by high-resolution transmission electron microscopy
[712-714]. Both processes appear to be roughly comparable and
were described in a four-step process. The first two steps include the
initial nucleation and formation of nano-sized particles of biological
apatite. They are followed by ribbon-like crystal formation, which
until recently was considered as the first step of biological crystal
formation [712-714]. These complicated processes, starting with
the heterogeneous nucleation of inorganic calcium orthophosphates
on an organic extracellular matrix, are controlled in both tissues
by the organic matrix and are under cellular control [715]. To
complicate the process even further, regular and discrete domains of
various charges or charge densities on the surface of apatite crystals
derived from the maturation stage of enamel development were
-(Ca,mg)
(PO
)
3
4
2
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