Biomineralization: Tooth Enamel Formation - Bioinspiration: From Nano to Micro Scales - page 162

5.2.3 Enamel HAP Formation

In the early stage of enamel formation, fine granules have been observed as an
initial cell product [ 42 , 43 ]. The process of thin ribbon-like crystallite formation
from the granules comprises five main steps [ 42 ]: (1) deposition of granules
(5-7 nm diameter) with low electron scattering; (2) linear assembly of granules into
fibers (5-10 nm

m) with 5-7 nm periodicity and much greater electron
scattering; (3) arrangement of fibers into parallel rows; (4) cross-linking of fibers
by joining of electron-dense granules, resulting in a ladder-like appearance; and (5)
deposition of many tiny crystallites, leading to the formation of ribbon-like crystals.
Except for the initial granules with low electron scattering, all the electron-dense
materials exhibit an apatitic electron diffraction pattern. This postulated process is
related to the HAP formation mechanisms described by Beniash et al .[ 44 ]. Briefly,
they observed that ACP particles form initially and that they assemble step-by-step
into thin plates and eventually transform into HAP crystallites. They propose that
the enamel proteins control the organized assemble of ACP particles and that the
transition starts with proper ionic alignments within the ACP, which could dictate
the direction of assembly. For details, see Chap. 4.6. Other candidates suggested as
the initial enamel mineral phases are octacalcium phosphate (OCP) [ 45 , 46 ], non-
apatitic calcium phosphate [ 47 ], and ACP [ 48 ].
The ribbon-like enamel crystals align with their c-axes parallel to each other
[ 49 - 51 ](Fig. 5.1 ). The thickness was measured to be larger than 2 nm with
distribution maxima at 2.5, 3, and 3.7 nm (newborn cat and 5-month-old human
fetuses) [ 52 ]. The crystal length, obtained from enamel with a density less than
1.8 g cm 3 , is at least 100

0.1-1.0

m, and the crystals are probably continuous from the
DEJ to Tomes' processes [ 53 ]. In the maturation stage, the crystals increase mainly
in thickness. As they do, they form plates characterized by flattened hexagonal
cross-sections (Fig. 5.4 c2). The thickening continues until the lateral faces of the
crystals fuse to each other. Mature human enamel crystal has a mean width of 68 nm
and a mean thickness of 26 nm [ 54 ]. Some researchers consider that the crystal
length reaches, at most, the thickness of the enamel matrix [ 1 ].

5.2.4 Microstructure of Enamel Matrix and Conformation

of Amelogenin

The morphological entities of the enamel matrix in the early formation stage have
been variously described as fibrillar, compartmental or tubule, helical, and lamellar.
These are reviewed by Nylen [ 55 ]. It is now generally accepted that amelogenin
molecules assemble into spheres of nanometer size, which were initially identified
as spherules with 15-20 nm diameter [ 20 , 21 ], and that these nanospheres are the
basic structural units of the enamel matrix (Fig. 5.2 a). Further studies have shown
that (1) the -NH 2 and -COOH groups [ 56 ] and hydrophobic regions [ 57 ]ofthe