Biomedical Engineering Reference
In-Depth Information
(DEJ). Needle-like enamel rods might be tens of microns long (up to
100 µm) but sometimes only 50 nm wide and 30 nm thick (Fig. 1.14)
[676, 677, 689-696]. They are quite different from the much smaller
crystals of dentine and bone (Table 1.2), but all of them consist of
biological apatite [438, 697, 698]. In cross section, an enamel rod is
best compared to a keyhole, with the top, or head, oriented toward
the crown of the tooth and the bottom, or tail, oriented toward the
root of the tooth.
Figure 1.14
Scanning electron micrograph of the forming enamel of a
continuously growing rat incisor showing ordered rods of
calcium orthophosphates. Scale bar: 10 μm. reprinted from
Ref. [454] with permission.
The arrangement of the crystals of biological apatite within
each enamel rod is highly complex. Enamel crystals in the head
of the enamel rod are oriented parallel to the long axis of the rod.
When found in the tail of the enamel rod, the crystals' orientation
diverges slightly from the long axis [676, 677]. The arrangement
of the enamel rods is understood more clearly than their internal
structure. Enamel rods are found in rows along the tooth (Fig. 1.14)
and, within each row, the long axis of the enamel rod is generally
perpendicular to the underlying dentine [676, 677, 689-693]. Recent
AFm study indicated that CDHA crystals in enamel exhibited regular
sub-domains or subunits with distinct chemical properties related to
topographical features and gave rise to patterned behavior in terms
of the crystal surface itself and the manner in which it responded to
low pH [699].
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