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Biomineralization
in the major lateral teeth of chitons
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Macey
D.J., Brooker L.R., Webb J., Lee, A.P., Wealthall, R.,
Griffin B.R. |
The
major lateral radula teeth of chitons (Mollusca: Polyplacophora)
are composite materials highly appropriate to their biological
function and, as such, provide inspiration for the design and
synthesis of novel composite materials. Due to both intrinsic
interest and possible commercial potential, their use of
biomineralized forms of both iron and calcium has provoked
considerable research over the years. Although some differences
occur between chiton species, the major lateral teeth consist of
an iron-oxide cap that covers the posterior (cutting) surface,
and extends to varying degrees over the anterior surface. This
cap overlies a softer central core which, depending on the
species, is composed of either a crystalline carbonated
hydroxyapatite or iron (III) phosphate. Much of our current
understanding of biomineralization in chiton teeth has derived
from electron microscopy, X-ray and electron diffraction and
elemental mapping via X-ray emission spectroscopy. In addition
to these techniques, we have recently initiated extensive
studies of biomineralization using laser Raman spectroscopy and
detailed SEM examination of ground embedded specimens. The
former is a particularly powerful technique for the analysis of
iron compounds because the majority of iron oxide and hydroxide
vibrational bands occur below 600 cm-1, which while
rendering routine infrared microscopy difficult, are well within
the range of Raman. These studies have revealed that the
deposition of magnetite in the posterior region of the tooth
cusp has been shown to occur on two fronts, from both the
anterior and posterior sides simultaneously. In contrast, in the
lepidocrocite region, the mineral is formed by aggregation over
the whole region. In addition, evidence is presented for the
existence of a thin veneer of ferrihydrite over the surface of
the magnetite region, which is suggested to preserve this region
from oxidation prior to use in the mature tooth. Data
is given that also suggests that the junction zone plays a vital
role in the overall biomineralization process, contributing
large amounts of iron at certain stages. Despite the presence of
these architecturally discrete regions of mineral formation, at
the microscopic level the whole tooth possess a very distinctive
overall structure composed of
‘rods and troughs’, which presumably gives the tooth
strength and prevents shattering when in use.
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