Biomedical Engineering Reference
In-Depth Information
To conclude this part, as ACPs represent a special class of
calcium orthophosphates, it is reasonably to presume that various
amorphous-to-crystalline transformation mechanisms might exist
due to the initial differences of the investigated ACPs. Furthermore,
various competitive processes might occur simultaneously and their
importance might depend on the conversion conditions. Further
details on this topic are available in literature [31, 123].
2.4
ACP
in vivo
As stated in the introduction, both physical and morphological
evidences for the presence of ACPs in skeletal tissue of mammals
have been difficult to establish directly and the validity of inferential
evidences for their presence and amounts has been the subject of
considerable debates. Indirect assessments, such as X-ray diffraction
methods, have produced widely varying estimates of the amorphous
content of bone mineral, placing it at less than ~1% [19] to more
than ~30% of the total mineral mass, the rest presumably being
poorly crystalline ion-substituted CDHA (biological apatite). Even if
ACPs only occur at a lower percentage, one would expect TEM to
reveal some evidences [9], but as stated earlier, most TEM studies of
bones and teeth do not even mention the existence of amorphous-like
structures that could be ascribed to ACPs. However, the absence of
such structures could have been a negative artifact caused by aqueous
dissolution of more labile ACPs during the sample preparation
for examination. A few early TEM studies [108, 109] that avoided
aqueous processing of bone specimens by directly embedding and
sectioning freeze-dried material revealed a zone of electron dense,
6- 20 µm diameter spheroidal bodies adjacent to crystal-rich areas
of bone tissue. Electron diffraction of these sites revealed a hazy,
diffuse pattern similar to that observed by Watson and Robinson
[98] in their synthetic, amorphous-like precipitates. Whether or not
these spheroidal particles represented ACPs has been debatable but
a labile nature of ACPs suggests the need to maintain carefully the
anhydrous conditions in preserving this phase in calcified material
for examinations [20].
Another possibility that could account for the inability to establish
with certainty the existence of ACPs in mammalian bones is that the
ACP does not exist as separate particles but, instead, as an amorphous
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