Biomedical Engineering Reference
In-Depth Information
growth mechanism [610]. The same was suggested for dentine and
enamel [611, 612] (see section
below), as well as for
more primitive living organisms. For example, in the shell of the fossil
marine animal
1.4.2. Teeth
Lingula brachiopod unguis
that consists of a biological
apatite, the crystal
-axes are oriented parallel to the β-chitin fibrils
[458, 613-616]. Therefore, the orientation of biological apatite
crystals parallel to the long axes of the organic framework could
be a general feature of calcium orthophosphate biomineralization.
However, the degree of biological apatite orientation appears to be
a useful parameter to evaluate
c
stress distribution, nano-scale
microstructure and the related mechanical function, the regenerative
process of the regenerated bone and to diagnose bone diseases such
as osteoarthritis [617, 618]. It is interesting to note, that contrary
to what might be expected in accordance with possible processes
of dissolution, formation and remineralization of hard tissues, no
changes in phase composition of mineral part, crystal sizes (length,
width and thickness) and arrangement of crystals on collagen fibers
were detected in abnormal (osteoporotic) human bones compared
to the normal ones [619].
Some animals, such as newts, are able to regenerate amputated
limbs. This is, of course, of a high interest in regenerative medicine.
Bone regeneration in the forelimbs of mature newts was studied by
noninvasive X-ray microtomography to image regenerating limbs
from 37 to 85 days. The missing limb skeletal elements were restored
in a proximal-to-distal direction, which reiterated the developmental
patterning program. However, in contrast to this proximal-distal
sequence, the portion of the humerus distal to the amputation site
was found to fail to ossify in synchrony with the regenerating radius
and ulna. This finding suggests that the replacement of cartilage
with mineralized bone close to the amputation site is delayed with
respect to other regenerating skeletal elements [620].
Unlike other mineralized tissues, bone continuously undergoes
a remodeling process, as it is resorbed by specialized cells called
osteoclasts and formed by another type of cells called osteoblasts (so
called “bone lining cells”) in a delicate equilibrium [533, 575, 578,
621, 622]. The purpose of remodeling is the release of calcium and
the repair of micro-damaged bones from everyday stress. Osteoblasts
are mononuclear cells primarily responsible for bone formation.
They contain alkaline phosphatase, which enzymatically produces
orthophosphate anions needed for the mineralization. In addition,
in vivo
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