Biomedical Engineering Reference
In-Depth Information
there is one more type of the cells called osteocytes that originate
from osteoblasts, which have migrated into, become trapped and
surrounded by bone matrix, which they themselves produce [533,
552-555, 575-578].
If osteoblasts are bone-forming cells, osteoclasts are multinuclear,
macrophage-like cells, which can be described as bone destroying
cells because they mature and migrate to discrete bone surfaces [578,
621, 622]. Upon arrival, active enzymes, such as acid phosphatase,
are secreted against the mineral substrate that causes dissolution.
This process, called bone resorption, allows stored calcium to be
released into systemic circulation and is an important process in
regulating calcium balance [621, 622]. The iteration of remodeling
events at the cellular level is influential on shaping and sculpting the
skeleton both during growth and afterwards. That is why, mature
bones always consist of a very complex mesh of bone patches, each of
which has both a slightly different structure and a different age [454,
533-535, 544, 546-549, 552-554, 575]. The interested readers are
suggested to read a review on the interaction between biomaterials
and osteoclasts [623].
Still there is no general agreement on the chemical mechanism of
bone formation. It is clear that the inorganic part of bone consists of
biological apatite, i.e. CDHA with ionic substitutions but without the
detectable amounts of hydroxide [624-628]. However, the recent
results of solid-state nuclear magnetic resonance on fresh-frozen and
ground whole bones of several mammalian species revealed that the
bone crystal OH
−
was readily detectable; a rough estimate yielded
−
content of human cortical bone of about 20% of the amount
expected in stoichiometric HA [629]. Various
an OH
experiments on
precipitation of CDHA and HA revealed that none of these compounds
is directly precipitated from supersaturated aqueous solutions
containing calcium and orthophosphate ions: some intermediate
phases (precursors) are always involved [27, 83, 84, 192-198,
270-274]. Depending on both the solution pH and crystallization
conditions, three calcium orthophosphates (DCPD, ACP and OCP) have
been discussed as possible precursors of CDHA precipitation
in vitro
.
Due to this reason, the same calcium orthophosphates are suggested
as possible precursors of biological apatite formation
in vitro
.
The transient nature of the precursor phase of bone, if it exists
at all, makes it very difficult to detect, especially
in vivo
[93].
However, in 1966 W. E. Brown proposed that OCP was the initial
in vivo
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