Biomedical Engineering Reference
In-Depth Information
In general, the biomineralization process (therefore, bone
formation) can happen in two basic ways: either the mineral
phase develops from the ambient environment as it would from a
supersaturated solution of the requisite ions, but requires the living
system to nucleate and localize mineral deposition, or the mineral
phase is developed under the direct regulatory control of the
organism, so that the mineral deposits are not only localized but may
be directed to form unique crystal habits not normally developed
by a saturated solution of the requisite ions. In a very famous paper
[669] and two extended elaborations [454, 670] the first type of
biomineralization was called “biologically induced” mineralization
and the second “(organic) matrix-mediated” biomineralization. In
some papers, the former process is called “passive” and the latter one
— “active” biomineralization [34]. Briefly, an “active process” means
the assembly of nano-sized crystals of calcium orthophosphate into
bones due to an activity of the suitable cells (e.g., osteoblasts), i.e.
within a matrix vesicle. Such structures have been discovered by
transmission electron microscopy for bone and teeth formation
[671, 672]. A “passive process” does not require involvement of
cells and means mineralization from supersaturated solutions with
respect to the precipitation of biological apatite. In the latter case,
thermodynamically, biomineralization might occur at any suitable
nucleus. The collagen fibrils have a specific structure with a 67
nm periodicity and 35-40 nm gaps or holes between the ends of
the collagen molecules where bone mineral is incorporated in the
mineralized fibril [454, 544, 545, 557, 576, 577]. Such a nucleation
within these holes would lead to discrete crystals with a size related
to the nucleating cavity in the collagen fibril (Fig. 1.11). It was
proposed that a temporary absence of the specific inhibitors might
regulate the process of bone formation [673-675].
To conclude the bone subject, let me briefly mention on the
practical application of bones. In the Stone Age, bones were used
to manufacture art, weapons, needles, catchers, amulets, pendants,
headdresses, etc. Nowadays, cut and polished bones from a variety
of animals are sometimes used as a starting material for jewelry and
other crafts. Ground cattle bone is occasionally used as a fertilizer.
Furthermore, in medicine, bones are used for bone graft substitutes,
e.g., allografts from cadavers.
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