Biomedical Engineering Reference
In-Depth Information
[590]. There is still no clear idea why the crystals of biological apatite
are platelet-shaped even though dahllite has hexagonal crystal
symmetry [454, 544, 552-556, 562-567, 569-573]. One possible
reason is that they grow via an OCP transition phase, which crystals
are plate-shaped [544].
The processes of bone formation (ossification) and growth are
very complicated ones and it is difficult to describe them without
making a deep invasion into biology. It has been studied for decades
[574] but still there are missing points. Very briefly, it is considered
that bones appear and grow as the result of calcification (or
biomineralization) of connective tissues, mainly cartilage [533, 575].
The ossified tissue is invaginated with blood vessels, which bring
ions of calcium and orthophosphate to be deposited in the ossifying
tissue. The biomineralization process is controlled to some extent
by cells and the organic matrices made by those cells facilitate the
deposition of crystals [578]. There is an opinion, that, initially, the
mineral crystals are formed in an environment rich in the so-called
SIBlING (Small Integrin-Binding ligand N-linked Glycoprotein)
proteins. As bone crystals grow, there is greater association with
proteins, such as osteocalcin, that regulate remodeling [606].
Thus,
formation of hard tissues always occurs by mineral
reinforcement of the previously formed network of soft tissues [533,
575-577, 602].
Cartilage is composed of cells (chondrocytes and their precursor
forms known as chondroblasts), fibers (collagen and elastic fibers)
and extracellular matrix (proteoglycans, which are a special class
of heavily glycosylated glycoproteins) [607-609]. The initial stage
involves the synthesis and extracellular assembly of the collagen
matrix framework of fibrils. At the second stage, the chondrocytes
calcify the matrix before undergoing the programmed cell death
(apoptosis). At this point, blood vessels penetrate this calcified
matrix, bringing in osteoblasts (they are mononuclear cells primarily
responsible for bone formation), which use the calcified cartilage
matrix as a template to build bone, thus completing ossification
[607-609].
During ossification, the crystals of biological apatite grow
with a specific crystalline orientation — the
in vivo
-axes of the crystals
are roughly parallel to the long axes of the collagen fibrils within
which they are deposited [533, 534, 544, 546-549, 552-554, 557,
575]. Earlier, it was believed that this process occurred via epitaxial
c
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