Biomedical Engineering Reference
In-Depth Information
the fluid shear stress induced by neighboring flow on the cellular activity was
proven in vitro the decade after [
4
,
18
,
65
,
160
].
• Pressure effects By comparing biochemical responses of osteoblast and osteo-
cytes when submitted to a fluid flow, it was showed that the shear stress induced
by the flow was the predominant mechanical effect felt by the cells [
65
]. Not-
withstanding this strong evidence, some studies still impute an important role of
the pressure in the bone cells behaviour [
104
,
105
].
• Flow shear stress In the footsteps of Piekarski and Munro [
121
], Weinbaum
et al. [
158
] proposed that the fluid flow due to physiological loading was the
primary stimulus that enabled osteocytes to sense and respond to their
mechanical environment. Their theoretical investigation predicted shear effects
induced by the fluid flow on the osteocyte process membrane of a few Pascals.
This value is roughly of the same level as for the endothelium in vascular
capillaries, and typically corresponds to in vitro measurements of bone fluid
shear stress [
65
]. The success of the ''flow shear stress'' as a valuable candidate
for the mechano-sensation role in bone tissue is still visible through the recent
works on bone remodelling based on this signal [
1
,
56
,
58
,
60
,
110
].
• Worship what you have burned, and burn what you have worshiped: shear force
versus drag force Even if the shear stress effects predicted in the lacuno-can-
alicular and vascular systems are comparable, the morphological differences
between dendritic cell and endothelial cell should induce different mechano-
sensation scenarios. Indeed, due to the densely packed central actin filament
bundle in the osteocyte process, this portion of the osteocyte should be much
more rigid than the endothelial cell body. Thus, the same group that made the
success of the shear effects within bone tissue proposed another point of view to
investigate the interactions between the interstitial fluid flow and the ultra-
structure of the osteocyte. Indeed, concomitantly to the progress in the imaging
of the canalicular structure [
97
,
166
], forgetting the shear effects, the Wein-
baum's group proposed that the stimulus was the drag force exerted by the fluid
flow on the pericellular matrix surrounding the cell processes. These forces
would be transmitted by tethering filaments and canalicular projections which
connect the membrane of the cell process to the canalicular wall, generating a
strain amplification of the cell membrane in the hoop direction [
46
,
156
,
165
].
Other recent studies provide evidence that primary cilia projecting from the
surface of cultured bone cells can translate fluid flow into cellular responses
[
90
]. Note that cilia are present only in lacuna in close proximity (25 lm) to the
periosteal surface, that these cilia are parallel and not perpendicular to the cell
body surface and, thus, could not bend in response to fluid flow under these
confined conditions [
159
].
• Cell-to-cell communication The transport of biochemical species within bone is
not only essential for the survival of osteocytes which are not directly in contact
with the vascular supply [
100
], but it is central in the paracrinal cell-to-cell
communication as well. Indeed, another stage in the pathway of bone mechano-
transduction is the production of signalling molecules by the osteocyte, which
can alter the bone remodelling activity of osteoclasts and osteoblasts. Even
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