It has been suggested that the osteocyte processes might be attached directly to the canalicular wall by β3 integrins at the apex of infrequent, previously

unrecognized canalicular projections [43]. A theoretical model was developed that predicts that the tensile forces acting on these integrins can be as large as 15 pN, Selleckchem AZD1208 and thus provide stable attachment in the range of physiological loading [20]. The model also predicts that axial strains caused by the sliding of actin microfilaments relative to the fixed attachments are two orders of magnitude greater than whole-tissue strains thereby producing local membrane strains in the cell process that can exceed 5%. In vitro experiments indicated that membrane strains of this order are large enough to open stretch-activated cation channels [74]. It is likely that stretch-activated ion channels play a role in the transduction of mechanical stimuli into a chemical response in osteocytes. A well known early response to mechanical stimulation of osteocytes and other mechanosensitive bone cells in vitro is an increase in intracellular calcium concentration [75], which could well be caused by opening of stretch-activated ion channels. Although no direct evidence exists that transient receptor potential (TRP) channels are stretch-activated ion channels, it is an idea that has often been

put forward. In MLO-Y4 osteocytes the Seliciclib price calcium response to mechanical stimulation can be partially blocked by Gd3 + [76], suggesting that some kind of TRP channel is involved in this response. Little is known about TRP channel expression in osteocytes, only TRPV6 is known to be expressed at low levels in murine osteocytes [77]. So far the involvement of specific ion channels in the mechanoresponse of osteocytes has not been elucidated. As described

above, actin microfilaments in the osteocyte cell extensions may slide relative to the fixed attachments. As a result, stretch-activated ion channels may be pulled open, since such channels are connected to the cytoskeleton. On the outside of the osteocyte next process any stretch-activated ion channels may also be connected to the extracellular matrix via tethers, further enabling the opening of ion channels. Such tethering filaments appear to be absent in the pericellular space surrounding the cell body, likely due to the wide pericellular space (~ 1 μm) between the cell membrane and the wall of the lacuna. In contrast, the pericellular space surrounding the cell process on average is 80 nm [32]. You et al. [78] were the first to propose that there were regularly spaced tethering filaments that attached the cell processes to the canalicular wall. They postulated that the flow through the pericellular matrix, which was supported by these tethers, would put them in tension creating a hoop strain on the cell process membrane.