In the latest study Price et al. [41] combined FRAP in a mouse tibia with computational modeling and was able to predict the peak computational fluid velocity during cyclic

loading, 60 μm/s, and also estimate the peak resultant fluid shear stress ~ 5 Pa. These predictions are based on a three compartment model which considers the pericellular matrix surrounding the osteocyte cell processes in their canaliculi. In the original fluid flow hypothesis [9] the activation of the osteocytes was proposed to be due to fluid shear stress acting on the cell process membrane. Numerous experimental studies were subsequently conducted exposing bone cells in culture to steady and pulsatile wall shear stresses in selleck the range 0.6 to 3.0 Pa predicted by the model in [9]. A typical in vitro study [42] is conducted in a two-dimensional (2D) environment on surface attached MLO-Y4 osteocyte-like cells as opposed to the three-dimensional (3D) in vivo environment of bone matrix where the osteocyte morphology and pericellular flow environment are different. There are several differences between the flow-induced activation Selleck Y 27632 of bone cells in vivo and in vitro. In vivo the cells are attached to their mineralized matrix either through tethering filaments, or perhaps through integrin-based focal adhesions. β3 integrins have been observed on the cell processes and β1 integrins are

found to be ubiquitous [43]. In vitro there is no pericellular matrix surrounding the cell and the attachments to the substrate are all integrin-based. The second difference is the flow environment itself. As shown in [41] the fluid drag forces on the pericellular matrix surrounding the cell process in vivo are 20-fold those of the fluid shear stress acting on the cell process membrane. In vitro the fluid shear stress in nearly all experiments is the same on the cell processes and the cell body. This raises the important issue, which part of the osteocyte is its mechanosensing selleck chemical organelle, its process or its cell body, which we discuss in the next paragraph.

Third, osteocytes seeded on a flat, stiff surface spread out and build up strong basal attachments to their substrate. It has been shown that round non-adherent osteocytes are an order of magnitude more sensitive to a mechanical stimulus than a flat adherent osteocyte [44]. The mechanosensitivity of osteocytes with a more 3D morphology, such as occurs in vivo, may thus differ from that of adherent osteocytes. In summary, experiments with osteocytes cultured in 2D on flat surfaces may not suffice to unravel the intricate mechanisms used by osteocytes to transduce a mechanical signal into a chemical response. However, in vitro experiments undoubtedly do provide valuable insights into which signaling molecules are produced by osteocytes in response to a mechanical stimulus.