Page 106 - Physico-Chemical Niche Conditions for Bone Cells

PFF affects pre-osteoblast behavior
Immediate impact of PFF on pre-osteoblasts predicted by FE modeling
Direct monitoring of fluid dynamics over cells in a parallel-plate flow chamber is almost impossible. FE modeling is a useful tool to obtain insight in local fluid dynamics for mechanobiological systems [27,28]. The modeling produces detailed quantitative spatial and temporal information of fluid velocity, fluid shear stress, and fluid pressure exerted directly by fluid flow acting on cells under dynamic conditions [29,30]. The level of insight offered by the modeling cannot be obtained by means of experiments alone.
Earlier we found that static fluid flow immediately, within seconds, causes pre- osteoblast deformation [31]. Furthermore, static fluid flow has been shown to decrease the apex height of myoblasts, i.e. the apex height decreases initially and returns to its original height [32]. This data is consistent with our finding showing osteoblast movement in vertical direction. To obtain information on the immediate impact of PFF on pre-osteoblast deformation, FE modeling was employed to analyze the distribution and magnitude of fluid velocity, fluid pressure, and fluid shear stress on the cell membrane. FE modeling confirmed that PFF immediately affected the distribution and magnitude of fluid velocity, fluid pressure, and fluid shear stress over an adherent pre-osteoblast during 5 seconds. Our results agree with previous studies suggesting that PFF has an immediate effect on fluid dynamics inside a parallel-plate flow chamber [21,33]. We found, similar to others [33,34], that fluid dynamics over a pre-osteoblast was oscillating between 0 and 5 mm/s at each pulse. The decrease in fluid pressure on an adherent pre-osteoblast after a few seconds of PFF can be explained by Bernoulli’s principle suggesting that along a horizontal fluid flow, points of high fluid speed have low fluid pressure and vice versa. Therefore, the highest fluid pressure on the pre- osteoblast was observed at time-point zero, when PFF did not yet enter the chamber, while the fluid pressure on the cell decreased after PFF started. The fluid velocity and fluid shear stress were also induced on the pre-osteoblast in the first time fraction after starting PFF, and then oscillated at each pulse. We found that fluid velocity, fluid pressure, and fluid shear stress distribution and magnitude over an adherent pre-osteoblast varied over time as a result of PFF. A model presented by Chen et al. showed that force application along a different cell axis results in a different cellular volume regulation response [35]. In our study, we showed that PFF altered the displacement of the live cell from 0-0.1 μm to 0-0.3 μm. In addition, based on the experimental cell deformation results (live cell video; Supplemental Video S1; https://figshare.com/s/10665c52af1d50f443a7; DOI: 10.6084/m9.figshare.14386730), we assumed that the glycocalyx of a pre-osteoblast is very flexible, since the cell deformed within a second after the start of PFF. We implicitly assumed that there were no viscous effects at the cell membrane and thus no boundary layer developed. Thus, the slip boundary condition was applied over the cell in the parallel-plate flow chamber. Flow controls cell behavior through numerous signaling pathways. Specific links between fluid flow, gene expression, and
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