Page 166 - Physico-Chemical Niche Conditions for Bone Cells
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General discussion
Changes in cell deformation and volume are believed to play an important role in the process of mechanotransduction [14]. In vivo, bone cells undergo four stages of terminal differentiation, including mature osteoblast, osteoblastic osteocyte, osteoid osteocyte, and mature osteocyte [21]. During these stages, the cell volume changes, e.g. from cuboidal, round, to more stellate or dendritic-shaped, leading to 70% volume-reduction in the mature osteocyte and 30% volume-reduction in the nascent osteocyte compared with the osteoblast cell volume [21]. In chapter 3 we found that mechanical loading changed the osteoblast cell surface area, cell body, and nucleus volume. These changes corresponded to alterations in paxillin, integrin-a5, and a-tubulin expression. Possibly, physical signals were transmitted from the membrane to the nucleus via the ECM-integrin-cytoskeletal axis, via the primary cilium and microtubules to the nucleus, or via the cytoskeleton and mitochondria to the nucleus [22–24], whereby even three mechanotransduction pipelines might have occured simultaneously, i.e. the ECM-integrin-cytoskeleton pipeline, the primary cilium-microtubules pipeline, and the cytoskeleton-mitochondria pipeline. During the process of mechanotransduction, intracellular organelles and (bio)chemical factors are affected, such as NO and prostaglandin production, as well as O2 and Ca2+ concentration, resulting in changes in cell behavior and function [14,17].
Mechanical forces derived by fluid flow can impose a biophysical cue on cells, which can be transduced to influence cell shape and behavior in the short-term. Fluid flow, a physical cue that naturally occurs in the bone microenvironment and is regulated by mechanical forces, can induce osteogenic differentiation of stem cells [25]. However, a better understanding of the effect of mechanical loading on the bone cell response and behavior in time is needed, as well as how bone cells are operating together within a niche and affect each other through (bio)chemical factors. In chapter 4 we demonstrated that mechanical stimulation by PFF modulated the distribution of flow dynamics (fluid velocity, fluid pressure, and fluid shear stress) over a pre-osteoblast (immediate effect), NO production, cell morphology and orientation (initial down-stream effect), cell behavior (e.g. metabolic activity and gene expression; short- term down-stream impact), and alkaline phosphatase (ALP) protein expression, collagen production, and mineralization (long-term down-stream impact). Mai and colleagues demonstrated that one single bout stress load promotes the rearrangement of F-actin stress fibers, osteogenic gene expression, ALP activity, collagen production, and mineralization through bone morphogenetic protein 2 (BMP2) and integrin b1 pathways in MC3T3-E1 osteoblasts [26]. In this chapter, we showed that PFF changed osteogenic gene expression and mineralization through NO production, Fgf2 expression, and Wnt signaling in MC3T3-E1 osteoblasts, but it did not affect the production of ALP protein and collagen. It would be
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