Page 40 - Physico-Chemical Niche Conditions for Bone Cells

Physicochemical niche conditions and mechanosensing
with the myofiber and strain the cell. The stretching will cause elongation of the cytoskeleton together with the nucleus. Such length changes have been shown in cardiac myocytes and myofibers [95]. During movement, skeletal muscle is subjected actively or passively to large excursions by myofiber contractions (e.g. eccentric or concentric ) or by external tensile forces, respectively [96]. Since muscle volume is constant [97], pressure within a muscle varies with its length. Moreover, due to myofascial connections between myofibers and between neighboring muscles, differences in length changes between myofibers or muscle induce shear forces onto myofibers and ECM. In addition, there exists an eccentric contraction can result in cytoskeleton and plasma membrane disruptions. In the beginning of eccentric contraction, fast glycolytic fibers fatigue. Then mitochondria of myocytes lose their calcium- buffering capacity as a result of their inability to regenerate ATP. Increased intracellular calcium activates calcium-activated lysosomal proteases, neutral proteases, and other cellular processes which are calcium mediated. It eventually causes myofiber damage [98]. Furthermore, the leakage of the enzymes and molecules may also affect the MuSCs [99].
In Vitro Experiments
How valid are in vitro experiments if cellular niche is indeed so important?
Several transmembrane molecules and one organelle (i.e. cilium) have been implicated as mediators of mechano-transduction based on experiments with osteocytes cultured on flat substrates. Such experiments provide valuable insights into which molecules are produced by osteocytes in response to a mechanical stimulus, but they may be less useful for unravelling the mechanism behind osteocyte mechanotransduction. Although focal adhesion kinase is known to play a role in mechanosensing by osteocytes in vitro, osteocytes in situ do not form the large, sharply demarcated focal adhesions that they exhibit on stiff and flat surfaces.
Osteocytes in situ are surrounded by a specialized pericellular matrix and may employ a different subset of integrins for anchorage to their ECM than osteocytes on artificial matrices in vitro. Furthermore, a mechanical stimulus applied to flat adherent osteocytes in vitro predominantly affects the osteocyte cell body, while physical laws dictate that the extracellular fluid surrounding osteocytes in vivo/in situ only flows over the cell extensions. It is a fundamental paradox that the strains applied to whole bones may be as much as 30-fold lower than the strains necessary to trigger signalling in 2D-cultured osteocytes. However, osteocytes on a flat surface spread out, and round non-adherent osteocytes are an order of magnitude more sensitive to a mechanical stimulus than flat osteocytes [100]. Higher mechanosensitivity of cells with a more 3D-morphology may thus provide part of the solution to the bone paradox. Taken together, it is still an enigma how osteocytes in situ transduce the minute mechanical stimuli that occur as a result of physical activity into a strong chemical response, and in vitro
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