The histological basis for acute osteocyte mechanosensitivity remains uncertain. A novel bone cell model of mechanotransduction and inorganic trafficking may be the powerful, silt-burrowing protozoan Spirostomum ambiguum which when being physically challenged fabricates within vesicles populations of bone-like calcium phosphate microspheres, about 1 μm in diameter. These not only attribute considerable compression-resilience but also resemble the Golgi-directed mineral assemblies we recently reported in osteocytes. Advantageously, calcification in the protozoan (confirmed by ultramicroscopy with EDX elemental microanalysis) enabled Golgi comparison under overt, natural phases of both high (i.e. silt-tunnelling) and low (i.e free-swimming) stress. Established hard-tissue microscopy techniques previously positive in bone cells included quantitative fluorescent tetracycline labelling for bone salt together with the same metazoan Golgi body marker (Green Fluorescent Protein-tagged mannosidase II construct). Organellar modulation was monitored by transfection of live organisms in situ (some post-stained with red nuclear fluorochrome TOPRO-3). Results showed that GFP-tagged Golgi fluorescence increased from swimmers (mean 74.5 ± SD 6.7 AU) to burrowers (mean 104.6 ± SD 2.7; p < 0.0001) synchronous with juxtanuclear tetracycline-labelled mineral fluorescence (swimmers, mean 89.7 ± SD 3.3 AU; burrowers, mean 138.0 ± SD 4.0; p < 0.0001). Intracellular dense microspheres, single or bridged, were harvested as pellets rich in Ca, P (Ca:P 0.98) and Si, their polarised alignment moving from transaxial in swimmers to axial in burrowers. It was concluded that Golgi-directed mineral fabrication in the large, accessible, silt-enclosed ciliate resembles that in the smaller, less-accessible bone cell and may be a conserved early mechanobiological intracellular development predicating force translation into compression-resistant mineral fabrication in loaded segments of the osteocyte syncitium.
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