Reduced local mechanical stimuli in spaceflight diminishes osteocyte lacunar morphometry and spatial heterogeneity in mouse cortical bone

ABSTRACT Three-dimensional (3D) imaging of osteocyte lacunae has recently substantiated the connection between lacunar shape and size, and osteocyte age, viability, and mechanotransduction. Yet it remains unclear why individual osteocytes reshape their lacunae and how networks of osteocytes change in response to local alterations in mechanical loads. We evaluated the effects of local mechanical stimuli on osteocyte lacunar morphometrics in tibial cortical bone from young female mice flown on the Space Shuttle for ∼13 days. We optimized scan parameters, using a laboratory-based submicrometer-resolution X-Ray Microscope, to achieve large ∼ 0.3 mm 3 fields of view with sufficient resolution (≥ 0.3 μm) to visualize and measure thousands of lacunae per scan. Our novel approach avoids large measurement errors that are inherent in 2D and enables a facile 3D solution as compared to the lower resolution from benchtop micro-computed tomography (CT) systems or the cost and inaccessibility of synchrotron-based CT. Osteocyte lacunae were altered following microgravity exposure in a region-specific manner: more elongated (+7.0% Stretch) in predominately tensile-loaded bone as compared to those in compressively-loaded regions. In compressively-loaded bone, lacunae formed in microgravity were significantly larger (+6.9% Volume) than in the same region formed on Earth. We also evaluated lacunar heterogeneity (i.e., spatial autocorrelation of lacunar morphometric parameters) via kriging models. These statistical models demonstrated that heterogeneity varied with underlying spatial contributors, i.e. the local mechanical and biological environment. Yet in the absence of gravitational loading, osteocyte lacunae in newly formed bone were larger and were collectively more homogenous than in bone formed on Earth. Overall, this study shows that osteocyte reshape their lacunae in response to changes, or absence, in local mechanical stimuli and different biological environments. Additionally, spatial relationships among osteocytes are complex and necessitate evaluation in carefully selected regions of interest and of large cell populations. GRAPHICAL ABSTRACT