Aviral Vatsa

Osteocyte morphology in fibula and calvaria --- is there a role for mechanosensing?

INTRODUCTION External mechanical forces on cells are known to influence cytoskeletal structure and thus cell shape. Mechanical loading in long bones is unidirectional along their long axes, whereas the calvariae are loaded at much lower amplitudes in different directions. We hypothesised that if osteocytes, the putative bone mechanosensors, can indeed sense matrix strains directly via their cytoskeleton, the 3D shape and the long axes of osteocytes in fibulae and calvariae will bear alignment to the different mechanical loading patterns in the two types of bone. MATERIALS AND METHODS We used confocal laser scanning microscopy and nano-computed tomography to quantitatively determine the 3D morphology and alignment of long axes of osteocytes and osteocyte lacunae in situ. RESULTS Fibular osteocytes showed a relatively elongated morphology (ratio lengths 5.9:1.5:1), whereas calvarial osteocytes were relatively spherical (ratio lengths 2.1:1.3:1). Osteocyte lacunae in fibulae had higher unidirectional alignment than the osteocyte lacunae in calvariae as demonstrated by their degree of anisotropy (3.33 and 2.10, respectively). The long axes of osteocyte lacunae in fibulae were aligned parallel to the principle mechanical loading direction, whereas those of calvarial osteocyte lacunae were not aligned in any particular direction. CONCLUSIONS The anisotropy of osteocytes and their alignment to the local mechanical loading condition suggest that these cells are able to directly sense matrix strains due to external loading of bone. This reinforces the widely accepted role of osteocytes as mechanosensors, and suggests an additional mode of mechanosensing besides interstitial fluid flow. The relatively spherical morphology of calvarial osteocytes suggests that these cells are more mechanosensitive than fibular osteocytes, which provides a possible explanation of efficient physiological load bearing for the maintenance of calvarial bone despite its condition of relative mechanical disuse.

Osteocyte morphology in human tibiae of different bone pathologies with different bone mineral density — Is there a role for mechanosensing?☆

Matrix strains due to external loading are different in bones of different pathologies with different bone mineral density (BMD), and are likely sensed by the osteocytes, the putative bone mechanosensors. The mechanosensitivity of osteocytes appears to be strongly influenced by their morphology. In this study, we explored the possibility that osteocyte morphology might play a role in various bone pathologies with different BMD. Confocal laser scanning microscopy and nano-CT were used to quantitatively determine 3D morphology and alignment of osteocytes and osteocyte lacunae in human proximal tibial bone with relatively low (osteopenic), medium (osteoarthritic), and high (osteopetrotic) BMD. Osteopenic osteocytes were relatively large and round (lengths 8.9:15.6:13.4 microm), osteopetrotic osteocytes were small and discoid shaped (lengths 5.5:11.1:10.8 microm), and osteoarthritic osteocytes were large and elongated (lengths 8.4:17.3:12.2 microm). Osteopenic osteocyte lacunae showed 3.5 fold larger volume and 2.2 fold larger surface area than osteoarthritic lacunae, whereas osteopetrotic lacunae were 1.9 fold larger and showed 1.5 fold larger surface area than osteoarthritic lacunae. Osteopetrotic osteocyte lacunae had lower alignment than osteopenic and osteoarthritic lacunae as indicated by their lower degree of anisotropy. The differences in 3D morphology of osteocytes and their lacunae in long bones of different pathologies with different BMD might reflect an adaptation to matrix strain due to different external loading conditions. Moreover, since direct mechanosensing of matrix strain likely occurs by the cell bodies, the differences in osteocyte morphology and their lacunae might indicate differences in osteocyte mechanosensitivity. The exact relationship between osteocyte morphology and bone architecture, however, is complex and deserves further study.

Bio imaging of intracellular NO production in single bone cells after mechanical stimulation

We show the intracellular upregulation of NO production after mechanical stimulation, an essential chemical signal in bone remodeling. This is done in real time using the fluorescent chromophore DAR‐4M AM. Differences in cellular response to mechanical stimulation of different regions of a single cell were observed.