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.

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.

Evaluation of pain behavior and bone destruction in two arthritic models in guinea pig and rat.

The primary aim of the study was to describe and correlate pain behavior and changes in bone morphology in animal models of arthritis both in rats and guinea pigs. Either complete Freund's adjuvant (CFA) or mono-iodoacetate (MIA) solution was injected into the left knee joint to obtain a model for rheumatoid arthritis and osteoarthritis, respectively. Subsequently, animals were behaviorally tested during a period of 12 days after CFA injection and at least 19 days after MIA injection. During these observation periods increasing pain behavior was observed, characterized by decreased von Frey mechanical thresholds and weight bearing on the affected limb. In Hargreaves' paw flick test slightly increased thermal hypersensitivity was observed in some instances in guinea pigs. In rats there was also decreased limb-use during forced ambulation. To evaluate bone destruction mu-computed tomography scans of the arthritic knee were taken on the last experimental day. Different bone parameters indicative of osteolysis and decreased trabecular connectivity were significantly correlated with the observed pain behavior. Detailed description of morphological changes in arthritic joints better characterizes the different animal models and might add to the knowledge on the working mechanisms of analgesic compounds that have an influence on bone structures in arthritis.

Bone cancer pain model in mice: evaluation of pain behavior, bone destruction and morphine sensitivity.

The primary aim of the study was to correlate pain development during bone cancer growth with objectively obtained tumor-induced changes in bone morphology. Additionally morphine sensitivity of this bone pain was evaluated. Mice were injected into the femur with osteolytic NCTC2472 cells, and behaviorally followed during a 3-week period. During the observation period increasing pain behavior was observed in tumor-bearing animals. Tumor mice exhibited spontaneous and movement-evoked lifting, the latter evoked through non-noxious palpation of the tumor. Limb use during forced ambulation on a rotarod decreased to substantial non-use of the affected limb by day 23. On day 23, micro-computer tomography scans of the tumor-bearing bones were evaluated for bone destruction. Different bone parameters indicative of osteolysis or fragmentation were significantly correlated with pain behavior. In a separate group of mice the effects of different morphine doses on pain behavior were evaluated on days 17 and 21 of tumor growth. Spontaneous lifting and movement-evoked lifting were sensitive to morphine treatment, although stress-induced analgesia due to repeated restraint might minimize movement-evoked lifting in mice. Limb use during forced ambulation was only slightly ameliorated by high morphine doses.