The early mouse 3D osteocyte network in the presence and absence of mechanical loading

2D vs. 3D Evaluation of Osteocyte Lacunae - Methodological Approaches, Recommended Parameters, and Challenges: A Narrative Review by the European Calcified Tissue Society (ECTS)

PURPOSE OF REVIEW

Quantification of the morphology of osteocyte lacunae has become a powerful tool to investigate bone metabolism, pathologies and aging. This review will provide a brief overview of 2D and 3D imaging methods for the determination of lacunar shape, orientation, density, and volume. Deviations between 2D-based and 3D-based lacunar volume estimations are often not sufficiently addressed and may give rise to contradictory findings. Thus, the systematic error arising from 2D-based estimations of lacunar volume will be discussed, and an alternative calculation proposed. Further, standardized morphological parameters and best practices for sampling and segmentation are suggested.

RECENT FINDINGS

We quantified the errors in reported estimation methods of lacunar volume based on 2D cross-sections, which increase with variations in lacunar orientation and histological cutting plane. The estimations of lacunar volume based on common practice in 2D imaging methods resulted in an underestimation of lacunar volume of up to 85% compared to actual lacunar volume in an artificial dataset. For a representative estimation of lacunar size and morphology based on 2D images, at least 400 lacunae should be assessed per sample.

Bundling of collagen fibrils influences osteocyte network formation during bone modeling

Osteocytes form a cellular network by gap junctions between their cell processes. This network is important since intercellular communication via the network is essential for bone metabolism. However, the factors that influence the formation of this osteocyte network remain unknown. As the early stage of osteocyte network formation occurs on the bone surface, we observed a newly formed trabecular bone surface by orthogonal focused ion beam-scanning electron microscopy. The embedding late osteoblast processes tended to avoid bundled collagen fibrils and elongate into sparse collagen fibrils. Then, we examined whether the inhibition of bundling of collagen fibrils using a potent lysyl oxidase inhibitor, β-aminopropionitrile (BAPN) changed the cellular network of the chick calvaria. The osteocyte shape of the control group was spindle-shape, while that of the BAPN group was sphere-shaped. In addition, the osteocyte processes of the control group were elongated vertically to the long axis of the cell body, whereas the osteocyte processes of the BAPN group were elongated radially. Therefore, it was suggested that the bundling of collagen fibrils influences normal osteocyte network formation during bone modeling.

Microstructural and cellular characterisation of the subchondral trabecular bone in human knee and hip osteoarthritis using synchrotron tomography

OBJECTIVE

It is unclear if different factors influence osteoarthritis (OA) progression and degenerative changes characterising OA disease in hip and knee. We investigated the difference between hip OA and knee OA at the subchondral bone (SCB) tissue and cellular level, relative to the degree of cartilage degeneration.

DESIGN

Bone samples were collected from 11 patients (aged 70.4 ± 10.7years) undergoing knee arthroplasty and 8 patients (aged 62.3 ± 13.4years) undergoing hip arthroplasty surgery. Trabecular bone microstructure, osteocyte-lacunar network, and bone matrix vascularity were evaluated using synchrotron micro-CT imaging. Additionally, osteocyte density, viability, and connectivity were determined histologically.

RESULTS

The associations between severe cartilage degeneration and increase of bone volume fraction (%) [- 8.7, 95% CI (-14.1, -3.4)], trabecular number (#/mm) [- 1.5, 95% CI (-0.8, -2.3)], osteocyte lacunar density (#/mm3) [4714.9; 95% CI (2079.1, 7350.6)] and decrease of trabecular separation (mm) [- 0.07, 95% CI (0.02, 0.1)] were found in both knee and hip OA. When compared to knee OA, hip OA was characterised by larger (µm3) but less spheric osteocyte lacunae [47.3; 95% CI (11.2, 83.4), - 0.04; 95% CI (-0.06, -0.02), respectively], lower vascular canal density (#/mm3) [- 22.8; 95% CI (-35.4, -10.3)], lower osteocyte cell density (#/mm2) [- 84.2; 95% CI (-102.5, -67.4)], and less senescent (#/mm2) but more apoptotic osteocytes (%) [- 2.4; 95% CI (-3.6, -1.2), 24.9; 95% CI (17.7, 32.1)], respectively.

CONCLUSION

SCB from hip OA and knee OA exhibits different characteristics at the tissue and cellular levels, suggesting different mechanisms of OA progression in different joints.

Alterations of bone material properties in growing Ifitm5/BRIL p.S42 knock-in mice, a new model for atypical type VI osteogenesis imperfecta

INTRODUCTION

Osteogenesis imperfecta (OI) is a heterogenous group of heritable connective tissue disorders characterized by high bone fragility due to low bone mass and impaired bone material properties. Atypical type VI OI is an extremely rare and severe form of bone dysplasia resulting from a loss-of-function mutation (p.S40L) in IFITM5/BRIL,the causative gene of OI type V and decreased osteoblast secretion of pigment epithelium-derived factor (PEDF), as in OI type VI. It is not yet known which alterations at the material level might lead to such a severe phenotype. We therefore characterized bone tissue at the micrometer level in a novel heterozygous Ifitm5/BRIL p.S42L knock-in murine model at 4 and 8 weeks of age.

METHODS

We evaluated in female mice, total body size, femoral and lumbar bone mineral density (BMD) by dual-energy X-ray absorptiometry. In the femoral bone we examined osteoid deposition by light microscopy, assessed bone histomorphometry and mineralization density distribution by quantitative backscattered electron imaging (qBEI). Osteocyte lacunae were examined by qBEI and the osteocyte lacuno-canalicular network by confocal laser scanning microscopy. Vasculature was examined indirectly by qBEI as 2D porosity in cortex, and as 3D porosity by micro-CT in third trochanter. Collagen orientation was examined by second harmonic generation microscopy. Two-way ANOVA was used to discriminate the effect of age and genotype.

RESULTS

Ifitm5/BRIL p.S42L female mice are viable, do not differ in body size, fat and lean mass from wild type (WT) littermates but have lower whole-body, lumbar and femoral BMD and multiple fractures. The average and most frequent calcium concentration, CaMean and CaPeak, increased with age in metaphyseal and cortical bone in both genotypes and were always higher in Ifitm5/BRIL p.S42L than in WT, except CaMean in metaphysis at 4 weeks of age. The fraction of highly mineralized bone area, CaHigh, was also increased in Ifitm5/BRIL p.S42L metaphyseal bone at 8 weeks of age and at both ages in cortical bone. The fraction of lowly mineralized bone area, CaLow, decreased with age and was not higher in Ifitm5/BRIL p.S42L, consistent with lack of hyperosteoidosis on histological sections by visual exam. Osteocyte lacunae density was higher in Ifitm5/BRIL p.S42L than WT, whereas canalicular density was decreased. Indirect measurements of vascularity revealed a higher pore density at 4 weeks in cortical bone of Ifitm5/BRIL p.S42L than in WT and at both ages in the third trochanter. Importantly, the proportion of bone area with disordered collagen fibrils was highly increased in Ifitm5/BRIL p.S42L at both ages.

CONCLUSIONS

Despite normal skeletal growth and the lack of a collagen gene mutation, the Ifitm5/BRIL p.S42L mouse shows major OI-related bone tissue alterations such as hypermineralization of the matrix and elevated osteocyte porosity. Together with the disordered lacuno-canalicular network and the disordered collagen fibril orientation, these abnormalities likely contribute to overall bone fragility.

A morphometric analysis of the osteocyte canaliculus using applied automatic semantic segmentation by machine learning

INTRODUCTION

Osteocytes play a role as mechanosensory cells by sensing flow-induced mechanical stimuli applied on their cell processes. High-resolution imaging of osteocyte processes and the canalicular wall are necessary for the analysis of this mechanosensing mechanism. Focused ion beam-scanning electron microscopy (FIB-SEM) enabled the visualization of the structure at the nanometer scale with thousands of serial-section SEM images. We applied machine learning for the automatic semantic segmentation of osteocyte processes and canalicular wall and performed a morphometric analysis using three-dimensionally reconstructed images.

MATERIALS AND METHODS

Six-week-old-mice femur were used. Osteocyte processes and canaliculi were observed at a resolution of 2 nm/voxel in a 4 × 4 μm region with 2000 serial-section SEM images. Machine learning was used for automatic semantic segmentation of the osteocyte processes and canaliculi from serial-section SEM images. The results of semantic segmentation were evaluated using the dice similarity coefficient (DSC). The segmented data were reconstructed to create three-dimensional images and a morphological analysis was performed.

RESULTS

The DSC was > 83%. Using the segmented data, a three-dimensional image of approximately 3.5 μm in length was reconstructed. The morphometric analysis revealed that the median osteocyte process diameter was 73.8 ± 18.0 nm, and the median pericellular fluid space around the osteocyte process was 40.0 ± 17.5 nm.

CONCLUSION

We used machine learning for the semantic segmentation of osteocyte processes and canalicular wall for the first time, and performed a morphological analysis using three-dimensionally reconstructed images.

Interactive effects of various loading parameters on the fluid dynamics within the lacunar-canalicular system for a single osteocyte

The osteocyte lacunar-canalicular system (LCS) serves as a mechanotransductive core where external loading applied to the skeleton is transduced into mechanical signals (e.g., fluid shear) that can be sensed by mechanosensors (osteocytes). The fluid velocity and shear stress within the LCS are affected by various loading parameters. However, the interactive effect of distinct loading parameters on the velocity and shear stress in the LCS remains unclear. To address this issue, we developed a multiscale modeling approach, combining a poroelastic finite element (FE) model with a single osteocytic LCS unit model to calculate the flow velocity and shear stress within the LCS. Next, a sensitivity analysis was performed to investigate individual and interactive effects of strain magnitude, strain rate, number of cycles, and intervening short rests between loading cycles on the velocity and shear stress around the osteocyte. Lastly, we developed a relatively simple regression model to predict those outcomes. Our results demonstrated that the strain magnitude or rate alone were the main factors affecting the velocity and shear stress; however, the combination of these two was not directly additive, and addition of a short rest between cycles could enhance the combination of these two related factors. These results show highly interactive effects of distinct loading parameters on fluid velocity and shear stress in the LCS. Specifically, our results suggest that an enhanced fluid dynamics environment in the LCS can be achieved with a brief number of load cycles combined with short rest insertion and high strain magnitude and rate.

Bone permeability and mechanotransduction: Some current insights into the function of the lacunar-canalicular network

Lacunar-canalicular (LC) permeability involves the passage of fluids, nutrients, oxygen, ions, and signalling molecules through bone tissue, facilitating the maintenance of bone vitality and function and responses to various physiological conditions and diseases. LC permeability and fluid flow-shear stress/drag force play important roles in mechanotransduction in bone tissue by inducing mechanical stimuli in osteocytes, modulating cellular functions, and determining bone adaptation. Alterations in LC structure may therefore influence the fluid flow pattern through the LC network, thereby affecting the ability of osteocytes to sense and translate mechanical signals and possibly contributing to bone remodelling. Several bone-health conditions are associated with changes in LC structure and function and may affect mechanotransduction and responses, although the mechanisms underlying these associations are still not fully understood. In this review, recent studies of LC networks, their formation and transfer mechanical stimuli, and changes in structure, functional permeability, and mechanotransduction that result from age, pathology, and mechanical loading are discussed. Additionally, applications of vibration and low-intensity pulsed ultrasound in bone healthcare and regeneration fields are also presented.

Tunnels in the rock: Dynamics of osteocyte morphogenesis

Osteocytes are dynamic, bone matrix-remodeling cells that form an intricate network of interconnected projections through the bone matrix, called the lacunar-canalicular system. Osteocytes are the dominant mechanosensory cells in bone and their mechanosensory and mechanotransductive functions follow their morphological form. During osteocytogenesis and development of the osteocyte lacunar-canalicular network, osteocytes must dramatically remodel both their cytoskeleton and their extracellular matrix. In this review, we summarize our current understanding of the mechanisms that govern osteocyte differentiation, cytoskeletal morphogenesis, mechanotransduction, and matrix remodeling. We postulate that the physiologic activation of matrix remodeling in adult osteocytes, known as perilacunar/canalicular remodeling (PLR) represents a re-activation of the developmental program by which the osteocyte network is first established. While much of osteocyte biology remains unclear, new tools and approaches make the present moment a particularly fruitful and exciting time to study the development of these remarkable cells.

Quantitative Evaluation of Osteocyte Morphology and Bone Anisotropic Extracellular Matrix in Rat Femur

Osteocytes are believed to play a crucial role in mechanosensation and mechanotransduction which are important for maintenance of mechanical integrity of bone. Recent investigations have revealed that the preferential orientation of bone extracellular matrix (ECM) mainly composed of collagen fibers and apatite crystallites is one of the important determinants of bone mechanical integrity. However, the relationship between osteocytes and ECM orientation remains unclear. In this study, the association between ECM orientation and anisotropy in the osteocyte lacuno-canalicular system, which is thought to be optimized along with the mechanical stimuli, was investigated using male rat femur. The degree of ECM orientation along the femur longitudinal axis was significantly and positively correlated with the anisotropic features of the osteocyte lacunae and canaliculi. At the femur middiaphysis, there are the osteocytes with lacunae that highly aligned along the bone long axis (principal stress direction) and canaliculi that preferentially extended perpendicular to the bone long axis, and the highest degree of apatite c-axis orientation along the bone long axis was shown. Based on these data, we propose a model in which osteocytes can change their lacuno-canalicular architecture depending on the mechanical environment so that they can become more susceptible to mechanical stimuli via fluid flow in the canalicular channel.

Impaired 1,25 dihydroxyvitamin D3 action and hypophosphatemia underlie the altered lacuno-canalicular remodeling observed in the Hyp mouse model of XLH

Osteocytes remodel the perilacunar matrix and canaliculi. X-linked hypophosphatemia (XLH) is characterized by elevated serum levels of fibroblast growth factor 23 (FGF23), leading to decreased 1,25 dihydroxyvitamin D₃ (1,25D) production and hypophosphatemia. Bones from mice with XLH (Hyp) have enlarged osteocyte lacunae, enhanced osteocyte expression of genes of bone remodeling, and impaired canalicular structure. The altered lacuno-canalicular (LCN) phenotype is improved with 1,25D or anti-FGF23 antibody treatment, pointing to roles for 1,25D and/or phosphate in regulating this process. To address whether impaired 1,25D action results in LCN alterations, the LCN phenotype was characterized in mice lacking the vitamin D receptor (VDR) in osteocytes (VDR^f/f;DMP1Cre+). Mice lacking the sodium phosphate transporter NPT2a (NPT2aKO) have hypophosphatemia and high serum 1,25D levels, therefore the LCN phenotype was characterized in these mice to determine if increased 1,25D compensates for hypophosphatemia in regulating LCN remodeling. Unlike Hyp mice, neither VDR^f/f;DMP1Cre+ nor NPT2aKO mice have dramatic alterations in cortical microarchitecture, allowing for dissecting 1,25D and phosphate specific effects on LCN remodeling in tibial cortices. Histomorphometric analyses demonstrate that, like Hyp mice, tibiae and calvariae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enlarged osteocyte lacunae (tibiae: 0.15±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.19±0.02μm²(VDR^f/f;DMP1Cre+), 0.12±0.02μm²(WT) vs 0.18±0.0μm²(NPT2aKO), calvariae: 0.09±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.11±0.02μm²(VDR^f/f;DMP1Cre+), 0.08±0.02μm²(WT) vs 0.13±0.02μm²(NPT2aKO), p<0.05 all comparisons) and increased immunoreactivity of bone resorption marker Cathepsin K (Ctsk). The osteocyte enriched RNA isolated from tibiae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enhanced expression of matrix resorption genes that are classically expressed by osteoclasts (Ctsk, Acp5, Atp6v0d2, Nhedc2). Treatment of Ocy454 osteocytes with 1,25D or phosphate inhibits the expression of these genes. Like Hyp mice, VDR^f/f;DMP1Cre+ and NPT2aKO mice have impaired canalicular organization in tibia and calvaria. These studies demonstrate that hypophosphatemia and osteocyte-specific 1,25D actions regulate LCN remodeling. Impaired 1,25D action and low phosphate levels contribute to the abnormal LCN phenotype observed in XLH.

Changes in the intra- and peri-cellular sclerostin distribution in lacuno-canalicular system induced by mechanical unloading

INTRODUCTION

Mechanical stimuli regulate Sclerostin (Scl), a negative regulator of bone formation, expression in osteocytes. However, the detailed Scl distribution in osteocytes in response to mechanical unloading remains unclear.

MATERIALS AND METHODS

Twelve-week-old male rats were used. The sciatic and femoral nerves on the right side were excised as mechanical unloading treatment. A sham operation was performed on the left side. One week after neurotrauma, the bone density of the femora was evaluated by peripheral quantitative computed tomography, and immunofluorescence was performed in coronal sections of the femoral diaphysis. The mean fluorescence intensity and fluorescent profile of Scl from the marrow to the periosteal side were analyzed to estimate the Scl expression and determine to which side (marrow or periosteal) the Scl prefers to distribute in response to mechanical unloading. The most sensitive region indicated by the immunofluorescence results was further investigated by transmission electron microscopy (TEM) with immunogold staining to show the Scl expression changes in different subcellular structures.

RESULTS

In femur distal metaphysis, neurotrauma-induced mechanical unloading significantly decreased the bone density, made the distribution of Scl closer to the marrow on the anterior and medial side, and increased the Scl expression only on the lateral side. TEM findings showed that only the expression of Scl in canaliculi was increased by mechanical unloading.

CONCLUSIONS

Our results showed that even short-term mechanical unloading is enough to decrease bone density, and mechanical unloading not only regulated the Scl expression but also changed the Scl distribution in both the osteocyte network and subcellular structures.

Loading history changes the morphology and compressive force-induced expression of receptor activator of nuclear factor kappa B ligand/osteoprotegerin in MLO-Y4 osteocytes

BACKGROUND

In this study, we investigated the effect of the mechanical loading history on the expression of receptor activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin (OPG) in MLO-Y4 osteocyte-like cells.

METHODS

Three hours after MLO-Y4 osteocytes were seeded, a continuous compressive force (CCF) of 31 dynes/cm2 with or without additional CCF (32 dynes/cm2) was loaded onto the osteocytes. After 36 h, the additional CCF (loading history) was removed for a recovery period of 10 h. The expression of RANKL, OPG, RANKL/OPG ratio, cell numbers, viability and morphology were time-dependently examined at 0, 3, 6 and 10 h. Then, the same additional CCF was applied again for 1 h to all osteocytes with or without the gap junction inhibitor to examine the expression of RANKL, OPG, the RANKL/OPG ratio and other genes that essential to characterize the phenotype of MLO-Y4 cells. Fluorescence recovery after photobleaching technique was also applied to test the differences of gap-junctional intercellular communications (GJIC) among MLO-Y4 cells.

RESULTS

The expression of RANKL and OPG by MLO-Y4 osteocytes without a loading history was dramatically decreased and increased, respectively, in response to the 1-h loading of additional weight. However, the expression of RANKL, OPG and the RANKL/OPG ratio were maintained at the same level as in the control group in the MLO-Y4 osteocytes with a loading history but without gap junction inhibitor treatment. Treatment of loading history significantly changed the capacity of GJIC and protein expression of connexin 43 (Cx43) but not the mRNA expression of Cx43. No significant difference was observed in the cell number or viability between the MLO-Y4 osteocyte-like cells with and without a loading history or among different time checkpoints during the recovery period. The cell morphology showed significant changes and was correlated with the expression of OPG, Gja1 and Dmp1 during the recovery period.

CONCLUSION

Our findings indicated that the compressive force-induced changes in the RANKL/OPG expression could be habituated within at least 11 h by 36-h CCF exposure. GJIC and cell morphology may play roles in response to loading history in MLO-Y4 osteocyte-like cells.

Using confocal imaging approaches to understand the structure and function of osteocytes and the lacunocanalicular network

Although overlooked in the past, osteocytes have come to the forefront of skeletal biology and are now recognized as a key cell type that integrates hormonal, mechanical and other signals to control bone mass through regulation of both osteoblast and osteoclast activity. With the surge of recent interest in osteocytes as bone regulatory cells and the discovery that they also function as endocrine regulators of phosphate homeostasis, there has been renewed interest in understanding the structure and function of these unique and relatively inaccessible cells. Osteocytes are embedded within the mineralized bone matrix and are housed within a complex lacunocanalicular system which connects them with the circulation and with other organ systems. This has presented unique challenges for imaging these cells. This review summarizes recent advances in confocal imaging approaches for visualizing osteocytes and their lacunocanalicular networks in both living and fixed bone specimens and discusses how computational approaches can be combined with live and fixed cell imaging techniques to generate quantitative outputs and predictive models. The integration of advanced imaging with computational approaches promises to lead to a more in depth understanding of the structure and function of osteocyte networks and the lacunocanalicular system in the healthy and aging state as well as in pathological conditions in bone.

Osteocyte lacunar strain determination using multiscale finite element analysis

Osteocytes are thought to be the primary mechanosensory cells within bone, regulating both osteoclasts and osteoblasts to control load induced changes in bone resorption and formation. Osteocytes initiate intracellular responses including activating the Wnt/β-catenin signaling pathway after experiencing mechanical forces. In response to changing mechanical loads (strain) the osteocytes signal to cells on the bone surface. However, this process of osteocyte activation appears heterogeneous since it occurs in sub-populations of osteocytes, even within regions predicted to be experiencing similar global strain magnitudes determined based on traditional finite element modeling approaches. Several studies have investigated the strain responses of osteocyte lacunae using finite element (FE) models, but many were limited by the use of idealized geometries (e.g., ellipsoids) and analysis of a single osteocyte. Finite element models by other groups included more details, such as canaliculi, but all were done on models consisting of a single osteocyte. We hypothesized that variation in size and orientation of the osteocyte lacunae within bone would give rise to micro heterogeneity in the strain fields that could better explain the observed patterns of osteocyte activation following load. The osteocytes in our microscale and nanoscale models have an idealized oval shape and some are based on confocal scans. However, all the FE models in this preliminary study consist of multiple osteocytes. The number of osteocytes in the 3D confocal scan models ranged from five to seventeen. In this study, a multi-scale computational approach was used to first create an osteocyte FE model at the microscale level to examine both the theoretical lacunar and perilacunar strain responses based on two parameters: 1) lacunar orientation and 2) lacunar size. A parametric analysis was performed by steadily increasing the perilacunar modulus (5, 10, 15, and 20 GPa). Secondly, a nanoscale FE model was built using known osteocyte dimensions to determine the predicted strains in the perilacunar matrix, fluid space, and cell body regions. Finally, 3-D lacunar models were created using confocal image stacks from mouse femurs to determine the theoretical strain in the lacunae represented by realistic geometries. Overall, lacunar strains decreased by 14% in the cell body, 15% in the fluid space region and 25% in the perilacunar space as the perilacunar modulus increased, indicating a stress shielding effect. Lacunar strains were lower for the osteocytes aligned along the loading axis compared to those aligned perpendicular to axis. Increases in lacuna size also led to increased lacunar strains. These finite element model findings suggest that orientation and lacunar size may contribute to the heterogeneous initial pattern of osteocyte strain response observed in bone following in vivo applied mechanical loads. A better understanding of how mechanical stimuli directly affect the lacunae and perilacunar tissue strains may ultimately lead to a better understanding of the process of osteocyte activation in response to mechanical loading.

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A multi-scale computational approach used to first create multiple osteocyte FE model at the microscale level

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3-D Lacuna model created using confocal image stacks from a mouse femur to determine the theoretical strain in the lacunae.

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Lacunar strains decreased as the perilacunar modulus increased.

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Lacunar strains were lower for the osteocytes aligned along the loading axis compared to those aligned perpendicular to axis.

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Increases in lacuna size also led to increased lacunar strains

Control of Bone Matrix Properties by Osteocytes

Osteocytes make up 90-95% of the cellular content of bone and form a rich dendritic network with a vastly greater surface area than either osteoblasts or osteoclasts. Osteocytes are well positioned to play a role in bone homeostasis by interacting directly with the matrix; however, the ability for these cells to modify bone matrix remains incompletely understood. With techniques for examining the nano- and microstructure of bone matrix components including hydroxyapatite and type I collagen becoming more widespread, there is great potential to uncover novel roles for the osteocyte in maintaining bone quality. In this review, we begin with an overview of osteocyte biology and the lacunar-canalicular system. Next, we describe recent findings from in vitro models of osteocytes, focusing on the transitions in cellular phenotype as they mature. Finally, we describe historical and current research on matrix alteration by osteocytes in vivo, focusing on the exciting potential for osteocytes to directly form, degrade, and modify the mineral and collagen in their surrounding matrix.

Selective patterning of netrin-1 as a novel guiding cue for anisotropic dendrogenesis in osteocytes

Although protein patterning approaches have found widespread applications in tuning surface characteristics of biomaterials, selective control of growth in cell body and dendrites utilizing such platforms remains difficult. The functional roles assumed by cell body and dendrites in a physiological milieu have extremely high specificity. In particular, osteocytes embedded inside the mineralized bone matrix are interconnected via dendritic cell processes characterized by an anisotropic arrangement of the lacunar-canalicular system, where the fluid-flow inside the canaliculi system regulates the mechanoresponsive functionalization of bone. Control of cellular networks connected by dendritic cell processes is, therefore, imperative for constructing artificially controlled bone-mimetic structures and as an extension, for gaining insights into the molecular mechanisms underlying dendrogenesis inside the mineralized bone matrix. Here, we report an innovative strategy to induce controlled elongation of cell body or dendritic process structures in selective directions by using the inkjet printing technique. Artificial runways employing netrin-1, inspired by neural architecture, were utilized to trigger controlled elongation in the osteocyte dendritic processes in desired directions. This is the first report, to the best of our knowledge, demonstrating that anisotropic dendrogenesis of osteocytes can be controlled with selective patterning of extracellular proteins, specifically via the axon guidance ligand netrin-1.

Increased Cellular Presence After Sciatic Neurectomy Improves the Bone Mechano-adaptive Response in Aged Mice

The mechano-adaptive response of bone to loading in the murine uniaxial tibial loading model is impaired in aged animals. Previous studies have shown that in aged mice, the amount of bone formed in response to loading is augmented when loads are applied following sciatic neurectomy. The synergistic effect of neurectomy and loading remains to be elucidated. We hypothesize that sciatic neurectomy increases cellular presence, thereby augmenting the response to load in aged mice. We examined bone adaptation in four groups of female C57BL/6J mice, 20-22 months old: (1) sham surgery + 9N loading; (2) sciatic neurectomy, sacrificed after 5 days; (3) sciatic neurectomy, sacrificed after 19 days; (4) sciatic neurectomy + 9N loading. We examined changes in bone cross sectional properties with micro-CT images, and static and dynamic histomorphometry with histological sections taken at the midpoint between tibiofibular junctions. The response to loading at 9N was not detectable with quantitative micro-CT data, but surface-specific histomorphometry captured an increase in bone formation in specific regions. 5 days following sciatic neurectomy, the amount of bone in the neurectomized leg was the same as the contralateral leg, but 19 days following sciatic neurectomy, there was significant bone loss in the neurectomized leg, and both osteoclasts and osteoblasts were recruited to the endosteal surfaces. When sciatic neurectomy and loading at 9N were combined, 3 out of 4 bone quadrants had increased bone formation, on the endosteal and periosteal surfaces (increased osteoid surface and mineralizing surface respectively). These data demonstrate that sciatic neurectomy increases cellular presence on the endosteal surface. With long-term sciatic-neurectomy, both osteoclasts and osteoblasts were recruited to the endosteal surface, which resulted in increased bone formation when combined with a sufficient mechanical stimulus. Controlled and localized recruitment of both osteoblasts and osteoclasts combined with appropriate mechanical loading could inform therapies for mechanically-directed bone formation.

Towards a Connectomic Description of the Osteocyte Lacunocanalicular Network in Bone

PURPOSE OF REVIEW

Osteocytes are the most abundant bone cells. They are completely encased in mineralized tissue, sitting inside lacunae that are connected by a multitude of canaliculi. In recent years, the osteocyte network has been shown to fulfill endocrine functions and to communicate with a number of other organs. This review addresses emerging knowledge on the connectome of the lacunocanalicular network in different types of bone tissue.

RECENT FINDINGS

Recent advances in three-dimensional imaging technology started to reveal parameters that are well known from general theory to characterize the function of networks, such as network density, degree of nodes, or shortest path length through the network. The connectome of the lacunocanalicular network differs in some aspects between lamellar and woven bone and seems to change with age. More research is needed to relate network structure to function, such as intercellular transport or communication and its role in mechanosensation, as well as to understand the effect of diseases.

Screening of key candidate genes and pathways for osteocytes involved in the differential response to different types of mechanical stimulation using a bioinformatics analysis

This study aimed to predict the key genes and pathways that are activated when different types of mechanical loading are applied to osteocytes. mRNA expression datasets (series number of GSE62128 and GSE42874) were obtained from Gene Expression Omnibus database (GEO). High gravity-treated osteocytic MLO-Y4 cell-line samples from GSE62128 (Set1), and fluid flow-treated MLO-Y4 samples from GSE42874 (Set2) were employed. After identifying the differentially expressed genes (DEGs), functional enrichment was performed. The common DEGs between Set1 and Set2 were considered as key DEGs, then a protein-protein interaction (PPI) network was constructed using the minimal nodes from all of the DEGs in Set1 and Set2, which linked most of the key DEGs. Several open source software programs were employed to process and analyze the original data. The bioinformatic results and the biological meaning were validated by in vitro experiments. High gravity and fluid flow induced opposite expression trends in the key DEGs. The hypoxia-related biological process and signaling pathway were the common functional enrichment terms among the DEGs from Set1, Set2 and the PPI network. The expression of almost all the key DEGs (Pdk1, Ccng2, Eno2, Egln1, Higd1a, Slc5a3 and Mxi1) were mechano-sensitive. Eno2 was identified as the hub gene in the PPI network. Eno2 knockdown results in expression changes of some other key DEGs (Pdk1, Mxi1 and Higd1a). Our findings indicated that the hypoxia response might have an important role in the differential responses of osteocytes to the different types of mechanical force.

Changes in the osteocyte lacunocanalicular network with aging

Osteoporosis is an aging-related disease of reduced bone mass that is particularly prevalent in post-menopausal women, but also affects the aged male population and is associated with increased fracture risk. Osteoporosis is the result of an imbalance whereby bone formation by osteoblasts no longer keeps pace with resorption of bone by osteoclasts. Osteocytes are the most abundant cells in bone and, although previously thought to be quiescent, they are now known to be active, multifunctional cells that play a key role in the maintenance of bone mass by regulating both osteoblast and osteoclast activity. They are also thought to regulate bone mass through their role as mechanoresponsive cells in bone that coordinate adaptive responses to mechanical loading. Osteocytes form an extensive interconnected network throughout the mineralized bone matrix and receive their nutrients as well as hormones and signaling factors through the lacunocanalicular system. Several studies have shown that the extent and connectivity of the lacunocanalicular system and osteocyte networks degenerates in aged humans as well as in animal models of aging. It is also known that the bone anabolic response to loading is decreased with aging. This review summarizes recent research on the degenerative changes that occur in osteocytes and their lacunocanalicular system as a result of aging and discusses the implications for skeletal health and homeostasis as well as potential mechanisms that may underlie these degenerative changes. Since osteocytes are such key regulators of skeletal homeostasis, maintaining the health of the osteocyte network would seem critical for maintenance of bone health. Therefore, a more complete understanding of the structure and function of the osteocyte network, its lacunocanalicular system, and the degenerative changes that occur with aging should lead to advances in our understanding of age related bone loss and potentially lead to improved therapies.

Visualizing the "sandwich" structure of osteocytes in their native environment deep in bone in vivo

Osteocytes are the most abundant cells in bone and always the focus of bone research. They are embedded in the highly scattering mineralized bone matrix. Consequently, visualizing osteocytes deep in bone with subcellular resolution poses a major challenge for in vivo bone research. Here we overcome this challenge by demonstrating 3-photon imaging of osteocytes through the intact mouse skull in vivo. Through broadband transmittance characterization, we establish that the excitation at the 1700-nm window enables the highest optical transmittance through the skull. Using label-free third-harmonic generation (THG) imaging excited at this window, we visualize osteocytes through the whole 140-μm mouse skull and 155 μm into the brain in vivo. By developing selective labeling technique for the interstitial space, we visualize the "sandwich" structure of osteocytes in their native environment. Our work provides novel imaging methodology for bone research in vivo.

Histomorphometry and cortical robusticity of the adult human femur

Recent quantitative analyses of human bone microanatomy, as well as theoretical models that propose bone microstructure and gross anatomical associations, have started to reveal insights into biological links that may facilitate remodeling processes. However, relationships between bone size and the underlying cortical bone histology remain largely unexplored. The goal of this study is to determine the extent to which static indicators of bone remodeling and vascularity, measured using histomorphometric techniques, relate to femoral midshaft cortical width and robusticity. Using previously published and new quantitative data from 450 adult human male (n = 233) and female (n = 217) femora, we determine if these aspects of femoral size relate to bone microanatomy. Scaling relationships are explored and interpreted within the context of tissue form and function. Analyses revealed that the area and diameter of Haversian canals and secondary osteons, and densities of secondary osteons and osteocyte lacunae from the sub-periosteal region of the posterior midshaft femur cortex were significantly, but not consistently, associated with femoral size. Cortical width and bone robusticity were correlated with osteocyte lacunae density and scaled with positive allometry. Diameter and area of osteons and Haversian canals decreased as the width of cortex and bone robusticity increased, revealing a negative allometric relationship. These results indicate that microscopic products of cortical bone remodeling and vascularity are linked to femur size. Allometric relationships between more robust human femora with thicker cortical bone and histological products of bone remodeling correspond with principles of bone functional adaptation. Future studies may benefit from exploring scaling relationships between bone histomorphometric data and measurements of bone macrostructure.

Analysis of Ca2+ response of osteocyte network by three-dimensional time-lapse imaging in living bone

Osteocytes form a three-dimensional (3D) cellular network within the mineralized bone matrix. The cellular network has important roles in mechanosensation and mechanotransduction related to bone homeostasis. We visualized the embedded osteocyte network in chick calvariae and observed the flow-induced Ca²⁺ signaling in osteocytes using 3D time-lapse imaging. In response to the flow, intracellular Ca²⁺ ([Ca²⁺]_i) significantly increased in developmentally mature osteocytes in comparison with young osteocytes in the bone matrix. To investigate the differences in response between young and developmentally mature osteocytes in detail, we evaluated the expression of osteocyte-related genes using the osteocyte-like cell line MLO-Y4, which was 3D-cultured within type I collagen gels. We found that the c-Fos, Cx43, Panx3, Col1a1, and OCN mRNA levels significantly increased on day 15 in comparison with day 7. These findings indicate that developmentally mature osteocytes are more responsive to mechanical stress than young osteocytes and have important functions in bone formation and remodeling.

3D micro structural analysis of human cortical bone in paired femoral diaphysis, femoral neck and radial diaphysis

Human bone is known to adapt to its mechanical environment in a living body. Both its architecture and microstructure may differ between weight-bearing and non-weight-bearing bones. The aim of the current study was to analyze in three dimensions, the morphology of the multi-scale porosities on human cortical bone at different locations. Eight paired femoral diaphyses, femoral necks, and radial diaphyses were imaged using Synchrotron Radiation µCT with a 0.7 µm isotropic voxel size. The spatial resolution facilitates the investigation of the multiscale porosities of cortical bone, from the osteonal canals system down to the osteocyte lacunar system. Our results showed significant differences in the microstructural properties, regarding both osteonal canals and osteocytes lacunae, between the different anatomical locations. The radius presents significantly lower osteonal canal volume fraction and smaller osteonal canals than the femoral diaphysis or neck. Osteocytes lacunae observed in the radius are significantly different in shape than in the femur, and lacunar density is higher in the femoral neck. These results show that the radius, a non-weight-bearing bone, is significantly different in terms of its microstructure from a weight-bearing bone such as the femur. This implies that the cortical bone properties evaluated on the femoral diaphysis, the main location studied within the literature, cannot be generalized to other anatomical locations.

Aging, Osteocytes, and Mechanotransduction

PURPOSE OF REVIEW

The bone is able to adapt its structure to mechanical signals via the bone remodeling process governed by mechanosensitive osteocytes. With aging, an imbalance in bone remodeling results in osteoporosis. In this review, we hypothesized that changes in lacunar morphology underlie the decreased bone mechanoresponsiveness to mechanical loading with aging.

RECENT FINDINGS

Several studies have reported considerable variations in the shape of osteocytes and their lacunae with aging. Since osteocytes can sense matrix strain directly via their cell bodies, the variations in osteocyte morphology may cause changes in osteocyte mechanosensitivity. As a consequence, the load-adaptive response of osteocytes may change with aging, even when mechanical loading would remain unchanged. Though extensive quantitative data is lacking, evidence exists that the osteocyte lacunae are becoming smaller and more spherical with aging. Future dedicated studies might reveal whether these changes would affect osteocyte mechanosensation and the subsequent biological response, and whether this is (one of) the pathways involved in age-related bone loss.

Administration frequency as well as dosage of PTH are associated with development of cortical porosity in ovariectomized rats

To investigate whether the administration frequency of parathyroid hormone (PTH) is associated with the development of cortical porosity, this study established 15 dosage regimens of teriparatide [human PTH(1–34), TPTD] with four distinct concentrations and four distinct administration frequencies of TPTD to 16-week-old ovariectomized rats. Our analyses demonstrated that the bone mineral density, mechanical properties, and bone turnover were associated with the total amount of TPTD administered. Our observations further revealed that the cortical porosity was markedly developed as a result of an increased administration frequency with a lower concentration of total TPTD administration in our setting, although the highest concentration also induced cortical porosity. Deconvolution fluorescence tiling imaging on calcein-labeled undecalcified bone sections also demonstrated the development of cortical porosity to be closely associated with the bone site where periosteal bone formation took place. This site-specific cortical porosity involved intracortical bone resorption and an increased number and proximity of osteocytic lacunae, occasionally causing fused lacunae. Taken together, these findings suggested the involvement of local distinctions in the rate of bone growth that may be related to the site-specific mechanical properties in the development of cortical porosity induced by frequent and/or high doses of TPTD.

Unloading-Induced Degradation of the Anisotropic Arrangement of Collagen/Apatite in Rat Femurs

The specific orientation of collagen and biological apatite (BAp) is an anisotropic feature of bone micro-organization; it is an important determinant of bone mechanical function and performance under anisotropic stress. However, it is poorly understood how this microstructure orientation is altered when the mechanical environment changes. We hypothesized that the preferential orientation of collagen/BAp would change in response to changes in mechanical conditions, similar to the manner in which bone mass and bone shape change. In the present study, we investigated the effect of unloading (removal of anisotropic stress) on the preferential orientation of collagen/BAp using a rat sciatic neurectomy model. Bone tissue that formed under unloaded conditions showed a more disordered collagen/BAp orientation than bone tissue that formed under physiological conditions. Coincidentally, osteocytes in unloaded bone displayed spherical morphology and random alignment. To the best of our knowledge, this study is the first to demonstrate the degradation of preferential collagen/BAp orientation in response to unloading conditions. In summary, we identified alterations in bone material anisotropy as an important aspect of the bone's response to unloading, which had previously been examined with regard to bone loss only.

Numerical simulation of osteocyte cell in response to directional mechanical loadings and mechanotransduction analysis: Considering lacunar-canalicular interstitial fluid flow

The osteocyte cell is a bone cell that also functions as a bone mechanosensor. In this work, a three-dimensional (3D) fluid-structure interaction (FSI) model of an osteocyte cell under different mechanical loading conditions was used to obtain a better understanding of osteocyte cell behavior under different physiological conditions. In the current study, both fluid and solid parts of osteocyte cell were considered in order to allow for more accurate results. Five different loading conditions have been applied to the osteocyte cell, and consequently the different interstitial fluid flow velocities and shear stresses have been investigated. Furthermore, using a mathematical model, the change in the stimulus function value with shear stress and NO enzyme was revealed. This work suggests that changes in osteocyte morphology and direction of loadings affect cell stimulation. It was found that cell is mostly stimulated and expanded in the direction experiencing the most shear stress. Finally, the amount of cell stimulation was shown quantitatively and there was strong dependency between stimulus function, shear stress, calcium, and NO concentration.

Impact of Non-Invasively Induced Motor Deficits on Tibial Cortical Properties in Mutant Lurcher Mice

It has been shown that Lurcher mutant mice have significantly altered motor abilities, regarding their motor coordination and muscular strength because of olivorecebellar degeneration. We assessed the response of the cross-sectional geometry and lacuno-canalicular network properties of the tibial mid-diaphyseal cortical bone to motor differences between Lurcher and wild-type (WT) male mice from the B6CBA strain. The first data set used in the cross-sectional geometry analysis consists of 16 mice of 4 months of age and 32 mice of 9 months of age. The second data set used in the lacunar-canalicular network analysis consists of 10 mice of 4 months of age. We compared two cross-sectional geometry and four lacunar-canalicular properties by I-region using the maximum and minimum second moment of area and anatomical orientation as well as H-regions using histological differences within a cross section. We identified inconsistent differences in the studied cross-sectional geometry properties between Lurcher and WT mice. The biggest significant difference between Lurcher and WT mice is found in the number of canaliculi, whereas in the other studied properties are only limited. Lurcher mice exhibit an increased number of canaliculi (p < 0.01) in all studied regions compared with the WT controls. The number of canaliculi is also negatively correlated with the distance from the centroid in the Lurcher and positively correlated in the WT mice. When the Lurcher and WT sample is pooled, the number of canaliculi and lacunar volume is increased in the posterior Imax region, and in addition, midcortical H-region exhibit lower number of canaliculi, lacuna to lacuna distance and increased lacunar volume. Our results indicate, that the importance of precise sample selection within cross sections in future studies is highlighted because of the histological heterogeneity of lacunar-canalicular network properties within the I-region and H-region in the mouse cortical bone.

A Single Chance to Contact Multiple Targets: Distinct Osteocyte Morphotypes Shed Light on the Cellular Mechanism Ensuring the Robust Formation of Osteocytic Networks

The formation of the complex osteocytic network relies on the emission of long cellular processes involved in communication, mechanical strain sensing, and bone turnover control. Newly deposited osteocytic processes rapidly become trapped within the calcifying matrix, and, therefore, they must adopt their definitive conformation and contact their targets in a single morphogenetic event. However, the cellular mechanisms ensuring the robustness of this unique mode of morphogenesis remain unknown. To address this issue, we examined the developing calvaria of the amphibian Xenopus tropicalis by confocal, two-photon, and super-resolution imaging, and described flattened osteocytes lying within a woven bone structured in lamellae of randomly oriented collagen fibers. While most cells emit peripheral and perpendicular processes, we report two osteocytes morphotypes, located at different depth within the bone matrix and exhibiting distinct number and orientation of perpendicular cell processes. We show that this pattern is conserved with the chick Gallus gallus and suggest that the cellular microenvironment, and more particularly cell-cell contact, plays a fundamental role in the induction and stabilization of osteocytic processes. We propose that this intrinsic property might have been evolutionarily selected for its ability to robustly generate self-organizing osteocytic networks harbored by the wide variety of bone shapes and architectures found in extant and extinct vertebrates.

Investigating histomorphometric relationships at the human femoral midshaft in a biomechanical context

Cortical bone histomorphometry utilised in human and animal bone biology studies has demonstrated that osteon densities and their geometric properties may be in a relationship with biomechanical load application. Further research is required to investigate mutual links between bone histological variables to elucidate their usefulness in future biomechanical studies. Here, a series of correlations exploring bone biology relationships at the human midshaft femur were performed using a large sample. Mean intact, fragmentary and total osteon population densities, Haversian canal diameter and area, osteon area, as well as osteocyte lacunae density were measured along the sub-periosteal cortex in sections removed from the posterior midshaft aspect of modern human male (n = 233) and female (n = 217) femora (total n = 450). Parametric and non-parametric correlations between the histology variables were sought in the entire sample, as well as within age and sex sub-groups. Several significant positive and negative correlations explaining a large proportion of data variation were found. Haversian canal area, diameter, and osteon area were positively correlated. As the density of osteocyte lacunae increased, Haversian canals and osteons became smaller. As osteons increased in density, so did osteocyte lacunae, but Haversian canal and osteon area became smaller. Results were consistent across age and sex groups. Findings suggest that an increased rate of bone remodelling is associated with a decrease in geometrical properties of osteons. An increased density of osteocyte lacunae and osteons indicates the involvement of bone maintenance cells in remodelling potentially induced by mechanical stimuli. Future histomorphometry studies will benefit from examining multiple bone histology variables due to many mutual bone biology relationships that exist at the human midshaft femur.

Altered mechanical environment of bone cells in an animal model of short- and long-term osteoporosis

Alterations in bone tissue composition during osteoporosis likely disrupt the mechanical environment of bone cells and may thereby initiate a mechanobiological response. It has proved challenging to characterize the mechanical environment of bone cells in vivo, and the mechanical environment of osteoporotic bone cells is not known. The objective of this research is to characterize the local mechanical environment of osteocytes and osteoblasts from healthy and osteoporotic bone in a rat model of osteoporosis. Using a custom-designed micromechanical loading device, we apply strains representative of a range of physical activity (up to 3000 με) to fluorescently stained femur samples from normal and ovariectomized rats. Confocal imaging was simultaneously performed, and digital image correlation techniques were applied to characterize cellular strains. In healthy bone tissue, osteocytes experience higher maximum strains (31,028 ± 4213 με) than osteoblasts (24,921 ± 3,832 με), whereas a larger proportion of the osteoblast experiences strains >10,000 με. Most interestingly, we show that osteoporotic bone cells experience similar or higher maximum strains than healthy bone cells after short durations of estrogen deficiency (5 weeks), and exceeded the osteogenic strain threshold (10,000 με) in a similar or significantly larger proportion of the cell (osteoblast, 12.68% vs. 13.68%; osteocyte, 15.74% vs. 5.37%). However, in long-term estrogen deficiency (34 weeks), there was no significant difference between bone cells in healthy and osteoporotic bone. These results suggest that the mechanical environment of bone cells is altered during early-stage osteoporosis, and that mechanobiological responses act to restore the mechanical environment of the bone tissue after it has been perturbed by ovariectomy.

Activation of HIFa pathway in mature osteoblasts disrupts the integrity of the osteocyte/canalicular network

The hypoxia-inducible factors (HIFs), HIF-1α and HIF-2α, are the central mediators of the homeostatic response that enables cells to survive and differentiate in low-oxygen conditions. Previous studies indicated that disruption of the von Hippel-Lindau gene (Vhl) coincides with the activation of HIFα signaling. Here we show that inactivation of Vhl in mature osteoblasts/osteocytes induces their apoptosis and disrupts the cell/canalicular network. VHL-deficient (ΔVHL) mice exhibited a significantly increased cortical bone area resulting from enhanced proliferation and osteogenic differentiation of the bone marrow stromal cells (BMSCs) by inducing the expression of β-catenin in the BMSC. Our data suggest that the VHL/HIFα pathway in mature osteoblasts/osteocytes plays a critical role in the bone cell/canalicular network and that the changes of osteocyte morphology/function and cell/canalicular network may unleash the bone formation, The underlying mechanism of which was the accumulation of β-catenin in the osteoblasts/osteoprogenitors of the bone marrow.

Functional heterogeneity of osteocytes in FGF23 production: the possible involvement of DMP1 as a direct negative regulator

Fibroblast growth factor 23 (FGF23) and dentin matrix protein (DMP1) are hallmarks of osteocytes in bone. However, the mechanisms underlying the actions of DMP1 as a local factor regulating FGF23 and bone mineralization are not well understood. We first observed spatially distinct distributions of FGF23- and DMP1-positive osteocytic lacunae in rat femurs using immunohistochemistry. Three-dimensional immunofluorescence morphometry further demonstrated that the distribution and relative expression levels of these two proteins exhibited reciprocally reversed patterns especially in midshaft cortical bone. These in vivo findings suggest a direct role of DMP1 in FGF23 expression in osteocytes. We next observed that the inoculation of recombinant DMP1 in UMR-106 osteoblast/osteocyte-like cells and long-cultured MC3T3-E1 osteoblastic cells showed significant downregulation of FGF23 production. This effect was rescued by incubation with an focal adhesion kinase (FAK) inhibitor or MEK (mitogen-activated protein kinase (MAPK)/extracellular signal regulated kinase (ERK)) inhibitor but not inhibitors of phosphoinositide 3-kinase or Rho kinase. Consistently, the levels of phosphorylated FAK, ERK and p38 were significantly elevated, indicating that exogenous DMP1 is capable of activating FAK-mediated MAPK signaling. These findings suggest that DMP1 is a local, direct and negative regulator of FGF23 production in osteocytes involved in the FAK-mediated MAPK pathway, proposing a relevant pathway that coordinates the extracellular environment of osteocytic lacunae and bone metabolism.

Analysis of Osteocyte Morphology in Terms of Sensation of In Vivo Stress Applied on Bone

Most bones are anisotropically loaded and seem to be adapted to the anisotropic stress or strain field by changing the anisotropy in their microstructure. Osteocyte (OCY) is believed to play an important role as a mechanosensor and regulator of modeling and/or remodeling orchestrating osteoblast and osteoclast activity to make bone suitable to resist the mechanical environment. In general, osteocytes sense magnitude of stress (strain) applied upon the bone and then work as a trigger to change bone mass to adjust bone’s mechanical function to the stress field. This structural optimization is an important aspect of the bone functional adaptation; another inevitable optimization might be achieved through the change in intrinsic material anisotropy including the preferential c-axis orientation of biological apatite (BAp) crystal. To achieve this adaptation through material anisotropy, osteocyte needs to be a mechanosensor which can detect anisotropic stress field. In the present study, osteocyte lacunae and canaliculi in the mid-diaphysis and the distal part of the rat femur were stained by a fluorescein dye for visualization and analysis. The mid-diaphysis shows greater degree of the preferential c-axis orientation of BAp crystal than the distal part in relation to the magnitude of uni-axial stress field. It was found that the osteocytes in long bone preferentially align along the bone long axis and the degree of alignment is greater in the mid-diaphysis than in the distal region, which seems to be effective for the sensation of the site-dependent specific stress field applied on the long bone.