Development of bone canaliculi during bone repair

Roles of Sp7 in osteoblasts for the proliferation, differentiation, and osteocyte process formation

Background

Zinc finger-containing transcription factor Osterix/Specificity protein-7 (Sp7) is an essential transcription factor for osteoblast differentiation. However, its functions in differentiated osteoblasts remain unclear and the effects of osteoblast-specific Sp7 deletion on osteocytes have not been sufficiently studied.

Methods

Sp7 floxneo/floxneo mice, in which Sp7 expression was 30 % of that in wild-type mice because of disturbed splicing by neo gene insertion, and osteoblast-specific knockout (Sp7 fl/fl;Col1a1-Cre) mice using 2.3-kb Col1a1 enhanced green fluorescent protein (EGFP)-Cre were examined by micro-computed tomography (micro-CT), bone histomorphometry, serum markers, and histological analyses. The expression of osteoblast and osteocyte marker genes was examined by real-time reverse transcription (RT)-PCR analysis. Osteoblastogenesis, osteoclastogenesis, and regulation of the expression of collagen type I alpha 1 chain (Col1a1) were examined in primary osteoblasts.

Results

Femoral trabecular bone volume was higher in female Sp7 floxneo/floxneo and Sp7 fl/fl;Col1a1-Cre mice than in the respective controls, but not in males. Bromodeoxyuridine (BrdU)-positive osteoblastic cells were increased in male Sp7 fl/fl;Col1a1-Cre mice, and osteoblast number and the bone formation rate were increased in tibial trabecular bone in female Sp7 fl/fl;Col1a1-Cre mice, although osteoblast maturation was inhibited in female Sp7 fl/fl;Col1a1-Cre mice as shown by the increased expression of an immature osteoblast marker gene, secreted phosphoprotein 1 (Spp1), and reduced expression of a mature osteoblast marker gene, bone gamma-carboxyglutamate protein/bone gamma-carboxyglutamate protein 2 (Bglap/Bglap2). Furthermore, alkaline phosphatase activity was increased but mineralization was reduced in the culture of primary osteoblasts from Sp7 fl/fl;Col1a1-Cre mice. Therefore, the accumulated immature osteoblasts in Sp7 fl/fl;Col1a1-Cre mice was likely compensated for the inhibition of osteoblast maturation at different levels in males and females. Vertebral trabecular bone volume was lower in both male and female Sp7 fl/fl;Col1a1-Cre mice than in the controls and the osteoblast parameters and bone formation rate in females were lower in Sp7 fl/fl;Col1a1-Cre mice than in Sp7 fl/fl mice, suggesting differential regulatory mechanisms in long bones and vertebrae. The femoral cortical bone was thin and porous in Sp7 floxneo/floxneo and Sp7 fl/fl;Col1a1-Cre mice of both sexes, the number of canaliculi was reduced, and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL)-positive lacunae and the osteoclasts were increased, whereas the bone formation rate was similar in Sp7 fl/fl;Col1a1-Cre and Sp7 fl/fl mice. The serum levels of total procollagen type 1 N-terminal propeptide (P1NP), a marker for bone formation, were similar, while those of tartrate-resistant acid phosphatase 5b (TRAP5b), a marker for bone resorption, were higher in Sp7 fl/fl;Col1a1-Cre mice. Osteoblasts were less cuboidal, the expression of Col1a1 and Col1a1-EGFP-Cre was lower in Sp7 fl/fl;Col1a1-Cre mice, and overexpression of Sp7 induced Col1a1 expression.

Conclusions

Our studies indicated that Sp7 inhibits the proliferation of immature osteoblasts, induces osteoblast maturation and Col1a1 expression, and is required for osteocytes to acquire a sufficient number of processes for their survival, which prevents cortical porosity.

The translational potential of this article

This study clarified the roles of Sp7 in differentiated osteoblasts in proliferarion, maturation, Col1a1 expression, and osteocyte process formation, which are required for targeting SP7 in the development of therapies for osteoporosis.

Multiscale morphological analysis of bone microarchitecture around Mg-10Gd implants

The utilization of biodegradable magnesium (Mg)-based implants for restoration of bone function following trauma represents a transformative approach in orthopaedic application. One such alloy, magnesium-10 weight percent gadolinium (Mg-10Gd), has been specifically developed to address the rapid degradation of Mg while enhancing its mechanical properties to promote bone healing. Previous studies have demonstrated that Mg-10Gd exhibits favorable osseointegration; however, it exhibits distinct ultrastructural adaptation in comparison to conventional implants like titanium (Ti). A crucial aspect that remains unexplored is the impact of Mg-10Gd degradation on the bone microarchitecture. To address this, we employed hierarchical three-dimensional imaging using synchrotron radiation in conjunction with image-based finite element modelling. By using the methods outlined, the vascular porosity, lacunar porosity and the lacunar-canaliculi network (LCN) morphology of bone around Mg-10Gd in comparison to Ti in a rat model from 4 weeks to 20 weeks post-implantation was investigated. Our investigation revealed that within our observation period, the degradation of Mg-10Gd implants was associated with significantly lower (p < 0.05) lacunar density in the surrounding bone, compared to Ti. Remarkably, the LCN morphology and the fluid flow analysis did not significantly differ for both implant types. In summary, a more pronounced lower lacunae distribution rather than their morphological changes was detected in the surrounding bone upon the degradation of Mg-10Gd implants. This implies potential disparities in bone remodelling rates when compared to Ti implants. Our findings shed light on the intricate relationship between Mg-10Gd degradation and bone microarchitecture, contributing to a deeper understanding of the implications for successful osseointegration.

Fam20C overexpression in odontoblasts regulates dentin formation and odontoblast differentiation

FAM20C phosphorylates secretory proteins at S-x-E/pS motifs, and previous studies of Fam20C-dificient mice revealed that FAM20C played essential roles in bone and tooth formation. Inactivation of FAM20C in mice led to hypophosphatemia that masks direct effect of FAM20C in these tissues, and consequently the direct role of FAM20C remains unknown. Our previous study reported that osteoblast/odontoblast-specific Fam20C transgenic (Fam20C-Tg) mice had normal serum phosphate levels and that osteoblastic FAM20C-mediated phosphorylation regulated bone formation and resorption. Here, we investigated the direct role of FAM20C in dentin using Fam20C-Tg mice. The tooth of Fam20C-Tg mice contained numerous highly phosphorylated proteins, including SIBLINGs, compared to that of wild-type mice. In Fam20C-Tg mice, coronal dentin volume decreased and mineral density unchanged at early age, while the volume unchanged and the mineral density elevated at maturity. In these mice, radicular dentin volume and mineral density decreased at all ages, and histologically, the radicular dentin had wider predentin and abnormal apical-side dentin with embedded cells and argyrophilic canaliculi. Immunohistochemical analyses revealed that abnormal apical-side dentin had bone and dentin matrix properties accompanied with osteoblast-lineage cells. Further, in Fam20C-Tg mice, DSPP content which is important for dentin formation, was reduced in dentin, especially radicular dentin, which might lead to defects mainly in radicular dentin. Renal subcapsular transplantations of tooth germ revealed that newly formed radicular dentin replicated apical abnormal dentin of Fam20C-Tg mice, corroborating that FAM20C overexpression indeed caused the abnormal dentin. Our findings indicate that odontoblastic FAM20C-mediated phosphorylation in the tooth regulates dentin formation and odontoblast differentiation.

Sp7 Transgenic Mice with a Markedly Impaired Lacunocanalicular Network Induced Sost and Reduced Bone Mass by Unloading

The relationship of lacunocanalicular network structure and mechanoresponse has not been well studied. The lacunocanalicular structures differed in the compression and tension sides, in the regions, and in genders in wild-type femoral cortical bone. The overexpression of Sp7 in osteoblasts resulted in thin and porous cortical bone with increased osteoclasts and apoptotic osteocytes, and the number of canaliculi was half of that in the wild-type mice, leading to a markedly impaired lacunocanalicular network. To investigate the response to unloading, we performed tail suspension. Unloading reduced trabecular and cortical bone in the Sp7 transgenic mice due to reduced bone formation. Sost-positive osteocytes increased by unloading on the compression side, but not on the tension side of cortical bone in the wild-type femurs. However, these differential responses were lost in the Sp7 transgenic femurs. Serum Sost increased in the Sp7 transgenic mice, but not in the wild-type mice. Unloading reduced the Col1a1 and Bglap/Bglap2 expression in the Sp7 transgenic mice but not the wild-type mice. Thus, Sp7 transgenic mice with the impaired lacunocanalicular network induced Sost expression by unloading but lost the differential regulation in the compression and tension sides, and the mice failed to restore bone formation during unloading, implicating the relationship of lacunocanalicular network structure and the regulation of bone formation in mechanoresponse.

Heterogeneity of the osteocyte lacuno-canalicular network architecture and material characteristics across different tissue types in healing bone

Various tissue types, including fibrous connective tissue, bone marrow, cartilage, woven and lamellar bone, coexist in healing bone. Similar to most bone tissue type, healing bone contains a lacuno-canalicular network (LCN) housing osteocytes. These cells are known to orchestrate bone remodeling in healthy bone by sensing mechanical strains and translating them into biochemical signals. The structure of the LCN is hypothesized to influence mineralization processes. Hence, the aim of the present study was to visualize and match spatial variations in the LCN topology with mineral characteristics, within and at the interfaces of the different tissue types that comprise healing bone. We applied a correlative multi-method approach to visualize the LCN architecture and quantify mineral particle size and orientation within healing femoral bone in a mouse osteotomy model (26 weeks old C57BL/6 mice). This approach revealed structural differences across several length scales during endochondral ossification within the following regions: calcified cartilage, bony callus, cortical bone and a transition zone between the cortical and callus region analyzed 21 days after the osteotomy. In this transition zone, we observed a continuous convergence of mineral characteristics and osteocyte lacunae shape as well as discontinuities in the lacunae volume and LCN connectivity. The bony callus exhibits a 34% higher lacunae number density and 40% larger lacunar volume compared to cortical bone. The presented correlations between LCN architecture and mineral characteristics improves our understanding of how bone develops during healing and may indicate a contribution of osteocytes to bone (re)modeling.

Overexpression of Fam20C in osteoblast in vivo leads to increased cortical bone formation and osteoclastic bone resorption

Fam20C, which phosphorylates many secretory proteins with S-x-E/pS motifs, is highly expressed in bone and tooth tissues, implying that Fam20C-mediated phosphorylation is critical for regulation of these mineralized tissues. Previous studies of Fam20C-deficient mice revealed that Fam20C plays important roles in bone formation and mineralization. However, Fam20C-deficient mice develop hypophosphatemia, a systemic factor that masks the local effect of Fam20C in the bone tissue; consequently, the local role of Fam20C remains unknown. To elucidate the local function of Fam20C in bone tissue, we studied osteoblast-specific Fam20C transgenic (Fam20C-Tg) mice, which have no alteration in serum calcium and phosphate levels. Fam20C-Tg mice had more highly phosphorylated proteins in bone tissue than wild-type mice. In cortical bone of Fam20C-Tg mice, bone volume, mineralization surface (MS/BS), and mineral apposition rate (MAR) were elevated; in addition, the transgenic mice had an elevated number of vascular canals, resulting in an increased cortical porosity. Osteocyte number was elevated in the transgenics, but osteoblast number was unchanged. The microstructure of bone matrix characterized by the preferential orientation of collagen and apatite, was degraded and thus the mechanical function of bone material was deteriorated. In trabecular bone of Fam20C-Tg mice, bone volume was reduced, whereas MS/BS and MAR were unchanged. Osteoclast number was elevated and eroded surface area was non-significantly elevated with an increased serum CTX-I level, whereas osteoblast number was unchanged. These findings indicated that Fam20C overexpression in osteoblasts promotes cortical bone formation by increasing MS/BS and MAR and promoting osteocyte differentiation, but does not affect trabecular bone formation. Furthermore, Fam20C overexpression indirectly promotes osteoclastic bone resorption in cortical and trabecular bones. Our findings show that osteoblastic Fam20C-mediated phosphorylation in bone tissue regulates bone formation and resorption, and bone material quality.

Diminished Capsule Exacerbates Virulence, Blood-Brain Barrier Penetration, Intracellular Persistence, and Antibiotic Evasion of Hyperhemolytic Group B Streptococci

Group B streptococci (GBS) are encapsulated, β-hemolytic bacteria that are a common cause of infections in human newborns and certain adults. Two factors important for GBS virulence are the sialic acid capsular polysaccharide that promotes immune evasion and the hemolytic pigment that induces host cell cytotoxcity. These virulence factors are often oppositely regulated by the CovR/CovS two-component system. Clinical GBS strains exhibiting hyperhemolysis and low capsule due to pathoadaptive covR/S mutations have been isolated from patients. Given the importance of capsule to GBS virulence, we predicted that a decrease or loss of capsule would attenuate the virulence of covR/S mutants. Surprisingly, hyperhemolytic GBS with low or no capsule exhibit increased virulence, intracellular persistence, and blood-brain barrier penetration, which was independent of a Trojan horse mechanism of barrier penetration. Additionally, intracellular persistence enabled both hemolytic and hyperhemolytic GBS to evade antibiotics routinely used to treat these infections. The finding that diminished capsule expression promotes GBS virulence, intracellular persistence, and antibiotic evasion has important implications for sustained antibiotic therapy and efficacy of capsule-based vaccines.

Osteocytic connexin 43 channels affect fracture healing

The cross-talk between cells is very critical for moving forward fracture healing in an orderly manner. Connexin (Cx) 43-formed gap junctions and hemichannels mediate the communication between adjacent cells and cells and extracellular environment. Loss of Cx43 in osteoblasts/osteocytes results in delayed fracture healing. For investigating the role of two channels in osteocytes in bone repair, two transgenic mouse models with Cx43 dominant negative mutants driven by a 10 kb-DMP1 promoter were generated: R76W (gap junctions are blocked, whereas hemichannels are promoted) and Δ130-136 (both gap junctions and hemichannels are blocked). R76W mice (promotion of hemichannels) showed a significant increase of new bone formation, whereas delayed osteoclastogenesis and healing was observed in Δ130-136 (impairment of gap junctions), but not in R76W mice (hemichannel promotion may recover the delay). These results suggest that gap junctions and hemichannels play some similar and cooperative roles in bone repair.

Interaction among Calcium Diet Content, PTH (1-34) Treatment and Balance of Bone Homeostasis in Rat Model: The Trabecular Bone as Keystone

The present study is the second step (concerning normal diet restoration) of the our previous study (concerning the calcium-free diet) to determine whether normal diet restoration, with/without concomitant PTH (1-34) administration, can influence amounts and deposition sites of the total bone mass. Histomorphometric evaluations and immunohistochemical analysis for Sclerostin expression were conducted on the vertebral bodies and femurs in the rat model. The final goals are (i) to define timing and manners of bone mass changes when calcium is restored to the diet, (ii) to analyze the different involvement of the two bony architectures having different metabolism (i.e., trabecular versus cortical bone), and (iii) to verify the eventual role of PTH (1-34) administration. Results evidenced the greater involvement of the trabecular bone with respect to the cortical bone, in response to different levels of calcium content in the diet, and the effect of PTH, mostly in the recovery of trabecular bony architecture. The main findings emerged from the present study are (i) the importance of the interplay between mineral homeostasis and skeletal homeostasis in modulating and guiding bone's response to dietary/metabolic alterations and (ii) the evidence that the more involved bony architecture is the trabecular bone, the most susceptible to the dynamical balance of the two homeostases.

Numerical modeling of bone as a multiscale poroelastic material by the homogenization technique

Bone is a complex material showing a hierarchical and porous structure but also a natural ability to remodel thanks to cells sensitive to fluid flows. Based on these characteristics, a multiscale numerical model has been developed in order to represent the bone response under mechanical solicitation. It relies on the homogenization technique, simulating bone as a homogeneous structure having a porous microstructure saturated with bone fluid. The numerical modeling of the loading of a finite volume of bone enables the determination of an equivalent poroelastic stiffness. Focusing on two extreme fluid boundary conditions, the study of the corresponding structural response provides an overview of the fluid contribution to the poroelastic behavior, impacting the stiffness of the considered material. This parameter is either reduced (when the fluid can flow out of the structure) or increased (when the fluid is kept inside the structure) and quantified through this model. The presented poroelastic numerical model is here developed in the perspective of providing a bio-reliable model of bones, to determine the critical parameters that might impact bone remodeling.

Individual response variations in scaffold-guided bone regeneration are determined by independent strain- and injury-induced mechanisms

This study explored the regenerative osteogenic response in the distal femur of sheep using scaffolds having stiffness values within, and above and below, the range of trabecular bone apparent modulus. Scaffolds 3D-printed from stiff titanium and compliant polyamide were implanted into a cylindrical metaphyseal defect 15 × 15 mm. After six weeks, bone ingrowth varied between 7 and 21% of the scaffold pore volume and this was generally inversely proportional to scaffold stiffness. The individual reparative response considerably varied among the animals, which could be divided into weak and strong responders. Notably, bone regeneration specifically within the interior of the scaffold was inversely proportional to scaffold stiffness and was strain-driven in strongly-responding animals. Conversely, bone regeneration at the periphery of the defect was injury-driven and equal in all scaffolds and in all strongly- and weakly-responding animals. The observation of the strain-driven response in some, but not all, animals highlights that scaffold compliance is desirable for triggering host bone regeneration, but scaffold permanence is important for the load-bearing, structural role of the bone-replacing device. Indeed, scaffolds may benefit from being nonresorbable and mechanically reliable for those unforeseeable cases of weakly responding recipients.

Formation of osteon-like structures in unidirectional porous hydroxyapatite substitute

Unidirectional porous hydroxyapatite (UDPHAp) bone substitute comprises a microstructure of cross‐sectionally oval pores with diameters ranging from 30 to 300 µm. Bone remodeling within the UDPHAp is expected upon implantation into bone; however, the mechanism and factors influencing this bone growth remain unclear. The objectives of the present study were to assess the vasculature and microstructure of newly formed bone and to determine how bone formation is affected by load transfer and UDPHAp pore size. Formation of osteon‐like structures, defined by the presence of lacunae, canaliculi and a central lumen containing capillaries, was observed within the implanted UDPHAp material in all animals after six weeks. The number of osteocytes and osteon‐like structures in areas adjacent to the cortex of recipient bone was significantly higher than in areas next to the medullary cavity throughout the recovery period. Notably, osteon‐like structures tended to form in smaller diameter pores. Continuous bone remodeling might be promoted by the rapid formation of unidirectional capillaries and the osteocyte lacunae‐canalicular system. Load transfer and smaller pore size could positively affect cortical bone regeneration. © 2018 The Authors Journal of Biomedical Materials Research Part B: Applied Biomaterials Published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2665–2672, 2018.

Flow velocity-driven differentiation of human mesenchymal stromal cells in silk fibroin scaffolds: A combined experimental and computational approach

Mechanical loading plays a major role in bone remodeling and fracture healing. Mimicking the concept of mechanical loading of bone has been widely studied in bone tissue engineering by perfusion cultures. Nevertheless, there is still debate regarding the in-vitro mechanical stimulation regime. This study aims at investigating the effect of two different flow rates (v_low = 0.001m/s and v_high = 0.061m/s) on the growth of mineralized tissue produced by human mesenchymal stromal cells cultured on 3-D silk fibroin scaffolds. The flow rates applied were chosen to mimic the mechanical environment during early fracture healing or during bone remodeling, respectively. Scaffolds cultured under static conditions served as a control. Time-lapsed micro-computed tomography showed that mineralized extracellular matrix formation was completely inhibited at v_low compared to v_high and the static group. Biochemical assays and histology confirmed these results and showed enhanced osteogenic differentiation at v_high whereas the amount of DNA was increased at v_low. The biological response at v_low might correspond to the early stage of fracture healing, where cell proliferation and matrix production is prominent. Visual mapping of shear stresses, simulated by computational fluid dynamics, to 3-D micro-computed tomography data revealed that shear stresses up to 0.39mPa induced a higher DNA amount and shear stresses between 0.55mPa and 24mPa induced osteogenic differentiation. This study demonstrates the feasibility to drive cell behavior of human mesenchymal stromal cells by the flow velocity applied in agreement with mechanical loading mimicking early fracture healing (v_low) or bone remodeling (v_high). These results can be used in the future to tightly control the behavior of human mesenchymal stromal cells towards proliferation or differentiation. Additionally, the combination of experiment and simulation presented is a strong tool to link biological responses to mechanical stimulation and can be applied to various in-vitro cultures to improve the understanding of the cause-effect relationship of mechanical loading.

Effect of combined treatment with zoledronic acid and parathyroid hormone on mouse bone callus structure and composition

In recent years, great interest in combined treatment of parathyroid hormone (PTH) with anti-resorptive therapy has emerged. PTH has been suggested to aid bridging of atrophic fractures and improve strength in closed fracture models. Bisphosphonate treatments typically result in a larger woven bone callus that is slower to remodel. The combination of both drugs has been demonstrated to be effective for the treatment of osteoporotic bone loss in many preclinical studies. However, the effect of combined treatment on fracture repair is still largely unexplored. In this study, we aimed to compare these drugs as single-agent and in combination in a murine closed fracture model. We wanted to assess potential differences in material properties, morphometry and in the development of the lacuno-canalicular network. A total of 40 female, 11-week-old wild type mice underwent a closed fracture on the midshaft of the tibia and were assigned to four groups (n=8-10 per group). Beginning on post-operative day 8, animals received different subcutaneous injections. Group 1 received a single injection of saline solution and Group 2 of zoledronic acid (ZA). Group 3 received daily dosing of PTH. Group 4 received a dual treatment, starting with a single dose of ZA followed by daily injection of PTH. Three weeks after fracture, all animals were euthanized and tibiae were assessed using micro-computed tomography (micro-CT), high-resolution micro-CT (HR micro-CT), Raman spectroscopy, quantitative histomorphometry, and deconvolution microscopy (DV microscopy). Combined treatment showed a significant increase of 41% in bone volume fraction and a significant decrease of 61% in the standard deviation of the trabecular spacing compared to vehicle, both known to be strong predictors of callus strength. An analysis via HR micro-CT showed similar results on all groups for lacunar numerical density, whereas mean lacuna volume was found to be higher compared to vehicle in treated groups, but only PTH mono-treatment showed a significant increase compared to vehicle (+45%). Raman spectroscopy did not reveal detectable changes in material properties of the bone calluses. Sclerostin staining, tartrate resistant acid phosphatase (TRAP) staining and canalicular analysis with DV microscopy on a subset of samples did not display distinctive difference in any of the treatments. We therefore consider PTH+ZA treatment beneficial for bone healing. No clear negative effect on bone quality was detected during this study.

PTH(1-34) effects on repairing experimentally drilled holes in rat femur: novel aspects - qualitative vs. quantitative improvement of osteogenesis

The timetable of effects on bone repair of the active fraction-parathyroid hormone, PTH(1-34), was analytically investigated from the morphometric viewpoint in 3-month-old male Sprague-Dawley rats, whose femurs were drilled at mid-diaphyseal level (transcortical holes). The animals were divided into groups with/without PTH(1-34) administration, and sacrificed at different times (10, 28, 45 days after surgery). The observations reported here need to be framed in the context of our previous investigations regarding bone histogenesis (Ferretti et al. Anat Embryol. 2002; 206: 21-29) in which we demonstrated the occurrence of two successive bone-forming processes during both skeletal organogenesis and bone repair, i.e. static and dynamic osteogenesis: the former (due to stationary osteoblasts, haphazardly grouped in cords) producing preliminary bad quality trabecular bone, the latter (due to typical polarized osteoblasts organized in ordered movable laminae) producing mechanically valid bone tissue. The primary function of static osteogenesis is to provide a rigid scaffold containing osteocytes (i.e. mechano-sensors) for osteoblast laminae acting in dynamic osteogenesis. In the present work, histomorphometric analysis revealed that, already 10 days after drilling, despite the holes being temporarily filled by the same amount of newly formed trabecular bone by static osteogenesis independently of the treatment, the extent of the surface of movable osteoblast-laminae (covering the trabecular surface) was statistically higher in animals submitted to PTH(1-34) administration than in control ones; this datum strongly suggests the effect of PTH(1-34) alone in anticipating the occurrence of dynamic osteogenesis involved in the production of good quality bone (with more ordered collagen texture) more suitable for loading. This study could be crucial in further translational clinical research in humans for defining the best therapeutic strategies to be applied in recovering severe skeletal lesions, particularly as regards the time of PTH(1-34) administration.

Overexpression of BCLXL in Osteoblasts Inhibits Osteoblast Apoptosis and Increases Bone Volume and Strength

The Bcl2 family proteins, Bcl2 and BclXL, suppress apoptosis by preventing the release of caspase activators from mitochondria through the inhibition of Bax subfamily proteins. We reported that BCL2 overexpression in osteoblasts using the 2.3 kb Col1a1 promoter increased osteoblast proliferation, failed to reduce osteoblast apoptosis, inhibited osteoblast maturation, and reduced the number of osteocyte processes, leading to massive osteocyte death. We generated BCLXL (BCL2L1) transgenic mice using the same promoter to investigate BCLXL functions in bone development and maintenance. Bone mineral density in the trabecular bone of femurs was increased, whereas that in the cortical bone was similar to that in wild-type mice. Osteocyte process formation was unaffected and bone structures were similar to those in wild-type mice. A micro-CT analysis showed that trabecular bone volume in femurs and vertebrae and the cortical thickness of femurs were increased. A dynamic bone histomorphometric analysis revealed that the mineralizing surface was larger in trabecular bone, and the bone-formation rate was increased in cortical bone. Serum osteocalcin but not TRAP5b was increased, BrdU-positive osteoblastic cell numbers were increased, TUNEL-positive osteoblastic cell numbers were reduced, and osteoblast marker gene expression was enhanced in BCLXL transgenic mice. The three-point bending test indicated that femurs were stronger in BCLXL transgenic mice than in wild-type mice. The frequency of TUNEL-positive primary osteoblasts was lower in BCLXL transgenic mice than in wild-type mice during cultivation, and osteoblast differentiation was enhanced but depended on cell density, indicating that enhanced differentiation was mainly owing to reduced apoptosis. Increased trabecular and cortical bone volumes were maintained during aging in male and female mice. These results indicate that BCLXL overexpression in osteoblasts increased the trabecular and cortical bone volumes with normal structures and maintained them majorly by preventing osteoblast apoptosis, implicating BCLXL as a therapeutic target of osteoporosis. © 2016 American Society for Bone and Mineral Research.

Novel approaches for two and three dimensional multiplexed imaging of osteocytes

Although osteocytes have historically been viewed as quiescent cells, it is now clear that they are highly active cells in bone and play key regulatory roles in diverse skeletal functions, including mechanotransduction, phosphate homeostasis and regulation of osteoblast and osteoclast activity. Three dimensional imaging of embedded osteocytes and their dendritic connections within intact bone specimens can be quite challenging and many of the currently available methods are actually imaging the lacunocanalicular network rather than the osteocytes themselves. With the explosion of interest in the field of osteocyte biology, there is an increased need for reliable ways to image these cells in live and fixed bone specimens. Here we report the development of reproducible methods for 2D and 3D imaging of osteocytes in situ using multiplexed imaging approaches in which the osteocyte cell membrane, nucleus, cytoskeleton and extracellular matrix can be imaged simultaneously in various combinations. We also present a new transgenic mouse line expressing a membrane targeted-GFP variant selectively in osteocytes as a novel tool for in situ imaging of osteocytes and their dendrites in fixed or living bone specimens. These methods have been multiplexed with a novel method for labeling of the lacunocanalicular network using fixable dextran, which enables aspects of the osteocyte cell structure and lacunocanalicular system to be simultaneously imaged. The application of these comprehensive approaches for imaging of osteocytes in situ should advance research into osteocyte biology and function in health and disease.

Core binding factor β of osteoblasts maintains cortical bone mass via stabilization of Runx2 in mice

Core binding factor beta (Cbfβ), the partner protein of Runx family transcription factors, enhances Runx function by increasing the binding of Runx to DNA. Null mutations of Cbfb result in embryonic death, which can be rescued by restoring fetal hematopoiesis but only until birth, where bone formation is still nearly absent. Here, we address a direct role of Cbfβ in skeletal homeostasis by generating osteoblast-specific Cbfβ-deficient mice (Cbfb(Δob/Δob) ) from Cbfb-floxed mice crossed with mice expressing Cre from the Col1a1 promoter. Cbfb(Δob/Δob) mice showed normal growth and development but exhibited reduced bone mass, particularly of cortical bone. The reduction of bone mass in Cbfb(Δob/Δob) mice is similar to the phenotype of mice with haploinsufficiency of Runx2. Although the number of osteoblasts remained unchanged, the number of active osteoblasts decreased in Cbfb(Δob/Δob) mice and resulted in lower mineral apposition rate. Immunohistochemical and quantitative real-time PCR analyses showed that the expression of osteogenic markers, including Runx2, osterix, osteocalcin, and osteopontin, was significantly repressed in Cbfb(Δob/Δob) mice compared with wild-type mice. Cbfβ deficiency also reduced Runx2 protein levels in osteoblasts. The mechanism was revealed by forced expression of Cbfβ, which increased Runx2 protein levels in vitro by inhibiting polyubiquitination-mediated proteosomal degradation. Collectively, these findings indicate that Cbfβ stabilizes Runx2 in osteoblasts by forming a complex and thus facilitates the proper maintenance of bone mass, particularly cortical bone.

Counter-effect of constrained dynamic loading on osteoporosis in ovariectomized mice

In recent years, dynamic mechanical loading has been shown to effectively enhance bone remodeling. The current study attempted to research the counter-effect of constrained dynamic loading on osteoporosis (OP) in ovariectomized (OVX) mice. Female Kunming (KM) mice were randomly divided into 2 groups: SHAM and OVX. The right ulnas of the OVX mice were subjected to a 4-week constrained dynamic loading protocol, and the mechanical properties, trabecular micromorphology parameters and biochemical indices of osteogenesis were evaluated. We detected higher levels of tissue alkaline phosphatase (AKP) and serum bone gamma-carboxyglutamic-acid-containing proteins (BGPs), better trabecular micromorphology parameters and ulnar mechanical properties in the loading group than in the nonloading group. In summary, constrained dynamic loading could prevent ovariectomy-induced osteoporosis by facilitating osteogenesis, improving trabecular microstructure and enhancing bone mechanical properties.

The cast imaging of the osteon lacunar-canalicular system and the implications with functional models of intracanalicular flow

A casting technique with methyl-methacrylate (MMA) was applied to the study of the osteon lacunar-canalicular network of human and rabbit cortical bone. The MMA monomer infiltration inside the vascular canals and from these into the lacunar-canalicular system was driven by capillarity, helped by evaporation and the resulting negative pressure in a system of small pipes. There was uniform, centrifugal penetration of the resin inside some osteons, but this was limited to a depth of four to five layers of lacunae. Moreover, not all of the osteon population was infiltrated. This failure can be the result of one of two factors: the incomplete removal of organic debris from the canal and canalicular systems, and lack of drainage at the osteon external border. These data suggest that each secondary osteon is a closed system with a peripheral barrier (represented by the reversal line). As the resin advances into the osteon, the air contained inside the canalicula is compressed and its pressure increases until infiltration is stopped. The casts gave a reliable visualization of the lacunar shape, position and connections between the lacunae without the need for manipulations such as cutting or sawing. Two systems of canalicula could be distinguished, the equatorial, which connected the lacunae (therefore the osteocytes) lying on the same concentric level, and the radial, which established connections between different levels. The equatorial canalicula radiated from the lacunar border forming ramifications on a planar surface around the lacuna, whereas the radial canalicula had a predominantly straight direction perpendicular to the equatorial plane. The mean length of the radial canalicula was 40.12 ± 10.26 μm in rabbits and 38.4 ± 7.35 μm in human osteons; their mean diameter was 174.4 ± 71.12 nm and 195.7 ± 79.58 nm, respectively. The mean equatorial canalicula diameter was 237 ± 66.04 nm in rabbit and 249.7 ± 73.78 nm in human bones, both significantly larger (P < 0.001) than the radial. There were no significant differences between the two species. The lacunar surface measured on the equatorial plane was higher in rabbit than in man, but the difference was not statistically significant. The cast of the lacunar-canalicular network obtained with the reported technique allows a direct, 3-D representation of the system architecture and illustrates how the connections between osteocytes are organized. The comparison with models derived by the assumption of the role of hydraulic conductance and other mechanistic functions provides descriptive, morphological data to the ongoing discussion on the Haversian system biology.

SP7 inhibits osteoblast differentiation at a late stage in mice

RUNX2 and SP7 are essential transcription factors for osteoblast differentiation at an early stage. Although RUNX2 inhibits osteoblast differentiation at a late stage, the function of SP7 at the late stage of osteoblast differentiation is not fully elucidated. Thus, we pursued the function of SP7 in osteoblast differentiation. RUNX2 induced Sp7 expression in Runx2^−/− calvarial cells. Adenoviral transfer of sh-Sp7 into primary osteoblasts reduced the expression of Alpl, Col1a1, and Bglap2 and mineralization, whereas that of Sp7 reduced Bglap2 expression and mineralization at a late stage of osteoblast differentiation. Sp7 transgenic mice under the control of 2.3 kb Col1a1 promoter showed osteopenia and woven-bone like structure in the cortical bone, which was thin and less mineralized, in a dose-dependent manner. Further, the number of processes in the osteoblasts and osteocytes was reduced. Although the osteoblast density was increased, the bone formation was reduced. The frequency of BrdU incorporation was increased in the osteoblastic cells, while the expression of Col1a1, Spp1, Ibsp, and Bglap2 was reduced. Further, the osteopenia in Sp7 or Runx2 transgenic mice was worsened in Sp7/Runx2 double transgenic mice and the expression of Col1a1 and Bglap2 was reduced. The expression of Sp7 and Runx2 was not increased in Runx2 and Sp7 transgenic mice, respectively. The expression of endogenous Sp7 was increased in Sp7 transgenic mice and Sp7-transduced cells; the introduction of Sp7 activated and sh-Sp7 inhibited Sp7 promoter; and ChIP assay showed the binding of endogenous SP7 in the proximal region of Sp7 promoter. These findings suggest that SP7 and RUNX2 inhibit osteoblast differentiation at a late stage in a manner independent of RUNX2 and SP7, respectively, and SP7 positively regulates its own promoter.

Overexpression of Bcl2 in osteoblasts inhibits osteoblast differentiation and induces osteocyte apoptosis

Bcl2 subfamily proteins, including Bcl2 and Bcl-X_L, inhibit apoptosis. As osteoblast apoptosis is in part responsible for osteoporosis in sex steroid deficiency, glucocorticoid excess, and aging, bone loss might be inhibited by the upregulation of Bcl2; however, the effects of Bcl2 overexpression on osteoblast differentiation and bone development and maintenance have not been fully investigated. To investigate these issues, we established two lines of osteoblast-specific BCL2 transgenic mice. In BCL2 transgenic mice, bone volume was increased at 6 weeks of age but not at 10 weeks of age compared with wild-type mice. The numbers of osteoblasts and osteocytes increased, but osteoid thickness and the bone formation rate were reduced in BCL2 transgenic mice with high expression at 10 weeks of age. The number of BrdU-positive cells was increased but that of TUNEL-positive cells was unaltered at 2 and 6 weeks of age. Osteoblast differentiation was inhibited, as shown by reduced Col1a1 and osteocalcin expression. Osteoblast differentiation of calvarial cells from BCL2 transgenic mice also fell in vitro. Overexpression of BCL2 in primary osteoblasts had no effect on osteoclastogenesis in co-culture with bone marrow cells. Unexpectedly, overexpression of BCL2 in osteoblasts eventually caused osteocyte apoptosis. Osteocytes, which had a reduced number of processes, gradually died with apoptotic structural alterations and the expression of apoptosis-related molecules, and dead osteocytes accumulated in cortical bone. These findings indicate that overexpression of BCL2 in osteoblasts inhibits osteoblast differentiation, reduces osteocyte processes, and causes osteocyte apoptosis.

New laboratory tools in the assessment of bone quality

Bone quality is a complex set of intricated and interdependent factors that influence bone strength. A number of methods have emerged to measure bone quality, taking into account the organic or the mineral phase of the bone matrix, in the laboratory. Bone quality is a complex set of different factors that are interdependent. The bone matrix organization can be described at five different levels of anatomical organization: nature (organic and mineral), texture (woven or lamellar), structure (osteons in the cortices and arch-like packets in trabecular bone), microarchitecture, and macroarchitecture. Any change in one of these levels can alter bone quality. An altered bone remodeling can affect bone quality by influencing one or more of these factors. We have reviewed here the main methods that can be used in the laboratory to explore bone quality on bone samples. Bone remodeling can be evaluated by histomorphometry; microarchitecture is explored in 2D on histological sections and in 3D by microCT or synchrotron. Microradiography and scanning electron microscopy in the backscattered electron mode can measure the mineral distribution; Raman and Fourier-transformed infra-red spectroscopy and imaging can simultaneously explore the organic and mineral phase of the matrix on multispectral images; scanning acoustic microscopy and nanoindentation provide biomechanical information on individual trabeculae. Finally, some histological methods (polarization, surface staining, fluorescence, osteocyte staining) may also be of interest in the understanding of quality as a component of bone fragility. A growing number of laboratory techniques are now available. Some of them have been described many years ago and can find a new youth; others having benefited from improvements in physical and computer techniques are now available.

Towards quantitative 3D imaging of the osteocyte lacuno-canalicular network

Osteocytes are the most abundant cells in bone and the only cells embedded in the bone mineral matrix. They form an extended, three-dimensional (3D) network, whose processes interconnecting the cell bodies reside in thin canals, the canaliculi. Together with the osteocyte lacunae, the canaliculi form the lacuno-canalicular network (LCN). As the negative imprint of the cellular network within bone tissue, the LCN morphology is considered to play a central role for bone mechanosensation and mechanotransduction. However, the LCN has neither been visualized nor quantified in an adequate way up to now. On this account, this article summarizes the current state of knowledge of the LCN morphology and then reviews different imaging methods regarding the quantitative 3D assessment of bone tissue in general and of the LCN in particular. These imaging methods will provide new insights in the field of bone mechanosensation and mechanotransduction and thus, into processes of strain sensation and transduction, which are tightly associated with osteocyte viability and bone quality.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

A method for observing silver-stained osteocytes in situ in 3-microm sections using ultra-high voltage electron microscopy tomography

Osteocytes are surrounded by hard bone matrix, and it has not been possible previously to directly observe the in situ architecture of osteocyte morphology in bone. Electron microscope tomography, however, is a technique that has the unique potential to provide three-dimensional (3D) visualization of cellular ultrastructure. This approach is based on reconstruction of 3D volumes from a tilt series of electron micrographs of cells, and resolution at the nanometer level has been achieved. We applied electron microscope tomography to thick sections of silver-stained osteocytes in bone using a Hitachi H-3000 ultra-high voltage electron microscope equipped with a 360 degrees tilt specimen holder, at an accelerating voltage of 2 MeV. Osteocytes with numerous processes and branches were clearly seen in the serial tilt series acquired from 3-microm-thick sections. Reconstruction of young osteocytes showed the 3D topographic morphology of the cell body and processes at high resolution. This morphological data on osteocytes should provide useful information to those who study osteocyte physiology and the several models used to explain their mechanosensory properties.

The material properties of acellular bone in a teleost fish

Several lineages of teleost fishes have independently derived skeletons composed solely of acellular bone, a tissue without obvious advantages over bone that has osteocytes in the matrix. There is no consensus for the functional role of acellular bone, as factors such as salinity, activity level and gross morphology have been shown to be poor predictors of acellularity. We used a three-point bending method to test the hypothesis that the material stiffness (elastic modulus) of acellular bone is higher than that of cellular bone, which could be evidence that material properties were a selective pressure in the evolution of this unusual skeletal material. The acellular ribs of Myoxocephalus polyacanthocephalus are curved, hollow beams that decrease in size both distally and posteriorly along the rib series. First and second moments of area decreased distally and caudally in all individuals. Young's modulus (E) ranged from 3.67 to 8.40 GPa, with a mean of 6.48 GPa. The flexural stiffness (EI) differed significantly between ribs, and the hollow cylinder morphology increased the flexural stiffness by 12.0% over a solid, circular cross-section rod with the same area. Contrary to our expectations, acellular bone is not stiffer by virtue of fewer lacunae but instead falls at the very low end of the range of stiffness seen in cellular bone. There remains the possibility that other properties(e.g. fatigue resistance, toughness) are higher in acellular bone.

Osteopontin is histochemically detected by the AgNOR acid-silver staining

Silver nitrate staining of decalcified bone sections is known to reveal osteocyte canaliculi and cement lines. Nucleolar Organising Regions (NOR) are part of the nucleolus, containing argyrophilic proteins (nucleoclin/C23, nucleophosmin/B23) that can be identified by silver staining at low pH. The aim of this study was to clarify the mechanism explaining why AgNOR staining also reveals osteocyte canaliculi. Human bone and kidney sections were processed for silver staining at light and electron microscopy with a modified method used to identify AgNOR. Sections were processed in parallel for immunohistochemistry with an antibody direct against osteopontin. Protein extraction was done in the renal cortex and decalcified bone and the proteins were separated by western blotting. Purified hOPN was also used as a control. Proteins were electro-transferred on polyvinylidene difluoride membranes and stained for AgNOR proteins. In bone, Ag staining identified AgNOR in cell nuclei, as well as in osteocyte canaliculi, cement and resting lines. In the distal convoluted tubules of the kidney, silver deposits were also observed in cytoplasmic granules on the apical side of the cells. Immunolocalization of osteopontin closely matched with all these locations in bone and kidney. Ag staining of membranes at low pH revealed bands for NOR proteins and 56 KDa (kidney), 60KDa (purified hOPN) and 75 KDa (bone) bands that corresponded to osteopontin. NOR proteins and osteopontin are proteins containing aspartic acid rich regions that can bind Ag. Staining protocols using silver nitrate at low pH can identify these proteins on histological sections or membranes.

Cyclooxygenase-2 inhibition delays the attainment of peak woven bone formation following four-point bending in the rat

Fracture healing is retarded in the presence of cyclooxygenase-2 (COX-2) inhibitors, demonstrating an important role of COX-2 in trauma-induced woven bone adaptation. The aim of this experiment was to determine the influence of COX-2 inhibition on the remodeling and consolidation of nontraumatic woven bone produced by mechanical loading. A periosteal woven bone callus was initiated in the right tibia of female Wistar rats following a single bout of four-point bending, applied as a haversine wave for 300 cycles at a frequency of 2 Hz and a magnitude of 65 N. Daily injections of vehicle (VEH, polyethylene glycol) or the COX-2 inhibitor 5,5-dimethyl-3-3(3 fluorophenyl)-4-(4-methylsulfonal)phenyl-2(5H)-furanone (DFU, 2.0 mg . kg(-1) and 0.02 mg . kg(-1) i.p.), commenced 7 days postloading, and tibiae were examined 2, 3, 4, and 5 weeks postloading. Tibiae were dissected, embedded in polymethylmethacrylate, and sectioned for histomorphometric analysis of periosteal woven bone. No significant difference in peak woven bone area was observed between DFU-treated and VEH rats. However, treatment with DFU resulted in a temporal defect in woven bone formation, where the achievement of peak woven bone area was delayed by 1 week. Woven bone remodeling was observed in DFU-treated rats at 21 days postloading, demonstrating that remodeling of the periosteal callus is not prevented in the presence of a COX-2 inhibitor in the rat. We conclude that COX-2 inhibition does not significantly disrupt the mechanism of woven bone remodeling but alters its timing.

The role of osteocytes and bone microstructure in preventing osteoporotic fractures

The skeleton alters its geometry following trauma, the introduction of artificial defects and of fatigue-induced microcracks. The precise mechanism by which the skeleton adapts remains unclear. Microcracks might directly affect the cell by damaging the osteocyte cell network or causing apoptosis. Bone microstructure may play an important role in these processes by diverting and arresting propagating microcracks and so prevent fracture failure. This paper discusses the effects of microstructure on propagating cracks, how microdamage may act as a stimulus for bone adaptation and its potential effects on bone biochemistry.

A crucial role for matrix metalloproteinase 2 in osteocytic canalicular formation and bone metabolism

Extracellular matrix production and degradation by bone cells are critical steps in bone metabolism. Mutations of the gene encoding MMP-2, an extracellular matrix-degrading enzyme, are associated with a human genetic disorder characterized by subcutaneous nodules, arthropathy, and focal osteolysis. It is not known how the loss of MMP-2 function results in the pathology. Here, we show that Mmp2(-/-) mice exhibited opposing bone phenotypes caused by an impaired osteocytic canalicular network. Mmp2(-/-) mice showed decreased bone mineral density in the limb and trunk bones but increased bone volume in the calvariae. In the long bones, there was moderate disruption of the osteocytic networks and reduced bone density throughout life, whereas osteoblast and osteoclast function was normal. In contrast, aged but not young Mmp2(-/-) mice had calvarial sclerosis with osteocyte death. Severe disruption of the osteocytic networks preceded osteocyte loss in Mmp2(-/-) calvariae. Successful transplantation of wild-type periosteum restored the osteocytic canalicular networks in the Mmp2(-/-) calvariae, suggesting local roles of MMP-2 in determining bone phenotypes. Our results indicate that MMP-2 plays a crucial role in forming and maintaining the osteocytic canalicular network, and we propose that osteocytic network formation is a determinant of bone remodeling and mineralization.

Polychrome labeling of bone with seven different fluorochromes: enhancing fluorochrome discrimination by spectral image analysis

Bone formation and remodeling in vivo can be assessed by polychrome labeling using calcium-binding fluorescent dyes. The number of fluorochromes, however, limits this technique due to the fact that with increasing number, fluorescent spectra inevitably overlap, which makes discrimination more difficult. In order to enhance discrimination, we performed spectral image analysis. Non-critical size defects of the femur of male Wistar rats served as a model for bone formation. Eight different fluorochromes (calcein blue, xylenol orange, calcein, alizarine complexone, doxycycline, rolitetracycline, hematoporphyrin, and BAPTA) were administered sequentially subcutaneously every third day starting at day 4 after surgery. Following, bone specimen were embedded in methylmethacrylate and analyzed by spectral image acquisition using a Sagnac type interferometer (ASI, Israel). Seven of the eight applied fluorochromes could be resolved using spectral image examination. With BAPTA, we present a new fluorochrome suitable for bone labeling. Due to the superior sensitivity of the spectral image acquisition, the thickness of the bone sections could be reduced so that 5 mum thick sections could be analyzed. Spectral decomposition and subsequent linear unmixing allows depiction of each individual fluorochrome without interference of any other, enabling a reliable and superior morphometric analysis of labeled regions.

Collagen fiber orientation near dental implants in human bone: Do their organization reflect differences in loading?

This study is the first to investigate the collagen fiber orientation in human bone around titanium dental implants after loading. Birefringence under circularly polarized light (CPL) and scanning electron microscopy (SEM) investigation were used to assess both collagen orientation and density. Twenty osseointegrated dental implants were retrieved from the mandible bone of 10 patients and were used in the present study. The implants were retrieved after 6 months of function. The birefringence measurements were performed on digitized images using both the peri-implant bone (PB) and the alveolar bone (AB) retrieved at the moment of implant placing. All images detected at 100x were measured using a software image analysis. The results showed that 32.96% (3.208 +/- 0.435 mm(2)) of the PB area was composed of transverse collagen fibers while only 19.70% (1.957 +/- 0.253 mm(2)) was composed of longitudinally collagen fibers. In the AB 26.99% (2.620 +/- 0.520 mm(2)) of the examined area was composed of transverse collagen fibers while 22.25% (2.160 +/- 0.320 mm(2)) was composed of longitudinally collagen fibers. The CPL measurements of the birefringence indicated that the difference in area between the two collagen fibers orientations was statistically significant (p = 0.03) for both AB and PB. In PB the transverse collagen fibers were significantly increased (p < 0.01) than in AB; also the longitudinal collagen fibers differ significantly in the two groups (p = 0.02). The transverse collagen fibers in PB are more present under the lower flank of the threads where the load acts with a compression vectors, while outside the tip of the implant threads the collagen fibers run more longitudinally due to the loads that act with a tensile vectors. SEM observations showed, in the first collagen layers facing the implant surface, a random direction of the collagen fibers, while in the areas away from the implant surface the collagen fibers were well oriented and run parallel. In conclusion, in these loaded dental implants, the bone adjacent to the first two implant threads shows a significant increase of the amount of transverse collagen fibers that were mainly associated with the lower flank of the threads where compressive load acts. The longitudinal collagen fibers also differs significantly from those found in the AB.

Effect of lacunocanalicular architecture on hydraulic conductance in bone tissue: implications for bone health and evolution

Bone tissue health depends largely on efficient fluid and solute transport between the blood supply and cells that are the living component of the tissue. We hypothesized that the lacunocanalicular hydraulic network, which is defined by the pericellular fluid space that is common to all bone tissue, is optimized to transport fluid and solutes between the blood supply and bone cells. An analytical study was carried out to evaluate the effect of osteonal architecture, including the osteon diameter, number of annular lamellar regions, and number and length of canalicular channels, on fluid transport between the blood supply and bone cells. On the basis of this analysis, we conclude that osteon size is limited to the distance over which fluid and solutes can be transported efficiently between the blood supply and cells. This analytic model suggests that hydraulic conductivity is highest in lamellar regions closest to the Haversian canal (HC) and decreases with increasing distance from the blood supply, reaching a plateau after the fifth lamella (169 micro m radius). Furthermore, an increase in the diameter of the HC, or a decrease in the length of canaliculi, reduces the hydraulic conductivity within the lacunocanalicular network. Applying the principle of minimal expenditure of energy to this analysis, the path distance comprising five or six lamellar regions represents an effective limit for fluid and solute transport between the blood supply and cells; beyond this threshold, hydraulic resistance in the network increases and additional energy expenditure is necessary for further transportation. This suggests that transport is optimized to meet metabolic demands concomitant with a minimal expenditure of energy. This fundamental new insight into bone structure and physiology may provide a new basis of understanding for tissue engineering, bone physiology in health and disease, and evolutionary biology.