Role Of Osteocytes Research Articles

A Trabecular Bone Explant Model of Osteocyte–Osteoblast Co-Culture for Bone Mechanobiology

The osteocyte network is recognized as the major mechanical sensor in the bone remodeling process, and osteocyte-osteoblast communication acts as an important mediator in the coordination of bone formation and turnover. In this study, we developed a novel 3D trabecular bone explant co-culture model that allows live osteocytes situated in their native extracellular matrix environment to be interconnected with seeded osteoblasts on the bone surface. Using a low-level medium perfusion system, the viability of in situ osteocytes in bone explants was maintained for up to 4 weeks, and functional gap junction intercellular communication (GJIC) was successfully established between osteocytes and seeded primary osteoblasts. Using this novel co-culture model, the effects of dynamic deformational loading, GJIC, and prostaglandin E(2) (PGE(2)) release on functional bone adaptation were further investigated. The results showed that dynamical deformational loading can significantly increase the PGE(2) release by bone cells, bone formation, and the apparent elastic modulus of bone explants. However, the inhibition of gap junctions or the PGE(2) pathway dramatically attenuated the effects of mechanical loading. This 3D trabecular bone explant co-culture model has great potential to fill in the critical gap in knowledge regarding the role of osteocytes as a mechano-sensor and how osteocytes transmit signals to regulate osteoblasts function and skeletal integrity as reflected in its mechanical properties.

Do osteocytes contribute to phosphate homeostasis?

Osteocytes, the terminally differentiated cell of the osteoblast lineage, account for over 90% of all bone cells. Due to their relative inaccessibility within mineralized matrix, little is known regarding their specific functions in comparison to the well studied surface bone cells, osteoblasts and osteoclasts. Furthermore, bone is often viewed as a mineral reservoir that passively releases calcium and phosphate in response to hormones secreted from remote organs. Noncollagenous matrix proteins produced in osteocytes, such as dentin matrix protein 1 (DMP1), have also been viewed as inert scaffolds for calcium-phosphate deposition. Recent discoveries of new genetic mutations in human diseases and development of genetically engineered animal models challenge these classic paradigms, suggesting that the osteocyte plays an active role in both mineralization and total systemic phosphate regulation. In this review, we will focus on roles of osteocytes in mineralization and particularly in phosphate regulation via the DMP1- FGF23 pathway.

Co–Cr–Mo alloy particles induce tumor necrosis factor alpha production in MLO-Y4 osteocytes: A role for osteocytes in particle-induced inflammation

Wear debris-induced osteolysis is purportedly the limiting problem affecting the long term results of joint arthroplasty. Pathogenic effects of wear debris in peri-implant cells such as macrophages, osteoblasts and osteoclasts have been well studied. In contrast, the effects of wear debris on osteocytes, which make up over 90% of all bone cells, remain unknown. We hypothesized that metal implant debris can induce the pro-inflammatory response in osteocytes. This study demonstrated the effects of cobalt–chromium–molybdenum alloy (Co–Cr–Mo) particles on a well-characterized MLO-Y4 osteocyte cell line. Co–Cr–Mo alloy particle treatment significantly ( p < 0.05) up-regulated tumor necrosis factor alpha (TNFα) gene expression after 3 and 6 h and TNFα protein production after 24 h, but down-regulated interleukin-6 (IL-6) gene expression after 6 h. Co–Cr–Mo alloy particle treatment also induced osteocyte apoptosis after 24 h. This apoptotic effect was partially (40%) dependent on TNFα. Therefore, our results suggest that osteocytes play a role in particle-induced inflammation and bone resorption following total joint arthroplasty by inducing pro-inflammatory cytokines and inducing osteocyte apoptosis.

The role of osteocytes in bone mechanotransduction

IntroductionBones are subjected to a variety of mechanical loads duringdaily activities. In the nineteenth century, Julius Wolffproposed that bones adapt their mass and 3D structure tothe loading conditions in order to optimize their load-bearing capacity, and that this process is driven bymechanical stress [1]. For the past centuries, an increasingnumber of theoretical and experimental results reveal thatosteocytes are the pivotal cells orchestrating this bio-mechanical regulation of bone mass and structure, whichis accomplished by the process of bone remodeling [2–5]Osteocytes are terminally differentiated cells of theosteogenic lineage that are derived from mesenchymalprecursor cells. A number of molecules have been iden-tified as important markers of osteocytes, such as matrixextracellular phosphoglycoprotein [6] sclerostin [7], dentinmatrix protein-1 [8], and phex protein [8]. The osteocytesare the most abundant cells in adult bone and are constantlyspaced throughout the mineralized matrix. Mature osteo-cytes have a characteristic dendritic cell shape, with pro-cesses radiating from the cell body through the canaliculi indifferent directions. These processes form an intercellularnetwork through gap and adherent junctions with surround-ing osteocytes, the cells lining the bone surface and bonemarrow. Through this unique 3D network, osteocytes areanatomically placed in a prime position not only to sensedeformations driven by stresses placed upon bone, butalso to respond with passage of signals to the neighboringcells [9].For more than a decade now, it is known that theosteocytes are very sensitive to stress applied to intact bonetissue [10–16]. Computer simulation models have shownthat mechanosensors lying at the surface of bone, asosteoblasts and bone lining cells do, would be less sensitiveto changes in the loading pattern than the osteocytes, lyingwithin the calcified matrix [3]. Interestingly, targetedablation of osteocytes in mice disturbs the adaptation ofbone to mechanical loading [16].Osteocytes as key players in the process of bonemechanotransductionIt is currently believed that when bones are loaded, theresulting deformation will drive the thin layer of interstitialfluid surrounding the network of osteocytes to flow fromregions under high pressure to regions under low pressure[17, 18]. This flow of fluid is sensed by the osteocyteswhich in turn produce signaling molecules that can regulatebone resorption through the osteoclasts, and bone formationthrough the osteoblasts, leading to adequate bone remodel-ing [17, 18]. This concept is known as the fluid flowhypothesis. Evidence has been increasing for the flow ofcanalicular interstitial fluid as the likely factor that informsthe osteocytes about the level of bone loading [2, 5, 17, 18].Nevertheless, Vatsa and colleagues [19, 20] proposed thatif osteocytes could sense matrix strains directly, the cellshape, cytoskeletal alignment and distribution of adhesionsites in osteocytes in situ would bear alignment to themechanical loading patterns. Indeed, it was shown that the

Osteocyte Density in the Peri‐Implant Bone of Immediately Loaded and Submerged Dental Implants

The role of osteocytes in bone structure and function remains partially unresolved. Their participation in mechanotransduction, i.e., the conversion of a physical stimulus into a cellular response, has been hypothesized. The present study was an evaluation of the osteocyte density in the peri-implant bone of immediately loaded and submerged dental implants. Fourteen male patients were included in the study; all of them were partially edentulous and needed a posterior mandibular restoration. Implants were inserted in these areas; half of the sample was loaded immediately (included in a fixed provisional prosthesis on the same day as implant surgery), whereas the other half was left to heal submerged. Fourteen implants (seven immediately loaded and seven unloaded) were retrieved with a trephine after a healing period of 8 weeks. The specimens were treated to obtain thin ground sections, and histomorphometry was used to evaluate the osteocyte index in the peri-implant bone. A higher and statistically significant number of osteocytes was found in the peri-implant bone around immediately loaded implants (P = 0.0081). A correlation between the percentage of bone-implant contact and osteocyte density was found for immediately loaded implants (P = 0.0480) but not for submerged implants (P = 0.2667). The higher number of osteocytes in the peri-implant bone around immediately loaded implants could be related to the functional adaptation required by the loading stimulus, which also explains the hypothesized involvement of the osteocytes in the maintenance of the bone matrix.

Role of osteocytes in the adaptation of bone to mechanical loading

The mechanical performance of our skeleton is secured by the constant adaptation of bone to its mechanical loading environment. Mechanical adaptation of bone is brought about by the coordinated actions of osteoclasts and osteoblasts, which are orchestrated by the most mechano-sensitive cells in bone, the osteocytes. Loading on bone generates a flow of interstitial fluid through the lacuno-canalicular network in which the osteocytes are positioned. This flow is sensed by the osteocytes, which respond by the release of signaling factors such as nitric oxide, prostaglandins and Wnts, which alter the recruitment and activity of osteoblasts and osteoclasts, thereby affecting bone mass. Thus, any factor that alters the response of osteocytes to mechanical loading potentially affects bone mass. This paradigm could have implications for the field of rheumatology, where proinflammatory cytokines might affect bone mass by altering the response of osteocytes to mechanical loading.

The role of osteocytes in bone metabolism

The role of osteocytes in bone metabolism

Differentiation of Osteoblasts and Osteocytes from Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are multipotent cells that arise from the mesenchyme during development. They reside in the bone marrow close to hematopoietic stem cell niches allowing them to maintain bone marrow homeostasis and to regulate the maturation of both hematopoietic and non-hematopoietic cells. MSCs possess an extensive potential to proliferate and differentiate e.g. into osteoblasts, osteocytes, adipocytes and chondrocytes. Nevertheless, there still are some open questions about the complex process of MSC differentiation involving different transcription factors and signaling pathways, which will be discussed in this review. We also shortly introduce the characteristics and function of bone-forming osteoblasts and their role in angiogenesis. MSCs are of interest in clinical applications, since they can be easily isolated from bone marrow aspirates and expanded in vitro. When the source of osteoprogenitors is compromised, cell-based therapies could provide a novel way to repair bone defects. Indeed, there is an increasing interest in the use of MSCs and more differentiated cells in clinical applications for bone repair, which will be introduced in this review. A major section of the review is dedicated to the functions of osteocytes in the regulation of bone remodeling. Finally, we present an original hypothesis about the possible role of osteocytes in future bone tissue engineering.

Osteocytes: Mechanosensors of Bone and Orchestrators of Mechanical Adaptation

Significant progress has been made in the field of mechanotransduction in bone cells. The knowledge about the role of osteocytes as the professional mechanosensor cells of bone as well as the lacuno-canalicular porosity as the structure that mediates mechanosensing is increasing. New insights might result in a paradigm for understanding the bone formation response to mechanical loading, and the bone resorption response to disuse. Under physiological loading conditions the strain-derived flow of interstitial fluid through the lacuno-canalicular porosity seems to mechanically activate the osteocytes, which subsequently alter the bone remodeling activity of osteoblasts and/or osteoclasts. Fatigue loading results in local microdamage, disruption of normal flow patterns, and osteocyte apoptosis. Apoptotic osteocytes likely attract osteoclasts to resorb the damaged bone. This concept allows explanation of local bone gain and loss, as well as remodeling in response to fatigue damage, as processes supervised by mechanosensitive osteocytes. Uncovering the cellular and mechanical basis of the osteocyte’s response to loading would greatly contribute to our understanding of the cellular basis for bone remodeling, and could contribute to the discovery of new treatment modalities for bone mass disorders, such as osteoporosis.

Maintaining the life-long vitality and integrity of skeletal tissue

Although decreased bone strength and a consequent increase in fragility fractures has been regarded as an inevitable consequence of aging, there is great variability in this process. Moreover there are new findings concerning the mechanisms by which the skeleton maintains it vitality and integrity that should change this view. There are four basic mechanisms that can lead to decreased bone mass and strength and an increase in fragility fractures. 1) Genetic and lifestyle factors that impair the ability to achieve optimal peak bone mass and strength during childhood and adolescence. 2) Loss of bone mass and microarchitectural deterioration due to increased bone resorption, for which estrogen deficiency is probably the most important cause. 3) Inadequate bone formation during remodeling, which appears to be age related but for which the specific causes are not known. 4) Increased propensity to fall and decreased mobility that may be associated both with age related impairment in neuromuscular function and the use of drugs that impair balance. Substantial advances have been made in our understanding of the bone remodeling cycle, particularly the interaction of osteoblasts and osteoclasts that initiates resorption. Inflammatory cytokines may be involved in this process, but their specific role in human osteoporosis has not been resolved. New information on the roles of osteocytes and the Wnt signaling pathway in the regulation of bone formation indicate that these may be critical in the regulation of skeleton vitality and integrity. Hence new interventions that affect regulators of bone growth and remodeling may be developed that can maintain skeletal tissue and prevent the development of osteoporosis and fragility fractures.

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.

The role of osteocytes in functional bone adaptation

The role of osteocytes in functional bone adaptation

The role of osteocytes in bone mechanotransduction

Purpose of review: Significant progress has been made in the field of mechanotransduction in bone cells. The increasing knowledge about the role of the osteocyte as the professional mechanosensor cell of bone, and the lacuno-canalicular porosity as the structure that mediates mechanosensing, might result in a new paradigm for understanding the bone formation response to mechanical loading, and the bone resorption response to disuse. Strain-derived flow of interstitial fluid through the lacuno-canalicular porosity seems to mechanically activate the osteocytes, as well as ensures transport of cell signaling molecules, nutrients, and waste products. This concept allows explanation of local bone gain and loss, as well as remodeling in response to fatigue damage, as processes supervised by mechanosensitive osteocytes. Recent findings: The osteogenic activity of bone cells in vitro have been quantitatively correlated with varying stress stimulations highlighting the importance of the rate of loading. Theoretically, a possible mechanism for the stress response by osteocytes is due to strain-amplification at the pericellular matrix. Hence, single cell studies on molecular responses of osteocytes are on their way. These findings contribute insight on local architectural alignment in bone during remodeling. Alignment seems to occur as a result of the osteocyte's sensing different canalicular flow patterns around cutting cone and reversal zone during loading, thus determining the bone's structure. Summary: Uncovering the cellular and mechanical basis of the osteocyte's response to loading represents a significant challenge to our understanding of cellular mechanotransduction and bone remodeling. © 2005 Lippincott Williams & Wilkins.

Bone Mechanics Handbook, 2nd Edition. -

Bone Mechanics Handbook, 2nd Edition. -

Osteocyte-Specific Monoclonal Antibody MAb OB7.3 Is Directed Against Phex Protein

Osteocytes are the most abundant cells in bone; however, relatively little is known about their properties and functions. The development of monoclonal antibody MAb OB7.3 directed against chicken osteocytes enabled us to purify osteocytes from enzymatically isolated bone cells. Cultures of purified osteocytes were used to gain better insight into the role of osteocytes in bone metabolism. Until now, the antigen of MAb OB7.3 has not been elucidated. In this study, we examined the antigen to which this osteocyte-specific antibody is directed. Immunoprecipitation and purification of the protein, followed by amino acid sequence analysis of two isolated peptides, revealed that the antigen has high homology to human and murine PHEX/Phex protein sequences (PHosphate-regulating gene with homology to Endopeptidases on the X chromosome). The OB7.3 antigen was therefore identified as chicken Phex protein. In addition, using suppression subtractive hybridization, we obtained a complementary DNA (cDNA) sequence of 502 base pairs (bp) with high homology to the human and murine PHEX/Phex genes. This method was applied to identify genes, which are differentially expressed in osteocytes compared with osteoblasts. The results also suggest that Phex is expressed at higher levels in chicken osteocytes compared with osteoblasts. Reverse-transcription polymerase chain reaction (RT-PCR) and Northern blot analyses supported these findings. The function of Phex is not completely understood. However, it is known that the gene is preferentially expressed in bone and that mutations in PHEX/Phex lead to X-linked hypophosphatemia and bone mineralization abnormalities. Our findings suggest that osteocytes play an important role in the Phex-regulated phosphate handling in the kidney and in bone.

Osteocytes and bone lining cells: Which are the best candidates for mechano-sensors in cancellous bone?

Previously, we have investigated the possible role of osteocytes as mechano-sensors, and mediators of bone turnover. It was found that the proposed regulatory mechanism produced morphologies of trabecular bone, under particular loading conditions, which were consistent with morphogenesis and adaptation as seen in reality. The main objective of this study was to descern whether lining cells or osteoblasts could possibly play a similar role as effectively with regard to their capacity for self-optimization of the trabecular architecture, in terms of a low apparent mass to stiffness ratio. For that purpose the earlier analyses with osteocytes as mechano-sensors, distributed throughout the bone, were repeated for mechano-sensors located at bone surfaces only. Compared to the osteocyte model, the surface cell remodelling algorithm was reluctant to change its architecture, which implies that it is less sensitive to changes in the loading pattern. This resulted in less efficient bone adaptation, which was reflected by a considerably higher relative mass for a similar apparent stiffness in the loading direction. In other words, more mass is needed to obtain an equally stiff structure, at the apparent level, with respect to the externally applied loads. Furthermore, stresses and strains at the tissue level vary across a much wider range, relative to the osteocyte model, where the higher incidence of elevated strains indicates an increased failure risk. Therefore, we conclude that mechanical information at the bone surface may not be sufficient to adequately regulate functional bone adaptation.

The role of osteocytes in bone resorption during lactation: Morphometric observations

It has been suggested that osteocytes may resorb bone during the calcium mobilization that occurs during lactation. To test this hypothesis morphometric methods were used to evaluate changes in mature nonmated female rats, 6-day postpartum nonlactating rats, 21-day postpartum nonlactating rats, 6-day lactating rats, 15-day lactating rats, and 21-day lactating rats using femur cross sections in the diaphyseal region. Osteoclast resorption per unit length of periosteal surface, as well as other measures of resorption activity, demonstrated that by 15 days of lactation a significant mobilization of calcium was occurring. The volume density of osteocyte lacunae and individual lacunar volumes from serial section reconstruction showed no increase in the lactating groups. Indeed, the only significant change in lacunae volume was a decrease apparently due to pregnancy. It is concluded from this study that osteocytes do not resorb bone during lactation.