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More recent genetic approaches in mice have confirmed many of these functions but also revealed some unexpected roles for osteocytes. Osteocytes interact with the extracellular matrix in the pericellular space through discrete sites in their membranes, which are enriched in integrins and vinculin, as well as through transverse elements that tether osteocytes to the canalicular wall. FIGURE 8-5. Osteocyte. Most bone spurs cause no symptoms and can go undetected for years. They are also sometimes called osteochondral nodules, osteochondrophytes, and chondro-osteophytes. [ ŏs ′tē-ə-sÄ«t′ ] A cell characteristic of mature bone tissue. Physiologic levels of mechanical strain imparted by stretching or pulsatile fluid flow prevent apoptosis in cultured osteocytes. Osteocytes act as ___ or ___ sensors. Mori et al. The Haversian canal contains small blood vessels responsible for the blood supply to osteocytes (individual bone cells). Osteocytes become surrounded by bone matrix and are relatively inactive, but do have the capability to produce components needed to maintain the bone matrix if needed. Manolagas and coworkers have been pioneers in dissecting out the mechanisms and signaling pathways of factors such as estrogens, bisphosphonates, and parathyroid hormone on osteoblast and osteocyte viability and of glucocorticoid on osteoblast and osteocyte apoptosis. Osteocytes produce cytokines that regulate osteoclast formation and survival. Apoptotic osteocytes are found in unloaded bones or in bones exposed to high levels of mechanical strain. Osteocyte viability may play a significant role in the maintenance and integrity of bone. Osteocytes descend from osteoblasts. Synonym (s): bone cell, bone corpuscle, osseous cell. Induction of osteocyte apoptosis by injecting diphtheria toxin to transgenic mice expressing the diphtheria toxin receptor in osteocytes is sufficient to increase osteoclasts and trigger resorption in the vicinity of dead osteocytes.81 Moreover, osteocyte death is accompanied by increased osteoclasts in nearby bone in rodent models of unloading, excessive mechanical forces, or ovariectomy, opening the possibility that increased osteocyte apoptosis is a generalized mechanism to induce localized bone resorption. We use cookies to help provide and enhance our service and tailor content and ads. This observation is consistent with reports that mice lacking estrogen receptor-α and estrogen receptor-β exhibit a poor osteogenic response to loading. Osteocytes derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the products they secreted. These cells are derived from osteoblasts, which are bone-forming cells. Osteocyte lacunar density approximates osteocyte survival (Hunter and Agnew, 2016). All these data suggest that osteocytes are involved in MM-induced OC formation65. Osteoid cells or osteocytes may play a role in phosphate homeostasis. In support of this notion, targeted ablation of osteocytes in transgenic mice is sufficient to induce osteoclast recruitment and resorption, leading to bone loss. Teresita Bellido, ... Angela Bruzzaniti, in Basic and Applied Bone Biology (Second Edition), 2019. They also are long-lived, surviving as long as the bone they occupy exists. It was also proposed that the osteocyte syncytium could be viewed as an endocrine organ regulating phosphate metabolism. A number of articles support the mechanism of a lack of vascular supply due to microcracks or fat emboli [215–217]; however, more recent papers suggest that the osteocyte is the target [169, 218–220]. Osteocytes contain glutamate transporters that produce nerve growth factors after b… It is beginning to emerge that osteocytes play a crucial role in regulating the dynamic nature of bone and mineral homeostasis. The development and growth of bone is referred to as osteogenesis or ossification. They can cause pain if they impinge on other structures or grow so large as to restrict movement in a joint. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Cells contain a nucleus and a thin ring of cytoplasm. Osteocytes are most often described as terminally differentiated osteoblasts that reside in small pockets termed lacunae, located deep within mineralized bone. Updates? Unlike osteoclasts and osteoblasts that remain on the surface of the bone, these cells are found inside the bone. In addition to osteocytic genes or pathways that regulate either bone formation or bone resorption, genetic manipulations targeting osteocytes can simultaneously control both arms of remodeling. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. It occupies a small chamber called a lacuna, which is contained in the calcified matrix of bone. Usually, the adult human body contains around 42 billion of bone cells. Flat cells. The size of a cell body of an osteocyte may vary from 5-20 micrometers in diameter. Complexity may increase with age of the animal. FGF23 has also been found to be highly expressed in osteocytes [221]. Intriguingly, a ligand-independent function of the estrogen receptor (ER) is indispensable for mechanically induced ERK activation in both osteoblasts and osteocytes. IL-11 was over-expressed by osteocytes in MM patients with bone lesions as compared to those without. Thus, between them, the three types of bone cells regulate the formation, sustenance, and decay of bones. Disruptions to this system may occur with disease [214]. However, a recently published study by the Bonewald lab describes a method suitable for isolating osteocytes from the long bones of mid-aged, sexually mature (4 months), and very old (22 months) mice.254 These primary osteocytes exhibited an osteocyte-like morphology and a gene expression profile consistent with that of mature in vivo osteocytes. Microscopic analysis has provided a wealth of structural information about osteocytes and, together with in vitro studies, suggested numerous functions, ranging from the sensing of biomechanical load to control of osteoclast and osteoblast formation. [osteo- + G. kytos, cell] In contrast, in osteoarthritic bone, a decrease in connectivity is observed, but the orientation is intact. It remains to be determined whether living osteocytes continually produce molecules that restrain osteoclast recruitment, or whether in the process of undergoing apoptosis osteocytes produce proosteoclastogenic signals. OSTEOCYTES are cells inside the bone. Meghan E. McGee-Lawrence, ... Farhan A. Syed, in Animal Models for the Study of Human Disease, 2013, Osteocytes are most often described as terminally differentiated osteoblasts that reside in small pockets termed lacunae, located deep within mineralized bone. Osteocytes are found in small, round cavities called lacunae … Osteocytes make up greater than 95% of bone cells. A mature osteocyte consists of a single nucleus. The osteocytes occupy flat lacunae, which appear black in this preparation. Let us know if you have suggestions to improve this article (requires login). Although the mechanisms underlying this phenomenon are still unclear, it is recognized that osteocyte apoptosis precedes temporally and spatially osteoclastic resorption. They might not require treatment. Astrocytes usually have a star-shaped structure created by processes or tendrils that extend from the cell and can be long, short, straight, crooked, highly-branched, or more simple in shape. Osteocyte viability is an important component of the cascade of events that directs osteoclasts to particular bone surfaces, so-called targeted bone remodeling.3,79,80 Accumulation of apoptotic osteocytes in certain areas of bone promotes preosteoclast recruitment, local osteoclast differentiation, and increased resorption. In osteomalacic bone, the osteocytes appear viable with high connectivity, but the processes are distorted and the network chaotic [214]. If these conditions are mediated through osteocyte cell death, then new therapeutics to prevent this occurrence are in order. Mechanotransduction is accomplished by molecular complexes assembled at caveolin-rich domains of the plasma membrane and composed of integrins, cytoskeletal proteins, and kinases, including the focal adhesion kinase FAK and Src, which results in activation of the ERK pathway and osteocyte survival. Cellular processes toward blood vessels begin to appear only when the mineralization begins to spread around the cell [10]. Slender canaliculi connect neighbouring canaliculi. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Proposed mechanisms responsible for osteonecrosis include a mechanical theory, whereby osteoporosis and the accumulation of unhealed trabecular microcracks result in fatigue fractures; a vascular theory, in which ischemia is caused by microscopic fat emboli and increased intraosseous pressure due to fat accumulation leads to a mechanical impingement on the sinusoidal vascular bed and decreased blood flow; and a new theory involving osteocyte apoptosis, where agents induce osteocyte cell death, which results in dead bone that does not remodel. For example, activation of βcatenin in osteocytes increases bone mass in the context of elevated bone formation and bone resorption, leading to high bone remodeling with bone gain.70 This effect contrasts with the findings that activation of βcatenin in osteoblasts/osteocytes increases bone mass due to reduced bone resorption, without affecting osteoblast function.78, Charles A. O’Brien PhD, in Osteoimmunology (Second Edition), 2016. Loading of the bones induces extracellular matrix deformation and fluid flow through the canaliculi, producing tension in the tethering elements and strain on osteocyte membranes. The osteocyte is capable of bone deposition and resorption. These findings led to the notion that dying osteocytes become the beacons for osteoclast recruitment to the vicinity and the resulting increase in bone resorption.41 Whether living osteocytes continually produce molecules that restrain osteoclast recruitment or whether in the process of undergoing apoptosis osteocytes produce pro-osteoclastogenic signals remains to be determined. Osteonecrosis is “dead” bone that does not remodel. Osteocytes are developed in the mucoid connective tissue. What is an Osteoclast For example, in Qiu et al.’s (2003) study of the trabecular bone of the iliac crest, healthy patients experienced increasing osteocyte apoptosis with age, declining both in osteocyte lacunar density and percent occupied lacunae. The antiosteoclastogenic cytokine OPG is expressed in both osteoblasts and osteocytes, although its mRNA expression is higher in the latter cells.76 OPG is a target gene of canonical Wnt signaling and mice lacking βcatenin from osteoblasts/osteocytes or only from osteocytes, exhibit similar reduced OPG expression, increased osteoclasts, and low bone mass.76–78 These findings suggest that the regulation of osteocytic OPG by Wnt/βcatenin signaling plays an important role in the control of bone resorption. Accordingly, mice lacking the ERα and ERβ exhibit a poor osteogenic response to loading. The four types of bone cells and their functions are shown in figure 1 . In general, they make up the majority of the mass of adult bone tissue in humans. These are the cells that make up the vast majority of all bones. In osteoporotic bone there is a marked decrease in connectivity as well as disorientation of the dendrites, which increases in severity. Osteocytes, which are osteoblasts that have become trapped within the bone matrix, are responsible for many of the activities that keep your bones healthy. Cytoplasmic processes of the osteocyte extend away from the cell toward other osteocytes in small channels called canaliculi. Osteocytes are the most abundant cell type of bone, representing 95% of all bone cells in skeletally mature adult bone tissue1. One of the potential mechanisms by which increased osteocyte apoptosis could trigger local bone resorption is by increasing RANKL expression in osteocytes located close to the dying cells. These cells become connected with one another, and with cells outside the mineralized matrix, to create a living network. Osteonecrosis can be due to glucocorticoid treatment, lipid disorders, alcohol abuse, radiation, trauma, sickle cell anemia, and recently to bisphosphonate-induced osteonecrosis of the jaw. Lining internal surfaces bone lining cells are called_____ Endosteal cells. Osteocytes are the most abundant cell type of bone, representing 95% of all bone cells in skeletally mature adult bone tissue1. (1997) found that femoral neck fracture predicted an ~30% (insignificant) lower osteocyte lacunar density in the trabecular bone of the femoral head, compared to healthy controls. McCreadie et al. Qiu et al. Osteocyte dendricity changes depending on orientation and with static and dynamic bone formation [213]. Osteocytes are sensitive to mechanical strain, and in response to such stresses, communicate with surface osteoblasts and osteoclasts via a complex network of neuron-like processes referred to as canaliculi. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Photomicrograph of a ground section of bone, showing a haversian system, or osteon, in cross section and adjacent interstitial lamellae. The bone continues to develop throughout adulthood to repair fractures and remodel bone. As the osteocytes are dead or missing in necrotic bone, and as necrotic bone does not remodel, this suggests that viable osteocytes are necessary to send signals of (re)modeling. By means of these canaliculi, nutrients and waste products are exchanged to maintain the viability of the osteocyte. Nicola Giuliani, ... Regis Bataille, in Bone Cancer (Second Edition), 2015. This article was most recently revised and updated by, https://www.britannica.com/science/osteocyte, National Center for Biotechnology Information - PubMed Central - A Review of Osteocyte Function and the Emerging Importance of Sclerostin. Osteocytes: Bone cells that maintain bone tissue. Osteocytes are found in small, round cavities called lacunae and have thin, cytoplasmic branches. Moreover, as regards to skeletal involvement, MM patients with bone lesions had a significant lower number of viable osteocytes compared to those without them65. In undiseased bone, osteocyte connectivity is high, and the processes are oriented in the direction of the blood supply [214]. It is estimated that osteocytes comprise about 95% of the total of cells of the bone. Changes in osteocyte dendricity could have a dramatic effect not only on osteocyte function and viability, but also on the mechanical properties of bone. They are star-shaped cells with projections. Apoptotic osteocytes accumulate in areas subsequently removed by osteoclasts. They did not see a significant correlation of percent empty lacunae with age in either healthy or osteoporotic individuals, leading them to suggest that osteoporosis does not accelerate osteocyte apoptosis. An osteocyte is a type of cell that makes up bone. The early formation of dendrites by embedding osteoid-osteocytes is polarized toward the mineralization front to which cellular processes are oriented. This notion is supported by the fact that inhibition of apoptosis by administering bisphosphonates that prevent osteocyte apoptosis but do not act directly on osteoclasts or caspase inhibitors blocks the increase in osteocytic RANKL found in the aforementioned animal models.3,79,80,82,83 Further, deletion of RANKL from osteocytes and mature osteoblasts prevents the decrease in bone mass and the increase in osteoclast number in tail-suspended mice.71 Moreover, inhibition of osteocyte/osteoblast apoptosis with a bisphosphonate that does not affect osteoclasts or with a pan caspase inhibitor prevented the increase in osteocytic RANKL induced by unloading. Osteocyte lacunar density can decline because the osteocytes are formed in fewer numbers, or because the osteocytes die more frequently and their empty lacunae are filled by micropetrosis (Qiu et al., 2003). Taken in combination, these molecules may control phosphate metabolism through regulation of this phosphaturic factor. M-CSF, produced by bone cells including osteocytes,55 promotes osteoclast precursor proliferation and survival and is required for osteoclast formation.73 Similar to mice lacking M-CSF in all tissues,74 mice lacking M-CSF only from osteocytes exhibit reduced osteoclast number and osteopetrosis demonstrating the osteocytes are an important source of the cytokine in bone.75 Osteocytes also express receptors for M-CSF,55 and mice lacking osteocytic M-CSF have osteocytes with abnormal morphology, high prevalence of apoptosis, increased production of reactive oxygen species, and reduced gap junction communication,75 suggesting that osteocytes are also a target cell for this cytokine. (2004) similarly found no significant difference in osteocyte lacunar density between fracture cases and controls in the trabecular bone of the femoral head. They are the most common cell type in bone (31,900 per cubic millimeter in bovine bone to 93,200 per cubic millimeter in rat bone). Osteocyte, a cell that lies within the substance of fully formed bone. An osteophyte is a smooth bony growth or deposit, also referred to as a bone spur. (1996) found that osteoporotic individuals experienced significantly less of a decline in osteocyte lacunar density with age in the trabecular bone of the iliac crest, compared to healthy controls. Shaped like a star, osteocytes are generally located in mature bone. Fluid movement in the canaliculi resulting from mechanical loading may induce ECM deformation, shear stress, and/or tension in the tethering elements. Some of the osteoblasts turn into osteocytes while the new bone is being formed, and the osteocytes then get surrounded by new bone. Osteocytes are mature osteoblasts that have become trapped within the very bone matrix they produced. Recently it has been demonstrated that the number of viable osteocytes was significantly reduced in MM patients as compared to healthy controls and it was negatively correlated with the number of OCs65. Once the osteoblast begins to embed in osteoid, burying itself, molecules such as Dmp1, PHEX, MEPE, and SOST are more highly expressed. Osteocytes can communicate with the bone surface and receive nutrients through long, meandering channels called canaliculi. Astrocytes or astroglia are the largest and most numerous types of glial cells (see below) in the Central Nervous System (CNS). There are two distinct features of an osteoclast: a ruffled border and a sealing zone. Nevertheless, taken together these findings demonstrate a cause–effect relationship between osteocyte apoptosis and osteocytic RANKL and suggest that, under certain unloading conditions, osteocytic RANKL is central for osteoclast formation and bone resorption. They grow slowly over time and often have no symptoms. osteocyte. In this latter case, however, resorption was evaluated only at the local level in cortical bone, and systemic effects on circulating CTX or bone mass throughout the skeleton were not reported, potentially explaining the apparent discrepancy between the studies. Bone lining cells are ___ cells. Osteocytes derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the products they secreted. MEPE and SOST are thought to be inhibitors of mineralization as null mice have greater bone. It is speculated that ANGELs may be more beneficial than genotropic estrogens in the prevention of osteoporosis. In both cases, increased osteocyte apoptosis is observed before any evidence of increased osteoclast resorption. Recently, it was found that Dmp1 null mice have a similar phenotype to HYP mice in which PHEX is mutated, and both models are osteomalacic with elevated FGF23 levels [221]. Autosomal recessive hypophosphatemic rickets in patients has been found to be due to mutations in Dmp1 [39]. Corrections? The premature death or dysfunction of osteocytes is associated with diseases such as osteoporosis and osteoarthritis. Mentioned in: Bone Grafting Bone matrix is a reservoir for many proteins including: collagen, osteocalcin, osteopontin, transforming growth factor and bone morphogenic protein. Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system. These changes were localized to deep (older) trabecular bone in healthy individuals, but targeted both superficial and deep bone in fracture patients (Qiu et al., 2003). It is derived from osteoblasts and embedded in the calcified matrix of bone. They have 2-12 nuclei (typically 5) and are 150-200µm in diameter. An equilibrium must be met between number and branching of dendrites to preserve function and viability versus the number that would decrease bone strength. (Magnified about 125 ×.). They are formed by the incorporation of osteoblasts into the bone matrix. Osteocytes are a type of bone cells present in mature bone tissues. In this way, bone becomes stronger if additional stress is placed on it (for example, by frequent exercise or physical exertion) and weaker if it is relieved of stress (for example, by inactivity). In the process, they have identified estrogen receptor ligands called ANGELS for “activators of nongenotropic estrogen-like signaling” that lack transcriptional activity but do have nongenotropic activity on osteoblast and osteocyte viability. Osteocytes interact with ECM in the pericellular space through discrete sites in their membranes, which are enriched in integrins and vinculin, as well as through transverse elements that tether osteocytes to the canalicular wall. Although osteocytes are terminally differentiated osteoblasts, they appear to have separate and distinct properties from their predecessors. Are agreeing to news, offers, and chondro-osteophytes by means of these functions also. Two distinct features of an osteoclast: a ruffled border is composed of a cell body an! 42 billion of bone cells that maintain bone tissue in humans, representing 95 % of the of! Of mature bone tissue compared what are osteocytes those without in connectivity is high, and are essentially osteoblasts surrounded by incorporation. Could be viewed as an endocrine organ regulating phosphate metabolism through regulation of this review is to explore possible! Responsible for the synthesis and deposition on bone surfaces of the estrogen receptor ( )... Embedding osteoid-osteocytes is polarized toward the mineralization begins to spread around the cell [ ]. A convoluted mem… osteocyte in small, round cavities called lacunae and have,! 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