What do bone cells do

Charles C. Thomas, Springfield, MA. Parfitt AM Osteonal and hemi-osteonal remodeling: the spatial and temporal framework for signal traffic in adult human bone.

J Cell Biochem 55 : — Cloning in vitro and retransplantation in vivo. Transplantation 17 : — Owen M Lineage of osteogenic cells and their relationship to the stromal system. Elsevier, Amsterdam, vol 3 : 1 — Triffitt JT The stem cell of the osteoblast.

Roodman GD Advances in bone biology: the osteoclast. Endocr Rev 17 : — Endocr Rev 13 : 66 — Science : — Clin Orthop : 81 — Manolagas SC , Jilka RL Bone marrow, cytokines, and bone remodeling—emerging insights into the pathophysiology of osteoporosis. J Bone Miner Res 15 : — Cell 89 : — Biochem Biophys Res Commun : — Mol Endocrinol 11 : — Biochemistry 37 : — Dev Biol : — Canalis E Skeletal Growth Factors.

Cell 86 : — J Neurosci 15 : — Nature : — J Exp Med : — Ann NY Acad Sci : — J Clin Invest 89 : — Franchimont N , Canalis E Platelet-derived growth factor stimulates the synthesis of interleukin-6 in cells of the osteoblast lineage.

Endocrinology : — Franchimont N , Durant D , Rydziel S , Canalis E Platelet-derived growth factor induces interleukin-6 transcription in osteoblasts through the activator protein-1 complex and activating transcription factor J Biol Chem : — Cell 74 : — Science : 92 — Blood 82 : — Science : 88 — EMBO J 13 : — J Clin Invest 97 : — Proc Assoc Am Physicians : — Mol Cell Biol 15 : — Development : — Bone Miner 16 : — J Bone Miner Res 10 : 59 — Trends Endocrinol Metab 9 : 6 — J Clin Invest 93 : — J Clin Invest 96 : — J Clin Invest 95 : — J Clin Endocrinol Metab 82 : 78 — Mol Cell Biol 10 : — J Bone Miner Res 9 : — Rodan GA Osteopontin overview.

Ann NY Acad Sci : 1 — 5. Yamate T , Mocharla H , Taguchi Y , Igietseme JU , Manolagas SC , Abe E Osteopontin expression by osteoclast and osteoblast progenitors in the murine bone marrow: demonstration of its requirement for osteoclastogenesis and its increase after ovariectomy. Zhao W , Krane S Inability of collagenase to cleave type I collagen in vivo is associated with osteocyte apoptosis. Bone S Abstract. J Cell Sci : — Ciba Found Symp : 42 — Westen H , Bainton DF Association of alkaline-phosphatase-positive reticulum cell in bone marrow with granulocyte precursors.

Weiss L Bone marrow. Urban and Schwarzenberg , Baltimore , pp — Br J Haematol 68 : — Rouleau MF , Mitchell J , Goltzman D In vivo distribution of parathyroid hormone receptors in bone: evidence that a predominant osseous target cell is not the mature osteoblast. Bone 14 : — J Cell Sci 99 : — J Bone Miner Res 13 : 96 — Genes Dev 9 : — Cell 87 : — Mol Cell Biol 16 : — EMBO J 10 : — Genes Dev 8 : — Cell 79 : — J Cell Biochem 74 : — J Cell Biol : — Ott SM Theoretical and methodological approach.

Elsevier , New York , pp — Mol Endocrinol 5 : — Rodan GA Coupling of bone resorption and formation during bone remodeling. J Cell Physiol : — Endocr Rev 20 : — Cell 93 : — Genes Dev 12 : — J Leukoc Biol 65 : — Manolagas SC Cell number vs. J Bone Miner Res 14 : — Crit Rev Eukaryot Gene Expr 8 : 1 — Academic Press, New York, pp 95 — Boskey AL Biomineralization: conflicts, challenges, and opportunities. J Cell Biochem Suppl 30— 83 — Boskey AL Matrix proteins and mineralization: an overview.

Connect Tissue Res 35 : — Whyte MP Hypophosphatasia and the role of alkaline phosphatase in skeletal mineralization. Endocr Rev 15 : — Ital J Miner Electro Metab 4 : 93 — Marotti G The structure of bone tissues and the cellular control of their deposition.

Ital J Anat Embryol : 25 — Weinstein RS , Jilka RL , Parfitt AM , Manolagas SC Inhibition of osteoblastogenesis and promotion of apoptosis of osteoblasts and osteocytes by glucocorticoids: potential mechanisms of their deleterious effects on bone.

J Clin Invest : — J Clin Endocrinol Metab 82 : — J Bone Miner Res 11 : — Nat Med 2 : — Steller H Mechanisms and genes of cellular suicide. Parfitt AM Bone-forming cells in clinical conditions. In: Hall BK ed Bone. The Osteoblast and Osteocyte. Frost HM In vivo osteocyte death. J Bone Joint Surg [Am] 42 : — Bone 20 : — Mol Pathol 50 : — Quarles LD , Siddhanti SR Guanine nucleotide binding- protein coupled signaling pathway regulation of osteoblast-mediated bone formation editorial.

J Bone Miner Res 12 : — J Cell Biochem Suppl 30— 62 — Life Sci 65 : — Lajtha L Stem cell concepts. Churchill Livingston , New York , pp 1 — J Pathol : 27 — Am J Med : — Dobnig H , Turner RT The effects of programmed administration of human parathyroid hormone fragment 1—34 on bone histomorphometry and serum chemistry in rats.

Manolagas SC Cellular and molecular mechanisms of osteoporosis. Aging Clin Exp Res 10 : — Jilka RL Cytokines, bone remodeling, and estrogen deficiency: a update.

Bone 23 : 75 — Pacifici R Cytokines, estrogen, and postmenopausal osteoporosis—the second decade. McDonnell DP , Norris JD Analysis of the molecular pharmacology of estrogen receptor agonists and antagonists provides insights into the mechanism of action of estrogen in bone. Osteoporos Int 7 Suppl 1 : S29 — S J Bone Miner Res 10 : — Bismar H , Diel I , Ziegler R , Pfeilschifter J Increased cytokine secretion by human bone marrow cells after menopause or discontinuation of estrogen replacement.

J Clin Endocrinol Metab 80 : — Cheleuitte D , Mizuno S , Glowacki J In vitro secretion of cytokines by human bone marrow: effects of age and estrogen status. J Clin Endocrinol Metab 81 : — J Clin Invest 89 : 46 — Blood 78 : — J Bone Miner Res 11 : 88 — J Clin Endocrinol Metab 84 : — J Clin Invest 98 : 30 — J Clin Invest 94 : — J Clin Invest 99 : — Calcif Tissue Res 26 : 13 — J Clin Endocrinol Metab 67 : — Mueller S , Glowacki J The effect of age on the osteogenic potential of human bone marrow stromal cells.

Banks LM , Lees B , MacSweeney JE , Stevenson JC Effect of degenerative spinal and aortic calcification on bone density measurements in post-menopausal women: links between osteoporosis and cardiovascular disease? Eur J Clin Invest 24 : — Clin Rheumatol 13 : — Mamm Genome 10 : 81 — Endocrine 7 : 87 — Rajaram S , Baylink DJ , Mohan S Insulin-like growth factor-binding proteins in serum and other biological fluids: regulation and functions.

Endocr Rev 18 : — Exp Hematol 17 : 34 — Bone 19 : — Tavassoli M Fatty involution of marrow and the role of adipose tissue in hemopoiesis. Humana Press , Clifton, NJ , pp — Fitzpatrick LA Glucocorticoid-induced osteoporosis. In: Marcus R ed Osteoporosis. Blackwell Science , Boston , pp — Mankin HJ Nontraumatic necrosis of bone osteonecrosis. J Cell Biochem 67 : — Canalis E Inhibitory actions of glucocorticoids on skeletal growth. Is local insulin-like growth factor I to blame?

Endocr Rev 14 : — Br Med J : — JAMA : — Lancet 1 : — Mol Pharmacol 54 : 70 — Dobnig H , Turner RT Evidence that intermittent treatment with parathyroid hormone increases bone formation in adult rats by activation of bone lining cells. Hock JM Stemming bone loss by suppressing apoptosis. Papapoulos SE Bisphosphonates: pharmacology and use in the treatment of osteoporosis. Fleisch H Bisphosphonates: mechanisms of action. Endocr Rev 19 : 80 — J Clin Invest 92 : — Storm T , Steiniche T , Thamsborg G , Melsen F Changes in bone histomorphometry after long-term treatment with intermittent, cyclic etidronate for postmenopausal osteoporosis.

J Bone Miner Res 8 : — Rosen C , Wuster C Growth hormone, insulin-like growth factors. Lieberherr M , Grosse B , Kachkache M , Balsan S Cell signaling and estrogens in female rat osteoblasts: a possible involvement of unconventional nonnuclear receptors.

Biochem Biophys Res Commun : 99 — EMBO J 18 : — Nature : 69 — Mol Endocrinol 13 : — Biochemistry 32 : — J Steroid Biochem Mol Biol 54 : — Neuroscience 84 : 7 — J Steroid Biochem Mol Biol 63 : — The fibrillar phase occurs when the supersaturation of calcium and phosphate ions inside the matrix vesicles leads to the rupture of these structures and the hydroxyapatite crystals spread to the surrounding matrix [ 44 , 45 ]. Mature osteoblasts appear as a single layer of cuboidal cells containing abundant rough endoplasmic reticulum and large Golgi complex Figures 2 a and 3 a.

Some of these osteoblasts show cytoplasmic processes towards the bone matrix and reach the osteocyte processes [ 46 ]. At this stage, the mature osteoblasts can undergo apoptosis or become osteocytes or bone lining cells [ 47 , 48 ]. These findings suggest that besides professional phagocytes, osteoblasts are also able to engulf and degrade apoptotic bodies during alveolar bone formation [ 49 ]. Light a and b and electron micrographs of portions of alveolar bone rats.

Osteoblasts Ob and bone lining cells BLC are present on bone surface while osteocytes Ot are observed entrapped in the bone matrix. BV: blood vessels. Note the cytoplasmic processes arrows of the osteocytes Ot connecting them with each other. Note that the cytoplasmic processes arrows are observed between the osteocytes Ot forming an interconnected network. Bone lining cells are quiescent flat-shaped osteoblasts that cover the bone surfaces, where neither bone resorption nor bone formation occurs [ 50 ].

These cells exhibit a thin and flat nuclear profile; its cytoplasm extends along the bone surface and displays few cytoplasmic organelles such as profiles of rough endoplasmic reticulum and Golgi apparatus [ 50 ] Figure 2 b. Some of these cells show processes extending into canaliculi, and gap junctions are also observed between adjacent bone lining cells and between these cells and osteocytes [ 50 , 51 ]. The secretory activity of bone lining cells depends on the bone physiological status, whereby these cells can reacquire their secretory activity, enhancing their size and adopting a cuboidal appearance [ 52 ].

Bone lining cells functions are not completely understood, but it has been shown that these cells prevent the direct interaction between osteoclasts and bone matrix, when bone resorption should not occur, and also participate in osteoclast differentiation, producing osteoprotegerin OPG and the receptor activator of nuclear factor kappa-B ligand RANKL [ 14 , 53 ].

Moreover, the bone lining cells, together with other bone cells, are an important component of the BMU, an anatomical structure that is present during the bone remodeling cycle [ 9 ].

Different from osteoblasts and osteoclasts, which have been defined by their respective functions during bone formation and bone resorption, osteocytes were earlier defined by their morphology and location. For decades, due to difficulties in isolating osteocytes from bone matrix led to the erroneous notion that these cells would be passive cells, and their functions were misinterpreted [ 55 ]. The development of new technologies such as the identification of osteocyte-specific markers, new animal models, development of techniques for bone cell isolation and culture, and the establishment of phenotypically stable cell lines led to the improvement of the understanding of osteocyte biology.

In fact, it has been recognized that these cells play numerous important functions in bone [ 8 ]. The osteocytes are located within lacunae surrounded by mineralized bone matrix, wherein they show a dendritic morphology [ 15 , 55 , 56 ] Figures 3 a — 3 d. The morphology of embedded osteocytes differs depending on the bone type. For instance, osteocytes from trabecular bone are more rounded than osteocytes from cortical bone, which display an elongated morphology [ 57 ].

Osteocytes are derived from MSCs lineage through osteoblast differentiation. In this process, four recognizable stages have been proposed: osteoid-osteocyte, preosteocyte, young osteocyte, and mature osteocyte [ 54 ]. At the end of a bone formation cycle, a subpopulation of osteoblasts becomes osteocytes incorporated into the bone matrix.

This process is accompanied by conspicuous morphological and ultrastructural changes, including the reduction of the round osteoblast size. The number of organelles such as rough endoplasmic reticulum and Golgi apparatus decreases, and the nucleus-to-cytoplasm ratio increases, which correspond to a decrease in the protein synthesis and secretion [ 58 ]. The mechanisms involved in the development of osteocyte cytoplasmic processes are not well understood.

Once the stage of mature osteocyte totally entrapped within mineralized bone matrix is accomplished, several of the previously expressed osteoblast markers such as OCN, BSPII, collagen type I, and ALP are downregulated. On the other hand, osteocyte markers including dentine matrix protein 1 DMP1 and sclerostin are highly expressed [ 8 , 62 — 64 ].

Whereas the osteocyte cell body is located inside the lacuna, its cytoplasmic processes up to 50 per each cell cross tiny tunnels that originate from the lacuna space called canaliculi, forming the osteocyte lacunocanalicular system [ 65 ] Figures 3 b — 3 d.

These cytoplasmic processes are connected to other neighboring osteocytes processes by gap junctions, as well as to cytoplasmic processes of osteoblasts and bone lining cells on the bone surface, facilitating the intercellular transport of small signaling molecules such as prostaglandins and nitric oxide among these cells [ 66 ]. In addition, the osteocyte lacunocanalicular system is in close proximity to the vascular supply, whereby oxygen and nutrients achieve osteocytes [ 15 ].

It has been estimated that osteocyte surface is fold larger than that of the all Haversian and Volkmann systems and more than fold larger than the trabecular bone surface [ 67 , 68 ]. The cell-cell communication is also achieved by interstitial fluid that flows between the osteocytes processes and canaliculi [ 68 ].

By the lacunocanalicular system Figure 3 b , the osteocytes act as mechanosensors as their interconnected network has the capacity to detect mechanical pressures and loads, thereby helping the adaptation of bone to daily mechanical forces [ 55 ]. By this way, the osteocytes seem to act as orchestrators of bone remodeling, through regulation of osteoblast and osteoclast activities [ 15 , 69 ].

Moreover, osteocyte apoptosis has been recognized as a chemotactic signal to osteoclastic bone resorption [ 70 — 73 ]. In agreement, it has been shown that during bone resorption, apoptotic osteocytes are engulfed by osteoclasts [ 74 — 76 ]. The mechanosensitive function of osteocytes is accomplished due to the strategic location of these cells within bone matrix.

Thus, the shape and spatial arrangement of the osteocytes are in agreement with their sensing and signal transport functions, promoting the translation of mechanical stimuli into biochemical signals, a phenomenon that is called piezoelectric effect [ 77 ]. The mechanisms and components by which osteocytes convert mechanical stimuli to biochemical signals are not well known. However, two mechanisms have been proposed.

The second mechanism involves osteocyte cytoskeleton components, including focal adhesion protein complex and its multiple actin-associated proteins such as paxillin, vinculin, talin, and zyxin [ 79 ]. Independently of the mechanism involved, it is important to mention that the mechanosensitive function of osteocytes is possible due to the intricate canalicular network, which allows the communication among bone cells.

Osteoclasts are terminally differentiated multinucleated cells Figures 4 a — 4 d , which originate from mononuclear cells of the hematopoietic stem cell lineage, under the influence of several factors. Among these factors the macrophage colony-stimulating factor M-CSF , secreted by osteoprogenitor mesenchymal cells and osteoblasts [ 81 ], and RANK ligand, secreted by osteoblasts, osteocytes, and stromal cells, are included [ 20 ]. Together, these factors promote the activation of transcription factors [ 81 , 82 ] and gene expression in osteoclasts [ 83 , 84 ].

Light a and c and electron micrographs b and d of portions of alveolar bone of rats. In a tartrate-resistant acid phosphatase TRAP activity in red color is observed in the cytoplasm of osteoclasts OC adjacent to the alveolar bone B surface. Note that in the opposite side of the bony trabecula B is covered by large and polarized osteoblasts Ob. Ot, osteocytes Ot ; BV: blood vessel. Several vacuoles V are observed in the cytoplasm adjacent to ruffled border RB.

A round cell Ap with condensed irregular blocks of chromatin, typical apoptotic cell, is observed inside a large vacuole of the Oc 1. B: bone matrix; Ot: osteocyte. Vacuoles V with varied size are present next to the ruffled border RB ; one of them contains a round cell with masses of condensed chromatin Ap , typical of cell undergoing apoptosis.

B: bone matrix; N: nucleus. M-CSF binds to its receptor cFMS present in osteoclast precursors, which stimulates their proliferation and inhibits their apoptosis [ 82 , 85 ]. RANKL is a crucial factor for osteoclastogenesis and is expressed by osteoblasts, osteocytes, and stromal cells. When it binds to its receptor RANK in osteoclast precursors, osteoclast formation is induced [ 86 ].

Schematic summary of bone tissue showing bone cells and the relationships among them and with bone matrix B. Then, osteoclast becomes polarized through its cytoskeleton reorganization; the ruffled border RB and clear zone CZ are membrane specializations observed in the portion of the osteoclast juxtaposed to the bone resorption surface, Howship lacuna HL. After dissolution of mineral phase, osteoclast Oc releases cathepsin Cp , matrix metalloproteinase-9 MMP-9 , and tartrate-resistant acid phosphatase TRAP that degrade the organic matrix.

Sema4D produced by osteoclasts inhibits osteoblasts, while Sema3A secreted by osteoblasts inhibits osteoclasts. Osteocytes Ot are located within lacunae surrounded by mineralized bone matrix B. Its cytoplasmic processes cross canaliculi to make connection with other neighboring osteocytes processes by gap junctions, mainly composed by connexin 43 Cx3 , as well as to cytoplasmic processes of osteoblasts Ob and bone lining cells BLC on bone surface.

Conversely, osteocytes produce OPG that inhibits osteoclastogenesis; moreover, osteocytes produce sclerostin and dickkopf WNT signaling pathway inhibitor DKK-1 that decrease osteoblast activity. By interacting with the transcription factors PU. Despite these osteoclastogenic factors having been well defined, it has recently been demonstrated that the osteoclastogenic potential may differ depending on the bone site considered.

It has been reported that osteoclasts from long bone marrow are formed faster than in the jaw. This different dynamic of osteoclastogenesis possibly could be, due to the cellular composition of the bone-site specific marrow [ 93 ]. During bone remodeling osteoclasts polarize; then, four types of osteoclast membrane domains can be observed: the sealing zone and ruffled border that are in contact with the bone matrix Figures 4 b and 4 d , as well as the basolateral and functional secretory domains, which are not in contact with the bone matrix [ 94 , 95 ].

Polarization of osteoclasts during bone resorption involves rearrangement of the actin cytoskeleton, in which an F-actin ring that comprises a dense continuous zone of highly dynamic podosome is formed and consequently an area of membrane that develop into the ruffled border is isolated. Ultrastructurally, the ruffled border is a membrane domain formed by microvilli, which is isolated from the surrounded tissue by the clear zone, also known as sealing zone.

The clear zone is an area devoid of organelles located in the periphery of the osteoclast adjacent to the bone matrix [ 98 ]. The maintenance of the ruffled border is also essential for osteoclast activity; this structure is formed due to intense trafficking of lysosomal and endosomal components.

In this region, protons and enzymes, such as tartrate-resistant acid phosphatase TRAP , cathepsin K, and matrix metalloproteinase-9 MMP-9 are transported into a compartment called Howship lacuna leading to bone degradation [ 94 , — ] Figure 5. The products of this degradation are then endocytosed across the ruffled border and transcytosed to the functional secretory domain at the plasma membrane [ 7 , 95 ]. Abnormal increase in osteoclast formation and activity leads to some bone diseases such as osteoporosis, where resorption exceeds formation causing decreased bone density and increased bone fractures [ ].

In some pathologic conditions including bone metastases and inflammatory arthritis, abnormal osteoclast activation results in periarticular erosions and painful osteolytic lesions, respectively [ 83 , , ].

In periodontitis, a disease of the periodontium caused by bacterial proliferation [ , ] induces the migration of inflammatory cells. As a result, an abnormal increased bone resorption occurs in the alveolar bone, contributing to the loss of the insertions of the teeth and to the progression of periodontitis [ 89 , ]. On the other hand, in osteopetrosis, which is a rare bone disease, genetic mutations that affect formation and resorption functions in osteoclasts lead to decreased bone resorption, resulting in a disproportionate accumulation of bone mass [ 17 ].

These diseases demonstrate the importance of the normal bone remodeling process for the maintenance of bone homeostasis. Furthermore, there is evidence that osteoclasts display several other functions. For example, it has been shown that osteoclasts produce factors called clastokines that control osteoblast during the bone remodeling cycle, which will be discussed below. Other recent evidence is that osteoclasts may also directly regulate the hematopoietic stem cell niche [ ]. These findings indicate that osteoclasts are not only bone resorbing cells, but also a source of cytokines that influence the activity of other cells.

Bone is composed by inorganic salts and organic matrix [ ]. There are also small leucine-rich proteoglycans including decorin, biglycan, lumican, osteoaderin, and seric proteins [ — ]. The inorganic material of bone consists predominantly of phosphate and calcium ions; however, significant amounts of bicarbonate, sodium, potassium, citrate, magnesium, carbonate, fluorite, zinc, barium, and strontium are also present [ 1 , 2 ].

Calcium and phosphate ions nucleate to form the hydroxyapatite crystals, which are represented by the chemical formula Ca 10 PO 4 6 OH 2. Together with collagen, the noncollagenous matrix proteins form a scaffold for hydroxyapatite deposition and such association is responsible for the typical stiffness and resistance of bone tissue [ 4 ]. Bone matrix constitutes a complex and organized framework that provides mechanical support and exerts essential role in the bone homeostasis.

The bone matrix can release several molecules that interfere in the bone cells activity and, consequently, has a participation in the bone remodeling [ ]. Once loss of bone mass alone is insufficient to cause bone fractures [ ], it is suggested that other factors, including changes in the bone matrix proteins and their modifications, are of crucial importance to the understanding and prediction of bone fractures [ ].

In fact, it is known that collagen plays a critical role in the structure and function of bone tissue [ ]. Accordingly, it has been demonstrated that there is a variation in the concentration of bone matrix proteins with age, nutrition, disease, and antiosteoporotic treatments [ , , ] which may contribute to postyield deformation and fracture of bone [ ].

For instance, in vivo and in vitro studies have reported that the increase in hyaluronic acid synthesis after parathyroid hormone PTH treatment was related to a subsequent bone resorption [ — ] suggesting a possible relationship between hyaluronic acid synthesis and the increase in osteoclast activity.

As previously discussed, bone matrix does not only provides support for bone cells, but also has a key role in regulating the activity of bone cells through several adhesion molecules [ , ]. Integrins are the most common adhesion molecules involved in the interaction between bone cells and bone matrix [ ].

Osteoblasts make interactions with bone matrix by integrins, which recognize and bind to RGD and other sequences present in bone matrix proteins including osteopontin, fibronectin, collagen, osteopontin, and bone sialoprotein [ , ].

These proteins also play an important role in osteoblast organization on the bone surface during osteoid synthesis [ ]. On the other hand, the interaction between osteoclasts and bone matrix is essential for osteoclast function, since as previously mentioned, bone resorption occurs only when osteoclasts bind to mineralized bone surface [ 97 ].

Despite these bindings, osteoclasts are highly motile even active resorption and, as migrating cells, osteoclasts do not express cadherins. However, it has been demonstrated that cadherins provide intimate contact between osteoclast precursors and stromal cells, which express crucial growth factors for osteoclast differentiation [ ]. Integrins play a mediating role in osteocyte-bone matrix interactions. These interactions are essential for the mechanosensitive function of these cells, whereby signals induced by tissue deformation are generated and amplified [ ].

These interactions occur between osteocyte body and the bone matrix of the lacuna wall as well as between canalicular wall with the osteocyte processes [ ]. Only a narrow pericellular space filled by a fluid separates the osteocyte cell body and processes from a mineralized bone matrix [ 58 ].

The space between osteocyte cell body and the lacunar wall is approximately 0. The chemical composition of the pericellular fluid has not been precisely defined. However, a diverse array of macromolecules produced by osteocytes such as osteopontin, osteocalcin, dentin matrix protein, proteoglycans, and hyaluronic acid is present [ , , ].

It has been suggested that perlecan is a possible compound of these tethers [ ]. Thus, these structures seem to play a key role in the mechanosensitive function of osteocytes, by sensing the fluid flux movements along with the pericellular space, provoked by mechanical load forces [ ].

In addition, the fluid flux movement is also essential for the bidirectional solute transport in the pericellular space, which influences osteocyte signaling pathways and communication among bone cells [ , ]. Bone remodeling is a highly complex cycle that is achieved by the concerted actions of osteoblasts, osteocytes, osteoclasts, and bone lining cells [ 3 ]. The formation, proliferation, differentiation, and activity of these cells are controlled by local and systemic factors [ 18 , 19 ].

The local factors include autocrine and paracrine molecules such as growth factors, cytokines, and prostaglandins produced by the bone cells besides factors of the bone matrix that are released during bone resorption [ 46 , ]. The systemic factors which are important to the maintenance of bone homeostasis include parathyroid hormone PTH , calcitonin, 1,dihydroxyvitamin D 3 calcitriol , glucocorticoids, androgens, and estrogens [ 16 , — ]. Estrogen plays crucial roles for bone tissue homeostasis; the decrease in estrogen level at menopause is the main cause of bone loss and osteoporosis [ 16 ].

The mechanisms by which estrogen act on bone tissue are not completely understood. Nevertheless, several studies have shown that estrogen maintains bone homeostasis by inhibiting osteoblast and osteocyte apoptosis [ — ] and preventing excessive bone resorption. The estrogen suppresses the osteoclast formation and activity as well as induces osteoclast apoptosis [ 16 , 76 , , ]. It has been suggested that estrogen decreases osteoclast formation by inhibiting the synthesis of the osteoclastogenic cytokine RANKL by osteoblasts and osteocytes.

Moreover, estrogen stimulates these bone cells to produce osteoprotegerin OPG , a decoy receptor of RANK in osteoclast, thus inhibiting osteoclastogenesis [ 19 , — ]. Moreover, it has been shown that osteoclast is a direct target for estrogen [ , ]. Moreover, the enhanced immunoexpression observed in TUNEL-positive osteoclasts indicates that estrogen participates in the control of osteoclast life span directly by estrogen receptors [ ]. These findings demonstrate the importance of estrogen for the maintenance of bone homeostasis.

The bone remodeling cycle takes place within bone cavities that need to be remodeled [ ]. In these cavities, there is the formation of temporary anatomical structures called basic multicellular units BMUs , which are comprised of a group of osteoclasts ahead forming the cutting cone and a group of osteoblasts behind forming the closing cone, associated with blood vessels and the peripheral innervation [ 11 , ].

It has been suggested that BMU is covered by a canopy of cells possibly bone lining cells that form the bone remodeling compartment BRC [ 13 ]. The BRC seems to be connected to bone lining cells on bone surface, which in turn are in communication with osteocytes enclosed within the bone matrix [ 13 , 14 ].

The bone remodeling cycle begins with an initiation phase, which consists of bone resorption by osteoclasts, followed by a phase of bone formation by osteoblasts but between these two phases, there is a transition or reversal phase. The cycle is completed by coordinated actions of osteocytes and bone lining cells [ 10 , 11 ].

In the initiation phase, under the action of osteoclastogenic factors including RANKL and M-CSF, hematopoietic stem cells are recruited to specific bone surface areas and differentiate into mature osteoclasts that initiate bone resorption [ , ].

It is known that during bone remodeling cycle, there are direct and indirect communications among bone cells in a process called coupling mechanism, which include soluble coupling factors stored in bone matrix that would be released after osteoclast bone resorption [ ].

This idea is supported by genetic studies in humans and mice as well as by pharmacological studies [ , ]. Recently, it has been suggested that another category of molecules called semaphorins is involved in the bone cell communication during bone remodeling [ ].

During the initial phase, osteoblast differentiation and activity must be inhibited, in order to completely remove the damaged or aged bone. The osteoclasts express a factor called semaphorin4D Sema4D that inhibits bone formation during bone resorption [ ]. Semaphorins comprise a large family of glycoproteins which are not only membrane-bound but also exist as soluble forms that are found in a wide range of tissues and shown to be involved in diverse biological processes such as immune response, organogenesis, cardiovascular development, and tumor progression [ , ].

In bone, it has been suggested that semaphorins are also involved in cell-cell communication between osteoclasts and osteoblasts during the bone remodeling cycle [ — ]. Sema4D expressed in osteoclasts binds to its receptor Plexin-B1 present in osteoblasts and inhibits IGF-1 pathway, essential for osteoblast differentiation [ ], suggesting that osteoclasts suppress bone formation by expressing Sema4D.

Conversely, another member of semaphorin family Sema3A has been found in osteoblasts and is considered an inhibitor of osteoclastogenesis [ ]. Thus, during the bone remodeling cycle, osteoclasts inhibit bone formation by expressing Sema4D, in order to initiate bone resorption, whereas osteoblasts express Sema3A that suppresses bone resorption, prior to bone formation [ ] Figure 5.

Recent studies also suggest the existence of other factors involved in the coupling mechanism during the bone remodeling cycle. One of these factors is ephrinB2, a membrane-bound molecule expressed in mature osteoclasts, which bind to ephrinB4, found in the plasma membrane of osteoblasts. In addition, it has been shown that ephrinB2 is also expressed in osteoblasts [ ].

Furthermore, mature osteoclasts secrete a number of factors that stimulate osteoblast differentiation such as the secreted signaling molecules Wnt10b, BMP6, and the signaling sphingolipid, sphingosinephosphate [ ]. Besides osteoclasts and osteoblasts, it has been demonstrated that osteocytes play key roles during the bone remodeling cycle [ 8 ].

In fact, under the influence of several factors, the osteocytes act as orchestrators of the bone remodeling process, producing factors that influence osteoblast and osteoclast activities [ 55 ] Figure 5. On the other hand, mechanical unloading downregulates anabolic factors and stimulates osteocytes to produce sclerostin and DKK-1, which are inhibitors of osteoblast activity [ — ], as well as specific factors that stimulate local osteoclastogenesis [ ].

Osteocyte apoptosis has been shown to act as a chemotactic signal for local osteoclast recruitment [ 70 , , , , ]. Moreover, it is reported that the osteoclastogenic factors is also produced by viable osteocytes nearby the dying osteocytes [ ]. There is evidence that osteocytes act as the main source of RANKL to promote osteoclastogenesis [ , ], although this factor has also been demonstrated to be produced by other cell types such as stromal cells [ ], osteoblasts, and fibroblasts [ 88 , 89 ].

Thus, there are still uncertainties about the precise osteoclastogenesis-stimulating factors produced by osteocytes. Recent reviews have focused on some molecules that may be candidates for signaling between osteocyte apoptosis and osteoclastogenesis [ 72 , 73 ]. It has been suggested that osteocytes act as the main source of RANKL to promote osteoclastogenesis [ , ].

High mobility group box protein 1 HMGB1 [ — ] and M-CSF [ ] have also been suggested to be produced by osteocytes that stimulate osteoclast recruitment during bone remodeling [ 72 , 73 ]. Thus, future studies are required to address this issue. The classical functions of bone tissue, besides locomotion, include support and protection of soft tissues, calcium, and phosphate storage and harboring of bone marrow. Additionally, recent studies have focused on the bone endocrine functions which are able to affect other organs [ ].

For instance, osteocalcin produced by osteoblasts has been shown to act in other organs [ ]. Osteocalcin can be found in two different forms: carboxylated and undercarboxylated. The carboxylated form has high affinity to the hydroxyapatite crystals, remaining into bone matrix during its mineralization.

The undercarboxylated form shows lower affinity to minerals, due to acidification of bone matrix during osteoclast bone resorption, and then it is ferried by the bloodstream, reaching other organs [ , ]. It has been shown that the undercarboxylated osteocalcin has some effects in pancreas, adipose tissue, testis, and the nervous system. In the adipose tissue, osteocalcin stimulates adiponectin gene expression that in turn enhances insulin sensitivity [ ]. In the testis, osteocalcin can bind to a specific receptor in Leydig cells and enhances testosterone synthesis and, consequently, increases fertility [ ].

Osteocalcin also stimulates the synthesis of monoamine neurotransmitters in the hippocampus and inhibits gamma-aminobutyric acid GABA synthesis, improving learning and memory skills [ ]. Another endocrine function of bone tissue is promoted by osteocytes. These cells are able to regulate phosphate metabolism by the production of FGF23, which acts on other organs including parathyroid gland and kidneys to reduce the circulating levels of phosphates [ , ].

Osteocytes also act on the immune system by modifying the microenvironment in primary lymphoid organs and thereby influencing lymphopoiesis [ ].

Not only osteocyte but also osteoblast and osteoclast activities are known to influence the immune system, mainly upon bone inflammatory destruction. Indeed, the discovery of communication interplay between skeletal and immune systems led to a new field of study called osteoimmunology [ ]. The knowledge of the structural, molecular, and functional biology of bone is essential for the better comprehension of this tissue as a multicellular unit and a dynamic structure that can also act as an endocrine tissue, a function still poorly understood.

In vitro and in vivo studies have demonstrated that bone cells respond to different factors and molecules, contributing to the better understanding of bone cells plasticity.

Additionally, bone matrix integrins-dependent bone cells interactions are essential for bone formation and resorption. Studies have addressed the importance of the lacunocanalicular system and the pericellular fluid, by which osteocytes act as mechanosensors, for the adaptation of bone to mechanical forces.

The osteoblast, osteoclast, osteocyte, and osteoprogenitor bone cells are responsible for the growing, shaping, and maintenance of bones. Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor or osteogenic cells. Each cell type has a unique function and is found in different locations in bones.

The osteoblast, the bone cell responsible for forming new bone, is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it.

How man nuclei does the osteoclast have? Osteoclasts are secretory, and have prominent Golgi apparatus, and vesicles. They secrete enzymes such as carbonic anhydrase which acidifies the matrix, and causes it to decalcify, and hydrolyses, which break down the matrix once it is decalcified.

Other cell types help to phagocytose and get rid of the debris. Osteoclasts are large multinucleated cells, with a 'ruffled border' that resorb bone matrix, as shown in the diagram above. They are important for remodelling, growth and repair of bone.