Normal Bone Formation Research Articles

MORPHOLOGICAL EVALUATION OF CONSEQUENCES OF LONG BONE OSTEOMYELITIS IN CHILDREN

The authors studied morphological changes in bone, cartilage and soft tissue from the zone of old inflammation and far from it in 36 children with pathological hip dislocation, in 23 - with limb shortening, in 13 - with pseudoarthrosis and long bone defects after osteomyelitis. Morphological assessment of proximal femur tissue in children, who had hematogenous osteomyelitis in early childhood, identified severe changes in cartilage and bone tissue which accompanied by muscle pathology. The state of the femoral head articular cartilage and acetabulum in patients with pathological hip dislocation is characterized by degenerative changes of varying severity (up to the formation of necrobiosis and necrosis focus), by the replacement of hyaline cartilage by tissue, formed by the type of fibrous cartilage and fibrous (scar) tissue. This picture did not have significant differences depending on the osteomyelitis prescription. Directly in the area of the former inflammatory process, as well as in the ends of pseudoarthrosis bone fragments of long bone the degenerative changes of bone tissue with the signs of resorption and low-grade osteogenic activity are marked. The morphological texture of the femur diaphysis, located far from the old inflammation, did not differ from normal. It may indicate its potency for normal bone formation. Muscle tissue in the area of old inflammation keeps the signs of unevenly expressed degeneration and atrophy. Far from this zone muscle tissue doesn’t differ from its usual structure. The described morphological changes can serve as justification for the improvement of methods of surgical treatment in children with consequences of osteomyelitis.

The RHO-Type Guanine Nucleotide Exchange Factor AKAP13 is Required for Normal Bone Formation and Expression of RUNX2

The Protein Kinase A Anchoring Protein 13 (AKAP13 or a.k.a. Brx) is a guanine nucleotide-exchange factor for Rho-type small GTP-binding (G), and functions as a cytoplasmic integrator or docking platform for multiple signaling cascades including those of the protein kinase A and nuclear hormone receptors, such as estrogen and glucocorticoid receptors. All these signaling cascades are important for bone metabolism, while Rho-type small G proteins are essential for mediating mechanical stress, an important stimulator of bone formation. Thus, we examined involvement of AKAP13 in the regulation of bone development by employing AKAP13 haploinsufficient mice and MC3T3-E1 cells, an established model of osteoblast differentiation.

ThehSSB1orthologueObfc2bis essential for skeletogenesis but dispensable for the DNA damage responsein vivo

Human single-stranded DNA-binding protein 1 (hSSB1), encoded by OBFC2B, was recently characterized as an essential factor for the initiation of DNA damage checkpoints and the maintenance of genomic stability. Here, we report that loss of Obfc2b in mice results in perinatal lethality characterized by growth delay and skeletal abnormalities. These abnormalities are associated with accumulation of γH2ax, apoptosis and defective pre-cartilage condensation, which is essential for normal bone formation. However, deficiency of Obfc2b does not affect the initiation of DNA damage checkpoints, Atm activation, or the maintenance of genomic stability in B lymphocytes and primary fibroblasts. Loss of Obfc2b results in increased expression of its homologue Obfc2a (hSSB2). In contrast to Obfc2b deficiency, depletion of Obfc2a in fibroblasts results in impaired proliferation, accumulation of γH2ax and increased genomic instability. Thus, the hSSB1 orthologue Obfc2b has a unique function during embryogenesis limited to cell types that contribute to bone formation. While being dispensable in most other cell lineages, its absence leads to a compensatory increase in Obfc2a protein, a homologue required for the maintenance of genomic integrity.

Transplantation of nano-bioglass/gelatin scaffold in a non-autogenous setting for bone regeneration in a rabbit ulna

Bioactive glass has been investigated for variety of tissue engineering applications. In this study, fabrication, in vitro and in vivo evaluation of bioactive glass nanocomposite scaffold were investigated. The nanocomposite scaffolds with compositions based on gelatin and bioactive glass nanoparticles were prepared. The apatite formation at the surface of the nanocomposite samples confirmed by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray powder diffraction analyses. The in vitro characteristics of bioactive glass scaffold as well as the in vivo bone formation capacity of the bioactive glass scaffold in rabbit ulnar model were investigated. The bioactive glass scaffold showed no cytotoxicity effects in vitro. The nanocomposite scaffold made from gelatin and bioactive glass nanoparticles could be deliberated as an extremely bioactive and prospective bone tissue engineering implant. Bioactive glass scaffolds were capable of guiding bone formation in a rabbit ulnar critical-sized-defect model. Radiographic evaluation indicated that successful bridging of the critical-sized defect on the sides both next to and away from the radius took place using bioactive glass scaffolds. X-ray analysis also proposed that bioactive glass scaffolds supported normal bone formation via intramembranous formation.

Effect of high fat diet on inflammatory markers and bone morphological microstructure in growing male mice

This study examined the change in inflammation makers and morphological microstructure of bones in mice fed on high‐fat diet. Four‐week‐old C57BL/6J male mice were divided into 2 groups: the control group was fed 10% kcal fat diet, and the high‐fat diet group was fed 45% kcal fat diet for 12 weeks. Inflammation markers were measured by using ELISA kits and microcomputed tomography (micro‐CT) was used to measure the morphological microstructure of the femoral bone. Leptin and IL‐6 levels were significantly higher in the high‐fat diet group (p < 0.01). Oosteocalcin was not significantly different between groups; however type I collagen C‐telopeptide was significantly higher in the high‐fat diet group (p < 0.01). The thickness of bone trabecula was significantly reduced in the high‐fat diet group as compared to the control group (p < 0.05), and the gap within the bone trabecula was significantly wider in the high‐fat diet group (p < 0.05). A positive correlation (p < 0.05) was observed between IL‐6 levels and the gap within the bone trabecula. These results showed that obesity induction in mice by feeding high‐fat diet during the growth phase resulted in a change in the bone microstructure and showed that bone microstructure was correlated with inflammation index. Therefore, restriction of excessive fat intake may be required to suppress inflammation reactions and promote normal bone formation.

The Stem Cell Niche as a Pharmaceutical Target for Prevention of Skeletal Metastases

Advanced cancers of the prostate and breast commonly progress by metastasizing to the skeleton, where they are incurable but cause serious morbidity and contribute to mortality. Growth of tumor in bone takes several years, opening a large window for pharmaceutical prevention of metastatic progression. Bone provides a unique microenvironment for tumor growth, including niches occupied by hematopoietic and mesenchymal stem cells. Recent data suggest that circulating tumor cells usurp these niches and compete with the normal stem cell occupants. Agents that encourage normal hematopoiesis or bone formation could inhibit colonization of bone by tumor stem cells and prevent or delay metastatic progression. It may be possible to develop high-throughput assays to test compounds for their ability to suppress tumor stem cell occupation of skeletal niches, thus decreasing metastatic progression in at-risk patients.

The molecular chaperone Hsp47 is essential for cartilage and endochondral bone formation

Heat shock protein 47 kDa (Hsp47) is considered as a molecular chaperone essential for the correct folding of type I and type IV procollagen in the ER. However, the function of Hsp47 for other types of procollagen and its importance for chondrogenesis have never been elucidated. To examine the function of Hsp47 in cartilage formation and endochondral ossification, we conditionally inactivated the Hsp47 gene in chondrocytes using Hsp47 floxed mice and mice carrying a chondrocyte-specific Col2a1-Cre transgene. Hsp47 conditional null mutant mice died just before or shortly after birth, and exhibited severe generalized chondrodysplasia and bone deformities with lower levels of type II and type XI collagen. Second-harmonic generation (SHG) analysis and electron microscopy revealed the accumulation of misaligned type I collagen molecules in the intervertebral discs and a substantial decrease in type II collagen fibers, respectively. Whole-mount skeletal staining showed no calcified region in the vertebral bodies of sacral vertebrae, and revealed that the endochondral bones were severely twisted and shortened. These results demonstrate that Hsp47 is indispensable for well-organized cartilage and normal endochondral bone formation.

Disseminated Prostate Cancer Cells Can Instruct Hematopoietic Stem and Progenitor Cells to Regulate Bone Phenotype

Prostate cancer metastases and hematopoietic stem cells (HSC) frequently home to the bone marrow, where they compete to occupy the same HSC niche. We have also shown that under conditions of hematopoietic stress, HSCs secrete the bone morphogenetic proteins (BMP)-2 and BMP-6 that drives osteoblastic differentiation from mesenchymal precursors. As it is not known, we examined whether metastatic prostate cancer cells can alter regulation of normal bone formation by HSCs and hematopoietic progenitor cells (HPC). HSC/HPCs isolated from mice bearing nonmetastatic and metastatic tumor cells were isolated and their ability to influence osteoblastic and osteoclastic differentiation was evaluated. When the animals were inoculated with the LNCaP C4-2B cell line, which produces mixed osteoblastic and osteolytic lesions in bone, HPCs, but not HSCs, were able to induced stromal cells to differentiate down an osteoblastic phenotype. Part of the mechanism responsible for this activity was the production of BMP-2. On the other hand, when the animals were implanted with PC3 cells that exhibits predominantly osteolytic lesions in bone, HSCs derived from these animals were capable of directly differentiating into tartrate-resistant acid phosphatase-positive osteoclasts through an interleukin-6-mediated pathway. These studies for the first time identify HSC/HPCs as novel targets for future therapy involved in the bone abnormalities of prostate cancer.

MicroRNA control of bone formation and homeostasis

MicroRNAs (miRNAs) repress cellular protein levels to provide a sophisticated parameter of gene regulation that coordinates a broad spectrum of biological processes. Bone organogenesis is a complex process involving the differentiation and crosstalk of multiple cell types for formation and remodeling of the skeleton. Inhibition of mRNA translation by miRNAs has emerged as an important regulator of developmental osteogenic signaling pathways, osteoblast growth and differentiation, osteoclast-mediated bone resorption activity and bone homeostasis in the adult skeleton. miRNAs control multiple layers of gene regulation for bone development and postnatal functions, from the initial response of stem/progenitor cells to the structural and metabolic activity of the mature tissue. This Review brings into focus an emerging concept of bone-regulating miRNAs, the evidence for which has been gathered largely from in vivo mouse models and in vitro studies in human and mouse skeletal cell populations. Characterization of miRNAs that operate through tissue-specific transcription factors in osteoblast and osteoclast lineage cells, as well as intricate feedforward and reverse loops, has provided novel insights into the supervision of signaling pathways and regulatory networks controlling normal bone formation and turnover. The current knowledge of miRNAs characteristic of human pathologic disorders of the skeleton is presented with a future goal towards translational studies.

Rare Genetic Cases of Heterotopic Ossification

Heterotopic Ossification (HO) is formation of normal bone in extraskeletal tissues, namely the skin, subcutaneous tissue and deep connective tissue. Heterotopic ossification can be hereditary or nonhereditary in nature. We report two cases of rare hereditary forms of heterotopic ossification namely Fibrodysplasia Ossificans Progressiva (FOP) and Progressive Osseous Heteroplasia (POH). Hereditary forms of heterotopic ossification are progressive and severely debilitating in nature. Though these disorders are rare, awareness and knowledge about these disorders will help in early identification of such unique cases, leading to appropriate referral and management.

Higher BMP receptor expression and BMP-2-induced osteogenic differentiation in tendon-derived stem cells compared with bone-marrow-derived mesenchymal stem cells

Surgical reattachment of tendon to bone often fails due to regeneration failure of the specialised tendon-bone junction (TBJ). The use of mesenchymal stem cells for TBJ regeneration has been reported with promising results. Tendon-derived stem cells (TDSCs) with high proliferative and multi-lineage differentiation potential have been isolated. As stem cells residing in tendons, TDSCs can be considered a new cell source for TBJ repair. Bone morphogenic protein 2 (BMP-2) is a potent osteogenic factor with roles in normal bone healing and pathological ectopic bone formation in soft tissues. The use of BMP-2 to promote TBJ repair has been well reported. This study aimed to compare TDSCs to the gold standard bone-marrow-derived mesenchymal stem cells (BMSCs) with respect to osteogenic response to BMP-2 in vitro. The clonogenicity and multi-differentiation potential of TDSCs and BMSCs were identified by colony-forming-unit assay, osteogenic, adipogenic and chondrogenic differentiation assays. Their osteogenic response to BMP-2 in vitro was examined by alkaline phosphatase (ALP) cytochemical staining, ALP activity assay and Alizarin red S staining of calcium nodule formation. Messenger RNA (mRNA) and BMP receptor (types IA, IB and II) protein expression were examined by quantitative real-time reverse-transcriptase polymerase chain reaction (qRT-PCR) and Western blotting. Our results showed that both TDSCs and BMSCs exhibited stem cell properties, including clonogenicity and multi-differentiation potential. TDSCs expressed higher mRNA and protein levels of BMP receptors IA, IB and II. They also exhibited higher osteogenic differentiation with and without BMP-2 stimulation compared with BMSCs. TDSCs with/without BMP-2 might be an attractive source for TBJ repair compared with BMSCs.

The anaphylatoxin receptor C5aR is present during fracture healing in rats and mediates osteoblast migration in vitro.

There is evidence that complement components regulate cytokine production in osteoblastic cells, induce cell migration in mesenchymal stem cells, and play a regulatory role in normal enchondral bone formation. We proved the hypothesis that complement might be involved in bone healing after fracture. We investigated the expression of the key anaphylatoxin receptor C5aR during fracture healing in rats by immunostaining after 1, 3, 7, 14, and 28 days. C5aR expression was additionally analyzed in human mesenchymal stem cells (hMSC) during osteogenic differentiation, in human primary osteoblasts, and osteoclasts by reverse transcriptase polymerase chain reaction and immunostaining. Receptor functionality was proven by the migratory response of cells to C5a in a Boyden chamber. C5aR was expressed in a distinct spatial and temporal pattern in the fracture callus by differentiated osteoblast, chondroblast-like cells in cartilaginous regions, and osteoclasts. In vitro C5aR was expressed by osteoblasts, osteoclasts, and hMSC undergoing osteogenic differentiation. C5aR was barely expressed by undifferentiated hMSC but was significantly induced after osteogenic differentiation. C5aR activation by C5a induced strong chemotactic activity in osteoblasts, and in hMSC, which had undergone osteogenic differentiation, being abolished by a specific C5aR antagonist. In hMSC, C5a induced less migration reflecting their low level of C5aR expression. Our in vitro and in vivo results demonstrated the presence of C5aR in bone forming osteoblasts and bone resorbing osteoclasts. It is suggested that C5aR might play a regulatory role in fracture healing in intramembranous and in enchondral ossification, one possible function being the regulation of cell recruitment.

The Effects of Smoke Carcinogens on Bone

The greatest cause of preventable morbidity and mortality is smoking, and one of the often-underappreciated effects of smoking is profound bone loss. The existing clinical paradigm for smoking is that there is a low turnover osteoporosis. This review highlights findings from recent clinical trials and animal research demonstrating either support or conflict with the existing paradigm. Clinically, it is noted that markers of bone formation are often normal in smokers; these clinical findings conflict with well-conducted animal research demonstrating that carcinogens acting on the aryl hydrogen receptor can significantly reduce osteoblast formation and function. Regarding bone resorption, highlights from recent clinical studies suggest that bone remodeling is increased in smokers. Directly contradicting this enhanced osteoclastogenesis are several animal studies all demonstrating significant inhibition of osteoclast formation and function upon exposure to smoke carcinogens. Future research is needed to clarify whether smoking is truly a low bone remodeling osteoporosis, or an osteoclast-driven bone destruction, with inappropriately normal bone formation.

Runx2 Protein Expression Utilizes the Runx2 P1 Promoter to Establish Osteoprogenitor Cell Number for Normal Bone Formation

The Runt-related transcription factor, Runx2, is essential for osteogenesis and is controlled by both distal (P1) and proximal (P2) promoters. To understand Runx2 function requires determination of the spatiotemporal activity of P1 and P2 to Runx2 protein production. We generated a mouse model in which the P1-derived transcript was replaced with a lacZ reporter allele, resulting in loss of P1-derived protein while simultaneously allowing discrimination between the activities of the two promoters. Loss of P1-driven expression causes developmental defects with cleidocranial dysplasia-like syndromes that persist in the postnatal skeleton. P1 activity is robust in preosteogenic mesenchyme and at the onset of bone formation but decreases as bone matures. Homozygous Runx2-P1(lacZ/lacZ) mice have a normal life span but exhibit severe osteopenia and compromised bone repair in adult mice because of osteoblastic defects and not increased osteoclastic resorption. Gene expression profiles of bone, immunohistochemical studies, and ex vivo differentiation using calvarial osteoblasts and marrow stromal cells identified mechanisms for the skeletal phenotype. The findings indicate that P1 promoter activity is necessary for generating a threshold level of Runx2 protein to commit sufficient osteoprogenitor numbers for normal bone formation. P1 promoter function is not compensated via the P2 promoter. However, the P2 transcript with compensatory mechanisms from bone morphogenetic protein (BMP) and Wnt signaling is adequate for mineralization of the bone tissue that does form. We conclude that selective utilization of the P1 and P2 promoters enables the precise spatiotemporal expression of Runx2 necessary for normal skeletogenesis and the maintenance of bone mass in the adult.

Effect of Reduced c‐Kit Signaling on Bone Marrow Adiposity

c-Kit (CD117) is required for normal differentiation of osteoblasts from bone marrow stromal cells and for normal bone formation. Osteoblasts and adipocytes originate from a common progenitor cell, and a reciprocal relationship in differentiation of the two lineages is often observed. Therefore, the effects of abnormal c-kit signaling on bone marrow adiposity and adipocyte precursor pool size were evaluated in mouse strains with loss of function mutations in kit receptor or kit ligand. Additionally, to determine whether short-duration pharmacological disruption of kit signaling influences bone marrow adiposity, we administered the kit receptor antagonist gleevec (imatinib mesilate) for 1 week to middle aged (13-month-old) male rats known to have high levels of bone marrow fat. Compared to wild-type littermates, adipocytes were absent and adipocyte precursors greatly reduced in bone marrow from kit receptor-deficient Kit(W/W-ν) mice. Administration of secreted kit ligand to membrane-associated kit ligand-deficient Kit(Sl/Sl-d) mice was ineffective in inducing bone marrow adipogenesis. These findings suggest that activation of kit receptor by the membrane-associated form of kit ligand is required for kit signaling to promote bone marrow adipogenesis in mice. Rats treated with gleevec had lower adipocyte density compared to age-matched controls, suggesting that kit signaling is required to maintain normal bone marrow adiposity. Taken together, our results indicate that c-Kit signaling plays an important but previously unsuspected role in regulating bone marrow adiposity.

Is It Possible to Shorten the Jaws Using Contraction Osteogenesis?

This study investigated whether shortening of osteotomized jaws is possible. Nine sheep were used (2 as controls and 7 as experimental subjects). Distraction devices that had previously been activated to 10 mm were fixed to the mandibles of all animals bilaterally and used in reverse as a contraction device. Control and experimental animals were sacrificed at 1 month and 3 months. Bone in the area of contraction was evaluated using radiodensitometry and microscopy. The mandibles were shortened an average of 5.5 mm. Exaggerated bone formation was seen around the osteotomized cortical bone. When histologic slices from experimental animals were examined 1 month after the contraction period, fibrous pseudoarthrosis formation was seen centrally, with hyaline cartilage around it, whereas normal bone formation was seen in the outer part. The hyaline cartilage had turned into normal bone 3 months after the end of contraction. It is possible to shorten bones using contraction osteogenesis.

Effects of Farnesyl Pyrophosphate Accumulation on Calvarial Osteoblast Differentiation

Statins, drugs commonly used to lower serum cholesterol, have been shown to stimulate osteoblast differentiation and bone formation. Statins inhibit 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A reductase (HMGCR), the first step of the isoprenoid biosynthetic pathway, leading to the depletion of the isoprenoids farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP). The effects of statins on bone have previously been attributed to the depletion of GGPP, because the addition of exogenous GGPP prevented statin-stimulated osteoblast differentiation in vitro. However, in a recent report, we demonstrated that the specific depletion of GGPP did not stimulate but, in fact, inhibited osteoblast differentiation. This led us to hypothesize that isoprenoids upstream of GGPP play a role in the regulation of osteoblast differentiation. We demonstrate here that the expression of HMGCR and FPP synthase decreased during primary calvarial osteoblast differentiation, correlating with decreased FPP and GGPP levels during differentiation. Zaragozic acid (ZGA) inhibits the isoprenoid biosynthetic pathway enzyme squalene synthase, leading to an accumulation of the squalene synthase substrate FPP. ZGA treatment of calvarial osteoblasts led to a significant increase in intracellular FPP and resulted in inhibition of osteoblast differentiation as measured by osteoblastic gene expression, alkaline phosphatase activity, and matrix mineralization. Simultaneous HMGCR inhibition prevented the accumulation of FPP and restored osteoblast differentiation. In contrast, specifically inhibiting GGPPS to lower the ZGA-induced increase in GGPP did not restore osteoblast differentiation. The specificity of HMGCR inhibition to restore osteoblast differentiation of ZGA-treated cultures through the reduction in isoprenoid accumulation was confirmed with the addition of exogenous mevalonate. Similar to ZGA treatment, exogenous FPP inhibited the mineralization of primary calvarial osteoblasts. Interestingly, the effects of FPP accumulation on osteoblasts were found to be independent of protein farnesylation. Our findings are the first to demonstrate that the accumulation of FPP impairs osteoblast differentiation and suggests that the depletion of this isoprenoid may be necessary for normal and statin-induced bone formation.

Correction of Alveolar Cleft With Calcium-Based Bone Substitutes

The criterion standard of alveolar cleft repair is iliac crest bone graft before secondary canine eruption. Tooth eruption has never been shown to occur in synthetic bone substitute, and there is no ideal autologous bone graft for primary repair. This prospective study evaluated alveolar cleft grafting with a calcium substitute before primary canine eruption. Ten consecutive patients with complete cleft lip, palate, and unilateral alveolar cleft with reasonably aligned arches were grafted beginning in January 2003 to March 2007. Mean age at surgery was 10.4 months. Follow-up ranged from 3 to 7 years. Radiologic evaluation of alveolar ridge was performed at the age of 4.All 10 patients were operated on by the same surgeon using the same technique, that is, conservative elevation of nasal, oral, and anterior alveolar mucosal flaps around the cleft, closure of nasal and oral flaps, placement of 1 to 3 mL of calcium substitute paste or crystals in the pocket, and closure of the anterior alveolar mucosa. All 10 patients healed without complication. Clinical evaluation revealed a well-healed arch with primary canine growth in the area of the previous cleft. Adequate normal bone formation and often a descending secondary canine were radiologically confirmed. Calcium substitutes offer significant advantages over other biomaterials as well as autologous bone grafts particularly in the primary alveolar cleft reconstruction. Our study has shown for the first time that teeth can erupt through this material, which turns into a normal functioning bone in the alveolar ridge.

Low dose parathyroid hormone maintains normal bone formation in adult male rats during rapid weight loss

A persistent negative energy balance results in bone loss. It is not clear whether the bone loss associated with chronic negative energy balance can be prevented. The objective of this study was to assess the efficacy of intermittent low dose parathyroid hormone (PTH) treatment in maintaining normal bone formation during severe energy restriction. Six-month-old male Fisher 344 rats were divided into 4 treatment groups: (1) baseline, (2) ad libitum (ad lib)-fed control, (3) energy-restricted (to consume 40% ad lib caloric intake), or (4) energy-restricted+low dose (1μg/kg/d) PTH. Severe energy restriction for 14d decreased body weight and serum leptin levels. Compared to ad lib-fed controls, energy-restricted rats had lower cancellous bone formation, higher osteoclast perimeter/bone perimeter and higher bone marrow adiposity in the proximal tibial metaphysis. Also, the energy-restricted rats had a lower periosteal bone formation rate at the tibia–fibula synostosis. Administration of PTH to energy-restricted rats had no effect on weight loss or osteoclast perimeter/bone perimeter. In contrast, energy-restricted rats treated with PTH had higher rates of cancellous and cortical bone formation compared to energy-restricted rats, and did not differ from the ad lib-fed control animals. Furthermore, PTH treatment maintained normal bone marrow adiposity. In conclusion, rapid weight loss in adult male rats was accompanied by decreased bone formation and increased bone marrow adiposity and these changes were prevented by low dose PTH treatment. Taken together, the results suggest that the energy cost of bone formation in adult rats is low and PTH therapy is effective in preventing the reduced bone formation associated with rapid weight loss.

Interleukin-33 is expressed in differentiated osteoblasts and blocks osteoclast formation from bone marrow precursor cells

Since the hematopoetic system is located within the bone marrow, it is not surprising that recent evidence has demonstrated the existence of molecular interactions between bone and immune cells. While interleukin 1 (IL-1) and IL-18, two cytokines of the IL-1 family, have been shown to regulate differentiation and activity of bone cells, the role of IL-33, another IL-1 family member, has not been addressed yet. Since we observed that the expression of IL-33 increases during osteoblast differentiation, we analyzed its possible influence on bone formation and observed that IL-33 did not affect matrix mineralization but enhanced the expression of Tnfsf11, the gene encoding RANKL. This finding led us to analyze the skeletal phenotype of Il1rl1-deficient mice, which lack the IL-33 receptor ST2. Unexpectedly, these mice displayed normal bone formation but increased bone resorption, thereby resulting in low trabecular bone mass. Since this finding suggested a negative influence of IL-33 on osteoclastogenesis, we next analyzed osteoclast differentiation from bone marrow precursor cells and observed that IL-33 completely abolished the generation of TRACP(+) multinucleated osteoclasts, even in the presence of RANKL and macrophage colony-stimulating factor (M-CSF). Although our molecular studies revealed that IL-33 treatment of bone marrow cells caused a shift toward other hematopoetic lineages, we further observed a direct negative influence of IL-33 on the osteoclastogenic differentiation of RAW264.7 macrophages, where IL-33 repressed the expression of Nfatc1, which encodes one of the key transciption factors of osteoclast differentiation. Taken together, these findings have uncovered a previously unknown function of IL-33 as an inhibitor of bone resorption.