骨痂组织冰冻切片前胶原及转化生长因子－β1基因表达的原位定位

目的观察骨痂组织中前胶原、转化生长因子－β１（ｔｒａｎｓｆｏｒｍｉｎｇｇｒｏｗｔｈｆａｃｔｏｒβ１,ＴＧＦ－β１）基因表达模式,分析ＴＧＦ－β１在骨折愈合过程中的作用,探索骨组织冰冻切片原位杂交技术。方法采用不脱钙的大鼠骨痂组织冰冻切片进行原位杂交,观察骨痂组织中前胶原和ＴＧＦ－β１基因的表达,并与先前研究作对照。结果杂交信号清晰,定位良好,特异性高。骨折第１周末,成纤维细胞的Ⅲ型前胶原基因表达占主导,Ⅰ型前胶原ｍＲＮＡ阳性成骨细胞也出现于膜内化骨区。ＴＧＦ－β１在分化、增殖的成骨细胞以及接近成熟的软骨细胞有显著表达。骨折第２周末,Ⅱ型前胶原和ＴＧＦ－β１ｍＲＮＡ在成熟的软骨细胞大量表达,而Ⅰ型前胶原ｍＲＮＡ表达也明显增加。骨折第４周末,软骨骨痂基本被骨组织替代,见散在Ⅰ型前胶原ｍＲＮＡ表达阳性的成骨细胞。同时证实共有表型表达的现象存在。结论实验结果与以往有关研究结果的吻合,提示ＴＧＦ－β１在骨折愈合过程中,尤其在细胞分化、增殖中,起重要的调节作用。同时,也说明了此方法是一种快捷、灵敏、又不失特异性的骨组织原位杂交方法。     

Expression of parathyroid hormone-related peptide and insulin-like growth factor I during rat fracture healing.

Parathyroid hormone-related peptide (PTHrP) and insulin-like growth factor I (IGF-I) are both involved in the regulation of bone and cartilage metabolisms and their interaction has been reported in osteoblasts. To investigate the interaction of PTHrP and IGF-I during fracture healing, the expression of mRNA for PTHrP and IGF-I, and receptors for PTH/PTHrP and IGF were examined during rat femoral fracture healing using an in situ hybridization method and an immunohistochemistry method, respectively. During intramembranous ossification, PTHrP mRNA, IGF-I mRNA and IGF receptors were detected in preosteoblasts, differentiated osteoblasts and osteocytes in the newly formed trabecular bone. PTH/PTHrP receptors were markedly detected in osteoblasts and osteocytes, but only barely so in preosteoblasts. During cartilaginous callus formation, PTHrP mRNA was expressed by mesenchymal cells and proliferating chondrocytes. PTH/PTHrP receptors were detected in proliferating chondrocytes and early hypertrophic chondrocytes. IGF-I mRNA and IGF receptor were co-expressed by mesenchymal cells, proliferating chondrocytes, and early hypertrophic chondrocytes. At the endochondral ossification front, osteoblasts were positive for PTHrP and IGF-I mRNA as well as their receptors. These results suggest that IGF-I is involved in cell proliferation or differentiation in mesenchymal cells, periosteal cells, osteoblasts and chondrocytes in an autocrine and/or paracrine fashion. Furthermore, PTHrP may be involved in primary callus formation presumably co-operating with IGF-I in osteoblasts and osteocytes, and by regulating chondrocyte differentiation in endochondral ossification.     

Colocalization of noggin and bone morphogenetic protein-4 during fracture healing.

The regulation of callus formation during fracture repair involves the coordinate expression of growth factors and their receptors. This article describes the temporal and spatial expression of noggin gene, an antagonist to bone morphogenetic protein (BMP), during the fracture repair process. Noggin expression was examined by means of Northern blotting and in situ hybridization and compared with the expression pattern of BMP-4 in a model of fracture repair in adult mice. Expression levels of noggin messenger RNA (mRNA) were enhanced in the early phase of fracture callus formation. The localization of the noggin mRNA was similar to that of BMP-4 mRNA. Distinct noggin mRNA signals were located predominantly in cells lining the periosteum and the cortical endosteum near the fracture site at 2 days after fracture. At 5, 10, and 21 days after fracture, noggin mRNA was detected in the chondrocytes and osteoblasts in the newly formed callus. The pattern of localization was indistinguishable from that of BMP-4. These results suggest that the noggin/BMP-4 balance could be an important factor in the regulation of callus formation during fracture healing.     

In situ localization of collagen production by chondrocytes and osteoblasts in fracture callus

An experimental model of fracture-healing was used to study the production of types-I and II collagen by in situ hybridization. The distribution of cartilage matrix in callus was determined by histochemical staining. Messenger RNA (mRNA) for cartilage-specific type-II collagen was detectable as early as the fifth day in a small number of cells that had acquired a chondrocyte phenotype but that also contained type-I collagen mRNA, suggesting an ongoing change in the expression of collagen genes. The location of the first chondrocytes, which were adjacent to cortical bone, suggested that they originated from cells that had derived from the periosteum by differentiation. On the seventh day of callus formation, the presence of both type-I and type-II collagen mRNA in chondrocytes of expanding cartilage suggested that most growth occurred by differentiation of mesenchymal cells and less by proliferation of differentiated chondrocytes. Expansion continued until the tenth to fourteenth day, after which the cartilage was replaced by woven bone. This was characterized by the presence of osteoblasts that were active in the synthesis of type-I collagen.     

Expression of metalloproteinase-13 (Collagenase-3) is induced during fracture healing in mice.

In fracture healing, a large amount of cartilage is formed, then rapidly replaced by osseous tissue. This process requires the transition of extracellular matrix component from type II to type I collagen. We investigated the expression of matrix metalloproteinase-13 (MMP-13), which has a high potential to cleave type II as well as type I collagen, during fracture repair in mouse ribs. In situ hybridization demonstrated that MMP-13 mRNA was present throughout the healing process. It was detected in the cells of the periosteum at day 1. As fracture callus grew, strong MMP-13 mRNA signals were detected in cells of the cartilaginous callus. In the reparative and remodeling phases, both hypertrophic chondrocytes and immature osteoblastic cells in the fracture callus expressed MMP-13 mRNA strongly. These cells were located adjacent to tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts at the sites of cartilage/bone transition. In osteoclasts, MMP-13 expression was not detected. The level of MMP-13 mRNA peaked at day 14 postfracture by northern blotting. Immunohistochemical staining showed that MMP-13 was detected primarily in hypertrophic chondrocytes. These results indicate that MMP-13 is induced during fracture healing. The site- and cell-specific expression of MMP-13 and its enzymatic property suggest that MMP-13 initiates the degradation of cartilage matrix, resulting in resorption and remodeling of the callus. In conclusion, MMP-13 plays an important role in the healing process of fractured bone in mice.     

Immature osteoblastic cells express the pro-alpha2(XI) collagen gene during bone formation in vitro and in vivo.

Type XI collagen is predominantly found in cartilage. However, expression of the pro-alpha2(XI) collagen gene (COL11A2) has recently been detected in various non-cartilaginous tissues. We identified the differentiation stage at which COL11A2 was expressed in cultured fetal rat calvarial (FRC) cells and in rat femoral fracture calluses in order to investigate the involvement of COL11A2 during bone formation in vitro and in vivo. We also studied the alternative splicing of exons 6-8 in FRC cells and fracture calluses. In FRC cells, mineralized nodules stained with von Kossa stain were observed from day 9 after confluence. COL11A2 was highly expressed on days 0 and 5, but the expression levels were rapidly decreased on day 9 by Northern blot analysis. During rat femoral fracture repair, intramembranous ossification proceeded and newly formed woven bone was observed on the cortex on day 7 after fracture. In situ hybridization showed that COL11A2 signals were detected in osteoblastic cells in the newly formed woven bone. According to the maturation and remodeling of the woven bone into the trabecular bone, the distribution of the signal for COL11A2 mRNA was limited to the superficial osteoblastic cells of the newly formed trabecular bone. These results demonstrated that COL11A2 was expressed in relatively immature osteoblastic cells during bone formation in vitro and in vivo. RT-PCR showed that the shortest band corresponding to mRNA lacking exons 6-8 was clearly detected when using RNA from soft calluses. In contrast, the largest band corresponding to mRNA with exons 6-8 was predominant when using RNA from FRC cells or from hard calluses on days 7 and 14. These results indicate that the splicing pattern of exons 6-8 in osteoblastic cells is different from the pattern in chondrocytes.     

Expression of connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24 (CTGF/Hcs24) during fracture healing

Localization and expression of connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24 (CTGF/Hcs24) during fracture healing in mouse ribs were investigated. In situ hybridization demonstrated that CTGF/Hcs24 mRNA was remarkably expressed, especially in hypertrophic chondrocytes and proliferating chondrocytes, in the regions of regenerating cartilage on days 8 and 14 after fracture. CTGF/Hcs24 mRNA was also expressed in proliferating periosteal cells in the vicinity of the fracture sites on days 2 and 8, and in cells in fibrous tissue around the callus on day 8. Northern blot analysis showed that expression of CTGF/Hcs24 mRNA was 3.9 times higher on day 2 of fracture healing than that on day 0. On day 8, it reached a peak of 8.6 times higher than that on day 0. It then declined to a lower level. Immunostaining showed that CTGF/Hcs24 was localized in hypertrophic chondrocytes and proliferating chondrocytes in the regions of regenerating cartilage, and in active osteoblasts in the regions of intramembranous ossification. Although CTGF/Hcs24 was abundant in the proliferating and differentiating cells (on days 8 and 14), immunostaining decreased as the cells differentiated to form bone (on day 20). CTGF/Hcs24 was also detected in cells in fibrous tissue, vascular endothelial cells in the callus, and periosteal cells around the fracture sites. These results suggest that CTGF/Hcs24 plays some role in fracture healing.     

Expression of Indian Hedgehog During Fracture Healing in Adult Rat Femora

Indian hedgehog (Ihh) has recently been shown to be expressed in prehypertrophic and hypertrophic chondrocytes during embryonic development, and it has been implicated in the regulation of terminal differentiation of chondrocytes. In this paper we examined the expression of Ihh during fracture healing in an adult rat model. A transverse diaphyseal fracture was made in the right femur, and the expression of Ihh in the fracture callus was examined at 1, 2, and 3 weeks after fracture. Northern blot analysis demonstrated the expression of Ihh mRNA in these tissues. Immunohistological analysis detected hedgehog protein in prehypertrophic chondrocytes in the fracture callus at 1 week after fracture. From 2 weeks and on, positive staining was observed in hypertrophic chondrocytes as well. At 3 weeks, some of the osteoblasts close to the endochondral ossification front were also stained positive for hedgehog protein. Our data indicate that Ihh is expressed in chondrocytes and osteoblasts during the process of fracture healing in adult rat femora, suggesting that Ihh, a regulator of endochondral ossification in embryonic development, may also play a role in the regulation of bone formation during fracture repair in adult animals. Received: 29 March 1999 / Accepted: 30 September 1999     

Expression of the fibroblast growth factor receptor genes in fracture repair

The spatial and temporal expression domains of the fibroblast growth factor receptor genes were examined in the healing rat femur fracture by in situ hybridization. Fibroblast growth factor receptor gene expression was detected in diverse fracture tissues throughout healing. Fibroblast growth factor receptor 1 and 2 expression was present throughout fracture repair, in the early proliferating periosteal mesenchyme, in the osteoblasts during intramembranous bone formation, and in the chondrocytes and osteoblasts during endochondral bone formation. Fibroblast growth factor receptor 3 expression colocalized with fibroblast growth factor receptor 1 and 2 expression in the chondrocytes and osteoblasts beginning at 10 days of healing, and persisted throughout endochondral bone formation. Fibroblast growth factor receptor 3 recapitulated its expression in fetal skeletal development, suggesting that it has a similar function in the control of endochondral bone growth during fracture repair. Fibroblast growth factor receptor 4 expression was not observed at any time. The extensive colocalized expression of the fibroblast growth factor receptors in healing indicates that fibroblast growth factor regulation of fracture callus maturation is extensive, and accurate identification of the receptor isoforms is necessary to establish the functions of fibroblast growth factor family members in fracture repair.     

Spatial and temporal gene expression for fibroblast growth factor type I receptor (FGFR1) during fracture healing in the rat.

Recent experiments have shown that exogenous basic fibroblast growth factor (bFGF) enlarges fracture callus and accelerates the healing of osteotomized long bones. The actions of bFGF are mediated by four different transmembrane receptors (FGFR1-4). Among them, FGFR1 has a high affinity for bFGF, and gain-of-function mutations of the FGFR1 gene cause craniosynostosis in humans. Gene expression for FGFR1 has been analyzed in embryogenesis; however, in skeletal repair, detailed expression of FGFR1 has not been fully established. In the present study, a rat model of closed femoral fracture healing was used to quantify mRNA encoding the FGFR1 and to characterize cells expressing FGFR1 by in situ hybridization. Gene expression for FGFR1 was rapidly upregulated after fracture; its mRNA level on day 1 was 3.4-fold higher than that of unfractured femora. At this stage, a moderate signal for FGFR1 was detected in periosteal osteoprogenitor cells, inflammatory cells near fracture sites, and cells among muscle layers. FGFR1 mRNA reached peak expression when callus remodeling actively progressed (6.8-fold on day 14), and remained elevated even in the later stages of healing (6.3-fold on day 28). During the intermediate stage of fracture healing, a strong signal for FGFR1 was diffusely distributed in mature osteoblasts in the hard callus, and mature osteoclasts also expressed a weak signal for FGFR1. These results suggest that FGF/FGFR1 signaling has multifunctional roles during fracture healing and may regulate both osteoblasts and osteoclasts, contributing to bone formation and callus remodeling.     

Expression of dentin matrix protein 1 (DMP1) during fracture healing

Dentin matrix protein 1 (DMP1) is one of the acidic phosphorylated extracellular matrix proteins called the SIBLING (small integrin-binding ligand, N-linked glycoproteins) family. Recent studies showed that DMP1 is expressed in the mineralized tissues and suggested that DMP1 is involved in the mineralization. We investigated the precise localization of DMP1 messenger RNA (mRNA) and protein during fracture healing. In situ hybridization demonstrated that DMP1 mRNA was strongly expressed in preosteocytes and osteocytes in the bony callus during intramembranous and endochondral ossification while DMP1 mRNA was not detected in osteoblasts and chondrocytes. During endochondral ossification, however, a low number of DMP1-expressing cells were identified in the cluster of hypertrophic chondrocytes. However, these DMP1-expressing cells were not hypertrophic and were likely to be osteoblast-lineage cells, which were embedded in the matrix of bone or cartilage, because type I collagen-expressing cells and invasion of capillary vessels were observed in the same area. Northern blot, in situ hybridization, and immunohistochemical analyses showed that DMP1 mRNA and protein expressions were increased until day 14 postfracture, when bony callus was formed, and then declined to a lower level during remodeling of the bony callus. Therefore, DMP1 is likely to play an important role in the mineralization of the bony callus.     

The angiogenic peptide pleiotrophin (PTN/HB-GAM) is expressed in fracture healing: an immunohistochemical study in rats

Introduction Formation of new blood vessels is essential for the process of fracture healing.Materials and methods We investigated the expression of the angiogenic factor pleiotrophin/HB-GAM in a closed fracture model in rats by immunohistochemical methods.Results Histologically, 5 days after fracture the callus was predominantly composed of fibrous tissue. On day 10 a prominent chondral callus connected both ends of the fractured tibia. There was a continuous transition from the chondral callus to the newly formed bone adjacent to the corticalis of the tibia. On day 15 the amount of woven bone had increased, and in 3 of 5 animals the proximal and distal tibiae were connected by a bridge of woven bone. Pleiotrophin could be immunostained in fibroblasts and endothelial cells of the fibrous tissue between the fractured tibia ends. The chondral callus remained largely pleiotrophin-negative. Only single chondrocytes adjacent to the newly formed bone were pleiotrophin-positive. On days 10 and 15 strong immunoreactivity for pleiotrophin in the well vascularized, newly formed, woven bone was detectable. Osteoblasts, endothelial cells and fibroblasts were strongly pleiotrophin-positive.Conclusions These results show the presence of the angiogenic peptide pleiotrophin during fracture healing.     

脑外伤对骨折愈合中骨形成蛋白2表达的影响

目的通过研究大鼠股骨干骨折合并脑外伤时骨痂组织内骨形成蛋白2(bone morphogenetic protein2,BMP-2)表达水平的变化,探讨脑外伤对骨折愈合的影响及作用机制。方法取12周雄性SD大鼠32只,体重368±25g。随机分成4组,每组8只:A组为骨折合并脑外伤1周组,B组为单纯骨折1周组,C组为骨折合并脑外伤2周组,D组为单纯骨折2周组。A、C组制作脑外伤和股骨干骨折模型,B、D组制作单纯股骨干骨折模型作对照。各组摄X线片后截取骨痂,行HE染色观察骨痂生长情况及其组织形态,免疫组织化学染色测定BMP-2表达,RT-PCR检测BMP-2mRNA水平。结果X线片示A组骨折端有较少量骨痂形成,骨折线较清晰;B组骨折线清晰;C组骨折端有较多骨痂形成,骨折线已模糊;D组仅有少量骨痂形成,骨折线较清晰。HE染色A组可见较多早期软骨细胞、成纤维细胞;B组骨折间隙可见成纤维细胞,仅夹杂有少量的早期软骨细胞;C组骨折端有新生骨小梁长入;D组未见有骨小梁形成。免疫组织化学染色可见成纤维细胞、间充质细胞、血管内皮细胞、早期软骨细胞、成骨细胞胞浆均出现广泛的强阳性反应,A组阳性细胞数量多于B组,并且显色强;C组阳性细胞数量多于D组,并且显色强。分析显示A、C组平均阳性细胞百分数为0.762%±0.052%、0.756%±0.079%,分别高于同一时间点B、D组的0.702%±0.052%、0.672%±0.044%,差异有统计学意义(P<0.05)。RT-PCR分析显示:A～D组骨痂BMP-2mRNA含量依次递减,A、C组骨痂BMP-2mRNA含量为1.07±0.13、0.78±0.11,均显著高于同一时间点B、D组的0.91±0.12、0.61±0.08,差异有统计学意义(P<0.05)。结论脑外伤对骨折愈合有促进作用,可能与脑外伤后BMP-2表达水平升高有关。     

Immunolocalization and expression of bone morphogenetic proteins 2 and 4 in fracture healing

Recently, it has become increasingly evident that fracture healing involves a complex interaction of many local and systemic regulatory factors. The roles of some of these growth factors have been described; however, little is understood about the presence of the bone morphogenetic proteins in fracture repair, despite the fact that they are the most potent osteoinductive proteins known. This study defines and characterizes the physiologic presence, localization, and chronology of the bone morphogenetic proteins in fracture healing with an established rat fracture healing model. With use of a recently developed monoclonal antibody against bone morphogenetic proteins 2 and 4 developed with standard avidin-biotin complex/immunoperoxidase protocols, frozen undecalcified fracture calluses were analyzed semiquantitatively for the percentage of various types of fracture cells staining positively. During the early stages of fracture healing, only a minimum number of primitive cells stained positively in the fracture callus. As the process of endochondral ossification proceeded, the presence of bone morphogenetic proteins 2 and 4 increased dramatically, especially in the primitive mesenchymal and chondrocytic cells. While the cartilaginous component of the callus matured with a concomitant decrease in the number of primitive cells, there was a concomitant decrease in both the intensity and the number of positively staining cells. As osteoblasts started to lay down woven bone on the chondroid matrix, these osteoblastic cells exhibited strong positive staining. The intensity of this staining decreased, however, as lamellar bone replaced the primitive woven bone. A similar observation was noted for the areas of the callus undergoing intramembranous ossification. Initially, within several days after the fracture, periosteal cells and osteoblasts exhibited intense staining for bone morphogenetic proteins 2 and 4. As the woven bone was replaced with mature lamellar bone, this staining decreased. These data, and the awareness of the strong osteoinductive capacities of bone morphogenetic protein, suggest that bone morphogenetic proteins 2 and 4 are important regulators of cell differentiation during fracture repair.     

Developmental and TGF-beta-mediated regulation of Ank mRNA expression in cartilage and bone

OBJECTIVES: Ank encodes a transmembrane protein that is involved in pyrophosphate (PPi) transport and mutations in the Ank gene have been associated with pathological mineralization in cartilage and bone. To understand how Ank works in normal skeletal development it is also important to know which cells within the developing skeleton express Ank. To this end, we examined the expression pattern of Ank mRNA during mouse embryonic development as well as in mouse hind limb joints with emphasis on the period when articular cartilage forms. Since it was previously shown that TGF-beta regulates PPi transport in cells in culture, we also tested the hypothesis that TGF-beta regulates Ank expression. METHODS: The localization of Ank mRNA was determined by radioactive in situ hybridization in E15.5 and E17.5 mouse embryos as well as in 1 and 3 week post-natal mice. Ank expression was compared to that of other cartilage markers. In situ hybridization and semi-quantitative RT-PCR were used to determine the effects of TGF-beta on Ank expression in metatarsal organ cultures. RESULTS: Ank expression was detected at high levels at sites of both endochondral and intramembranous bone development. In endochondral bones, expression was detected in a subset of hypertrophic cells at ossification centers. Expression was also detected in osteogenic/chondrogenic cells of the perichondrium/periosteum lining the metaphysis, an area associated with the formation and extension of the bone collar. High levels of expression were also detected in non-mineralized tissues of the skeletal system including tendons and the superficial layer of the articular cartilage. Treatment with TGF-beta resulted in an approximately four-fold induction of Ank mRNA in prehypertrophic chondrocytes and perichondrium of metatarsal cultures.CONCLUSIONS: The expression pattern of Ank suggests an important role both in inhibiting and regulating mineralization in the developing skeletal system. In addition, TGF-beta1 is able to mediate Ank mRNA expression in chondrocytes suggesting a possible role for TGF-beta and Ank in the regulation of normal mineralization.     

Acidic fibroblast growth factor (aFGF) injection stimulates cartilage enlargement and inhibits cartilage gene expression in rat fracture healing

The effect of the administration of acidic fibroblast growth factor (aFGF) on normal fracture healing was examined in a rat fracture model. One microgram of aFGF was injected into the fracture site between the first and the ninth day after fracture either every other day or every day. aFGF-injected calluses were significantly larger than control calluses, although this does not imply an increased mechanical strength of the callus. Histology showed a marked increase in the size of the cartilaginous soft callus. Total DNA and collagen content in the cartilaginous portion of the aFGF-injected calluses were greater than those of controls, although the collagen content/DNA content ratio was not different between the aFGF-injected and control calluses. Fracture calluses injected with aFGF remained larger than controls until 4 weeks after fracture. The enlarged cartilaginous portion of the aFGF-injected calluses seen at 10 days after fracture was replaced by trabecular bone at 3 and 4 weeks. Northern blot analysis of total cellular RNA extracted separately from the cartilaginous soft callus and the bony hard callus showed decreased expression of type II procollagen and proteoglycan core protein mRNA in the aFGF-injected calluses when compared with controls. A slight decrease in types I and III procollagen mRNA expression was also observed. We concluded that aFGF injections induced cartilage enlargement and decreased mRNA expression for type II procollagen and proteoglycan core protein.     

碱性成纤维细胞生长因子在骨愈合过程中的表达与合成

目的 通过对骨愈合过程中内源性碱性成纤维细胞生长因子（ｂａｓｉｃ ｆｉｂｒｏｂｌａｓｔ ｇｒｏｗｔｈ ｆａｃｔｏｒ,ｂＦＧＦ）合成变化的观察,探讨其成此过程的作用,以此加深对骨生理学的认识,同时为干预骨愈合提供理论依据。方法 选用ＳＤ大鼠并将其胫骨造成骨缺损,用免疫组织化学及原位杂交技术检测ｂＦＧＦ基因和蛋白质在骨闸组织成熟过程不同时期的变化情况。结果 免疫组织化学染色显示,从骨折后４天～３人,ｂ     

Expression profiles of receptor activator of nuclear factor kappaB ligand, receptor activator of nuclear factor kappaB, and osteoprotegerin messenger RNA in aged and ovariectomized rat bones.

The receptor activator of nuclear factor-kappaB ligand (RANKL; also known as tumor necrosis factor-related activation-induced cytokine [TRANCE], osteoprotegerin ligand [OPGL], and osteoclast differentiation factor [ODF]) is a transmembrane ligand expressed in osteoblasts and bone marrow stromal cells. It binds to RANK, which is expressed in osteoclast progenitor cells, and induces osteoclastogenesis. OPG, a decoy receptor for RANKL, also binds to RANKL, and competitive binding of RANKL with RANK or OPG is thought to regulate bone metabolism. To investigate roles of the RANKL/RANK/OPG system in pathophysiological conditions, the expression of RANKL, RANK, and OPG messenger RNA (mRNA) was analyzed in bones of aged and ovariectomized rats by means of in situ hybridization. In the control 8-week-old male and sham-operated female rat bones, the expression of RANKL mRNA was detected in hypertrophic chondrocytes of the growth plate and some periosteal and endosteal mesenchymal cells. The expression of RANK mRNA was detected in osteoclast-like cells and mononuclear cells in contact with the cortical and trabecular bones. The expression of OPG mRNA was detected in proliferating chondrocytes and osteocytes. In the 2.5-year-old rat bones, the expression of RANKL, RANK, and OPG mRNA tended to decrease except for the endosteal region. In the ovariectomized rat bones, the expression of RANKL, RANK, and OPG mRNA increased, and high expression of OPG mRNA was induced in resting chondrocytes and osteocytes. These results suggest that estrogen deficiency stimulates the RANKL/RANK/OPG system and induces OPG in cells that have been thought to be less important for bone metabolism.