Mechanism of osteoclastic bone resorption: A new hypothesis

Summary Osteoclastic bone resorption involves the solubilization of the mineral salts and the degradation of noncollagen bone matrix and collagen fibrils. As no recognizable collagen fibrils have ever been reported within cytoplasmic vacuoles in osteoclasts, it is generally assumed that the collagen fibrils are digested extracellularly in the resorption zone. The extent to which lysis occurs extracellularly and whether or not the osteoclasts phagocytose the degradation products remain to be established.In the present communication, a hypothesis is presented suggesting the possibility that osteoclastic resorption of bone involves the participation of two different cell types. According to this hypothesis, osteoclastic bone resorption is initiated by osteoclasts that demineralize areas of bone and degrade noncollagen bone matrix. After the osteoclasts have moved away or become partially detached from the demineralized site, the exposed collagen fibrils are phagocytosed by mononuclear, fibroblast-like or monocyte-derived cells.     

Trabecular and endocortical bone remodeling in postmenopausal osteoporosis: Comparison with normal postmenopausal women

To assess the bone turnover abnormalities which characterize postmenopausal osteoporosis with vertebral fractures (PMOp), a transiliac bone biopsy was performed after double labeling of the mineralizing front with tetracycline in 50 untreated PMOp patients who were compared with 13 healthy age-matched volunteer females. The analysis of bone remodeling and structure parameters demonstrated that PMOp is a disease affecting both the cancellous and the endocortical envelopes and characterized by increased resorption and by a marked decrease in the osteoblastic apposition rate due to a reduced duration of bone formation. This induces a decrease in the width of both individual osteons and trabeculae. In PMOp, the wide spectrum of bone turnover as compared with the controls, associated with the typical bimodal distribution of cancellous osteoid perimeter, allowed us to identify two subsets, one with normal turnover (NT) and one with high turnover (HT) representing 30% of the cases. When compared to NT, HT was characterized by increased osteoclast number, lower bone volume, thinner osteons, increased formation at the tissue-level and markedly decreased duration of formation. In HT the marked decrease in the duration of activity of osteoblasts and the markedly increased number of osteoclasts induced a greater decrease in bone volume, despite the increase of bone formation at the tissue level. These subsets could not be distinguished by any clinical or biochemical parameter except for serum bone gla protein (osteocalcin) which was significantly higher (as a group) in HT than in NT. The underlying cause for these two subsets is unknown. We conclude that PMOp affects the cancellous and the endocortical bone. Bone loss results from a wide spectrum of bone turnover abnormalities, with two distinct subsets, one with normal turnover and one with high turnover.     

Mechanisms of altered bone remodeling in children with type 1 diabetes

Bone loss associated with type 1 diabetes mellitus (T1DM) begins at the onset of the disease, already in childhood, determining a lower bone mass peak and hence a greater risk of osteoporosis and fractures later in life. The mechanisms underlying diabetic bone fragility are not yet completely understood. Hyperglycemia and insulin deficiency can affect the bone cells functions, as well as the bone marrow fat, thus impairing the bone strength, geometry, and microarchitecture. Several factors, like insulin and growth hormone/insulin-like growth factor 1, can control bone marrow mesenchymal stem cell commitment, and the receptor activator of nuclear factor-κB ligand/osteoprotegerin and Wnt-b catenin pathways can impair bone turnover. Some myokines may have a key role in regulating metabolic control and improving bone mass in T1DM subjects. The aim of this review is to provide an overview of the current knowledge of the mechanisms underlying altered bone remodeling in children affected by T1DM.     

骨源性外泌体在骨重建中的作用研究进展

骨代谢活动中骨吸收和骨形成在时间和空间上的紧密偶联过程称为骨重建。骨重建是一个非常精密并且程序化的过程,主要是由于骨吸收的破骨作用和骨形成的成骨作用之间在骨表面相继发生并配对调节的一过程,从而在骨损伤的相关部位产生新骨用以替代旧骨。通过这一方式,能够修复或靶向重建骨的微损伤,从而预防骨组织疲劳损伤的累积、保持骨损伤相关部位生物力学的功能及维持体内矿物质平衡。外泌体通常指的是30 nm~100 nm的细胞外囊泡,广泛存在于生物体之中,并能参与细胞间的物质与信息交换,能够调节细胞的增殖与分化,从而控制疾病的发生与发展。以往骨重建调节研究主要集中在骨组织的自分泌、旁分泌等方面,而本文通过整理中外学者对外泌体在骨重建相关细胞的研究报道,旨在简明阐述骨源性外泌体和骨重建之间的关系。     

Cyclic bone remodeling in deer

Biopsy samples of rib, metacarpus, metatarsus and tibia were obtained from deer at five intervals during the annual antler growth cycle. Tissues were examined by either histometric methods or by densitometric measurements followed by mineral analysis using atomic absorption spectrophotometry. Little remodelling occurred in bones during the period of antler quiescence, but during the peak of antler growth in June, internal bone remodelling reached a peak. The highest resorption of 22.99% occurred in ribs with lower levels of 13.10% for metacarpus and 10.37% for metatarsus. At the cessation of antler growth in the fall, remodelling again decreased to the low level found preceeding antler growth. In ribs lowest values of ash weight per unit volume occurred during the middle of antler growth but no cyclic variations were found for these values in metacarpus, metatarsus and tibia. Percentage of calcium, magnesium and phosphorus in bone ash did not vary during the antler growth cycle indicating no change in bone composition during periods of high remodelling.This work was supported by training grant 5-Tl-DEl 30 from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md.     

Influence of estrogens on bone resorption in organ culture

Estradiol benzoate, ethinyl estradiol, and human calcitonin were compared for their ability to inhibit the spontaneous or parathyroid extract-induced bone resorption in organ cultures of 19-day fetal rat fibulae. The criteria used for the assessment of bone resorption were: the release of⁴⁵Ca from paired prelabeled bone rudiments into the culture medium, the dry weight, and the number of osteoclasts per bone. Estradiol benzoate at concentrations of 1.3×10^–5 to 2.6×10^–4 M had no effect on the release of⁴⁵Ca from fetal fibulae or on the dry weight. Histologically, treated bones were well preserved and resembled the controls. Parathyroid extract (PTE) alone caused extensive bone resorption with numerous osteoclasts and enhanced⁴⁵Ca release without weight gain. The addition of estradiol benzoate to the culture medium did not prevent the resorptive action of PTE. Ethinyl estradiol alone provoked a dose-related inhibition of⁴⁵Ca release at concentrations of 3.4×10^–5 to 1.7×10^–4 M. However, this inhibition was accompanied by a decrease in bone dry weight and by various degrees of cellular damage. The same phenomenon was observed in PTE-treated fibulae. Human calcitonin, on the contrary, inhibited the release of⁴⁵Ca by decreasing the number of osteoclasts while the weight of the fibulae increased. The inhibitory action of ethinyl estradiol appears to be caused by a toxic effect on bone cells. It is concluded that estrogens have no direct physiological effect on bone resorption in vitro.     

Trabecular bone remodeling and balance in primary hyperparathyroidism

The total remodeling sequences in 19 primary hyperparathyroid patients and 16 approximately age-matched and sex-matched controls were reconstructed from histomorphometric analyses of bone specimens obtained after intravital tetracycline double labeling. In the primary hyperparathyroid group the total amount of work performed by resorptive cells was reduced, as indicated by the significantly lower three-dimensional mononuclear and preosteoblast-like cell resorption depths (35.8 microns vs 44.5 microns in normals, P less than 0.01 and 45.3 microns vs 56.6 microns in normals, P less than 0.01, respectively). The active resorption period (i.e., the function period for osteoclasts and mononuclear cells) was reduced to 19 days compared to 29 days in normals (P less than 0.05), but no difference with respect to bone resorption rates could be demonstrated between the two groups. The median bone formation period (Sigmaf) in primary hyperparathyroid patients was not different from the value obtained in normals (172 days vs 134 days, respectively), and the matrix appositional rate (Ama), as well as the mineralization lag time (tm), were also unchanged. The initial mineralization rate (Ami(i)) was not significantly different from the value obtained in normals, but averaged over the total bone formation period, a reduced mineralization rate could be demonstrated (0.32 micron3/micron2 per day vs 0.46 micron3/micron2 per day in normals, P less than 0.01). The measured final three-dimensional thickness of bone formed during Sigmaf (mcwTm) was reduced in the primary hyperparathyroid group (51.1 microns vs 55.9 microns in normals, P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphology of bone development and bone remodeling in embryonic chick limbs

Staged embryos from White Leghorn chicken eggs were used to assemble a detailed morphological sequence of events occurring in long bone development from Hamburger-Hamilton stage 32 through stage 44 and 2 days post hatching. The detailed patterning of osteoblasts, osteoid, mineral, and vasculature were observed at the mid-diaphysis of the tibia. At stage 32, the cartilage core is composed of hypertrophie chondrocytes and is surrounded by a continuous ring of mineralized osteoid on which osteoblasts and vasculature reside. At stage 35, the vasculature and associated cell types invade the cartilage core region. By stage 37, marrow occupies the entire cartilage core region at the mid-diaphysis. Anastamosing channels, containing vasculature, interconnect with each other and the marrow region to the inside and the periosteal region to the outside. Clearly, the cartilage is replaced by marrow, not bone.

Mineral deposition at the periosteal surface continues through stage 44 as does mineral resorption on the endosteal surface, although the rate of mineral deposition and resorption varies at different developmental stages. Vasculature plays an important role in the pattern formation of the trabeculae and their channels as can be seen in the developmental sequence within one bone (the tibia) or comparisons between two bones (the tibia and fibula). A model is presented which considers the possibility that osteoprogenitor cells are formed as early as the chondroprogenitor cells. This model also emphasizes the observation that cartilage is not replaced by bone but is replaced by marrow.     

The role of pleiotrophin in bone repair

Bone has an enormous capacity for growth, regeneration, and remodelling, largely due to induction of osteoblasts that are recruited to the site of bone formation. Although the pathways involved have not been fully elucidated, it is well accepted that the immediate environment of the cells is likely to play a role via cell–matrix interactions, mediated by several growth factors. Formation of new blood vessels is also significant and interdependent to bone formation, suggesting that enhancement of angiogenesis could be beneficial during the process of bone repair. Pleiotrophin (PTN), also called osteoblast-specific factor 1, is a heparin-binding angiogenic growth factor, with a well-defined and significant role in both physiological and pathological angiogenesis. In this review we summarise the existing evidence on the role of PTN in bone repair.     

Medullary bone of laying hens during calcium depletion and repletion

Bone cell activity and the composition of the femur of laying hens were studied during 7 days of calcium depletion on a 0.13% calcium diet and 7 days of calcium repletion on a 3.2% calcium diet. Histologically, only cortical bone showed clear signs of bone resorption and osteoclastic activity during the depletion period. The number of osteoclasts in medullary bone varied little from control values throughout both calcium depletion and repletion, except for a significant increase on the first day of depletion. The major histologicalchange in medullary bone was a marked increase in the number of osteoblasts on the third, fifth and, to a lesser extent, seventh, day of depletion. The number of osteoblasts in medullary bone was positively correlated with its osteoid content and negatively correlated with its degree of calcification. Alkaline phosphatase activity of medullary bone increased with the time the hens had been on the calcium-deficient diet. Returning the hens to the 3.2% calcium ration caused, within one day, a significant increase in medullary bone calcification, a decrease of osteoblast numbers to, or below, control levels, and a drastic reduction in alkaline phosphatase activity of medullary bone. The significance of these findings in relation to the control of bone cell populations and the functions of medullary bone is discussed.Authorized as paper no. 3510 in the Journal Series of the Pennsylvania Agricultural Experiment Station. This investigation was supported in part by U. S. Public Health Service Research Grant AM-04362 from the National Institutes of Health.     

A quantitative study of the effects of prolonged calcitonin treatment on alveolar bone remodelling in the golden hamster

This study was carried out on adult golden hamsters, to correlate the effects of porcine calcitonin on serum Ca and P concentrations with changes in osteoclastic resorption of bone. After 1 month of treatment with 5 MRC units/kg/day, there was little effect on these parameters. On the other hand, the hormone appears to have an effect on the remodelling sequence, as indicated by a large and very significant decrease in the extent of resorption lacunae unoccupied by osteoclasts. It is suggested that the duration of the reversal phase, separating the end of active resorption from the beginning of active bone formation in each remodelling focus, is greatly decreased. This shortening is associated with a prolongation oft he bone formation phase, and the extent of osteoid tissue is markedly and proportionally increased in the treated animals. After a prolonged administration, calcitonin does not seem to impede the formation of new osteoclasts, their number being only slightly diminished, but it remains possible that it does continue to inactivate these cells. The effect on the bone remodelling sequence could be either due to direct action on the osteoblasts or their precursor cells, or an indirect action via the osteoclasts.     

Runx2基因对骨代谢调控的研究进展

人体骨代谢是一个复杂的过程,是破骨细胞（ osteoclast ,OC）吸收旧骨和成骨细胞（ osteoblast ,OB）形成新骨的动态平衡的过程。 Runx2（core binding factor alphal 1,核心结合因子a1）是调控成骨细胞和破骨细胞的分化促进骨形成的关键调控因子,通过调控成骨细胞特异性细胞外基质蛋白基因的表达和成骨细胞周期参与成骨细胞的分化过程,促进骨形成和抑制骨吸收。本文就Runx2在骨代谢中的作用作一综述。     

Resorption of bone collagen by multinucleated cells

Young donor rats of an isogenously related strain were injected with³H proline (1 Ci/g) and killed from 6 hours to 28 days later. The scapulae were removed, decalcified with EDTA and implanted subcutaneously into the backs of recipient rats. They were removed 14 days later with the surrounding tissue, sectioned, processed for autoradiography and stained with hematoxylin and eosin. In all cases there was a giant cell response round the bone. In bones removed from the donor animals 6 hours after proline injections, the label was on the edge of the appositional side of the bone and the giant cells did not remove it. By 28 days after proline administration when, due to apposition and resorption, the label was on the resorptive side of the bone, giant cells were seen removing the label, which however they did not ingest. It thus appears, as has been suggested in the literature, that recently formed bone collagen is removed with difficulty, but older collagen can be resorbed.This work was supported in part by Research Grant No. DE-1592 from the National Institute for Dental Research. National Institutes of Health.     

Osteoprotegerin deficiency attenuates strontium‐mediated inhibition of osteoclastogenesis and bone resorption

Strontium (Sr) exerts an anabolic and antiresorptive effect on bone, but the mechanism remains unknown. Osteoprotegerin (OPG) expressed by osteoblasts plays an important role in regulating bone homeostasis by inhibiting osteoclastogenesis and bone resorption. This study aims at evaluating the role of OPG in Sr‐mediated inhibition of osteoclastogenesis and bone resorption. Six‐week‐old Opg knockout (KO) male mice and their wild‐type (WT) littermates were treated orally with vehicle (Veh) or Sr compound (4 mmol/kg) daily for 8 weeks. Bone mass and microstructure in the lumbar spine (L₄) and proximal tibia were analyzed with micro–computed tomography (µCT). Bone remodeling was evaluated with serum biochemical analysis and static and dynamic bone histomorphometry. Osteoclast differentiation potential and gene expression were analyzed in bone marrow cells. The findings demonstrate that Sr compound treatment results in greater bone volume and trabecular number than Veh treatment in WT mice. The anabolic response of trabecular bone to Sr treatment is attenuated in KO mice. Although Sr treatment significantly decreases in vitro osteoclastogenesis and bone resorption in WT mice, these effects are attenuated in KO mice. Furthermore, Sr treatment profoundly increases Opg gene expression in the tibias and OPG protein levels in the sera of WT mice. This study concludes that the inhibition of osteoclastogenesis and bone resorption is possibly associated with OPG upregulation by Sr treatment. © 2011 American Society for Bone and Mineral Research.     

甲亢治疗前、后骨代谢的临床观察

目的 了解甲亢合并骨质疏松病人治疗前、后生化指标，骨密度的变化，观察甲亢治疗1年后骨代谢的情况。方法 选择未治疗的甲亢病人88例与甲亢治疗1年后病人的BMD，血Ca、P、AKP、BGP、尿HOP的各项指标进行对比，进行临床分析和研究。结果 甲亢治疗前FT3、FT4、AKP、BGP均高于治疗组，TSH明显低于治疗组。甲亢治疗后骨量减少与骨质疏松均低于治疗前组。结论 甲亢经治疗后，甲状腺功能恢复正常，骨矿含量增加。     

Rapid induction of localized bone resorption in the auditory bulla of the mongolian gerbil,Meriones unguiculatus,by increased air presure

Summary Osteoclastic bone resorption can be induced within the auditory bulla of the Mongolian gerbil by minute increases in air pressure. The objective of this study was to determine the rate at which osteoclasts are induced in areas of resorption and to compare the rate of osteoblast recruitment. Stainless steel tubes were implanted into the middle ears of 31 male gerbils through which a static pressure of 10 mm Hg above atmospheric was applied. The animals were killed at intervals (0, 1, 2, 4, 6, and 8 days) and the ventral wall of the bulla was studied histomorphometrically. The osteoclast area and osteoclast number were measured along the inner surface of the bone of the bulla; the activity of osteoblasts of the outer surface was also estimated. A significant increase in osteoclasts was found over time (P<0.0001). There was no significant increase in osteoclasts at 1 day (P=0.5) but at 2, 4, 6, and 8 days there was a significant increase (P<0.03). Osteoblast activity on the outside of the bulla wall paralleled osteoclast activity. In this quantifiable, experimental system, a localized modeling process may be induced within 2 days without the confounding elements of localized trauma or inflammation inherent in other models of localized bone remodeling.     

Uptake and loss of plutonium from osteoclasts and macrophages in the mandibular condyle of the rat

Summary Female rats were used to study the kinetics of plutonium transfer from the bone surfaces of the mandibular condyle to osteoclasts and macrophages. This study was made using autoradiographs prepared from plastic sections of the mineralized bones of animals which had been injected with²⁴¹ Pu citrate. Measurements of the concentration of plutonium in the osteoclasts and macrophages at different times after the injection of plutonium showed that plutonium was concentrated by osteoclasts from bone surfaces and was retained with a half-time of ∼ 70 h. Subsequently, plutonium appeared to be transferred to macrophages. The results showed that plutonium was unlikely to be accumulated by macrophages as a result of their participation in bone resorption.     

Formation of bone by isolated,cultured osteoblasts in millipore diffusion chambers

Summary Osteoblast-like and osteoclast-like cells freed from neonatal calvaria by sequential enzymatic digestion after 6–7 days in culture were placed in diffusion chambers and implanted in the peritoneal cavities of CD-1 mice. About half of the chambers also contained a dead calvarium to test for the need of an “inducer.” After 20 days, 11 of 18 chambers containing the osteoblast-like cells formed large foci of mineralized bone that corresponded to alkaline phosphatase activity throughout the chambers. Moreover, only type I (i.e., bone) collagen was formed. Occasional deposits of bone were found in only 3 of 22 chambers containing the osteoclast-like cells. The presence of dead bone did not affect any of the results. These data confirm the osteoblast-like nature of the isolated cell populations and demonstrate that these cells retain their differentiated function in culture.     

骨髓单个核细胞和骨髓源内皮祖细胞移植促进血流重建的比较研究

目的 比较骨髓单个核细胞(MNCs)和骨髓源内皮祖细胞(EPCs)移植促进血流重建的效果,探讨非内皮祖细胞在血流重建中的作用.方法 获取Lewis大鼠骨髓MNCs,部分MNCs在体外诱导分化为EPCs.采用Lewis大鼠建立单侧后肢缺血模型.建模后3 d,将模型鼠随机分为3组:(1)对照组(n=6),将0.8 mL D-Hank's液注入对照组大鼠缺血侧后肢;(2)MNC组(n=6),将8×10~6个骨髓MNC植入MNC组大鼠缺血侧后肢;(3)EPC组(n=6),将体外培养的8 × 10~6个EPC植入EPC组大鼠缺血侧后肢.细胞移植后3周行大鼠后肢动脉造影,检测缺血侧后肢侧支血管数;切取缺血侧后肢腓肠肌,分别行CD31和α-SMA免疫组化染色,计算毛细血管密度和小动脉密度.结果 MNC组毛细血管密度与EPC组差异无统计学意义[(31.67 ± 7.87)个/HP vs.(32.83±5.38)个/HP,P＞0.05].而均高于对照组(19.67 ± 4.80个/HP)(P＜0.05);MNC组侧支血管数与EPC组差异无统计学意义[(4.17±0.75)个vs.(4.50 4±1.38)个,P＞0.05],但均高于对照组[(2.50 ± 1.5)个](P＜0.05);MNC组小动脉密度与EPC组差异无统计学意义[(4.83 ± 1.47)个vs.(5.50 ± 2.35)个,P＞0.05],亦均高于对照组[(2.17±0.98)个](P＜0.05).结论 在骨髓干细胞移植治疗肢体缺血性疾病中,非内皮祖细胞在血流重建中所起的作用与EPC相似.     

Osteoclastic resorption of3H-proline labelled bone,dentine and cementum in the rat

Six litter mate Wistar rats were injected intraperitoneally with 2 Ci/g body weight³H-proline at the age of 15 and 20 days. Six to seven weeks later resorption of cementum, dentine and bone was caused experimentally by luxation of the first right mandibular molar. The animals were killed 10 days later in glutaraldehyde perfusion fixation and 5 serial sections of the mandibles were processed for radioautography. Cementum, dentine and bone showed bands of labelling corresponding to the stage of odontogenesis and osteogenesis at the time of injection. Osteoclasts were found in contact with labelled cementum, dentine and bone but the number of grains over osteoclasts was insignificant.