Altered lacunar and vascular porosity in osteogenesis imperfecta mouse bone as revealed by synchrotron tomography contributes to bone fragility

Osteogenesis imperfecta (brittle bone disease) is caused by mutations in the collagen genes and results in skeletal fragility. Changes in bone porosity at the tissue level indicate changes in bone metabolism and alter bone mechanical integrity. We investigated the cortical bone tissue porosity of a mouse model of the disease, oim, in comparison to a wild type (WT-C57BL/6), and examined the influence of canal architecture on bone mechanical performance.High-resolution 3D representations of the posterior tibial and the lateral humeral mid-diaphysis of the bones were acquired for both mouse groups using synchrotron radiation-based computed tomography at a nominal resolution of 700 nm. Volumetric morphometric indices were determined for cortical bone, canal network and osteocyte lacunae. The influence of canal porosity architecture on bone mechanics was investigated using microarchitectural finite element (μFE) models of the cortical bone. Bright-field microscopy of stained sections was used to determine if canals were vascular.Although total cortical porosity was comparable between oim and WT bone, oim bone had more numerous and more branched canals (p < 0.001), and more osteocyte lacunae per unit volume compared to WT (p < 0.001). Lacunae in oim were more spherical in shape compared to the ellipsoidal WT lacunae (p < 0.001). Histology revealed blood vessels in all WT and oim canals. μFE models of cortical bone revealed that small and branched canals, typical of oim bone, increase the risk of bone failure. These results portray a state of compromised bone quality in oim bone at the tissue level, which contributes to its deficient mechanical properties.     

Increased osteocyte death and mineralization inside bone after parathyroidectomy in patients with secondary hyperparathyroidism

In order to gain insight into the mechanisms underlying the dynamic changes in bone metabolism and bone quality after parathyroidectomy (PTX) in secondary hyperparathyroid patients with high levels of parathyroid hormone (PTH), we performed bone histomorphometric analysis with tetracycline labeling in iliac bone biopsy specimens taken before and after PTX, with special attention paid to osteocytes. At 2 to 4 weeks after PTX, PTH concentrations decreased markedly with evident reductions in bone turnover markers. Histomorphometry revealed that at 2 to 4 weeks following PTX, the osteoclast surface decreased to nearly 0%, with a substantial increase in osteoid volume and a reduction in fibrosis volume. Labeling with tetracycline was observed not only at the mineralization front on the bone surface but also around the osteocyte lacunar walls and canaliculi within both the basic multicellular units (BMUs) and bone structural units (BSUs), suggesting that mineralization was taking place along the lacunocanalicular system after PTX. The tetracycline‐labeled area was much greater in the BSUs than in the BMUs and at the mineralization front, and the tetracycline labeling in the BSUs was markedly increased after PTX compared with that in the low‐ and high‐PTH control groups without PTX. The osteocyte number was decreased significantly after PTX, concomitant with an increase in the number of empty lacunae and a reduction of lacunar volume. Thus the increased osteocyte death and mineralization around the lacunocanalicular system in association with a rapid decline in PTH may underlie the changes in bone metabolism and quality that occur following PTX. © 2010 American Society for Bone and Mineral Research.     

Loss of osteocyte integrity in association with microdamage and bone remodeling after fatigue in vivo.

As a result of fatigue, bone sustains microdamage, which is then repaired by bone-remodeling processes. How osteoclastic activity is targeted at the removal of microdamaged regions of bone matrix is unknown. In the current studies, we tested the hypothesis that changes in osteocyte integrity, through the initiation of regulated cell death (apoptosis), are associated with fatigue-related microdamage and bone resorption. Ulnae of adult rats were fatigue-loaded to produce a known degree of matrix damage. Osteocyte integrity was then assessed histomorphometrically from terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-nick end labeling (TUNEL)-stained sections to detect cells undergoing DNA fragmentation associated with apoptosis; toluidine blue-stained sections were used for secondary morphological confirmation. Ten days after loading, large numbers of TUNEL-positive osteocytes were found in bone surrounding microcracks and in bone surrounding intracortical resorption spaces (approximately 300% increases over controls, p < 0.005). TUNEL labeling in loaded ulnae at sites distant from microcracks or resorption foci did not differ from that in control bone. Osteocytes in toluidine blue-stained sections showed equivalent trends to TUNEL-stained sections, with significant increases in pyknotic nuclei and empty lacunae associated with microcracks and intracortical resorption spaces. TUNEL-positive osteocytes were observed around bone microdamage by 1 day after loading (p < 0.01 relative to baseline), and their number remained elevated throughout the entire experimental period. Increases in empty lacunae and decreases in normal osteocyte numbers were observed over time as well. These studies show that (1) osteocyte apoptosis is induced by bone fatigue, (2) this apoptosis is localized to regions of bone that contain microcracks, and (3) osteoclastic resorption after fatigue also coincides with regions of osteocyte apoptosis. The strong associations between microdamage, osteocyte apoptosis, and subsequent bone remodeling support the hypothesis that osteocyte apoptosis provides a key part of the activation or signaling mechanisms by which osteoclasts target bone for removal after fatigue-induced matrix injury.     

Fragile X Messenger Ribonucleoprotein 1(FMR1), a novel inhibitor of osteoblast/osteocyte differentiation,regulates bone formation,mass, and strength in young and aged male and female mice

<正>Fragile X Messenger Ribonucleoprotein 1(FMR1) gene mutations lead to fragile X syndrome, cognitive disorders, and, in some individuals,scoliosis and craniofacial abnormalities. Four-month-old(mo) male mice with deletion of the FMR1 gene exhibit a mild increase in cortical and cancellous femoral bone mass. However, consequences of absence of FMR1 in bone of young/aged male/female mice and the cellular basis of the skeletal phenotype remain unknown. We found that absence of FMR1 results i...     

Microtubule assembly affects bone mass by regulating both osteoblast and osteoclast functions: Stathmin deficiency produces an osteopenic phenotype in mice

Cytoskeleton microtubules regulate various cell signaling pathways that are involved in bone cell function. We recently reported that inhibition of microtubule assembly by microtubule‐targeting drugs stimulates osteoblast differentiation and bone formation. To further elucidate the role of microtubules in bone homeostasis, we characterized the skeletal phenotype of mice null for stathmin, an endogenous protein that inhibits microtubule assembly. In vivo micro–computed tomography (µCT) and histology revealed that stathmin deficiency results in a significant reduction of bone mass in adult mice concurrent with decreased osteoblast and increased osteoclast numbers in bone tissues. Phenotypic analyses of primary calvarial cells and bone marrow cells showed that stathmin deficiency inhibited osteoblast differentiation and induced osteoclast formation. In vitro overexpression studies showed that increased stathmin levels enhanced osteogenic differentiation of preosteoblast MC3T3‐E1 cells and mouse bone marrow–derived cells and attenuated osteoclast formation from osteoclast precursor Raw264.7 cells and bone marrow cells. Results of immunofluorescent studies indicated that overexpression of stathmin disrupted radial microtubule filaments, whereas deficiency of stathmin stabilized the microtubule network structure in these bone cells. In addition, microtubule‐targeting drugs that inhibit microtubule assembly and induce osteoblast differentiation lost these effects in the absence of stathmin. Collectively, these results suggest that stathmin, which alters microtubule dynamics, plays an essential role in maintenance of postnatal bone mass by regulating both osteoblast and osteoclast functions in bone. © 2011 American Society for Bone and Mineral Research     

Increased intra-cortical porosity reduces bone stiffness and strength in pediatric patients with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disease occurring in one out of every 20,000 births. Although it is known that Type I collagen mutation in OI leads to increased bone fragility, the mechanism of this increased susceptibility to fracture is not clear. The aim of this study was to assess the microstructure of cortical bone fragments from patients with osteogenesis imperfecta (OI) using polarized light microscopy, and to correlate microstructural observations with the results of previously performed mechanical compression tests on bone from the same source. Specimens of cortical bone were harvested from the lower limbs of three (3) OI patients at the time of surgery, and were divided into two groups. Group 1 had been subjected to previous micro-mechanical compression testing, while Group 2 had not been subjected to any prior testing. Polarized light microscopy revealed disorganized bone collagen architecture as has been previously observed, as well as a large increase in the areal porosity of the bone compared to typical values for healthy cortical bone, with large (several hundred micron sized), asymmetrical pores. Importantly, the areal porosity of the OI bone samples in Group 1 appears to correlate strongly with their previously measured apparent Young's modulus and compressive strength. Taken together with prior nanoindentation studies on OI bone tissue, the results of this study suggest that increased intra-cortical porosity is responsible for the reduction in macroscopic mechanical properties of OI cortical bone, and therefore that in vivo imaging modalities with resolutions of ~ 100 μm or less could potentially be used to non-invasively assess bone strength in OI patients. Although the number of subjects in this study is small, these results highlight the importance of further studies in OI bone by groups with access to human OI tissue in order to clarify the relationship between increased porosity and reduced macroscopic mechanical integrity.     

Enhanced bone regeneration associated with decreased apoptosis in mice with partial HIF-1alpha deficiency.

HIF-1alpha activates genes under hypoxia and was hypothesized to regulate bone regeneration. Surprisingly, HIF-1alpha+/- fracture calluses are larger, stronger, and stiffer than HIF-1alpha+/+ calluses because of decreased apoptosis. These data identify apoptosis inhibition as a means to enhance bone regeneration. INTRODUCTION: Bone regeneration subsequent to fracture involves the synergistic activation of multiple signaling pathways. Localized hypoxia after fracture activates hypoxia-inducible factor 1alpha (HIF-1alpha), leading to increased expression of HIF-1 target genes. We therefore hypothesized that HIF-1alpha is a key regulator of bone regeneration. MATERIALS AND METHODS: Fixed femoral fractures were generated in mice with partial HIF-1alpha deficiency (HIF-1alpha+/-) and wildtype littermates (HIF-1alpha+/+). Fracture calluses and intact contralateral femurs from postfracture days (PFDs) 21 and 28 (N=5-10) were subjected to microCT evaluation and four-point bending to assess morphometric and mechanical properties. Molecular analyses were carried out on PFD 7, 10, and 14 samples (N=3) to determine differential gene expression at both mRNA and protein levels. Finally, TUNEL staining was performed on PFD 14 samples (N=2) to elucidate differential apoptosis. RESULTS: Surprisingly, fracture calluses from HIF-1alpha+/- mice exhibited greater mineralization and were larger, stronger, and stiffer. Microarray analyses focused on hypoxia-induced genes revealed differential expression (between genotypes) of several genes associated with the apoptotic pathway. Real-time PCR confirmed these results, showing higher expression of proapoptotic protein phosphatase 2a (PP2A) and lower expression of anti-apoptotic B-cell leukemia/lymphoma 2 (BCL2) in HIF-1alpha+/+ calluses. Subsequent TUNEL staining showed that HIF-1alpha+/+ calluses contained larger numbers of TUNEL+ chondrocytes and osteoblasts than HIF-1alpha+/- calluses. CONCLUSIONS: We conclude that partial HIF-1alpha deficiency results in decreased chondrocytic and osteoblastic apoptosis, thereby allowing the development of larger, stiffer calluses and enhancing bone regeneration. Furthermore, apoptosis inhibition may be a promising target for developing new treatments to accelerate bone regeneration.     

HO-1基因修饰对急性肾损伤微环境下骨髓间充质干细胞增生分化的影响

目的观察经血红素加氧酶-1(HO-1)基因修饰的骨髓间充质干细胞(BMSCs)在急性肾损伤(AKI)微环境下的增生分化,并探讨其机制。 方法Gateway技术构建含HO-1目的基因的质粒,同时构建仅含示踪基因eGFP的对照载体;脂质体法转染293FT细胞获得慢病毒原液lenti-HO-1-eGFP和lenti-eGFP;稀释后分别感染BMSCs获得HO-1-BMSCs和eGFP-BMSCs(空载体对照),检测转染细胞的活力、分化潜能。制作缺血再灌注诱导AKI大鼠的肾脏匀浆上清(AKI-KHS),以正常大鼠肾脏匀浆上清(N-KHS)作为对照,分别干预BMSCs、eGFP-BMSCs和HO-1-BMSCs,构成空白组(BMSCs组)、对照组(BMSCs/N-KHS组)、BMSCs/AKI-KHS组、eGFP-BMSCs/AKI-KHS组和HO-1-BMSCs/AKI-KHS组,于37℃、5% CO₂培养箱中培养3 d。MTT法检测培养BMSCs的生长抑制率,流式细胞仪检测细胞凋亡,透射电镜观察细胞超微结构变化,免疫组化法检测细胞内角蛋白18(CK18)表达,Western印迹对各组细胞内HO-1、磷酸化丝氨酸/苏氨酸蛋白激酶(pAKT)、磷酸化细胞外信号调节激酶(pERK)水平进行检测。用SPSS19.0统计软件进行统计学分析。 结果基因修饰并未改变BMSCs活力及多向分化潜能。与对照组比,BMSCs/AKI-KHS组的细胞生长抑制率和凋亡阳性细胞比例显著增加(t＝12.581,t＝16.283;P<0.05);经HO-1基因修饰后,HO-1-BMSCs/AKI-KHS组的细胞生长抑制率和凋亡阳性细胞比例均有显著下降(t＝5.958,t＝7.957;P<0.05)。AKI-KHS可诱导BMSCs出现肾小管上皮细胞样分化的超微结构改变,胞浆中可观察到CK18表达,以HO-1-BMSCs/AKI-KHS组的CK18^＋细胞比例最高(t＝4.057,P<0.05)。与BMSCs/AKI-KHS组相比,HO-1-BMSCs/AKI-KHS组的细胞内HO-1蛋白水平显著增高(t＝4.163,P<0.05),并伴随着细胞内pAKT、pERK蛋白水平显著增高(t_pAKT＝14.305,t_pERK＝7.148;P<0.05)。 结论HO-1基因修饰可改善AKI微环境下培养BMSCs的增殖,细胞凋亡减轻,向肾小管上皮样细胞转分化能力增加,HO-1的过表达及其下游的AKT、ERK为发挥作用的可能信号通路。     

Cell line IDG‐SW3 replicates osteoblast‐to‐late‐osteocyte differentiation in vitro and accelerates bone formation in vivo

Osteocytes are the most abundant cells in bone yet are the most challenging to study because they are embedded in a mineralized matrix. We generated a clonal cell line called IDG‐SW3 (for Immortomouse/Dmp1‐GFP‐SW3) from long‐bone chips from mice carrying a Dmp1 promoter driving GFP crossed with the Immortomouse, which expresses a thermolabile SV40 large T antigen regulated by interferon γ (IFN‐γ). Cells from these mice can be expanded at 33 °C in the presence of IFN‐γ and then allowed to resume their original phenotype at 37 °C in the absence of IFN‐γ. IDG‐SW3 cells are Dmp1‐GFP^? and T antigen⁺ under immortalizing conditions but Dmp1‐GFP⁺ and T antigen^? under osteogenic conditions. Like osteoblasts, they express alkaline phosphatase and produce and mineralize a type 1 collagen matrix containing calcospherulites. Like early osteocytes, they express E11/gp38, Dmp1, MEPE, and Phex. Like late osteocytes, they develop a dendritic morphology and express SOST/sclerostin and fibroblast growth factor 23 (FGF‐23), regulated by parathyroid hormone (PTH) and 1,25‐dihydroxyvitamin D₃. When cultured on 3D matrices, they express Dmp1‐GFP and sclerostin. When the 3D cultures are implanted in calvarial defects in vivo, they accelerate bone healing. This cell line should prove useful for studying osteoblast‐to‐osteocyte transition, mechanisms for biomineralization, osteocyte function, and regulation of SOST/sclerostin and FGF‐23. © 2011 American Society for Bone and Mineral Research     

Remodeling of fracture callus in mice is consistent with mechanical loading and bone remodeling theory

During the remodeling phase of fracture healing in mice, the callus gradually transforms into a double cortex, which thereafter merges into one cortex. In large animals, a double cortex normally does not form. We investigated whether these patterns of remodeling of the fracture callus in mice can be explained by mechanical loading. Morphologies of fractures after 21, 28, and 42 days of healing were determined from an in vivo mid‐diaphyseal femoral osteotomy healing experiment in mice. Bone density distributions from microCT at 21 days were converted into adaptive finite element models. To assess the effect of loading mode on bone remodeling, a well‐established remodeling algorithm was used to examine the effect of axial force or bending moment on bone structure. All simulations predicted that under axial loading, the callus remodeled to form a single cortex. When a bending moment was applied, dual concentric cortices developed in all simulations, corresponding well to the progression of remodeling observed experimentally and resulting in quantitatively comparable callus areas of woven and lamellar bone. Effects of biological differences between species or other reasons cannot be excluded, but this study demonstrates how a difference in loading mode could explain the differences between the remodeling phase in small rodents and larger mammals. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 664–672, 2009     

Testosterone therapy increases sexual desire in ageing men with low-normal testosterone levels and symptoms of androgen deficiency

Although decline in sexual function is a common reason for ageing men to seek advice regarding testosterone therapy, placebo-controlled trial data have been unable to show a consistent, beneficial role for testosterone. The objective of this study was to determine the effect of testosterone therapy on sexual function in non-obese ageing men with symptoms of androgen deficiency and low-normal serum testosterone levels. A total of 60 men aged 55 years or older in good general health with total testosterone (TT) levels <15 nM, and with symptoms suggestive of androgen deficiency, were randomized in a double-blinded protocol to transdermal testosterone patches or placebo for 12 months. Sexual function was assessed using the International Index of Erectile Function at weeks 0, 26 and 52. In men receiving testosterone TT levels increased by 30% (P=0.01) and luteinizing hormone decreased by 50% (P<0.001). Relative to placebo, testosterone therapy improved sexual desire (P=0.04); however other parameters of sexual function including erectile function were unaffected by the treatment. Ageing men in good general health and with symptoms of androgen deficiency and low-normal serum testosterone levels receiving 12 months of transdermal testosterone therapy experienced, relative to placebo, improved sexual desire but no effect on other parameters of sexual function.     

Longitudinal elastic properties and porosity of cortical bone tissue vary with age in human proximal femur

Tissue level structural and mechanical properties are important determinants of bone strength. As an individual ages, microstructural changes occur in bone, e.g., trabeculae and cortex become thinner and porosity increases. However, it is not known how the elastic properties of bone change during aging. Bone tissue may lose its elasticity and become more brittle and prone to fractures as it ages. In the present study the age-dependent variation in the spatial distributions of microstructural and microelastic properties of the human femoral neck and shaft were evaluated by using acoustic microscopy. Although these properties may not be directly measured in vivo, there is a major interest to investigate their relationships with the linear elastic measurements obtained by diagnostic ultrasound at the most severe fracture sites, e.g., the femoral neck. However, before the validity of novel in vivo techniques can be established, it is essential to understand the age-dependent variation in tissue elastic properties and porosity at different skeletal sites. A total of 42 transverse cross-sectional bone samples were obtained from the femoral neck (Fn) and proximal femoral shaft (Ps) of 21 men (mean ± SD age 47.1 ± 17.8, range 17–82 years). Samples were quantitatively imaged using a scanning acoustic microscope (SAM) equipped with a 50 MHz ultrasound transducer. Distributions of the elastic coefficient (c₃₃) of cortical (Ct) and trabecular (Tr) tissues and microstructure of cortex (cortical thickness Ct.Th and porosity Ct.Po) were determined. Variations in c₃₃ were observed with respect to tissue type (c_33Tr < c_33Ct), location (c₃₃(Ct.Ps) = 37.7 GPa > c₃₃(Ct.Fn) = 35.3 GPa > c₃₃(Tr.Ps) = 33.8 GPa > c₃₃(Tr.Fn) = 31.9 GPa), and cadaver age (R² = 0.28–0.46, p < 0.05). Regional variations in porosity were found in the neck (superior 13.1%; inferior 6.1%; anterior 10.1%; posterior 8.6%) and in the shaft (medial 9.5%; lateral 7.7%; anterior 8.6%; posterior 12.0%). In conclusion, significant variations in elastic coefficients were detected between femoral neck and shaft as well as between the quadrants of the cross-sections of neck and shaft. Moreover, an age-related increase in cortical porosity and a stiffening of the bone tissue were observed. These findings may explain in part the increase in susceptibility to suffer low energy fractures during aging and highlight the potential of ultrasound in clinical osteoporosis diagnostics.     

Effects of selenium and iodine deficiency on bone, cartilage growth plate and chondrocyte differentiation in two generations of rats

OBJECTIVE: The purpose of the current study was to investigate the roles of combined selenium and iodine deficiency in bone development as a possible experimental model of Kashin-Beck osteoarthropathy. METHODS: Sprague-Dawley rats (n=48) were randomly divided into selenium deficiency (-Se+I), iodine deficiency (+Se-I), combined selenium and iodine deficiency (-Se-I), and selenium and iodine sufficient (+Se+I) groups. Growth of bone and cartilage, and the expression of type X collagen (ColX) and parathyroid hormone-related peptide (PTHrP) were measured in two generations of rats (F(0) and F(1)). RESULTS: The tibial length in -Se-I rats was significantly shorter in F(1) generation. In +Se-I of F(1) rats, the thickness of the growth plate cartilage, and the proliferative zone was smaller, while in -Se-I rats the growth plate, and the proliferative and hypertrophic zones were also thinner in F(1) generation. In articular cartilage, ColX expression was increased in the deep zone in -Se-I rats of F(0) generation, and in -Se+I, +Se-I and -Se-I rats of F(1) generation. PTHrP expression was increased in the middle zone of -Se+I, +Se-I and -Se-I rats of both F(0) and F(1) generations. In the growth plate cartilage, ColX and PTHrP were expressed in the hypertrophic zone. ColX expression was significantly weaker in -Se+I and -Se-I rats in both F(0) and F(1) generations, while PTHrP expression was stronger in -Se+I, +Se-I and -Se-I rats in both F(0) and F(1) animals. CONCLUSIONS: Combined selenium and iodine deficiency impaired the growth of bone and cartilage. The changes in the expression of ColX and PTHrP induced by combined selenium and iodine deficiency were compatible to measurements of ColX and PTHrP in Kashin-Beck osteoarthropathy.     

Evaluation of vascular and metabolic deficiency in patients with large leg ulcers.

A consecutive series of 50 patients with large leg ulcers (surface area > 100 cm2) were investigated for evidence of arterial, venous and nutritional problems. Arterial insufficiency was found in 34%, venous reflux in 50%. A group of eight patients had no arterial or venous problem but had serious deficiencies of vitamin C and zinc. Arterial bypass was performed successfully in 15 of the 17 patients with arterial disease. All patients had a mesh split-skin graft. The 25 with venous incompetence had compression bandaging; in these patients the ulcer had healed on discharge but 10 had recurrent ulceration within 6 months. The leg ulcers in patients with corrected arterial insufficiency healed significantly more rapidly than those with venous incompetence. The ulcers in those with nutritional deficiency healed promptly after skin grafting and correction of the deficiency. It is important to be aware of arterial insufficiency and nutritional deficiency in patients with leg ulcers, as such deficiencies may contribute to the non-healing of an apparently straightforward leg ulcer.     

Stk11 (Lkb1) deletion in the osteoblast lineage leads to high bone turnover,increased trabecular bone density and cortical porosity

The mTOR pathway couples energy homeostasis to growth, division and survival of the cell. Stk11/Lkb1 is a critical serine–threonine protein kinase in the inhibition of mTOR pathway action. In the mammalian skeleton, Stk11 regulates the transition between immature and hypertrophic chondrocytes. Here, we have focused on the action of Stk11in the osteoblast lineage through osteoblast specific-removal of Stk11 activity. In the mouse model system, specification and primary organization of the neonatal boney skeleton is independent of Stk11. However, histological, molecular and micro-CT analysis revealed a marked perturbation of normal bone development evident in the immediate post-natal period. Cortical bone was unusually porous displaying a high rate of turnover with new trabeculae forming in the endosteal space. Trabecular bone also showed enhanced turnover and marked increase in the density of trabeculae and number of osteoclasts. Though mutants showed an expansion of bone volume and trabecular number, their bone matrix comprised large amounts of osteoid and irregularly deposited woven bone highlighted by diffuse fluorochrome labeling. Additionally, we observed an increase in fibroblast-like cells associated with trabecular bone in Stk11 mutants. Stk11 down-regulates mTORC1 activity through control of upstream modulators of the AMP kinase family: an increase in the levels of the phosphorylated ribosomal protein S6, a target of mTORC1-mediated kinase activity, on osteoblast removal of Stk11 suggests deregulated mTORC1 activity contributes to the osteoblast phenotype. These data demonstrate Stk11 activity within osteoblasts is critical for the development of normally structured bone regulating directly the number and coordinated actions of osteoblasts, and indirectly osteoclast number.