Determination of bone volume by osteocyte population

During development and growth, biological tissues and organisms can control their size and mass by regulating cell number (Raff, 1992; Conlon and Raff, 1999). Later in life both cell number and organ mass decrease (Buetow, 1985). We demonstrate that the number density of bone cells buried in the calcified matrix (osteocyte lacunar density) predicts extracellular matrix volume for both cancellous and cortical bone in a broad cross-section of the population (males and females, age range 23-91 years, r(2) = 0.98). Our hypothesis is that bone mass is determined by the control of osteocyte number, and that this is a particular instance of the control of organ size through the social controls on cell survival and death (Raff, 1992; Conlon and Raff, 1999).     

Histomorphometric study on the osteocyte lacuno-canalicular network in animals of different species. II. Parallel-fibered and lamellar bones

The shape, size and density of osteocyte lacunae in parallel-fibered and lamellar bone were histomorphometrically analyzed in relation to the organization of the collagen fiber texture and the animal species (frog, sheep, dog, bovine, horse and man). The following parameters were measured under the light microscope (LM) by a computer-assisted image analyzer: 1) shape, size and distribution of osteocyte lacunae; 2) osteocyte lacuno-canalicular density. In close agreement with our previous studies, which includes woven bone, it resulted that in all animals (even in frog) osteocyte lacunae have a rounded globous shape in woven bone and an oval shape in both parallel-fibered and lamellar bone; in the latter, however, they are more flattened, only located in loose lamellae and thus regularly distributed in rows. Osteocyte lacunar density is higher in woven-fibered, intermediate in parallel-fibered and lower in lamellar bone, whereas no correlation seems to exist with the animal species. In conclusion, these results suggest that osteocyte shape, size and density seem to depend mainly on collagen fiber texture rather than on the animal species. The role of osteocyte-recruitment on the spatial organization of collagen fibers in bone tissues is discussed.     

Collagen for bone tissue regeneration

In the last decades, increased knowledge about the organization, structure and properties of collagen (particularly concerning interactions between cells and collagen-based materials) has inspired scientists and engineers to design innovative collagen-based biomaterials and to develop novel tissue-engineering products. The design of resorbable collagen-based medical implants requires understanding the tissue/organ anatomy and biological function as well as the role of collagen's physicochemical properties and structure in tissue/organ regeneration. Bone is a complex tissue that plays a critical role in diverse metabolic processes mediated by calcium delivery as well as in hematopoiesis whilst maintaining skeleton strength. A wide variety of collagen-based scaffolds have been proposed for different tissue engineering applications. These scaffolds are designed to promote a biological response, such as cell interaction, and to work as artificial biomimetic extracellular matrices that guide tissue regeneration. This paper critically reviews the current understanding of the complex hierarchical structure and properties of native collagen molecules, and describes the scientific challenge of manufacturing collagen-based materials with suitable properties and shapes for specific biomedical applications, with special emphasis on bone tissue engineering. The analysis of the state of the art in the field reveals the presence of innovative techniques for scaffold and material manufacturing that are currently opening the way to the preparation of biomimetic substrates that modulate cell interaction for improved substitution, restoration, retention or enhancement of bone tissue function.     

A comparison of osteocyte bioactivity in fine particulate bone powder grafts vs larger bone grafts in a rat bone repair model

The osteogenic potential for bone grafts is based on numbers and activities of cells that survive transplantation. In this study, we compared the bioactivity of osteocytes in 300–500 μm fine particulate bone powder grafts to 2 mm larger bone grafts in a rat radial defect model. Expression levels of bone morphogenetic protein-2 (BMP-2), transforming growth factor-beta 1 (TGF-β1), alkaline phosphatase (ALP), and collagen I were semi-quantified by both immunohistochemistry and RT-PCR at days 1 and 4, as well as weeks 1, 2, 4, 6 and 10 post-transplantation. Within two weeks post-transplantation, more cells stained positively for BMP-2, TGF-β1, ALP, and collagen I within the bone grafts and in the surrounding tissues in the group transplanted with the fine particulate bone powder grafts than in those with larger bone grafts (P < 0.05). The mRNA levels of all four markers in the group transplanted with fine particulate bone powder graft peaked earlier and were expressed more highly than in the larger bone graft group, suggesting that fine particulate bone powder grafts provide more viable and active osteocytes to accelerate bone defect healing than larger bone grafts.     

Sustained swimming increases the mineral content and osteocyte density of salmon vertebral bone

This study addresses the effects of increased mechanical load on the vertebral bone of post-smolt Atlantic salmon by forcing them to swim at controlled speeds. The fish swam continuously in four circular tanks for 9 weeks, two groups at 0.47 body lengths (bl) × s(-1) (non-exercised group) and two groups at 2 bl × s(-1) (exercised group), which is just below the limit for maximum sustained swimming speed in this species. Qualitative data concerning the vertebral structure were obtained from histology and electron microscopy, and quantitative data were based on histomorphometry, high-resolution X-ray micro-computed tomography images and analysis of bone mineral content, while the mechanical properties were tested by compression. Our key findings are that the bone matrix secreted during sustained swimming had significantly higher mineral content and mechanical strength, while no effect was detected on bone in vivo architecture. mRNA levels for two mineralization-related genes bgp and alp were significantly upregulated in the exercised fish, indicating promotion of mineralization. The osteocyte density of the lamellar bone of the amphicoel was also significantly higher in the exercised than non-exercised fish, while the osteocyte density in the cancellous bone was similar in the two groups. The vertebral osteocytes did not form a functional syncytium, which shows that salmon vertebral bone responds to mechanical loading in the absence of an extensive connecting syncytial network of osteocytic cell processes as found in mammals, indicating the existence of a different mechanosensing mechanism. The adaptive response to increased load is thus probably mediated by osteoblasts or bone lining cells, a system in which signal detection and response may be co-located. This study offers new insight into the teleost bone biology, and may have implications for maintaining acceptable welfare for farmed salmon.     

Structure of human vertebral lamellar bone in age-associated involution and osteoporosis

We compared the structure of lamellar bone in anterolateral zones of vertebral bodies in adult and senile subjects in health and osteoporosis. The appearance of structural signs of age-related osteopenia and enlargement of coplanar consolidations of the bone matrix crystals were noted. Osteopenia was more pronounced in osteoporosis, while the structural organization of the mineral component did not differ from that in age-matched controls.     

A study of lamellar organisation in juvenile and adult human bone

Summary The scanning electron microscope (SEM) has been used to study the three-dimensional organisation of collagen in slices of human rib and femur which were etched by chick osteoclasts, mechanically isolated and grown on their surfaces in vitro. Collagen organisation in the two bones showed a spectrum of appearances, ranging from lamellae of approximately equal thickness, but alternating fibre orientations, to an almost exclusive orientation of collagen apparently in a longitudinal direction. The rib contained a smaller component of transversely oriented collagen which may be related to a different functional loading. The thickness of circumferential lamellae was less than that of osteonal lamellae in the two adult ribs examined. Also, in the rib there was a trend towards increased average lamellar thickness with age in the range studied. This may be related to the fact that more of the lamellae in the rib cortex in children have been formed circumferentially. Correlation of results obtained with the SEM and the polarised light microscope (PLM) from the same substratum demonstrated that the latter grossly exaggerated the apparent component of collagen with a transverse orientation. This will always be true unless sections comparable with the lamellar thickness are used with the PLM.     

骨细胞网络结构的研究进展

骨细胞是成熟骨中最丰富的细胞,它在骨组织中的作用已达成共识,但其作用方式和原理仍未完全阐明。为更好认识骨细胞在骨组织中的作用,作者对骨细胞在骨组织中的网络结构进行了综述。     

Sexual dimorphism and age dependence of osteocyte lacunar density for human vertebral cancellous bone

The sexual dimorphism in age-related loss of human vertebral cancellous bone is not fully understood and could be related to dimorphism in the bone cell populations. The objective of this study was to investigate age- and gender-related differences in the osteocyte population and its relationship with bone volume fraction for human vertebral cancellous bone. Histomorphometric techniques were used to quantify osteocyte lacunae (a measure of osteocyte population) and bone volume fraction in male and female human T12 vertebrae, the most common site of vertebral fracture. Two measures of osteocyte population [number of osteocytes per bone area (OtLcDn) and number of osteocytes per total area (OtLcN/TA)] and their relationships with age and bone volume fraction were found to be sexually dimorphic. Dimorphism in osteocyte density may explain the dimorphic patterns of bone loss in human vertebrae due to the sensory and signal communication functions that osteocytes perform.     

成骨细胞和骨细胞的力学信号转导

背景:骨骼具有功能适应性的特点,骨骼细胞是力学信号敏感细胞,但细胞的力学信号转导功能是如何实现的,对骨骼如何调控仍不明确。 目的：了解成骨细胞和骨细胞的力学信号转导途径,为利用力学信号改善骨骼功能提供理论依据。 方法：应用计算机检索PubMed数据库2000-01/2011-03相关文献。英文检索词为“osteoblast,osteocyte,bone cells,mechanical stress”, 根据纳入标准共69篇文章进行综述,以此对骨骼细胞力学信号转导相关内容进行总结。 结果与结论：骨骼具有功能适应性的特点,骨骼细胞是力学信号敏感细胞,但细胞的力学信号转导功能是如何实现的,对骨骼具有怎样的调控仍不明确。研究表明,由于骨骼的结构特点和细胞位置,成骨细胞和骨细胞是最重要的力学敏感性细胞。力学信号在骨骼内的转导过程分为4个阶段：①力学偶联。②生化偶联。③信号的传递。④效应性细胞的反应。通过这4个阶段的作用,作用在骨骼上的应力信号转导为生物化学信号,并影响细胞的功能,最终导致骨骼组织出现相应的结构变化以适应应力环境的需要。对于力学信号在骨髓间充质干细胞中的调控机制还有待继续深入探索。     

Collagen fibril arrangement and size distribution in monkey oral mucosa

Collagen fibre organisation and fibril size were studied in the buccal gingival and hard palate mucosa of Macacus rhesus monkey. Light and electron microscopy analysis showed connective papillae exhibiting a similar inner structure in the different areas examined, but varying in distribution, shape and size. Moving from the deep to surface layers of the buccal gingival mucosa (free and attached portions), large collagen fibril bundles became smaller and progressively more wavy with decreasing collagen fibril diameter. This gradual diameter decrease did not occur in the hard palate mucosa (free portion, rugae and interrugal regions) where the fibril diameter remained constant. A link between collagen fibril diameter and mechanical function is discussed.     

Calcium signaling in osteocyte network

<正>Introduction Osteocytes are interconnected through numerous intercellular processes,forming extensive cell networks throughout the bone tissue[1]. It has been shown that osteocyte density is an important physiological parameter,which decreases with     

Collagen type distribution in healing of synthetic arterial prostheses

Layers of tissue encapsulating vascular prostheses recovered from humans were extracted and analyzed by SDS-polyacrylamide gel electrophoresis to determine the distribution of genetically distinct collagen types. Type V collagen was in maximal concentration in extracts of tissues nearest to the prosthesis lumen, type III in extracts of chronically inflamed tissue filling the interstices of the porous prosthesis, and type I in extracts of fibrous occlusive or outer capsule tissue. This pattern of distribution of collagen types across the prosthesis wall may have arisen due to the influence of modulating factors originating in the blood flowing through the prosthesis, and factors produced by inflammatory cells chronically present at the tissue-biomaterial interface. The increased proportion of type V collagen at or near the lumen may contribute to the recognized antithrombogenic properties of human pseudointima.     

Enamel-calibrated lamellar bone reveals long period growth rate variability in humans

Mammalian teeth exhibit incremental structures representing successive forming fronts of enamel at varying time scales, including a short daily increment called a cross striation and a long period called a stria of Retzius, the latter of which, in humans, occurs on average every 8-9 days. The number of daily increments between striae is called the repeat interval, which is the same period as that required to form one increment of bone, i.e. the lamella, the fundamental - if not archetypal - unit of bone. Lamellae of known formation time nevertheless vary in width, and thus their measures provide time-calibrated growth rate variability. We measured growth rate variability for as many as 6 years of continuously forming primary incremental lamellar bone from midshaft femur histological sections of sub-Saharan Africans of Bantu origin and known life history. We observed periodic growth rate variability in approximately 6- to 8-week intervals, and in some cases annual rhythms were visible. Endogenous biological periodicities, cycles manifest in the external environment, and/or perturbations of development are all potentially contained within growth rate variability studies of lamellar incremental patterns. Because lamellae are formed within defined periods of time, quantitative measures of widths of individual lamellae provide time-resolved growth rate variability that may reveal rhythms in human bone growth heretofore unknown.     

Residual stress distribution in a lamellar model of the arterial wall

Excessive wall circumferential stress in arteries caused by luminal pressure leads to endothelial damage and clinical consequences. In addition to circumferential stress, arterial wall contains residual stress with compressive and tensile components on intima and adventitia sides. The intimal compressive component compensates part of tensile stress induced by blood pressure, hence reduces severity of endothelial tension. The opening angle caused by radial cut of arterial ring defines residual stress. In this study, finite element modelling is used to evaluate residual stress in a lamellar model of human aorta with differing opening angle and elastic modulus. Results show non-linear residual stress profiles across wall thickness, influenced by structural and mechanical parameters. Elevation of opening angle from 50° to 90° leads to increase of intimal compressive component compensating up to 32.6% of the pressure-induced tensile stress. Results may be applied in study of endothelial injury caused by excessive stress in situations such as aging, hypertension and atherosclerosis.     

Resistance of bone marrow mesenchymal stem cells in a stressed environment - Comparison with osteocyte cells

Introduction: Recently, the efficacy of mesenchymal stem cells (MSCs) mediated by their tissue repair and anti-inflammatory actions in the prevention and therapy of various disorders has been reported. In this research, our attention was focused specifically on the prevention and therapy of glucocorticoid-induced osteonecrosis. We investigated the stress resistance of MSC against glucocorticoid administration and hypoxic stress, which are factors known to induce osteocytic cell death.Materials and Methods: Mouse bone cells (MLO-Y4) and bone-marrow derived mouse MSCs were exposed to dexamethasone (Dex), hypoxia of 1% oxygen or both in vitro. Mitochondrial membrane potentials were estimated by mitochondria labeling with a cell-permeant probe (Mito tracker red); expression of these apoptosis-inducing molecules, oxidative stress marker (8-hydroxy-2''-deoxyguanosine), caspase-3, -9, and two apoptosis-inhibiting molecules, energy-producing ATP synthase (ATP5A) and X-linked inhibitor of apoptosis protein (XIAP), were analyzed by both immunofluorescence and western blot.Results: With exposure to either dexamethasone or hypoxia, MLO-Y4 showed reduced mitochondrial membrane potential, ATP5A and upregulation of 8-OHdG, cleaved caspases and XIAP. Those changes were significantly enhanced by treatment with dexamethasone plus hypoxia. In MSCs, however, mitochondrial membrane potentials were preserved, while no significant changes in the pro-apoptosis or anti-apoptosis molecules analyzed were found even with exposure to both dexamethasone and hypoxia. No such effects induced by treatment with dexamethasone, hypoxia, or both were demonstrated in MSCs at all.Discussion: In osteocyte cells subjected to the double stresses of glucocorticoid administration and a hypoxic environment osteocytic cell death was mediated via mitochondria. In contrast, MSC subjected to the same stressors showed preservation of mitochondrial function and reduced oxidative stress. Accordingly, even under conditions sufficiently stressful to cause the osteocytic cell death in vivo, it was thought that the function of MSC could be preserved, suggesting that in the case of osteonecrosis preventative and therapeutic strategies incorporating their intraosseous implantation may be promising.     

Collagen and bone viscoelasticity: a dynamic mechanical analysis.

The purpose of this study was to explore the effects of changes in Type I collagen on the viscoelasticity of bone. Bone coupons were heated at either 100 or 200 degrees C to induce the thermal denaturation of Type I collagen. Half of these specimens were rehydrated after heat treatment; the other half were tested in a dry condition. The degree of denatured collagen (DC%) was analyzed by a selective digestion technique with the use of alpha-chymotrypsin. Isothermal (37 degrees C) and variable temperature tests (scans from 35 to 200 degrees C) were performed with the use of a dynamic mechanical analyzer to evaluate changes in bone viscoelastic properties as a function of collagen damage, specifically, changes in the loss factor (tan delta) and storage modulus (E') were assessed. Significant collagen denaturation occurred only when bone was heated at 200 degrees C irrespective of the hydration condition. Also, DC% did not show a significant effect on tan delta. However, higher values of tan delta were observed in wet samples compared to dry specimens. The temperature-scan tests revealed that the hydration condition, but not DC%, significantly affected the behavior of tan delta. However, E' was not strongly influenced either by DC% or by water content. These results suggest that at a constant frequency the denaturation of collagen triple-helical molecules may have few effects on the viscoelasticity of bone, but moisture may play a prominent role in determining this property.