Osteocytes: a proposed multifunctional bone cell.
ABSTRACT Most cell types are ascribed a single function. The osteoclast holds the unique distinction of performing only one function in the body - that of resorbing bone. The osteoblast has been ascribed the major function of bone matrix production. Other less well-defined cell types include progenitor cells and the nebulous cell type that can support osteoclast formation upon stimulation with various bone resorbing cytokines. Obviously, these cells could have other functions. The definition of an osteocyte is descriptive of its location - cells surrounded by mineralized matrix - not its function. For this year's Sun Valley Workshop on osteocytes, several proposed functions will be presented. First, a general consensus exists that osteocytes are most likely sensitive to mechanotransduction and translate mechanical strain into biochemical signals. Consensus does not exist on the nature of the mechanical strain, the form of the biochemical signals, the target cell(s), or the viability status of the osteocyte. Second, it is also proposed that this cell is incredibly adaptable and expresses plasticity in response to mechanical stimuli. In other words, this cell can readjust its responses to strain in the presence of other bone agents such as hormones and bone factors. Third, it will also be presented that osteocytes maintain systemic mineral homeostasis by regulating mineral release and deposition over the enormous surface area over which these cells interface with the surrounding matrix. Although osteocytes are terminally differentiated osteoblasts, they appear to have separate and distinct properties from their predecessors. Bone cell biologists loaded with an arsenal of bone anabolic and catabolic factors are examining the expression and effects of these factors on osteocytes. Engineers trained in mathematical modeling have generated new models of strain and connectivity to be tested. The unique morphology of osteocytes suggests that the cytoskeleton in these cells may function differently from osteoblasts and other cell types. Osteocytes may consist of different subpopulations; some that possess receptors for parathyroid hormone (PTH) and others that only express receptors for carboxyl terminal PTH suggesting different functions and responses. Osteocytes may respond rapidly to strain through glutamate receptor-like mechanisms, through calcium influxes, through gap junctions, and less rapidly through the production of small molecules and factors. Strain may take the form of substrate stretching and/or fluid flow. Osteocytes may communicate with other osteocytes and/or bone surface cells such as lining cells, stromal cells, osteoblasts, and/or osteoclasts and their precursors. The viability status of the osteocyte may determine the type of signals sent from these cells. If the cells are deprived of oxygen or nutrients, the apoptotic cells may send signals for initiation of resorption. If the cells and/or their dendritic process are ripped or torn by microdamage, they may send signals of both resorption and formation. If the majority of these theories are correct, then the osteocyte is the 'smart' cell that can direct or orchestrate the bone resorbing and bone forming cells even in its death and dying.
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ABSTRACT: The proliferative impulse of the growth plate cartilage and related structures and its effect on the dimensions of long bones are well documented. The modulation of shape, however, is less known, and in general, it is referred to the coupled resorption/apposition process of bone modelling. A morphometric study was carried out on rabbit tibiae comparing size increments and shape changes in relation to age. Utilizing measurements made using dried bones, radiography and computerized tomography, it was possible to perform a three-dimensional analysis of shape modulation occurring during a period of growth extending from 3 months to 1 year of age. The dynamics of the shape changes related to growth were studied with a fluorescent tetracycline labelling. This enabled correlation of shape modulation with the 3-D distribution of apposition and resorption. The current thinking behind the influences and mechanical forces affecting bone architecture was discussed in the light of these findings. Several factors play a role in the structural organization of the human and upper vertebrates' skeleton, whose shape is genetically determined in the complex process usually referred as 'modelling'. This does not conflict with the existing evidence of remodelling being influenced by mechanical stimuli, but the unsolved question remains how physical forces (strains) act on the biological substrate of cartilage and bone cells.Anantomia Histologia Embryologia 01/2012; 41(3):217-26. · 0.88 Impact Factor
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ABSTRACT: Cyclic mechanical loading is perhaps the most important physiological factor regulating bone mass and shape in a way which balances optimal strength with minimal weight. This bone adaptation process spans multiple length and time scales. Forces resulting from physiological exercise at the organ scale are sensed at the cellular scale by osteocytes, which reside inside the bone matrix. Via biochemical pathways, osteocytes orchestrate the local remodeling action of osteoblasts (bone formation) and osteoclasts (bone resorption). Together these local adaptive remodeling activities sum up to strengthen bone globally at the organ scale. To resolve the underlying mechanisms it is required to identify and quantify both cause and effect across the different scales. Progress has been made at the different scales experimentally. Computational models of bone adaptation have been developed to piece together various experimental observations at the different scales into coherent and plausible mechanisms. However additional quantitative experimental validation is still required to build upon the insights which have already been achieved. In this review we discuss emerging as well as state of the art experimental and computational techniques and how they might be used in a mechanical systems biology approach to further our understanding of the mechanisms governing load induced bone adaptation, i.e., ways are outlined in which experimental and computational approaches could be coupled, in a quantitative manner to create more reliable multiscale models of bone.Annals of biomedical engineering 05/2012; 40(11):2475-87. · 2.41 Impact Factor
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ABSTRACT: Osteoporosis is characterised by low bone mass and structural deterioration of bone tissue, resulting in increased fragility and susceptibility to fracture. Osteoporotic fractures are a significant cause of morbidity and mortality. Direct medical costs from such fractures in the UK are currently estimated at over two billion pounds per year, resulting in a substantial healthcare burden that is expected to rise exponentially due to increasing life expectancy. Currently bone mineral density is the WHO standard for diagnosis of osteoporosis, but poor sensitivity means that potential fractures will be missed if it is used alone. During the past decade considerable progress has been made in the identification and characterisation of specific biomarkers to aid the management of metabolic bone disease. Technological developments have greatly enhanced assay performance producing reliable, rapid, non-invasive cost effective assays with improved sensitivity and specificity. We now have a greater understanding of the need to regulate pre-analytical sample collection to minimise the effects of biological variation. However, bone turnover markers (BTMs) still have limited clinical utility. It is not routinely recommended to use BTMs to select those at risk of fractures, but baseline measurements of resorption markers are useful before commencement of anti-resorptive treatment and can be checked 3--6 months later to monitor response and adherence to treatment. Similarly, formation markers can be used to monitor bone forming agents. BTMs may also be useful when monitoring patients during treatment holidays and aid in the decision as to when therapy should be recommenced. Recent recommendations by the Bone Marker Standards Working Group propose to standardise research and include a specific marker of bone resorption (CTX) and bone formation (P1NP) in all future studies. It is hoped that improved research in turn will lead to optimised markers for the clinical management of osteoporosis and other bone diseases.Journal of Translational Medicine 08/2013; 11(1):201. · 3.46 Impact Factor