Osteoblast and osteoclast precursors in primary cultures of calvarial bone cells

Why Animal Experiments Are Still Indispensable in Bone Research: A Statement by the European Calcified Tissue Society

Major achievements in bone research have always relied on animal models and in vitro systems derived from patient and animal material. However, the use of animals in research has drawn intense ethical debate and the complete abolition of animal experimentation is demanded by fractions of the population. This phenomenon is enhanced by the reproducibility crisis in science and the advance of in vitro and in silico techniques. 3D culture, organ-on-a-chip, and computer models have improved enormously over the last few years. Nevertheless, the overall complexity of bone tissue cross-talk and the systemic and local regulation of bone physiology can often only be addressed in entire vertebrates. Powerful genetic methods such as conditional mutagenesis, lineage tracing, and modeling of the diseases enhanced the understanding of the entire skeletal system. In this review endorsed by the European Calcified Tissue Society (ECTS), a working group of investigators from Europe and the US provides an overview of the strengths and limitations of experimental animal models, including rodents, fish, and large animals, as well the potential and shortcomings of in vitro and in silico technologies in skeletal research. We propose that the proper combination of the right animal model for a specific hypothesis and state-of-the-art in vitro and/or in silico technology is essential to solving remaining important questions in bone research. This is crucial for executing most efficiently the 3R principles to reduce, refine, and replace animal experimentation, for enhancing our knowledge of skeletal biology, and for the treatment of bone diseases that affect a large part of society. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Endocrinology of bone mineralization: an update

Throughout the world, millions of people suffer from fragilizing osteopathies such as osteomalacia and osteoporosis.Osteomalacia is a rare disorder, corresponding to mineralization abnormalities in adult bone, as opposed to rickets in children. Renal phosphate loss and hypophosphatasia are the main causes of vitamin-resistant osteomalacia. Diagnosis is based on clinical history, phosphocalcic metabolism assessment and, if necessary, molecular characterization, and must be rapid in order to initiate the most appropriate treatment and consider new treatments such as burosumab if necessary.Osteoporosis is characterized by reduced bone mass and strength, which increases the risk of fragility fracture. Fracture-related burden is expected to increase over the coming decades linked to the aging of population and a treatment gap. In order to reduce this treatment gap, it is important to develop two strategies: improvement of screening and of treatment. Systematic screening using the FRAX® fracture risk assessment tool could be useful to increase anti-osteoporosis medical treatment and reduce fracture rates. The question of treatment sequencing in osteoporosis is another challenge, notably after denosumab cessation, complicated by a decrease in bone mineral density and increased risk of fracture. New treatments are also available, including romosozumab, a humanized monoclonal antibody which promotes bone formation and inhibits bone resorption by inhibiting sclerostin. Romosozumab is approved in several countries, including France, for treating severe osteoporosis in postmenopausal women at high risk of fracture and free of cardiovascular comorbidity.Endocrinologists need to be aware of these fragilizing osteopathies in order to improve both diagnosis and treatment.

Maturation of cortical bone suppresses periosteal osteoprogenitor proliferation in a paracrine manner

Periosteum contains enriched pools of osteogenic progenitors and is highly proliferative, thus giving this tissue a pivotal role in maintaining the diameter of the diaphyseal cortex and in recovery from fractures. Although periosteal proliferation has not been detected in normal bone, intense periosteal proliferation has been observed in pathologic states such as fracture, inflammation, and bone tumors. However, the mechanism by which periosteal osteoprogenitor proliferation is regulated remains poorly understood. To investigate this regulation mechanism, osteoblast/osteocyte-specific conditional knockout mice were developed lacking Smad4 and Osx, two factors that are essential for osteoblast differentiation and matrix mineralization. In Smad4 (Col) and Osx (Col) mice, osteocalcin, Dmp-1, and sclerostin expression were significantly decreased in the cortical bone. Interestingly, although Cre activity was not observed in the periosteum, the proliferation of periosteal osteoprogenitors was enhanced in Smad4 (Col) and Osx (Col) mice, as assessed by 5'-bromo-2'deoxyuridine incorporation and proliferating cell nuclear antigen localization. Since Wnt signaling is a major factor affecting periosteal proliferation, we evaluated Wnt signaling in the periosteum. The expression levels of β-catenin and Lef-1 were increased in the periosteal osteoprogenitors. Moreover, the mRNA levels of β-catenin, cyclin D1, Lef-1, and Axin2, all of which are Wnt target genes, were significantly increased in the periosteum of both Smad4 (Col) and Osx (Col) mice. These results indicated that extracellular proteins secreted by mature osteoblasts and osteocytes suppress the proliferation of periosteal osteoprogenitors by blocking Wnt signaling in a paracrine manner. Our data suggest a new concept of periosteal bone healing and periosteal bone formation.

Osteoblast isolation from murine calvaria and long bones

This chapter describes the isolation of primary mouse osteoblasts from adult mouse calvaria and long bones, as well as the process of isolation of bone cells from neonatal mouse calvaria. Osteoblasts from adult mouse bone are obtained as outgrowth from collagenase-treated bone pieces. Isolation of osteoblasts from neonatal calvaria is achieved by sequential enzymatic digestion of the bone matrix. Because of differences in origin and isolation method, each of the primary bone cell cultures described will have their own characteristics.

Application of biomarkers in the clinical development of new drugs for chondroprotection in destructive joint diseases: a review

Emerging evidence supports the concept that biochemical markers are clinically useful non-invasive diagnostic tools for the monitoring of changes in cartilage turnover in patients with destructive joint diseases such as osteoarthritis (OA) and rheumatoid arthritis (RA). Epidemiological studies demonstrated that measurements of different degradation products of proteins in the extracellular matrix of hyaline cartilage in urine or serum samples are (1) increased in OA or RA patients compared with healthy individuals, (2) correlate with disease activity, and (3) are predictive for the rate of changes in radiographic measures of cartilage loss. The present review provides an updated list of available biomarkers and summarize the research data arguing for their clinical utility. In addition, it addresses the question whether or not the monitoring of biomarkers during different treatment modalities could be a useful approach to characterize the chondro-protective effects of approved and candidate drugs. Finally, it briefly reviews the in vitro/ex vivo experimental settings - isolated chondrocyte cultures and articular cartilage explants - that can assist in the verification of novel markers, but also studies assessing direct effects of drug candidates on chondrocytes. Collectively, biomarkers may acquire a function as established efficacy parameters in the clinical development of novel chondro-protective agents.

Osteoblastic differentiation

The fully differentiated osteoblast may be easily recognized in bone tissue. Its cuboidal shape, its position directly opposed to the bone surface and its capacity to produce calcified bone matrix are characteristic. Three other differentiation stages are also reasonably well defined--the preosteoblast, the osteocyte and the lining cell. These differentiation stages are preceded by an unknown number of precursor, progenitor and stem cell stages. Little is known about the regulation of the transitions between the various osteogenic phenotypes and their reversibility or irreversibility. One of the reasons for this is the lack of adequate tools with which to recognize the various differentiation stages. We have developed a number of monoclonal antibodies (in bone) specifically directed against osteocytes, osteoblasts and as yet unidentified cells in the periosteum. The anti-osteocyte monoclonals were used to recognize osteocytes in bone cell cultures and we obtained purified osteocyte populations for metabolic studies. Osteocytes were shown to have binding sites for parathyroid hormone. The antibodies directed against osteoblasts showed that at present our culture conditions are inadequate to allow osteoblast differentiation in vitro.

In Vitro, Ex Vivo, andIn VivoMethodological Approaches for Studying Therapeutic Targets of Osteoporosis and Degenerative Joint Diseases: How Biomarkers Can Assist?

Although our approach to the clinical management of osteoporosis (OP) and degenerative joint diseases (DJD)-major causes of disability and morbidity in the elderly-has greatly advanced in the past decades, curative treatments that could bring ultimate solutions have yet to be found or developed. Effective and timely development of candidate drugs is a critical function of the availability of sensitive and accurate methodological arsenal enabling the recognition and quantification of pharmacodynamic effects. The established concept that both OP and DJD arise from an imbalance in processes of tissue formation and degradation draws attention to need of establishing in vitro, ex vivo, and in vivo experimental settings, which allow obtaining insights into the mechanisms driving increased bone and cartilage degradation at cellular, organ, and organism levels. When addressing changes in bone or cartilage turnover at the organ or organism level, monitoring tools adequately reflecting the outcome of tissue homeostasis become particularly critical. In this context, bioassays targeting the quantification of various degradation and formation products of bone and cartilage matrix elements represent a useful approach. In this review, a comprehensive overview of widely used and recently established in vitro, ex vivo, and in vivo set-ups is provided, which in many cases effectively take advantage of the potentials of biomarkers. In addition to describing and discussing the advantages and limitations of each assay and their methods of evaluation, we added experimental and clinical data illustrating the utility of biomarkers for these methodological approaches.

“Culture shock” from the bone cell's perspective: emulating physiological conditions for mechanobiological investigations

Bone physiology can be examined on multiple length scales. Results of cell-level studies, typically carried out in vitro, are often extrapolated to attempt to understand tissue and organ physiology. Results of organ- or organism-level studies are often analyzed to deduce the state(s) of the cells within the larger system(s). Although phenomena on all of these scales—cell, tissue, organ, system, organism—are interlinked and contribute to the overall health and function of bone tissue, it is difficult to relate research among these scales. For example, groups of cells in an exogenous, in vitro environment that is well defined by the researcher would not be expected to function similarly to those in a dynamic, endogenous environment, dictated by systemic as well as organismal physiology. This review of the literature on bone cell culture describes potential causes and components of cell “culture shock,” i.e., behavioral variations associated with the transition from in vivo to in vitro environment, focusing on investigations of mechanotransduction and experimental approaches to mimic aspects of bone tissue on a macroscopic scale. The state of the art is reviewed, and new paradigms are suggested to begin bridging the gap between two-dimensional cell cultures in petri dishes and the three-dimensional environment of living bone tissue.

Effects of leukemia inhibitory factor and oncostatin M on bone mineral formed in in vitro rat bone-marrow stromal cell culture: physicochemical aspects

Leukemia inhibitory factor (LIF) and oncostatin M (OSM), two pleiotropic cytokines involved in bone remodeling, have both anabolic and catabolic activities. This study analyzed the effects of LIF and OSM on the physicochemical characteristics of mineral phases formed in a rat bone-marrow stromal cell culture model. Stromal cells were cultured for three weeks in the presence of 10(-8) M dexamethasone, 50 microgram/mL ascorbic acid and 10 mM Na beta-glycerophosphate with or without 10 ng/ml LIF or OSM. Subsequently, the physicochemical characteristics of the mineralization nodules formed were analyzed by energy dispersive X ray microanalysis (EDX) and Fourier transform-infrared (FT-IR) and FT-Raman spectroscopy. EDX and FT-IR spectroscopy revealed the influence of LIF and OSM on the physicochemical characteristics of mineral phases. FT-Raman spectroscopy showed modifications of the main vibrational modes of the organic matrix. These alterations induced by growth factors could help define new strategies for the prevention and treatment of skeletal disorders.

Calcitonin receptor binding properties in bone and kidney of the chicken during the oviposition cycle

The binding property of calcitonin (CT) in the membrane fraction of calvaria and kidney of egg-laying and nonlaying hens was analyzed using a [125I] CT binding assay system. Binding properties of CT receptors in both tissues satisfy the authentic criteria of a receptor-ligand interaction in terms of specificity, reversibility, and saturation. Scatchard plots revealed a single class of binding sites. Values of the equilibrium dissociation constant (Kd) and binding capacity (Bmax) in laying hens showed a decrease during the period between 3 h before and 2 h after oviposition. No change was observed in nonlaying hens. In vivo administration of 17beta-estradiol or progesterone caused the decrease in Kd and Bmax values. The results suggest that the binding affinity and capacity of the CT receptor in the calvaria and the kidney of the hen may be modulated by the ovarian steroid hormone.

Enhancement by sodium orthovanadate of the formation and mineralization of bone nodules by chick osteoblasts in vitro

Orthovanadate is a known inhibitor of phosphotyrosyl protein phosphatase and is reported to stimulate osteogenic cell proliferation and differentiation when administered during the logarithmic growth phase and to potentiate the mitogenic effects of several growth factors. There is little information concerning the effects of orthovanadate on bone matrix deposition and mineralization, although there is some evidence that it increases collagen synthesis by osteogenic cells. To test the effects of orthovanadate on bone nodule formation and mineralization, osteogenic cells were exposed to 5-50 microM orthovanadate or 10(-7) M insulin-like growth factor-1 for 3, 7, and 21 days after plating. Exposure to orthovanadate produced differential effects on cellular proliferation and alkaline phosphatase activity following completion of the logarithmic growth phase, and on resultant bone nodule formation and mineralization by these populations. The effects of orthovanadate on osteogenic cultures were concentration dependent: 5 microM concentrations produced by a relatively large quantity of poorly mineralized matrix, while 30-50 microM concentrations produced a smaller quantity of heavily mineralized matrix. Thus, orthovanadate could possibly be used as a growth factor for bone, if administered at the critical dosage at the proper stage of the life cycle of the osteoblast.

The effects of smokeless tobacco extract on bone nodule formation and mineralization by chick osteoblasts in vitro

Short-term exposure to smokeless tobacco extracts (STE) reportedly inhibits osteoblast metabolism. The objective of this study was to determine the effects of serial dilutions of a water-soluble extract of smokeless tobacco on osteoblast proliferation and their potential to form and mineralize bone nodules. STE significantly stimulated cell proliferation when diluted 10(2)-10(4) times; 10(3) and 10(4) dilutions produced the greatest effect. 10(2)-10(4) STE dilutions significantly increased alkaline phosphatase activity at day 7 but 10(6) STE dilutions significantly decreased it. 10(3) and 10(4) dilutions significantly increased bone nodule formation, but inhibited their mineralization. In contrast, 10(5) and 10(6) dilutions significantly decreased bone nodule formation, but increased their mineralization. Stimulation of in vitro bone nodule formation by STE was similar to that produced by 10(-7) M insulin-like growth factor 1 (IGF-1) in vivo. Heat and acid treatment of STE significantly reduced its beneficial effect on cell proliferation, suggesting that a peptide within STE may be responsible for enhancement of osteogenic cell proliferation. Thus, STE may contain a peptide capable of significantly stimulating osteoblast proliferation, differentiation and metabolism, similar to the effects of IGF-1. This peptide could have potential therapeutic benefits.

What's new in osteoclast ontogeny?

The osteoclast (OC) is a multinuclear bone-resorbing cell which shares several characteristics with cells of the mononuclear phagocyte system. Unlike terminally differentiated macrophages, OCs possess specialized characteristics such as tartrate resistant acid phosphatase activity and the presence of calcitonin receptors. It appears that myeloid progenitor cells, probably granulocyte-macrophage colony-forming units, generate OC precursors which then differentiate and fuse into OCs under the regulation of osteotropic hormones, cytokines and other local factors. Parathyroid hormone and 1,25 dihydroxy Vitamin D3 induce both the formation and fusion of OC precursors, while calcitonin inhibits fusion. Osteoblasts also produce factor(s) which regulate OC precursor differentiation and therefore bone resorption; the nature of these factor(s), however, is unknown. In addition, the OC surface interacts specifically with a range of cellular and extracellular matrix-associated ligands which influence OC differentiation. The precise regulation of OC formation, however, is complex and awaits further investigation.

Mitogenic action(s) of fluoride on osteoblast line cells: determinants of the response in vitro

Clinically effective (osteogenic) concentrations of fluoride (5-30 microM) also have direct effects on skeletal tissues in vitro, to increase bone formation and osteoblast line cell proliferation. The effect on cell proliferation was specific for bone cells, modulated by systemic skeletal effectors, and dependent on (a) the [Pi] in the medium, (b) the presence of a bone cell mitogen, and (c) mitogen-responsive osteoprogenitor cells. Together, these data indicate that fluoride increases bone formation in vitro by increasing osteoprogenitor cell proliferation and that fluoride increases osteoprogenitor cell proliferation by enhancing the activity of bone cell mitogens.

A proposed mechanism of the mitogenic action of fluoride on bone cells: inhibition of the activity of an osteoblastic acid phosphatase

Fluoride (F) is a potent inhibitor of osteoblastic acid phosphatase activity with an apparent Ki value (10 to 100 mumol/L) that corresponds to F concentrations that increase bone cell proliferation and bone formation in vivo and in vitro. This high sensitivity of acid phosphatase to F inhibition appeared to be specific for skeletal tissues. Mitogenic concentrations of F did not increase cellular cAMP levels but significantly stimulated net protein phosphorylation in intact calvarial cells and in isolated calvarial membranes. These concentrations of F also stimulated net membrane-mediated phosphorylation of angiotensin II (which contains tyrosyl but no seryl or threonyl residues), suggesting that some of the F-stimulated protein phosphorylations could occur on tyrosyl residues. F had no apparent effect on thiophosphorylation of membrane proteins, suggesting that the F-stimulated net protein phosphorylation in bone cells was probably not mediated via activation of protein kinases. Orthovanadate or molybdate at concentrations that inhibit bone acid phosphatase activity also stimulated bone cell proliferation, supporting the idea that inhibition of bone acid phosphatase would lead to stimulation of bone cell proliferation. Mitogenic concentrations of F potentiated the mitogenic activities of insulin, EGF, and IGF-1 (ie, growth factors the receptors of which are tyrosyl kinases) to a greater extent than they potentiated the action of basic FGF (a growth factor that does not appear to stimulate tyrosyl protein phosphorylation). Based on these findings, a model is proposed for the biochemical mechanism of the osteogenic action of F in which F stimulates bone cell proliferation by a direct inhibition of an osteoblastic acid phosphatase/phosphotyrosyl protein phosphatase activity, which in turn increases overall cellular tyrosyl phosphorylation, resulting in a subsequent stimulation of bone cell proliferation.

Expression of the chondrogenic phenotype by mineralizing cultures of embryonic chick calvarial bone cells

Cells released by sequential enzymatic digestion of 18-day chick calvariae were cultured over a 4-5 week period in Alpha modified Eagles medium. In some cultures the medium was supplemented with ascorbate and/or Na-beta-glycerophosphate. Microscopic examination of these cultures showed both polygonal and spindle-shaped cells. The biochemical nature of these cells was investigated by incubating the cultures with radiolabelled proline and subsequently analysing the medium and cell layer proteins by SDS/PAGE and fluorography. Osteoblast and chondrocyte-containing cultures were clearly distinguished in this way as the former cells secreted type I collagen while the latter secreted types II and X collagens as the major medium macromolecules. Type X collagen synthesis occurred after 14 days, but only in cultures supplemented with both ascorbate and Na-beta-glycerophosphate, and was maintained for the duration of the culture period. Unsupplemented cultures and those containing either ascorbate alone or Na-beta-glycerophosphate alone failed to synthesize type X collagen after 28 days. Isolated cells pulsed with radiolabelled proline at confluence and organ cultures of embryonic chick calvaria synthesized types I and V collagens only. These data demonstrate that the expression of phenotype by heterogeneous populations of bone cells could be modulated by a combination of culture conditions including the length of time in culture and conditions favourable for the formation of a mineralized matrix.

Bone formation by rat calvarial cells grown at high density in organoid culture

Calvarial cells from day 21 rat fetuses were isolated by enzymatic digestion and grown at high density in an organoid culture system at the medium/air interface. In this type of culture, mineralization occurred as early as 7 days in vitro, as revealed by light and electron microscopic means. After about 18 days in vitro, most of the culture consisted of mineralized tissue. Mineralization was also achieved without beta-glycerophosphate, but it was delayed by 2 to 3 days. Maximal alkaline phosphatase activity occurred at days 8 to 12 in vitro and then declined continuously during further cultivation. Two types of mineralization could be observed: (1) mineralization of a collagen-rich osteoid by typical apatite crystals; (2) mineralization of a nearly collagen-free matrix by amorphous material which was possibly secreted by the cells. The importance of higher cell densities for cell differentiation and formation of histotypic tissue in vitro is apparent, and it is indicated that cell-cell contacts and cell-matrix interactions may be prerequisites for the development of histotypic conditions similar to the in vivo situation.

Bone formation in vitro by stromal cells obtained from bone marrow of young adult rats

Cells from fetal or neonatal skeleton can synthesize bone-like tissue in vitro. In contrast, formation of bone-like tissue in vitro by cells derived from adult animals has rarely been reported and has not been achieved using cells from bone marrow. We have explored development of bone-like tissue in vitro by bone marrow stromal cells. Marrow stromal cells obtained from 40-43-day-old Wistar rats were grown in primary culture for 7 days and then subcultured for 20-30 days. Cells were cultured in either alpha-minimal essential medium containing 15% fetal bovine serum, antibiotics, and 50 micrograms/ml ascorbic acid, or the above medium supplemented with either 10 mM Na-beta-glycerophosphate, 10(-8) M dexamethasone, or a combination of both. Cultures were examined using phase-contrast microscopy, undemineralized and demineralized tissue histology, histochemistry (for alkaline phosphatase activity), immunohistochemistry (for collagen type, osteonectin, and bone Gla-protein), scanning and transmission electron microscopy, energy dispersive X-ray microanalysis, and X-ray diffraction. Collagenous, mineralized nodules exhibiting morphological and ultrastructural characteristics similar to bone were formed in the cultures, but only in the presence of both beta-glycerophosphate and dexamethasone. Cells associated with the nodules exhibited alkaline phosphatase activity. The matrix of the nodules was composed predominantly of type-I collagen and both osteonectin and Gla-protein were present. X-ray microanalysis showed the presence of Ca and P, and X-ray diffraction indicated the mineral to be hydroxyapatite. The nodules were also examined for bone morphogenetic protein-like activity. Paired diffusion chambers containing partly demineralized nodules and fetal muscle were implanted intraperitonealy in rats. Induction of cartilage in relation to muscle was observed histologically after 40 days in the chambers. This finding provided further support for the bone-like nature of the nodules. The observations show that bone-like tissue can be synthesized in vitro by cells cultured from young-adult bone marrow, provided that the medium contains both beta-glycerophosphate and, particularly, dexamethasone.

Evidence that fluoride-stimulated 3[H]-thymidine incorporation in embryonic chick calvarial cell cultures is dependent on the presence of a bone cell mitogen, sensitive to changes in the phosphate concentration, and modulated by systemic skeletal effectors

In previous studies we have shown that clinically effective concentrations of fluoride (5 to 30 mumol/L) could also have direct effects in vitro on skeletal tissues to increase embryonic chick bone formation and bone cell proliferation (3[H]-thymidine incorporation into DNA). From these observations, we hypothesized that fluoride-stimulated bone formation might be mediated by a direct effect of fluoride to increase bone cell proliferation. The current studies were intended to investigate the mechanism of fluoride-stimulated 3[H]-thymidine incorporation, in chick calvarial cell cultures, by assessing mitogenic interactions between fluoride and inorganic phosphate, bone-derived growth factors, and systemic skeletal effectors. With respect to fluoride-phosphate interactions, the results of our studies indicate that the effect of fluoride was dependent on the phosphate concentration in the medium. Fluoride did not increase 3[H]-thymidine incorporation in BGJb medium containing 1 mmol/L (total) phosphate; but, in 1.6 mmol/L phosphate medium, fluoride caused a dose-dependent increase in 3[H]-thymidine incorporation, between 1 and 20 mumol/L (P less than .001). The action of fluoride was also dependent on the presence of a bone cell mitogen. Fluoride increased 3[H]-thymidine incorporation when added to calvarial cell cultures in the cell-conditioned medium, but had no effect in unconditioned (ie, fresh) medium. The action of fluoride could be restored by adding an exogenous growth factor (ie, concentrated cell-conditioned medium, bone-derived growth factors, or a systemic bone cell mitogen) to the unconditioned culture medium, P less than .05 for each effector.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Characterization of endosteal osteoblastic cells isolated from mouse caudal vertebrae

We have developed a reliable procedure for isolating endosteal osteoblasts from mouse trabecular bone. Endosteal osteoblasts were obtained by migration and proliferation of the cells from the metaphyseal bone surface of caudal vertebrae onto nylon meshes. The isolated cells were cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum. The cell population consisted of 95% alkaline-phosphatase-positive cells. The cell level of alkaline phosphatase was elevated (1.19 +/- 0.26 (SD) mumol PNP/mn/mg protein) and the enzyme activity was heat-inhibitable, indicating its skeletal origin. Light and electron microscopic observation revealed that cells have morphologic and ultrastructural appearance of typical osteoblasts with high protein synthesis activity. Osteoblasts grown in multilayers in the presence of 50 micrograms/ml ascorbic acid produced within 4 days an abundant fibrous intercellular collagenous matrix forming nodules in which osteocyte-like cells were embedded. Immunolabeling revealed synthesis of type I collagen but no detectable type III collagen. In presence of 7 mM beta-glycerophosphate the matrix became mineralized after 14-21 days of culture. Mineralization could not be induced by mouse skin fibroblasts cultured under similar conditions. The mineral deposits were closely associated with the collagen matrix, consisted of EDTA-removable, Von Kossa and alizarin red S stainable material and were composed of hydroxyapatite crystals identified by X-ray electron probe microanalysis. The isolated endosteal osteoblasts also displayed an intense (+457%) increase in intracellular cAMP production in response to human (1-34) PTH (2 x 10(-8) M) stimulation. The confluent cells responded to 20 nM 1,25(OH)2D3 by a significant 45% reduction in heat labile alkaline phosphatase activity. This procedure allowed us to isolate from trabecular bone a cell population that differentiates into osteoblasts in vitro, respond to calcitropic hormones and that retains its capacity to form a calcified bone tissue in culture. This method provided us a culture system for investigating the differentiation and metabolism of endosteal osteoblastic bone forming cells.

In vitro evidence that local and systemic skeletal effectors can regulate 3[H]-thymidine incorporation in chick calvarial cell cultures and modulate the stimulatory actions(s) of embryonic chick bone extract

These investigations were intended to determine whether local and systemic skeletal effectors--3'5'-cyclic adenosine monophosphate (cAMP), prostaglandin E2 (PGE2), parathyroid hormone (PTH), 1,25-dihydroxyvitamin D (1,25(OH)2D), calcitonin, and NaF--could regulate 3[H]-thymidine incorporation (i.e., into DNA) in serum-free, monolayer cultures of embryonic chick calvarial cells, and/or modulate the activity of embryonic chick bone extracts to increase 3[H]-thymidine incorporation. In the absence of added bone extract, we found that calcitonin (0.1 U/ml), NaF (100 microM) and low-dose PTH (0.1 nM) stimulated 3[H]-thymidine incorporation, P less than .05 for each; isobutylmethylxanthine (IBMX--1 mM), 1,25OHD (10 nM), and high-dose PTH (10 nM) decreased 3[H]-thymidine incorporation; and PGE2 (1 microM) had no effect. The stimulatory actions of calcitonin, fluoride, and low-dose PTH were inductive, and the inhibitory actions of IBMX and 1,25(OH)2D were acute. PTH had complex time-dependent actions on 3[H]-thymidine incorporation, being inhibitory after 4-8 hours of exposure and stimulatory after 20-24 hours (P less than .001 for each). The effects of calcitonin, fluoride, and low-dose PTH to increase 3[H]-thymidine incorporation were greater in calvarial cell cultures enriched for undifferentiated osteoprogenitor cells than in cultures enriched for differentiated osteoblastlike cells. PTH inhibited 3[H]-thymidine incorporation in the latter (i.e., osteoblastlike) cultures (P less than .005). The inhibitory actions of IBMX and 1,25(OH)2D were independent of cell differentiation. Additional studies further revealed that these local and systemic skeletal effectors could also modulate the activity of embryonic chick bone extracts to increase 3[H]-thymidine incorporation in calvarial cell cultures. We found that calcitonin, fluoride, and low-dose PTH enhanced the effect of the extracts to increase 3[H]-thymidine incorporation (P less than .001 for each). These activations were noncompetitive, indicating (1) mechanistic differences between the stimulatory actions of the effectors and the chick bone extract (i.e., different rate-limiting steps for the effects of each on 3[H]-thymidine incorporation); and (2) that neither calcitonin, fluoride, nor 0.1 nM PTH altered the apparent affinity of the cells for stimulatory activity(s) in the extract. High-dose PTH was a noncompetitive inhibitor with respect to bone extract activity, indicating that the effect of 10 nM PTH to decrease 3[H]-thymidine incorporation was mechanistically distinct from the effect of the bone extract to increase 3[H]-thymidine incorporation.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of osteocytes in osteoblast-like cell cultures using a monoclonal antibody specifically directed against osteocytes

The development of a monoclonal antibody, OB 7.3, directed against a cell surface antigenic site on osteocytes is described. Osteoblast-like cells were enzymatically isolated from calvaria of chicken embryos after removal of the periostea. The cells were cultured for 6 days, harvested and used to immunize mice. One of the monoclonal antibodies obtained, OB 7.3, reacted specifically with the cell surface of osteocytes. In frozen sections of bone only osteocytes were stained, all other cells present, including mature osteoblasts, were negative. Liver, kidney, spleen, intestine, bloodvessel and skin were also completely negative. Using the monoclonal OB 7.3, positive cells could be demonstrated in sparse osteoblast-like cell cultures. The OB 7.3 positive cells had a stellate morphology and were therefore identified as osteocytes. They behaved in culture as osteocytes in bone tissue in that they formed a network of cell processes connecting osteocytes with each other or with other neighbouring cells. Monoclonal OB 7.3 offers the possibility of isolating osteocytes thereby providing the means for a detailed study of their biochemical properties.