A quantitative description of components of in vitro morphometric change in the rat osteoclast model: relationships with cellular function

Predictors of COVID-19 Hospital Treatment Outcome

Background

There are more than 228,394,572 confirmed cases and 4,690,186 confirmed deaths caused by COVID-19 worldwide. The magnitude of the COOVID-19 pandemic has stimulated research on the treatment and diagnosis of COVID-19 patients.

Objective

In this report, a battery of specific parameters was used to develop a model that allows prediction of the outcome of the COVID-19 treatment. These parameters are C-reactive protein, procalcitonin, fibrinogen, D-dimers, immature granulocytes, and interleukin-6.

Methods

The study was carried out on a sample of N = 49 survivors (22 men, 27 women) and 83 deceased patients (62 men, 21 women). The distribution of means and differences in means of the parameters studied between survivors and deceased patients were evaluated using the bootstrap method.

Results

A mathematical model that allows for the prediction of hospitalization outcome was obtained using the Naive Bayes model. The results demonstrated a statistically significant difference between survivors and deceased patients in all parameters studied. A mathematical model employing a battery of parameters provided a 97% precision in predicting the outcome of hospitalization.

Conclusion

This study showed that the cross-correlation of survivability with absolute levels of C-reactive protein, procalcitonin, fibrinogen, D-dimers, immature granulocytes, and interleukin-6 could be used successfully in the hospital setting as a diagnostic tool.

Regulatory actions of 3',5'-cyclic adenosine monophosphate on osteoclast function: possible roles of Epac-mediated signaling

Alterations in cellular levels of the second messenger 3',5'-cyclic adenosine monophosphate ([cAMP]_i ) regulate a wide range of physiologically important cellular signaling processes in numerous cell types. Osteoclasts are terminally differentiated, multinucleated cells specialized for bone resorption. Their systemic regulator, calcitonin, triggers morphometrically and pharmacologically distinct retraction (R) and quiescence (Q) effects on cell-spread area and protrusion-retraction motility, respectively, paralleling its inhibition of bone resorption. Q effects were reproduced by cholera toxin-mediated G_s -protein activation known to increase [cAMP]_i , unaccompanied by the [Ca²⁺ ]_i changes contrastingly associated with R effects. We explore a hypothesis implicating cAMP signaling involving guanine nucleotide-exchange activation of the small GTPase Ras-proximate-1 (Rap1) by exchange proteins directly activated by cAMP (Epac). Rap1 activates integrin clustering, cell adhesion to bone matrix, associated cytoskeletal modifications and signaling processes, and transmembrane transduction functions. Epac activation enhanced, whereas Epac inhibition or shRNA-mediated knockdown compromised, the appearance of markers for osteoclast differentiation and motility following stimulation by receptor activator of nuclear factor kappa-Β ligand (RANKL). Deficiencies in talin and Rap1 compromised in vivo bone resorption, producing osteopetrotic phenotypes in genetically modified murine models. Translational implications of an Epac-Rap1 signaling hypothesis in relationship to N-bisphosphonate actions on prenylation and membrane localization of small GTPases are discussed.

Endogenous calcitonin regulates lipid and glucose metabolism in diet-induced obesity mice

Calcitonin (CT) plays an important role in calcium homeostasis, and its precursor, proCT, is positively associated with the body mass index in the general human population. However, the physiological role of endogenous CT in the regulation of metabolism remains unclear. Knockout mice with gene-targeted deletion of exon 4 of Calca (CT KO) were generated by targeted modification in embryonic stem cells. Male mice were used in all experiments and were fed a slightly higher fat diet than the standard diet. The CT KO mice did not exhibit any abnormal findings in appearance, but exhibited weight loss from 15 months old, i.e., significantly decreased liver, adipose tissue, and kidney weights, compared with wild-type control mice. Furthermore, CT KO mice exhibited significantly decreased fat contents in the liver, lipid droplets in adipose tissues, serum glucose, and lipid levels, and significantly increased insulin sensitivity and serum adiponectin levels. CT significantly promoted 3T3-L1 adipocyte differentiation and suppressed adiponectin release. These results suggested that CT gene deletion prevents obesity, hyperglycemia, and hyperlipidemia in aged male mice. This is the first definitive evidence that CT may contribute to glucose and lipid metabolism in aged male mice, possibly via decreased adiponectin secretion from adipocytes.

Serum C-reactive protein and procalcitonin kinetics in patients undergoing elective total hip arthroplasty

BACKGROUND

The sensitivity and the specificity of different methods to detect periprosthetic infection have been questioned. The current study aimed to investigate the kinetics of C-reactive protein (CRP) and procalcitonin (PCT) in patients undergoing uncomplicated elective total hip arthroplasty (THA), to provide a better interpretation of their levels in noninfectious inflammatory reaction.

METHODS

A total of 51 patients were included. Serum CRP and PCT concentrations were obtained before surgery, on the 1st, 3rd, and 7th postoperative days and after discharge on the 14th and 30th days and at 2 years.

RESULTS

Both markers were confirmed to increase after surgery. The serum CRP showed a marked increase on the 3rd postoperative day while the peak of serum PCT was earlier, even if much lower, on the first day. Then, they declined slowly approaching the baseline values by the second postoperative week. PCT mean values never exceed concentrations typically related to bacterial infections.

CONCLUSIONS

CRP is very sensitive to inflammation. It could be the routine screening test in the follow-up of THA orthopaedic patients, but it should be complemented by PCT when there is the clinical suspicion of periprosthetic infection.

Real-time activity bioassay of single osteoclasts using a silicon nanocrystal-impregnated artificial matrix

The lack of an in vitro real-time osteoclast (OC) activity assay has hampered mechanistic studies of bone resorption. Such an assay is developed, employing a hydroxyapatite matrix impregnated with alkyl-capped silicon nanocrystals, which is capable of monitoring the time-course of resorption by single osteoclasts. Resorption of the matrix by OC releases the nanocrystals, which are internalized by the cell and detected as an increase in OC luminescence. This particular choice of nanocrystals is motivated by their bright pH-independent luminescence, proportional to concentration, and by their rapid uptake without cytotoxicity. In this in vitro assay, OCs are inhibited by calcitonin (CT) and methyl-β-cyclodextrin (MCD), and stimulated by receptor activator of nuclear factor kappa-B ligand (RANKL) in the expected manner. The kinetics of the assay exhibit a lag phase representing cell attachment and commencement of resorption processes, followed by a growth of cell luminescence intensity, and the whole time-course is satisfactorily described by the logistic equation.

Regulation of bone turnover by calcium-regulated calcium channels

Calcium plays multiple roles in osteoclast formation, survival, and activity. Intracellular calcium is determined both by the release of intracellular stores and the influx of extracellular calcium through a variety of calcium channels. Osteoclasts express several classes of calcium channels, including ryanodine receptors (RyRs), inositol-1,4,5-trisphosphate receptors (IP(3)Rs), and calcium release-activated calcium channels (CRACs), which respond to depletion of intracellular stores. IP(3)R2 is expressed in osteoclast precursors and activated by cytokines that stimulate osteoclast differentiation. In mature osteoclasts, the IP(3)R1 isoform is highly expressed and is implicated in nitric oxide-cGMP-stimulated processes. RyR calcium channels may contribute to the release of intracellular calcium stores, while RyR2 in the plasma membrane may act to limit osteoclast activity based on extracellular calcium concentration. Orai, through regulation by endoplasmic reticular store-sensing proteins, including Stim-1, may also mediate calcium influx and act as a signal amplifier for calcium release by other calcium channels. Together, these receptors allow intracellular Ca(2+) signals to modulate bone turnover and, through calcium-sensing functions, allow coupling of osteoclast activity to extracellular conditions and integrating additional cytokine and nitric oxide signals via transient intracellular calcium signals.

Procalcitonin in sepsis and systemic inflammation: a harmful biomarker and a therapeutic target

The worldwide yearly mortality from sepsis is substantial, greater than that of cancer of the lung and breast combined. Moreover, its incidence is increasing, and its response to therapy has not appreciably improved. In this condition, the secretion of procalcitonin (ProCT), the prohormone of calcitonin, is augmented greatly, attaining levels up to thousands of fold of normal. This hypersecretion emanates from multiple tissues throughout the body that are not traditionally viewed as being endocrine. The serum values of ProCT correlate with the severity of sepsis; they recede with its improvement and worsen with exacerbation. Accordingly, as highlighted in this review, serum ProCT has become useful as a biomarker to assist in the diagnosis of sepsis, as well as related infectious or inflammatory conditions. It is also a useful monitor of the clinical course and prognosis, and sensitive and specific assays have been developed for its measurement. Moreover, it has been demonstrated that the administration of ProCT to septic animals greatly increases mortality, and several toxic effects of ProCT have been elucidated by in vitro experimental studies. Antibodies have been developed that neutralize the harmful effects of ProCT, and their use markedly decreases the symptomatology and mortality of animals that harbour a highly virulent sepsis analogous to that occurring in humans. This therapy is facilitated by the long duration of serum ProCT elevation, which allows for a broad window of therapeutic opportunity. An experimental groundwork has been established that suggests a potential applicability of such therapy in septic humans.

Rapid inactivation and apoptosis of osteoclasts in the maternal skeleton during the bone remodeling reversal at the end of lactation

There is a rapid reversal in maternal skeletal metabolism and bone remodeling from accelerated bone resorption during lactation to skeletal rebuilding after lactation. The purpose was to determine the changes that occur in maternal osteoclasts during the transition from lactation to postlactation. Skeletal samples were taken from female rats on days 10 and 19 of lactation and 1 and 7 days after lactation. The pups were weaned on day 20. There was a rapid change in the osteoclast population after weaning, resulting in less resorption surface. Osteoclasts detached from bone surfaces, lost their ruffled borders, and became fragmented with immunocytochemical evidence of apoptosis within 24 hr after lactation. Concomitant with the rapid regression in the osteoclast population was an over fivefold increase in maternal calcitonin (CT) levels at 24 hr after weaning. Serum calcium and estrogen (E2) increased, but prolactin (PRL) and PTH decreased after weaning. The hormone changes, particularly that of CT, are consistent with the rapid regression of the osteoclast population at the end of lactation. These changes are similar to a reversal phase of a bone remodeling cycle where bone formation commences when resorption ceases on bone surfaces and suggests that the fate of osteoclasts during bone remodeling is programmed cell death. These results also suggest that bone remodeling is well synchronized prior to, during, and after lactation to accommodate the mineral requirements of the offspring as well as the mother.

NO-dependent osteoclast motility: reliance on cGMP-dependent protein kinase I and VASP

The osteoclast degrades bone in cycles; between cycles, the cell is motile. Resorption occurs by acid transport into an extracellular compartment defined by an alphavbeta3 integrin ring. NO has been implicated in the regulation of bone turnover due to stretch or via estrogen signals, but a specific mechanism linking NO to osteoclastic activity has not been described. NO stimulates osteoclast motility, and at high concentrations NO causes detachment and terminates resorption. Here we demonstrate that NO regulates attachment through the cGMP-dependent protein kinase I (PKG I) via phosphorylation of the intermediate protein VASP. VASP colocalized with the alphavbeta3 ring in stationary cells, but alternating bands of VASP and alphavbeta3 occurred when motility was induced by NO donors or cGMP. Redistribution of VASP correlated with its phosphorylation. Dependency of NO-induced motility on PKG I and on VASP was shown by siRNA knockdown of each protein. VASP knockdown also altered distribution of alphavbeta3 at the attachment site. We conclude that PKG I and VASP are essential for reorganization of attachment and cytoplasmic proteins in motility induced by NO or by cGMP.

Calcitonin therapy in osteoporosis

Osteoporosis is the most prevalent metabolic bone disease and is characterized by diminished bone strength predisposing to an increased risk of fracture. Its incidence is particularly high in postmenopausal women but it can also affect other groups, such as men and patients receiving corticosteroid therapy. Calcitonin is a naturally occurring peptide which acts via specific receptors to strongly inhibit osteoclast function. It has been used in the treatment of osteoporosis for many years. Historically, calcitonin was administered as a parenteral injection, but the intranasal formulation is now the most widely used because of its improved tolerability. New approaches are currently being investigated to enhance the bioavailability and effects of calcitonin, including oral, pulmonary, and transdermal routes of administration, and novel allosteric activators of the calcitonin receptor. Several controlled trials have reported that calcitonin stabilizes and in some cases produces a short-term increase in bone density at the lumbar spine level. The most relevant clinical trial to evaluate the effect of calcitonin in the prevention of fractures was the Prevent Recurrence of Osteoporotic Fractures (PROOF) study, a 5-year double-blind, randomized, placebo-controlled trial showing that salmon calcitonin nasal spray at a dosage of 200 IU/day can reduce the risk of vertebral osteoporotic fractures by 33% (relative risk [RR] = 0.67; 95% CI 0.47, 0.97; p = 0.03). However, the 100 and 400 IU/day dosages did not significantly reduce vertebral fracture risk. Effects on nonvertebral fractures were not significant (RR = 0.80; 95% CI 0.59, 1.09; p = 0.16). There is mounting evidence to show that calcitonin diminishes bone pain in osteoporotic vertebral fractures, which may have clinical utility in vertebral crush fracture syndrome. A recent study suggests that nasal salmon calcitonin appears to be a promising therapeutic approach for the treatment of men with idiopathic osteoporosis, although long-term trials are necessary to confirm these results and evaluate fracture rate as an endpoint in men. The role of calcitonin in corticosteroid-induced osteoporosis remains controversial, hence it can only be considered a second-line agent for the treatment of patients with low bone mineral density who are receiving long-term corticosteroid therapy.

Effects on Bone

The actions of amylin on bone have been reviewed in several publications (MacIntyre, 1992a,b; MacIntyre et al., 1991; Reid and Cornish, 1996; Tamura et al., 1992a,b; Zaidi et al., 1990a,c, 1993b). MacIntyre proposed that amylin or its derivatives or agonists would be useful for treating bone disorders, such as osteoporosis, Paget's disease, or bone loss resulting from malignancy, endocrine disorders, autoimmune arthritides, breakage and fracture, immobility and disease, or hypercalcaemia (MacIntyre, 1995).

Calcitonin: physiological actions and clinical applications

Calcitonin (CT) was first reported as a hypocalcemic principle, initially thought to originate from the parathyroid gland, a view subsequently corrected to an origin from parafollicular C-cells. Human CT is a 32 amino acid peptide with an N-terminal disulphide bridge and a C-terminal prolineamide residue, shown to potently inhibit bone resorption. More recent studies have demonstrated that this may take place through a direct osteoclastic action. A number of osteoclast CT receptors have subsequently been characterized and particular receptor regions necessary for ligand binding and intracellular signaling identified. Its potent anti-resorptive effect has led to its use in treating Paget's bone disease, osteoporosis, hypercalcaemia and osteogenesis imperfecta. This review summarises some key aspects of its synthesis, structure and its actions at the cellular and molecular levels, and leads on to its therapeutic uses that have emerged since its discovery as well as possibilities for future clinical applications.

Calcium sensing and cell signaling processes in the local regulation of osteoclastic bone resorption

The skeletal matrix in terrestrial vertebrates undergoes continual cycles of removal and replacement in the processes of bone growth, repair and remodeling. The osteoclast is uniquely important in bone resorption and thus is implicated in the pathogenesis of clinically important bone and joint diseases. Activated osteoclasts form a resorptive hemivacuole with the bone surface into which they release both acid and osteoclastic lysosomal hydrolases. This article reviews cell physiological studies of the local mechanisms that regulate the resorptive process. These used in vitro methods for the isolation, culture and direct study of the properties of neonatal rat osteoclasts. They demonstrated that both local microvascular agents and products of the bone resorptive process such as ambient Ca2+ could complement longer-range systemic regulatory mechanisms such as those that might be exerted through calcitonin (CT). Thus elevated extracellular [Ca2+], or applications of surrogate divalent cation agonists for Ca2+, inhibited bone resorptive activity and produced parallel increases in cytosolic [Ca2+], cell retraction and longer-term inhibition of enzyme release in isolated rat osteoclasts. These changes showed specificity, inactivation, and voltage-dependent properties that implicated a cell surface Ca2+ receptor (CaR) sensitive to millimolar extracellular [Ca2+]. Pharmacological, biophysical and immunochemical evidence implicated a ryanodine-receptor (RyR) type II isoform in this process and localized it to a unique, surface membrane site, with an outward-facing channel-forming domain. Such a surface RyR might function either directly or indirectly in the process of extracellular [Ca2+] sensing and in turn be modulated by cyclic adenosine diphosphate ribose (cADPr) produced by the ADP-ribosyl cyclase, CD38. The review finishes by speculating about possible detailed models for these transduction events and their possible interactions with other systemic mechanisms involved in Ca2+ homeostasis as well as the possible role of the RyR-based signaling mechanisms in longer-term cell regulatory processes.

Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics

Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H+-ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.

Calcitonin: the other thyroid hormone

Calcitonin was originally discovered as a hypocalcemic factor synthesized by thyroid parafollicular C cells. Early experiments demonstrated that calcitonin inhibited bone resorption and decreased calcium efflux from isolated cat tibiae and subsequent histologic and culture studies confirmed the osteoclast as its major site of action. Its potent antiresorptive effect and analgesic action have led to its clinical use in treatment of Paget's bone disease, osteoporosis, and hypercalcemia of malignancy. This review surveys the cellular and molecular basis of these physiologic and clinical actions.

Forty years of calcitonin--where are we now? A tribute to the work of Iain Macintyre, FRS

Calcitonin was discovered as a hypocalcemic principal that was initially thought to originate from the parathyroid gland. This view was corrected subsequently, and an origin from the thyroid C cells was documented. The purification and sequencing of various calcitonins soon followed. Calcitonin is a 32-amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The peptide was shown to potently inhibit bone resorption; however, a direct osteoclastic action of the peptide was confirmed only in the early 1980s. Several osteoclast calcitonin receptors have subsequently been cloned and sequenced. Specific regions of the receptor necessary for ligand binding and intracellular signaling through cyclic AMP and calcium have been identified through systematic deletion mutagenesis and chimeric receptor studies. Calcitonin's potent antiresorptive effect has led to its use in treating Paget's disease of bone, osteoporosis, and hypercalcemia. This review retraces key aspects of the synthesis and structure of calcitonin, its cellular and molecular actions, and its therapeutic uses as they have emerged over the 40 years since its discovery. The review also examines the implications of these findings for future clinical applications as a tribute to early workers to whom credit must be given for creation of an important and expanding field. Notable are the new approaches currently being used to enhance calcitonin action, including novel allosteric activators of the calcitonin receptor, modulation of the release of endogenous calcitonin by calcimimetic agents, as well as the development of oral calcitonins.

CD38/ADP-ribosyl cyclase: A new role in the regulation of osteoclastic bone resorption

The multifunctional ADP-ribosyl cyclase, CD38, catalyzes the cyclization of NAD(+) to cyclic ADP-ribose (cADPr). The latter gates Ca(2+) release through microsomal membrane-resident ryanodine receptors (RyRs). We first cloned and sequenced full-length CD38 cDNA from a rabbit osteoclast cDNA library. The predicted amino acid sequence displayed 59, 59, and 50% similarity, respectively, to the mouse, rat, and human CD38. In situ RT-PCR revealed intense cytoplasmic staining of osteoclasts, confirming CD38 mRNA expression. Both confocal microscopy and Western blotting confirmed the plasma membrane localization of the CD38 protein. The ADP-ribosyl cyclase activity of osteoclastic CD38 was next demonstrated by its ability to cyclize the NAD(+) surrogate, NGD(+), to its fluorescent derivative cGDP-ribose. We then examined the effects of CD38 on osteoclast function. CD38 activation by an agonist antibody (A10) in the presence of substrate (NAD(+)) triggered a cytosolic Ca(2+) signal. Both ryanodine receptor modulators, ryanodine, and caffeine, markedly attenuated this cytosolic Ca(2+) change. Furthermore, the anti-CD38 agonist antibody expectedly inhibited bone resorption in the pit assay and elevated interleukin-6 (IL-6) secretion. IL-6, in turn, enhanced CD38 mRNA expression. Taken together, the results provide compelling evidence for a new role for CD38/ADP-ribosyl cyclase in the control of bone resorption, most likely exerted via cADPr.

Emerging insights into the role of calcium ions in osteoclast regulation

Osteoclasts are exposed to unusually high, millimolar, Ca2+ concentrations and can "sense" changes in their ambient Ca2+ concentration during resorption. This results in a sharp cystolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The rise in cystolic Ca2+ is transduced finally into an inhibition of bone resorption. We have shown that a type 2 ryanodine receptor isoform, expressed uniquely in the osteoblast plasma membrane, functions as a Ca2+ influx channel, and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily microsomal membrane Ca2+ release channels. They have only recently been shown to be expressed a other sites, including nuclear membranes. At the latter site, ryanodine receptors gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+, in turn, regulates key nuclear processes, including gene expression and apoptosis. Here, we review potential mechanisms underlying the recognition, movement, and actions of Ca2+ in the osteoclast.

Mode of action of interleukin-6 on mature osteoclasts. Novel interactions with extracellular Ca2+ sensing in the regulation of osteoclastic bone resorption

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti- IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 microg/l), but not IL-11 (10 and 100 microg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 microg/l). Additionally, IL-6 (5 pg/l to 10 microg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.

The effect of tetracyclines on quantitative measures of osteoclast morphology

We report the effects of the tetracycline analogues 4-dedimethylaminotetracycline (CMT-1) and minocycline on osteoclast spreading and motility. Both agents influenced the morphometric descriptor of cell spread area, rho, producing cellular retraction or an R effect (half-times: 30 and 44 minutes for CMT-1 and minocycline, respectively). At the concentrations employed, the tetracycline-induced R effects were significantly slower than, but were qualitatively similar to, those resulting from Ca2+ "receptor" activation through the application of 15 mM-[Ca2+] (slopes: -1.25, -0.18, and -4.40/minute for 10 mg/l-[CMT-1], 10 mg/l-[minocycline] and 15 mM-[Ca2+], respectively). In contrast, the same tetracycline concentrations did not influence osteoclast margin ruffling activity as described by mu, a motility descriptor known to be influenced by elevations of cellular cyclic AMP. Thus, the tetracyclines exert morphometric effects comparable to changes selectively activated by occupancy of the osteoclast Ca2+ "receptor" which may act through an increase in cytosolic [Ca2+].

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.