Evidence of coated membranes in the ruffled border of the osteoclast

Surface distribution of heterogenous clathrin assemblies in resorbing osteoclasts

Osteoclasts seeded on either glass coverslips or apatite pellets have at least two morphologically distinct substrate adhesion sites: actin-based adhesion structures including podosome belts and sealing zones, and adjacent clathrin sheets. Clathrin-coated structures are exclusively localized at the podosome belts and sealing zone, in both of which the plasma membrane forms a tight attachment to the substrate surface. When cultured on apatite osteoclasts can degrade the apatite leading to the formation of resorption lacunae. The sealing zone divides the ventral membrane into different domains, outside and inside of the sealing zones. The former facing the smooth-surfaced intact apatite contains relatively solitary or networks of larger flat clathrin structures; and the latter, facing the rough-surfaced degraded apatite in the resorption lacunae contain clathrin in various shapes and sizes. Clathrin assemblies on the membrane domain facing not only a resorption lacuna, or trails but also intact apatite indeed were observed to be heterogeneous in size and intensity, suggesting that they appeared to follow variations in the surface topography of the apatite surface. These results provide a detailed insight into the flat clathrin sheets that have been suggested to be the sites of adhesion and mechanosensing in co-operation with podosomes.

Septins are critical regulators of osteoclastic bone resorption

Septins are known to play key roles in supporting cytoskeletal stability, vesicular transport, endo-/exocytosis, stabilizing cellular membranes and forming diffusion barriers. Their function in mammalian cells is poorly investigated. The osteoclast offers an interesting tool to investigate septins because all cellular activities septins were reported to be involved in are critical for osteoclasts. However, the existence of septins in osteoclasts has not even been reported. Here we show that the SEPT9 gene and Septin 9 (SEPT9) protein are expressed and synthesized during differentiation of human osteoclasts. Pharmacological stabilization of septin filaments dose dependently inhibits bone resorption of human osteoclasts in vitro suggesting a role for septins in bone resorption. Attesting to this, conditional deletion of Sept9 in mice leads to elevated levels of trabecular bone and diminished femoral growth in vivo. Finally, systematic interrogation of the spatial organization of SEPT9 by confocal microscopy reveals that SEPT9 is closely associated to the structures known to be critical for osteoclast activity. We propose that septins in general and SEPT9 in particular play a previously unappreciated role in osteoclastic bone resorption.

A mathematical model of osteoclast acidification during bone resorption

Bone resorption by osteoclasts occurs through the creation of a sealed extracellular compartment (ECC), or pit, adjacent to the bone that is subsequently acidified through a complex biological process. The low pH of the pit dissolves the bone mineral and activates acid proteases that further break down the bone matrix. There are many ion channels, transporters, and soluble proteins involved in osteoclast mediated resorption, and in the past few years, there has been an increased understanding of the identity and properties of some key proteins such as the ClC-7 Cl^-/H⁺ antiporter and the H_V1 proton channel. Here we present a detailed mathematical model of osteoclast acidification that includes the influence of many of the key regulatory proteins. The primary enzyme responsible for acidification is the vacuolar H⁺-ATPase (V-ATPase), which pumps protons from the cytoplasm into the pit. Unlike the acidification of small lysosomes, the pit is so large that protons become depleted from the cytoplasm. Hence, proton buffering and production in the cytoplasm by carbonic anhydrase II (CAII) is potentially important for proper acidification. We employ an ordinary differential equations (ODE)-based model that accounts for the changes in ionic species in the cytoplasm and the resorptive pit. Additionally, our model tracks ionic flow between the cytoplasm and the extracellular solution surrounding the cell. Whenever possible, the properties of individual channels and transporters are calibrated based on electrophysiological measurements, and physical properties of the cell, such as buffering capacity, surface areas, and volumes, are estimated based on available data. Our model reproduces many of the experimental findings regarding the role of key proteins in the acidification process, and it allows us to estimate, among other things, number of active pumps, protons moved, and the influence of particular mutations implicated in disease.

Intracellular membrane trafficking in bone resorbing osteoclasts

There is ample evidence now that the two major events in bone resorption, namely dissolution of hydroxyapatite and degradation of the organic matrix, are performed by osteoclasts. The resorption cycle involves several specific cellular activities, where intracellular vesicular trafficking plays a crucial role. Although details of these processes started to open up only recently, it is clear that vesicular trafficking is needed in several specific steps of osteoclast functioning. Several plasma membrane domains are formed during the polarization of the resorbing cells. Multinucleated osteoclasts create a tight sealing to the extracellular matrix as a first indicator of their resorption activity. Initial steps of the sealing zone formation are alpha(v)beta(3)-integrin mediated, but the final molecular interaction(s) between the plasma membrane and mineralized bone matrix is still unknown. A large number of acidic intracellular vesicles then fuse with the bone-facing plasma membrane to form a ruffled border membrane, which is the actual resorbing organelle. The formation of a ruffled border is regulated by a small GTP-binding protein, rab7, which indicates the late endosomal character of the ruffled border membrane. Details of specific membrane transport processes in the osteoclasts, e.g., the formation of the sealing zone and transcytosis of bone degradation products from the resorption lacuna to the functional secretory domain remain to be clarified. It is tempting to speculate that specific features of vesicular trafficking may offer several potential new targets for drug therapy of bone diseases.

Chronic extracellular acidosis induces plasmalemmal vacuolar type H+ ATPase activity in osteoclasts

Proton extrusion into an extracellular resorption compartment is an essential component of bone degradation by osteoclasts. Chronic metabolic acidosis is known to induce negative calcium balance and bone loss by stimulating osteoclastic bone resorption, but the underlying mechanism is not known. The present studies were undertaken to evaluate whether chronic acidosis affects proton extrusion mechanisms in osteoclasts cultured on glass coverslips. Acidosis, mimicked experimentally by maintaining the cells at extracellular pH 6.5, rapidly lowered intracellular pH to 6.8. However, after 2 hours, a proportion of cells demonstrated the capacity to restore intracellular pH to near normal levels. To define the mechanism responsible for this recovery, the activity of individual H+ transport pathways was analyzed. We found that chronic acid treatment for up to 6 h did not significantly affect the cellular buffering power or Na+/H+ antiport activity. In contrast, chronic acidosis activated vacuolar H+ pumps in the osteoclasts. Although only approximately 5% of the control cells displayed proton pump activity, about 40% of cells kept at extracellular pH 6. 5 for 4-6 h were able to recover from the acute acid load by means of bafilomycin A1-sensitive proton extrusion. Conversely, the H+-selective conductance recently described in the plasma membrane of osteoclasts was clearly inhibited in the cells exposed to chronic acidosis. Following acid treatment, the activation threshold of the H+ conductance was shifted to more positive potentials, and the current density was significantly reduced. Considered together, these results suggest that induction of plasmalemmal vacuolar type ATPase activity by chronic acidosis, generated either systemically due to metabolic disease or locally at sites of inflammation, is likely to stimulate osteoclastic bone resorption and thus to promote bone loss.

Activation of proton pumping in human neutrophils occurs by exocytosis of vesicles bearing vacuolar-type H+-ATPases

Proton pump activity is not measurable in the plasma membrane of unstimulated neutrophils but becomes readily detectable upon activation by soluble agonists. The mechanism of pump activation was investigated in this report. V-type H+ pump activity, estimated as a bafilomycin A1-sensitive elevation of the cytosolic pH, was stimulated in suspended neutrophils by chemotactic peptides and by phorbol esters. Stimulation of pump activity induced by the agonists was greatly enhanced by cytochalasin B, an agent known to potentiate granular secretion in neutrophils. We therefore compared the rate and extent of pump activation with the pattern of exocytosis of the four types of secretory organelles present in neutrophils, using flow cytometry and enzyme-linked immunosorbent assay. The kinetics of exocytosis of secretory vesicles and secondary and tertiary granules but not primary granules paralleled the appearance of pump activity. The subcellular localization of the pump was defined by cellular fractionation and immunoblotting using an antibody to the C subunit of the V-type ATPase. The pump was abundant in tertiary granules, with significant amounts present also in primary granules and secretory vesicles. The pump was scarce in secondary granules and not detectable in the cytosol. Finally, the agonists failed to stimulate pump activity in neutrophil cytoplasts, which are intact cell fragments devoid of acidic granules. Together, our results suggest that the V-type H+-ATPase is not constitutively present in the plasma membrane of neutrophils but is delivered to the surface membrane by exocytosis during cellular activation. Tertiary granules and secretory vesicles are the most likely source of V-ATPases. Following insertion in the plasma membrane, the pump is poised to effectively extrude the excess metabolic acid that is generated during chemotaxis and bacterial killing.

Regulation of cytoplasmic pH in osteoclasts. Contribution of proton pumps and a proton-selective conductance

Osteoclasts resorb bone by secreting protons into an extracellular resorption zone through vacuolar-type proton pumps located in the ruffled border. The present study was undertaken to evaluate whether proton pumps also contribute to intracellular pH (pHi) regulation. Fluorescence imaging and photometry, and electrophysiological methods were used to characterize the mechanisms of pH regulation in isolated rabbit osteoclasts. The fluorescence of single osteoclasts cultured on glass coverslips and loaded with a pH-sensitive indicator was measured in nominally HCO(3-)-free solutions. When suspended in Na(+)-rich medium, the cells recovered from an acute acid load primarily by means of an amiloride-sensitive Na+/H+ antiporter. However, rapid recovery was also observed in Na(+)-free medium when K+ was used as the substitute. Bafilomycin-sensitive, vacuolar-type pumps were found to contribute marginally to pH regulation and no evidence was found for K+/H+ exchange. In contrast, pHi recovery in high K+ medium was largely attributed to a Zn(2+)-sensitive proton conductive pathway. The properties of this conductance were analyzed by patch-clamping osteoclasts in the whole-cell configuration. Depolarizing pulses induced a slowly developing outward current and a concomitant cytosolic alkalinization. Determination of the reversal potential during ion substitution experiments indicated that the current was due to H+ (equivalent) translocation across the membrane. The H+ current was greatly stimulated by reducing pHi, consistent with a homeostatic role of the conductive pathway during intracellular acidosis. These results suggest that vacuolar-type proton pumps contribute minimally to the recovery of cytoplasmic pH from intracellular acid loads. Instead, the data indicate the presence of a pH- and membrane potential-sensitive H+ conductance in the plasma membrane of osteoclasts. This conductance may contribute to translocation of charges and acid equivalents during bone resorption and/or generation of reactive oxygen intermediates by osteoclasts.

Electron microscopic and histochemical studies of the mononuclear odontoclast of the human

BACKGROUND

Osteoclasts and odontoclasts are multinucleated giant cells which resorb hard tissue by the ruffled borders. Recently, the authors reported the presence of a mononuclear osteoclast with a ruffled border in vitro. However, its presence in vivo has not been shown. To demonstrate the presence of a mononuclear odontoclast in humans, the present study used human deciduous teeth.

METHODS

After fixation and decalcification, tartrate-resistant acid phosphatase (TRACPase) activity was detected with the azo dye method, and then TRACPase-positive cells were observed on resorbing areas of teeth. TRACPase-positive cells could be distinguished from other cells by light microscopy, and the cells for investigation were serially sectioned by alternating semithin and ultrathin sections to observe their ultrastructure and three-dimensional organization.

RESULTS

TRACPase activity was detected in both multinucleated odontoclasts and a mononuclear cell from serial sections. By electron microscopy, most of the multinucleated odontoclasts had ruffled borders and clear zones. A mononuclear TRACPase-positive cell with a ruffled border and clear zone was reconstructed three-dimensionally by NIKON COSMO-ZONE 2SA. The reconstruction showed that this cell had one irregularly shaped nucleus and a wide ring-shaped clear zone and a small ruffled border. Under the ruffled border, this cell formed a small lacuna on the dentin surface. The results suggested that this cell was a mononuclear odontoclast.

CONCLUSIONS

The present study concludes that cells with ruffled borders and clear zones observed by transmission electron microscopy can be identified as odontoclasts or osteoclasts irrespective of the number of nuclei.

Proton pumps populate the contractile vacuoles of Dictyostelium amoebae

Amoebae of the eukaryotic microorganism Dictyostelium discoideum were found to contain an interconnected array of tubules and cisternae whose membranes were studded with 15-nm-diameter "pegs." Comparison of the ultrastructure and freeze-fracture behavior of these pegs with similar structures found in other cells and tissues indicated that they were the head domains of vacuolar-type proton pumps. Supporting this identification, the pegs were observed to decorate and clump when broken amoebae were exposed to an antiserum against the B subunit of mammalian vacuolar H(+)-ATPase. The appearance of the peg-rich cisternae in quick-frozen amoebae depended on their osmotic environment: under hyperosmotic conditions, the cisternae were flat with many narrow tubular extensions, while under hypo-osmotic conditions the cisternae ranged from bulbous to spherical. In all cases, however, their contents deep etched like pure water. These properties indicated that the interconnected tubules and cisternae comprise the contractile vacuole system of Dictyostelium. Earlier studies had demonstrated that contractile vacuole membranes in Dictyostelium are extremely rich in calmodulin (Zhu, Q., and M. Clarke, 1992, J. Cell Biol. 118: 347-358). Light microscopic immunofluorescence confirmed that antibodies against the vacuolar proton pump colocalized with anti-calmodulin antibodies on these organelles. Time-lapse video recording of living amoebae imaged by interference-reflection microscopy, or by fluorescence microscopy after staining contractile vacuole membranes with potential-sensitive styryl dyes, revealed the extent and dynamic interrelationship of the cisternal and tubular elements in Dictyostelium's contractile vacuole system. The high density of proton pumps throughout its membranes suggests that the generation of a proton gradient is likely to be an important factor in the mechanism of fluid accumulation by contractile vacuoles.

Electron microscopic and histochemical studies of the mononuclear osteoclast of the mouse

Osteoclasts collected from the long bones of mice were cultured on dentin slices. To identify osteoclasts, the tartrate-resistant acid phosphatase (TRACPase) activity of cultured cells was histochemically examined by the azo dye method. The TRACPase-positive cells could be distinguished from other cells by light microscopy. The cells were sectioned by alternating semithin and ultrathin sections to observe their ultrastructure and three-dimensional structure. TRACPase activity was detected both in multi-nucleated osteoclasts and in mononuclear cells. Most of the mononuclear TRACPase-positive cells had features similar to preosteoclasts. A mononuclear TRACPase-positive cell was a ruffled border and clear zone was reconstructed three-dimensionally by NIKON COSMOZONE 2SA. The reconstruction showed that this cell possessed a large clear zone and small ruffled border. Under the ruffled border, no lacuna was apparent; but there was disruption of the dentin surface. The results suggest that this cell was a mononuclear osteoclast and that it might have been in the process of making a new lacuna.

Osteoclasts: structure and function

Osteoclasts are multinucleated giant cells showing specialized membrane structures, clear zones and ruffled borders, which are responsible for the process of bone resorption. These cells arrive at the resorption site via the bloodstream as mononuclear cells, derived from haemopoietic precursors in the spleen or bone marrow, which fuse prior to resorption. The osteoclast may share an early progenitor cell, the granulocyte macrophage colony-forming unit (GM-CFU) with monocytes, macrophages and granulocytes, implying that osteoclasts share the pluripotent haemopoietic stem cell with all other haemopoietic cells. In the past, elucidation of the structure of these cells relied upon traditional ultrastructural techniques. Transmission electron microscopic studies revealed details of the unique ultrastructure of these cells and, in combination with stereological techniques, showed the response of cells to various hormonal stimuli. Scanning electron microscopy not only demonstrated the surface appearance of osteoclasts, and their predilection for spreading on various substratum components, but has also been used as an adjunct in resorption assays in which areas of resorption lacunae are measured as indicators of cell activity. Recent advances in fields such as immunocytochemistry and freeze fracture techniques have contributed towards a more detailed delineation of antigenic profile, cytoskeletal structure and localization of enzymatic pathways. The osteoclast is subject to extensive regulatory mechanisms and it has been established that the osteoblast plays a major rôle in mediating the effects of osteotropic hormones and local mediators on these cells. Hence, research aimed at elucidating the coupling mechanisms between these two cells may result in new therapies for bone disease.

Membrane modifications in chick osteoclasts revealed by freeze-fracture replicas

Remarkable differences among various membranes of bone cells became evident by examination of freeze-fracture replicas. In osteoclasts, three types of intramembranous particles (IMPs) were identified based on their size and shape: two sizes of isolated globular particles (8 and 12 nm in diameter) and rod-shaped, linear aggregates (8 x 30 nm in dimension). Furthermore, the density and distribution pattern of these IMPs enabled us to distinguish three different domains of membranes of osteoclasts including ruffled border, clear zone, and basolateral regions, as were also observed in thin sections. The highest density of IMPs was 3,500-4,000/microns2 in the ruffled border membrane, and these IMPs included linear aggregates among the usual globular particles. Linear aggregated particles were also observed in the membrane of cytoplasmic vesicles in the vicinity of the ruffled border region, but not in this membrane in other bone cells. In attached osteoclasts, the distribution patterns and densities of IMPs in each ruffled-finger and -plate were extremely variable, from closely to the loosely packed membrane particles. Focal aggregates of membrane particles were also frequently encountered. An important outcome of the present study was the finding that the presence of linear aggregated particles proved to be an additional criterion for distinguishing membrane domains in freeze-replicas of osteoclasts. The surface of the clear zone membrane was not smooth in profile, but revealed a number of eminences that were almost free of particles. Basolateral membranes exhibited a particle density of 2,400/microns2. Globular particles were homogeneously scattered in random fashion on their exposed fracture faces. In some cases, aggregates of IMPs on the basolateral membranes were encountered. In comparison with the ruffled fingers, microprojections from the basolateral surface showed a lesser density of IMPs and were devoid of rod-shaped or linear aggregated particles. Differences between osteoblasts and osteocytes were apparent in the density and the size of IMPs. The membranes of osteoblasts and osteocytes contained the same types of globular particles as seen in osteoclasts. Various sizes of gap junctions were located only on basolateral membranes of the osteoblasts. In contrast, no cellular junctions were observed between osteoclasts and any other type of cells.

Multinucleated cells formed on calcified dentine from mouse bone marrow cells treated with 1 alpha,25-dihydroxyvitamin D3 have ruffled borders and resorb dentine

Osteoclast-like multinucleated cells were formed from mouse bone marrow mononuclear cells, and their morphology on coverslips and on calcified dentine slices was compared by means of transmission electron microscopy. Addition of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] to bone marrow cells cultured on coverslips greatly stimulated the formation of multinucleated cells within 8 days. These multinucleated cells had the cytological features of osteoclasts (abundant pleomorphic mitochondria, a large number of vacuoles and lysosomes, many stacks of Golgi membranes, and an extensive canalicular system), but they developed neither ruffled borders nor clear zones. The multinucleated cells appeared to result from direct fusion of mononuclear progenitor cells, whose structural features were similar to those of multinucleated cells. Like isolated osteoclasts, both multinucleated cells and their precursors exhibited an intense reaction for tartrate-resistant acid phosphatase (TRACP) in the cisterns of endoplasmic reticulum and lysosomes. Multinucleated cells formed from alveolar macrophages in the presence of 1 alpha,25(OH)2D3 were totally negative for TRACP reaction. When marrow cells were cultured on dentine slices in the presence of 1 alpha,25(OH)2D3, some of the multinucleated cells were located in the shallow resorption lacunae of dentine surfaces, and they developed the characteristic ruffled borders and clear zones. The narrow extracellular spaces of the ruffled borders, the adjacent pale endocytotic vacuoles, and the dark lysosomes located in the perinuclear cytoplasm of the multinucleated cells contained numerous apatite crystals delete in resorption lacunae. These results indicate that 1) the multinucleated cells formed on coverslips from mouse marrow cells treated with 1 alpha,25(OH)2D3 exhibit non-functional basic features of osteoclast morphology, and 2) differentiation of the multinucleated cells into functional osteoclasts requires some components of calcified dentine.

Molecular mechanisms of bone resorption by the osteoclast

The osteoclast is a multinucleated cell that is actively engaged in the synthesis of lysosomal enzymes, their vectorial transport toward the apical membrane, and the secretion of these enzymes at its apical pole. These secreted enzymes are targeted to the apical ruffled-border membrane by mechanisms that involve cation-independent mannose-6-phosphate receptors. These receptors bind to an enzyme-linked mannose-6-phosphate recognition marker in the Golgi complex, and the enzyme-ligand-receptor complex, carried within small coated transport vesicles, dissociates upon reaching the low pH established in the bone-resorbing compartment by the osteoclast. The apical bone-resorbing compartment is sealed off by the attachment of the osteoclast to the calcified matrix and is actively acidified by the osteoclast. The plasma membrane of the cell is divided into distinct domains. The apical membrane at the ruffled-border shares common antigenic determinants with lysosomal and endosomal membranes, including a 100 kD protein and proton pumps that may be involved in the acidification of the extracellular resorbing compartment. The basolateral membrane is highly enriched in carbonic anhydrase, and bicarbonate-chloride exchange appears to regulate the intracellular pH of this cell. These observations are consistent with a scheme in which, in the low pH environment of the bone-resorbing lacuna produced by the osteoclast, the mineral phase dissolves, exposing the organic matrix to the action of the secreted enzymes. The activity of these enzymes is in turn presumably favored by the acidic milieu. All constituents of the matrix, whether mineral or organic, then would be reduced to their elemental forms (ions and amino acids) extracellularly.(ABSTRACT TRUNCATED AT 250 WORDS)

A histochemical localization on Maclura pomifera lectin during osteogenesis

Mandibular condyles of 4-week-old Wistar strain rats and mandibles of ICR strain mice from 14 days gestation stage to 2 days postnatal stage were used to investigate the localization of Maclura pomifera lectin (MPA) during two modes of osteogenesis. During endochondral ossification of the mandibular condyle, MPA was only localized at the peripheral regions of calcified cartilage after the destruction of chondrocyte lacunae. Bone extracellular matrix (ECM) was not reacted with MPA. In intramembranous ossification of mice mandibles, MPA was stained intensively in the early bone ECM. The intensity of the MPA reaction decreased during bone development. In both cases of osteogenesis, chondroclasts and osteoclasts showed the strong affinity to MPA. These results indicated that the time- and position-specific changes within ECM proceeded during osteogenesis and that MPA was the useful probe to detect chondroclasts and osteoclasts.

Surface modifications at the periosseous region of chick osteoclast as revealed by freeze-substitution

Improved preservation of osteoclast fine structure can be achieved by quick freezing, freeze-substitution, or detergent extraction. With such techniques the ruffled border mainly contains a disorganized, interconnected meshwork of microfilaments (5-7 nm in diameter), whereas in the clear zone a few ordered arrays of intermediate-type filaments (10-12 nm in diameter) are detectable among the network of microfilaments. In well-frozen samples, well-preserved matrix may have occluded the cytoskeleton; detergent extraction permits visualization of the cytoskeletal components. In fresh-frozen cells an extracellular fuzzy coat overlays the ruffled border. At the site of attachment of the clear zone to the bone surface, extracellular cementing material is detected only after quick freezing. The superiority of quick freezing to preserve ultrastructure is shown in various cytoplasmic organelles. Most vesicles and vacuoles found close to the ruffled border seemed not to make contact with the extracellular matrix. Anhydrous procedures using quick freezing and freeze-substitution stabilize bone mineral in some vacuoles and in the channels of the ruffled border.

Dentin resorption mediated by odontoclasts in physiological root resorption of human deciduous teeth

Extracted human deciduous teeth undergoing physiological root resorption were fixed with a mixture of formaldehyde and glutaraldehyde and processed for scanning (SEM) and analytical transmission electron microscopy, as well as acid trimetaphosphatase cytochemistry. The granulated tissues, which are rich in multinucleated odontoclasts and capillary vessels, formed various resorption lacunae on the resorbing dentin surfaces. SEM observations of dentin surfaces treated with sodium hypochlorite revealed two types of resorption lacunae: deep, round lacunae in which the peritubular matrix of dentinal tubules was strongly dissolved; and shallow, irregular lacunae with intact peritubular matrix. In trypsin-treated materials, the resorption surfaces were characterized by the presence of numerous collagen fibers in both the peritubular and intertubular matrices, suggesting demineralization of the surface dentin. Odontoclasts were characterized by the presence of abundant mitochondria, perinuclear stacks of Golgi membranes, various lysosomes, numerous endocytotic vacuoles, and a well-developed ruffled border against the resorption lacunae. Most endocytotic vacuoles were distributed in the cytoplasm between the ruffled border and the nuclei. In undemineralized ultrathin sections, the surface dentin of resorption lacunae consisted of collagen fibers and apatite crystals and had a lower packing density than those in unresorbed, deeper dentin. Many apatite crystals were demonstrated to be present in the extracellular channels of the ruffled border and in adjacent endocytotic vacuoles derived from it. Lysosomes located in the perinuclear cytoplasm of odontoclasts contained amorphous dense material and/or a small amount of crystals. An energy-dispersive x-ray microanalysis of apatite crystals in undemineralized sections indicated that the energy spectrum peaks of Ca and P detected from crystals in resorbing dentin were much lower than those in unresorbed dentin. Similarly, lower spectrum peaks of Ca and P were obtained from crystals found in the ruffled border and endocytotic vacuoles of odontoclasts. A slight trace Ca peak also was detected in the amorphous dense material in lysosomes of odontoclasts. The enzyme cytochemistry of lysosomal acid trimetaphosphatase indicated that odontoclasts had intense enzymatic activity in the Golgi membranes, endoplasmic reticulum cisternae, lysosomes, and endocytotic vacuoles. Dense reaction precipitates of enzymatic activity also were found along the dentin surfaces of resorption lacunae occupied by odontoclast ruffled borders.(ABSTRACT TRUNCATED AT 400 WORDS)

Osteoclastic features of cells that resorb bone implants in rats

The subcutaneous implantation of devitalized, mineral-containing bone particles in rats elicits the recruitment and differentiation of multinucleated cells and the rapid resorption of the bone. Cytochemical and ultrastructural characteristics were studied 12 days after implantation of bone. Tartrate-resistant acid phosphatase activity was demonstrated in these cells, especially at the cell/bone interface. Mast cells were seen vicinal to, but not in contact with the osteoclasts and the bone particles. Electron microscopic evaluation revealed that many multinucleated cells in lacunae on the surfaces of the bone particles displayed features characteristic of in osso osteoclasts, including clear zones of attachment to the bone substrate, and ruffled borders overlying bone with frayed collagen fibers. Many cells contained multiple giant centrospheres; this finding suggests that fusion can occur between multinucleated cells. This model may be useful to characterize the differentiation and regulation of bone-resorbing cells.

Evidence for capping of Fc gamma receptors on osteoclasts

Fc receptors, cell surface structures which bind the Fc portion of immunoglobulins, facilitate endocytosis and mediate triggered enzyme release in the phagocytic cells of the mononuclear phagocyte system (MPS). The osteoclast and the macrophage share many similarities, and it has been suggested that they are both derived from a common MPS precursor cell. Nevertheless, Fc gamma receptors, found on both primitive and well-differentiated cells of the mononuclear phagocyte system, have yet to be demonstrated on the osteoclast cell membrane. The aim of the present study was first, to study the distribution of rat endogenous IgG around resorbing bone using immunohistochemistry and the avidin-biotin-peroxidase (ABC) technique; and second, to determine if rat osteoclasts express Fc gamma receptors, using a recently described technique based on the ABC method. Observations that endogenous IgG accumulated at osteoclast/bone interfaces, and that Fc gamma receptors, expressed by osteoclasts, "capped" in the same area, have important implications regarding the origin of this cell and its mechanisms of bone attachment and resorption.

The rapid appearance of acid phosphatase activity at the developing ruffled border of parathyroid hormone activated medullary bone osteoclasts

The localization of acid phosphatase (ACPase) activity in and near parathyroid hormone (PTH) activated osteoclasts was investigated using electron microscopic cytochemical methods. At 3 hours after oviposition in Japanese Quail hens, medullary bone osteoclasts were highly reactive for ACPase but lacked ruffled borders. There was no evidence of extracellular ACPase activity associated with these osteoclasts. At 20 minutes after PTH administration, osteoclasts had developing ruffled borders and ACPase activity was found in the matrix and extracellular space adjacent to most of these ruffled borders. ACPase activity was seldom observed beyond the resorption zone delineated by the osteoclast clear zones. These results provide direct cytochemical evidence that the ruffled border functions in the release and/or activation of ACPase. In addition, these results show that ACPase localization is rapidly responsive to exogenous PTH.

The origin of osteoclasts: evidence, clinical implications and investigative challenges of an extra-skeletal source

Recent evidence for an extraskeletal origin of osteoclasts and the historical record of the genesis of osteoclasts are examined critically. Reviews of the structure, function and development of osteoclasts from mononuclear precursors, the local regulation of bone resorption and the coupling of bone formation to preceding resorption are presented as a background for discussing the clinical implications for management of osteolytic bone diseases. The roles of osteoclasts and macrophages as phagocytes are compared and contrasted, and recent evidence for macrophage heterogeneity resulting from site-specific monoblastic precursors is reviewed. The implications of these recent developments in macrophage biology are extrapolated to osteoclasts and the existence of site-specific, extraskeletal osteoclast precursors is proposed. Finally, the investigative challenges inherent in these perspectives are discussed.

Mechanism of osteoclastic bone resorption: a new hypothesis

Osteoclastic bone resorption involves the solubilization of the mineral salts and the degradation of noncollagen bone matrix and collagen fibrils. As no recognizable collagen fibrils have ever been reported within cytoplasmic vacuoles in osteoclasts, it is generally assumed that the collagen fibrils are digested extracellularly in the resorption zone. The extent to which lysis occurs extracellularly and whether or not the osteoclasts phagocytose the degradation products remain to be established. In the present communication, a hypothesis is presented suggesting the possibility that osteoclastic resorption of bone involves the participation of two different cell types. According to this hypothesis, osteoclastic bone resorption is initiated by osteoclasts that demineralize areas of bone and degrade noncollagen bone matrix. After the osteoclasts have moved away or become partially detached from the demineralized site, the exposed collagen fibrils are phagocytosed by mononuclear, fibroblast-like or monocyte-derived cells.

Uptake of peroxidase by calcitonin inhibited osteoclasts

The present electron microscopic histochemical study demonstrates that osteoclasts from calcitonin treated bone are able to take up organic macromolecules even though the ruffled border has disapppeared. The absorption occurs around the entire periphery of the osteoclast, but the amount of absorbed peroxidase seems to be reduced in comparison with that of untreated cells. It is concluded that the effect of calcitonin on osteoclasts is primarily a cessation of the exocytosis and the concomitant disappearance of the ruffled border.

Isolation of osteoclasts by velocity sedimentation at unit gravity

A method is presented for separating osteoclasts from the heterogeneous population of bone and marrow cells. Cell suspensions were prepared from femora of young rabbits by mechanical dispersion. The starting cell suspension typically contained only 1.0% +/- 0.5 osteoclasts. Following an initial 45 min of unit gravity sedimentation in a lucite chamber osteoclasts were primarily distributed in fractions 2-5. A second 45-min sedimentation of these pooled fractions yielded cell suspensions containing greater than 30% osteoclasts (as much as a 50-fold increase over starting percentages). Linear scan analysis, however, revealed that osteoclasts accounted for 73.14% +/- 0.58 of the cell colume. Subsequent in vitro experiments demonstrated linear incorporation of 3H-leucine into TCA precipitable protein for cells comprising the osteoclast fraction. Concomitant radioautographs revealed radioactive label in isolated osteoclasts.

Osteoclast ultrastructure in Paget's disease

The ultrastructural study of osteoclasts in biopsies from 12 patients with Paget's disease reveals several cytological anomalies. In Particular, nuclear inclusions, consisting essentially of striated filaments about 150 A in diameter, often organized in bundles and sometimes in paracrystalline arrays, appear to be specific to the disease. Morphologically, the inclusions are remarkably similar to those observed in various cases of viral attack, and the hypothesis of the action of a possible external agent in Paget's disease cannot be ruled out.