Immunocytochemical localization of cathepsin D in the rat osteoclast

Summary We performed immunocytochemical localization of cathepsin D in osteoclasts of the proximal growth plate of the rat femurs using both the avidin-biotin-peroxidase complex method for cryo-semi-thin (1 m) sections and the colloidal gold-labeled IgG method for K4M ultra-thin sections.At the light microscopic level, cathepsin D immunoreactivity in the osteoclasts appeared at the vesicles, granules, and/or small vacuoles. They were distributed throughout the cytoplasm of each cell and were relatively numerous close to the bone surface. This antigen could not be detected at the eroded bone surface. As for other cells, immunoreactivity was seen only in the lysosomes of osteoblast-like cells. Immunoreactivity in the osteoclasts was stronger and greater in the density and number than in osteoblast-like cells. At the electron microscopic level, osteoclasts with well-developed ruffled border possessed numerous cathepsin D-containing lysosomes, vacuoles, and coated vesicle-like structures. Cathepsin D-containing lysosomes fused with cathepsinnegative vacuoles and formed large secondary lysosomes. Osteoclasts with poorly developed ruffled border possessed fewer cathepsin D-containing lysosomes than those with well-developed ruffled border. No immunogold particles were seen in vacuole-like channel expansions of the ruffled borders, between the channels of the ruffled borders, or on the eroded bone surface.These findings demonstrate that osteoclasts contain a large amount of cathepsin D. They suggest that cathepsin D is necessary for osteoclastic bone resorption, that it plays an indirect rather than direct role.     

Localization of cathepsins B,D, and L in the rat osteoclast by immuno-light and -electron microscopy

The localization of cathepsins B, D, and L was studied in rat osteoclasts by immuno-light and-electron microscopy using the avidin-biotin-peroxidase complex (ABC) method. In cryosections prepared for light microscopy, immunoreactivity for cathepsin D was found in numerous vesicles and vacuoles but was not detected along the resorption lacunae of osteoclasts. However, immunoreactivity for cathepsins B and L occurred strongly along the lacunae, and only weak intracellular immunoreactivity was observed in the vesicles and peripheral part of the vacuoles near the ruffled border. In control sections that were not incubated with the antibody, no cathepsins were found in the osteoclasts or along the resorption lacunae of osteoclasts. At the electron microscopic level, strong intracellular reactivity of cathepsin D was found in numerous vacuoles and vesicles, while extracellular cathepsin D was only slightly detected at the base of the ruffled border but was not found in the eroded bone matrix. Most osteoclasts showed strong extracellular deposition of cathepsins B and L on the collagen fibrils and bone matrix under the ruffled border. The extracellular deposition was stronger for cathepsin L than for cathepsin B. Furthermore cathepsins B and L immunolabled some pits and part of the ampullar extracellular spaces, appearing as vacuoles in the sections. Conversely, the intracellular reactivity for cathepsins B and L was weak: cathepsin-containing vesicles and vacuoles as primary and secondary lysosomes occurred only sparsely. These findings suggest that cathepsins B and L, unlike cathepsin D, are rapidly released into the extracellular matrix and participate in the degradation of organic bone matrix containing collagen fibrils near the tip of the ruffled border. Cathepsin L may be more effective in the degradation of bone matrix than cathepsin B.     

Specific immunocytochemical localization of cathepsin E at the ruffled border membrane of active osteoclasts

The immunocytochemical localization of cathepsin E, a non-lysosomal aspartic proteinase, was investigated in rat osteoclasts using the monospecific antibody to this protein. At the light-microscopic level, the preferential immunoreactivity for cathepsin E was found at high levels in active osteoclasts in the physiological bone modeling process. Neighboring osteoblastic cells were devoid of its immunoreactivity. At the electron-microscopic level, cathepsin E was exclusively confined to the apical plasma membrane at the ruffled border of active osteoclasts and the eroded bone surface. Cathepsin E was also concentrated in some endocytotic vacuoles of various sizes in the vicinity of the ruffled border membrane, some of which appeared to be secondary lysosomes containing the phagocytosed materials. These results strongly suggest that this enzyme is involved both in the extracellular degradation of the bone organic matrix and in the intracellular breakdown of the ingested substances in osteoclasts.     

Cystein-proteinase localization in osteoclasts: An immunocytochemical study

Summary Cysteine-proteinases such as cathepsin B and G were localized in rat osteoclasts, by an indirect protein A-immunogold labeling technique, on post-embedded ultrathin sections. In osteoclasts, specific immunogold labeling of both anti-cathepsin B and G was localized in Golgi vesicles, lysosomes, pale vacuoles of various sizes, and the extracellular canals of ruffled borders; no immunoreactivity was seen in the cytoplasmic matrix, mitchondria, cisterns of the rough endoplasmic reticulum, or nuclei. The presence of immunolabeling of cathepsins in osteoclasts and in the subosteoclastic compartment suggests that these enzymes are involved in the extracellular degradation of collagen and other noncollagenous bone matrix proteins.     

Osteoclast cell-surface changes during the egg-laying cycle in Japanese quail

The medullary bone serves as a source of labile calcium mobilized during calcification of the egg shell in birds. Quantitative histological methods demonstrate that the numbers of medullary bone osteoclasts and nuclei per osteoclast remain unchanged during the egg cycle in the Japanese quail (Coturnix). Therefore, cyclic changes in bone resorption cannot be explained by modulations of osteoclasts from and into other bone cells, a mechanism previously suggested for certain species of birds. Rather, dramatic changes in osteoclast cell-surface features occur during the egg cycle, which might account for cyclic variations in resorptive activity. During egg shell calcification, osteoclasts with ruffled borders are closely apposed to bone surfaces; the cytoplasm is rich in vacuoles that contain mineral crystals and seem to derive from the ruffled border. At the completion of egg shell calcification, the ruffled borders and vacuoles move away from the bone surface, although the osteoclast remains attached to the bone along the filamentous or "clear" zone. Associated with the disappearance of the ruffled borders is the appearance of extensive interdigitated cell processes along the peripheral surface of the osteoclast away from the bone. These unusual structures, which may serve as a reservoir of membrane, largely disappear when ruffled borders and associated structures reappear. Therefore, in these hens, the osteoclasts modulate their cell surface rather than their population during the egg cycle.     

Limited and selective localization of the lysosomal membrane glycoproteins LGP85 and LGP96 in rat osteoclasts

 Monospecific antibodies against two major glycoproteins of rat lysosomal membranes with apparent molecular masses of 96 and 85 kDa, termed LGP96 and LGP85, respectively, were used as probes to determine the expression and distribution of lysosomal membranes in rat osteoclasts. At the light microscopic level, the preferential immunoreactivity for both proteins was found at high levels at the side facing bone of actively bone-resorbing osteoclasts. Osteoclasts detached from bone surface were devoid of immunoreactivity for each protein. At the electron microscopic level, both proteins were exclusively confined to the apical plasma membrane at the ruffled border of active osteoclasts with well-developed ruffled border membrane. No immunolabeling for both proteins was observed in the basolateral membrane and the clear zone of bone-resorbing osteoclasts. The plasma membrane of preosteoclasts and post- and/or resting osteoclasts showed little or no reactivity against these two antibodies. The results indicate that lysosomal membrane glycoproteins are actively synthesized in active osteoclasts, rapidly transported to the ruffled border area, and contribute to the formation and maintenance of the acidic resorption lacuna of osteoclasts. Accepted: 9 December 1998     

Studies of the mechanism of spleen cell cure for osteopetrosis in ia rats: appearance of osteoclasts with ruffled borders

After ia (osteopetrotic) rats receive whole body radiation and an injection of spleen cells from a normal littermate, the dense, sclerotic skeleton characteristic of osteopetrosis is rapidly remodeled and becomes normal in appearance radiographically and histologically within three weeks. The mechanism of this skeletal transformation has been explored in cured ia rats by light and electron microscopic examination of osteoclasts. In ia rats less than 25 days of age, osteoclasts viewed by electron microscopy lack a ruffled border - the extensive elaboration of plasma membrane next to the bone surface. Cured ia rats have osteoclasts with ruffled borders indistinguishable from those of normal littermates. In ia rats that receive only 600 rads whole body radiation, osteoclasts are still present three weeks later, but appear abnormal by light microscopy, with dense nuclei and lacking cytoplasmic vacuoles next to the bone surface. Cured ia rats have two types of osteoclasts, one type indistinguishable from osteoclasts of normal littermates by light microscopy, the other resembling osteoclasts of ia rats that received radiation only. These data indicate that the mechanism of the spleen cell cure for osteopetrosis in ia rats is rapid remodeling of the skeleton produced by osteoclasts with ruffled borders. Whether normal spleen cells produce these osteoclasts directly by cell division or indirectly by elaboration of some unknown local factor required for formations of ruffled borders by ia osteoclasts is not known.     

Osteoclasts and their relationship to bone as studied by electron microscopy

Summary Osteoclasts in metaphyses from young rats were systematically sectioned at different levels. Two types of osteoclasts were recognized. One type had no ruffled border while the other, and predominant type contained a ruffled border in a part of its length; some of the latter contained two ruffled borders. The closest contact between osteoclast and bone occurred at the level of the ruffled border and this bone under the border showed characteristic changes indicative of resorption. In some osteoclasts the ruffled border consisted of numerous slender cytoplasmic projections separated by very narrow spaces or channels while in other osteoclasts it was more open. The ruffled border was commonly surrounded by a transitional zone containing numerous thin filaments. The osteoclast usually had its greatest dimension at the level of the ruffled border and the cytoplasm here contained many bodies and vacuoles but a sparse endoplasmic reticulum. Away from the level of the ruffled border the cytoplasmic vacuoles and bodies were fewer while the endoplasmic reticulum was often more pronounced. Parts of the osteoclasts were usually situated close to a vessel. It is suggested that there is a correlation between the development of the ruffled border and the degree of bone resorption and that osteoclasts without a ruffled border are, at least temporarily, inactive with respect to bone resorption. The numerous cytoplasmic bodies, interpreted as lysosomes, are presumed to be important in the resorption process. The closely adjacent positioning of osteoclasts and vessels may facilitate the transport of resorption products to the blood.This research was supported by the Danish Research Council. Grant no. 512–727, 512–819 and 512–1545.I wish to thank Professor Arvid B. Maunsbach for valuable discussions.     

Ultrastructure of the giant cell infiltrate of subcutaneously implanted bone particles in rats and mice

The giant cells of soft tissues and those of mineralized tissues (osteoclasts) have distinctly different cell surface receptors and ultrastructural characteristics. Recently, the removal of dead bone particles in a subcutaneous environment has been described as a prototype of bone resorption, and a major issue is whether the giant cells that surround these ectopic bone implants and the processes involved in the disruption of bone surfaces are the same as those in the skeleton. We have compared the cytology and ultrastructure of giant cells recruited to subcutaneously implanted isogeneic bone particles with similar features of osteoclasts in metaphyseal bone of young normal rats and mice. Giant cells on surfaces of bone particles 2, 3, and 4 weeks after implantation were multinucleated, had a homogeneous, nonvacuolated cytoplasm, and had a bone surface interface unremarkable by light microscopy. In a few cells randomly distributed, small cytoplasmic vacuoles were present and large vacuoles were noted next to the bone surface at high magnification. By transmission electron microscopy, folded membrane configurations forming extensive interdigitations with adjacent cells were prominent features on most surfaces of giant cells. In instances where these interdigitations abutted bone surfaces, configuration resembling a ruffled border were noted, but these regions were always part of two different cells when examined at lower magnification or in serial sections. Breakdown of bone particles appeared to be by phagocytosis of small pieces and subsequent intracellular digestion in electron-dense cytoplasmic vacuoles. Osteoclasts from these same young animals were smaller with fewer nuclei, had cytoplasmic vacuoles concentrated next to bone surfaces, and had characteristic ruffled borders and clear zones. These results confirm those of others that native osteoclasts and multinucleated giant cells on dead bone particles are distinctly different with respect to both ultrastructure and mechanism of disruption of bone surfaces.     

Immunohistochemical localization of cathepsins B,D and L in the rat osteoclast

Immunohistochemical localization of cathepsins B, D and L in the osteoclasts of rat alveolar and femoral bones was investigated by using the avidin-biotin-peroxidase complex method for semithin, 1-m-thick cryosections. Extracellular immunoreactivity for cathepsins B and L was clearly demonstrated along the bone resorption lacunae; the intensity of the extracellular immunoreactivity of cathepsin L was stronger than that of cathepsin B. However, the intracellular immunoreactivity of both cathepsins was weak compared with that of cathepsin D. The intracellular immunoreactivity of cathespin D in the osteoclasts was clearly observed in the granules and/or vacuoles, but extracellular cathepsin D immunoreactivity was either negligible or not detected along the resorption lacunae. In the adjacent sections stained with anti-cathepsin L or D, extensive extracellular deposition of cathepsin L was found along the bone resorption lacunae, with or without osteoclasts, although the intracellular reactivity of cathepsin L was weak. This is the first morphological study in which cathepsins B and L have been demonstrated to be produced in the osteoclasts and extensively secreted into resorption lacunae, and in which cathepsin D was found to be present in the cells but scantily secreted into the lacunae. These findings suggest that cathepsins B and L directly and effectively participate in the degradation of the bone matrix.     

Increased synthesis and specific localization of a major lysosomal membrane sialoglycoprotein (LGP107) at the ruffled border membrane of active osteoclasts

The immunocytochemical localization was investigated of a major lysosomal membrane sialoglycoprotein with a molecular mass of 107 kDa, which was designated as LGP107. The study utilized rat osteoclasts with different bone resorbing activity and osteoclast precursors at various stages of differentiation and maturation together with monospecific antibodies to this protein. Despite its localization primarily in lysosomes and endosomes in the other cell types examined, LGP107 was exclusively confined to the apical plasma membrane at the ruffled border of the active osteoclast, where the osteoclast is in contact with the bone surface. The protein was also concentrated in a number of endocytic vacuoles in the vicinity of the ruffled border membrane. However the labeling was not found in the basolateral membranes of the active osteoclast. The ruffled border membrane detached from the bone surface showed a marked decrease in the extent of the immunolabeling. The post-and/or resting osteoclasts, which were located away from the bone surface, were totally devoid of the membraneous localization of LGP107. No definite immunolabeling was found in the immature preosteoclasts. These results indicate that the protein is largely synthesized in the active osteoclast and rapidly translocated to the ruffled border membrane by vectorial vesicle transport. LGP107 is suggested to contribute to the formation and maintenance of the specialized acidic environment for bone resorption.     

Osteoclasts from mice deficient in tartrate-resistant acid phosphatase have altered ruffled borders and disturbed intracellular vesicular transport

Tartrate-resistant acid phosphatase (TRAP) is an enzyme highly expressed in osteoclasts (OC) and chondroclasts. As an approach to pinpoint the function of TRAP in bone-resorbing osteoclasts, the morphological phenotypic alterations of bone and osteoclasts in mice with targeted disruption of the TRAP gene were assessed by quantitative histomorphometry and immunocytochemistry at the light microscopic and ultrastructural levels. TRAP-deficient mice display alterations in the epiphyseal growth plates as evidenced by increased height with disorganized columns of chondrocytes, in particular affecting the zone of hypertrophic chondrocytes, consistent with a disturbance of chondrocyte maturation and chondroclastic resorption at the epiphyseal/metaphyseal junction. TRAP -/- mice express an early onset osteopetrotic bone phenotype, apparent already at 4 weeks of age. The differentiation of OCs was apparently normal; however, the osteoclasts in TRAP-deficient mice were less active in terms of degradation or release of the resorption marker C-terminal type I collagen cross-linked peptide, indicative of an intrinsic defect. Ultrastructural morphometry disclosed that OCs from TRAP-deficient young mice exhibited an increased relative area of ruffled borders. Moreover, mutant OC accumulated cytoplasmic vesicles 200-500 nm in size in both ruffled border and basolateral parts of the cytoplasm, reflecting disturbed intracellular transport. The accumulated vesicles were not likely derived from the secretory pathway, since cathepsin K was detected at normal levels in the ruffled border area and matrix in TRAP -/- mice. In summary, the resorptive defect in TRAP-deficient OCs is reflected by a disturbance at the level of ruffled borders and intracellular transport vesicles. Consequently, accumulation of vesicles in the cytoplasm of mutant OCs indicates a novel function for TRAP in modulating intracellular vesicular transport in osteoclasts.     

THE EFFECTS OF PARATHYROID HORMONE, COLCHICINE, AND CALCITONIN ON THE ULTRASTRUCTURE AND THE ACTIVITY OF OSTEOCLASTS IN ORGAN CULTURE

The ultrastructure of osteoclasts was examined in fetal rat bones after stimulation or inhibition of resorption in culture. A central ruffled border area completely encircled by a clear zone was considered to represent the resorbing system of the cell. The proportion of ruffled border and clear zone in osteoclast cross sections was compared with changes in bone resorption as measured by the release of previously incorporated radioactive calcium (⁴⁵Ca). In control cultures 55% of the osteoclast cross sections showed an area closely apposed to bone and this consisted mainly of clear zone; only 11% showed ruffled borders. Treatment with parathyroid hormone (PTH) increased ⁴⁵Ca release, increased the frequency of finding areas closely apposed to bone (79%), and markedly increased the frequency of the ruffled border area (64%). Colchicine given concurrently with PTH decreased the number of osteoclasts. Colchicine or calcitonin treatment after PTH stimulation decreased the proportion of ruffled border area significantly by 1 h; this was followed by a decrease in ⁴⁵Ca release. These inhibited osteoclasts resembled osteoclasts from control, unstimulated cultures, suggesting that the cells had returned to their inactive state. Colchicine-treated osteoclasts also showed a loss of microtubules and a massive accumulation of 100 Å filaments, suggesting that synthesis of microtubular subunits had increased.     

Transcytosis of calcium from bone by osteoclast-like cells evidenced by direct visualization of calcium in cells

'Transcytosis' of calcium (Ca) from bone by osteoclasts was identified by using a newly developed method that uses fixed or living osteoclast-like cells previously differentiated in vitro, a Ca-specific cell-membrane-impermeable fluorescent dye, and confocal laser scanning microscopy. This method, called the cell-membrane-impermeable dye method, revealed that in fixed osteoclast-like cells, a large quantity of Ca was confined within vacuoles and transported toward the apical cell membrane in the cells. These Ca-confined vacuoles were co-localized with marker proteins of both ruffled border and lysosome. The vacuoles were disrupted when treated with an inhibitor of ruffled border ATPase. In living osteoclast-like cells, Ca-confined vacuoles were again preferentially located at the central region and near the apical cell membrane. These results suggest actual transcytosis of Ca from bone by osteoclasts, and are the first direct evidence of the significant role of osteoclasts in the entire process of Ca metabolism in bone.     

Three-dimensional distribution of the clear zone of migrating osteoclasts on dentin slices in vitro

Osteoclasts are cells that dynamically alternate resorption and migration on bone surfaces, and have the special structure called ruffled borders and clear zones by transmission electron microscopy (TEM). However, TEM features, especially the distribution of the clear zone of osteoclasts during migration, remains unclear. This study aimed to examine osteoclasts cultured on dentin slices by TEM and clarify the features of migrating osteoclasts, especially the three-dimensional distribution of clear zones. Osteoclasts obtained from mice were cultured with dentin slices for 72 h, and then cells were fixed and the tartrate-resistant acid phosphatase (TRAP) activity was detected. Specimens were embedded in Epon, then TRAP-positive cells were serially sectioned by alternating semithin and ultrathin sections. The cells were examined by TEM and the three-dimensional structures were reconstructed by computer. By TEM, most TRAP-positive cells were resorbing osteoclasts with ruffled borders and a clear zone. There were osteoclasts without ruffled borders, and these cells had clear zone-like structures and lamellipodia. The three-dimensional reconstruction showed that resorbing osteoclasts had rounded contours and ring-shaped clear zones encircling ruffled borders, and that osteoclasts without ruffled borders had irregular and flat shapes; the clear zone-like structures showed a dot or patch-like distribution. The presence of lamellipodia of the osteoclasts without ruffled borders shows that the cells are migrating osteoclasts. These results suggest that dot or patch-like distribution is the feature of the clear zone of osteoclasts during migration, and that these structures play the role of focal contacts and adhesion to the dentin surfaces during cell migration.     

Rapid activation of the medullary bone osteoclast cell surface by parathyroid hormone

Quantitative transmission electron microscope methods were used to determine the response of functionally inactive avian medullary bone osteoclasts to parathyroid hormone (PTH). Egg-lying Japanese quail were used during a period of the egg cycle when medullary bone was not being resorbed for egg shell calcification and when medullary bone osteoclasts were functionally inactive. Ruffled borders adjacent to bone surfaces were rarely, if ever, found on these cells. 20 min after the administration of PTH, over 70% of the osteoclast profiles had ruffled borders adjacent to bone surfaces. These ruffled borders were bounded by filamentous-rich "clear zones" and resembled ruffled borders found on functionally active cells. There was also a marked increase in plasma calcium levels after PTH administration. This study demonstrates that PTH stimulates the de novo generation of ruffled borders on osteoclasts in vivo and suggests that osteoclasts may be involved in the acute regulation of calcium metabolism by exogenous PTH.     

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 in teleost fish: Light-and electron-microscopical observations

Summary This paper reports the common occurrence of osteoclasts during normal and experimental bone resorption in a number of teleost fishes. Light-microscopical observations on osteoclasts are presented in resorption areas on perichondral bone (mandibula and pharyngeal jaws of cichlids and vertebrae of gymnotids), on dermal bone (mandibula of salmonids and characoids and frontal bone of cichlids), on chondroid bone (pharyngeal jaws of cichlids), and on elasmoid body scales (eichlids and gymnotids). Osteoclasts acting along the bone surface usually lie in a Howship's lacuna whereas others are wrapped around bone extremities. Electronmicroscopical observations reveal that teleost osteoclasts show features similar to those of higher vertebrate osteoclasts, c.g., the presence of a ruffled border and the occurrence of numerous vacuoles, lysosomes and mitochondria. The multinucleated aspect that characterizes osteoclasts in other vertebrate groups is not a distinct feature of teleost osteoclasts since some are possibly mononucleated. Teleost osteoclasts are also able to resorb uncalcified tissues adjoining bone resorption areas, either as a primary process directed toward the tissue (basal plate of elasmoid scale) or as a secondary phenomenon (cartilage).     

Ultrastructural and lanthanum tracer examination of rapidly resorbing rat alveolar bone suggests that osteoclasts internalize dying bone cells

Glutaraldehyde-formaldehyde fixed undecalcified alveolar bone from 7-day-old rats was prepared for light and electron microscopy. Colloidal lanthanum was used as an ultrastructural tracer, and both random and semi-serial sections were examined. Lanthanum penetrated the infoldings of the ruffled border and some nearby vacuoles and vesicles. The majority of vacuoles and vesicles were lanthanum-free. Some osteoclast profiles contained a large vacuole with a cell enclosed in its interior. The enclosed cell exhibited an irregular nucleus containing condensed peripheral chromatin, intact cytoplasmic organelles, conspicuous rough endoplasmic reticulum and large blebs on the cell surface. These features are characteristic of osteoblasts or bone-lining cells or immature osteocytes which may be undergoing apoptosis or necrosis. The observation of remnants of cellular structures within internalized osteoclast vacuoles, together with the above results, suggests that osteoclasts engulf and probably degrade dying osteoblasts/bone-lining cells or immature osteocytes. Received: 29 April 1997 / Accepted: 20 February 1998     

The Foreign Body Giant Cell Cannot Resorb Bone,But Dissolves Hydroxyapatite Like Osteoclasts

Foreign body multinucleated giant cells (FBGCs) and osteoclasts share several characteristics, like a common myeloid precursor cell, multinuclearity, expression of tartrate-resistant acid phosphatase (TRAcP) and dendritic cell-specific transmembrane protein (DC-STAMP). However, there is an important difference: osteoclasts form and reside in the vicinity of bone, while FBGCs form only under pathological conditions or at the surface of foreign materials, like medical implants. Despite similarities, an important distinction between these cell types is that osteoclasts can resorb bone, but it is unknown whether FBGCs are capable of such an activity. To investigate this, we differentiated FBGCs and osteoclasts in vitro from their common CD14⁺ monocyte precursor cells, using different sets of cytokines. Both cell types were cultured on bovine bone slices and analyzed for typical osteoclast features, such as bone resorption, presence of actin rings, formation of a ruffled border, and characteristic gene expression over time. Additionally, both cell types were cultured on a biomimetic hydroxyapatite coating to discriminate between bone resorption and mineral dissolution independent of organic matrix proteolysis. Both cell types differentiated into multinucleated cells on bone, but FBGCs were larger and had a higher number of nuclei compared to osteoclasts. FBGCs were not able to resorb bone, yet they were able to dissolve the mineral fraction of bone at the surface. Remarkably, FBGCs also expressed actin rings, podosome belts and sealing zones—cytoskeletal organization that is considered to be osteoclast-specific. However, they did not form a ruffled border. At the gene expression level, FBGCs and osteoclasts expressed similar levels of mRNAs that are associated with the dissolution of mineral (e.g., anion exchange protein 2 (AE2), carbonic anhydrase 2 (CAII), chloride channel 7 (CIC7), and vacuolar-type H⁺-ATPase (v-ATPase)), in contrast the matrix degrading enzyme cathepsin K, which was hardly expressed by FBGCs. Functionally, the latter cells were able to dissolve a biomimetic hydroxyapatite coating in vitro, which was blocked by inhibiting v-ATPase enzyme activity. These results show that FBGCs have the capacity to dissolve the mineral phase of bone, similar to osteoclasts. However, they are not able to digest the matrix fraction of bone, likely due to the lack of a ruffled border and cathepsin K.