Isolation of osteoclasts by velocity sedimentation at unit gravity

Isolation and Generation of Osteoclasts

This chapter describes the isolation, culture, and staining of osteoclasts. The key advantages of this assay are that it allows direct measurement of osteoclast number, bone resorption, as well as yielding good quantities of osteoclasts at defined stages of formation for molecular analysis. An additional focus of this chapter will be the generation of osteoclasts from less conventional animal species and cell lines.

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

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

Adhesive and bone resorptive activities of isolated osteoclasts from hen medullary bone

In the present study, osteoclasts were isolated from hen medullary bones at the formative and resorptive phases. The cells were cultured on glass culture dishes and bone slices. After culturing, the adhesion activity of the isolated osteoclasts with the substrates was estimated with a light microscope, and the surfaces of the bone slices were observed with a scanning electron microscope. The results showed that the adhesion activity of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts is higher at the bone resorptive phase than at the bone formative phase, and this tendency in isolated osteoclasts was observed more frequently on the bone slices than on the glass culture dishes. Furthermore, scanning electron microscopy showed that the isolated osteoclasts in the bone resorptive phase adhered to the bone surface with developed-cytoplasmic projections and formed broad pits where collagen fibrils were exposed. On the other hand, isolated osteoclasts in the bone formative phase adhered to the bone slice with board-shaped cytoplasmic projections and did not form any pits. These results suggest that isolated osteoclasts in the bone resorptive phase have a high level of adhesion activity and actively resorb the bone, whereas isolated osteoclasts in the bone formative phase have a low level of adhesion activity and cease bone resorption. The procedure reported here is useful for studying the bone-resorptive mechanism of authentic osteoclasts.

Long-term culture of disaggregated rat osteoclasts: inhibition of bone resorption and reduction of osteoclast-like cell number by calcitonin and PTHrP[107-139]

The isolated osteoclast bone resorption assay has proved to be a useful means of examining the response of mammalian and avian osteoclasts to a variety of stimuli. The assay has traditionally been performed over a period of 24 hours. By extending the duration of the osteoclast bone resorption assay, we have been able to assess the long-term effects of carboxyl-terminal parathyroid hormone-related protein (hPTHrP[107-139]), salmon calcitonin (sCT) and hPTH[1-34] on bone resorption and TRACP-positive osteoclast-like cell number. We found that, in control cultures over a period of up to 144 hours, the osteoclast-like cells not only remained viable but their numbers also increased. The number of mononucleated and multinucleated osteoclast-like cells doubled in the first 48 hours before stabilizing over the remainder of the incubation period. Osteoblasts also proliferated, resulting in a resorption response to hPTH[1-34] being evident from 48 hours onward. hPTHrP]107-139] persistently inhibited basal and PTH-stimulated bone resorption for at least 96-144 hours, whereas "escape" from the inhibitory effect of sCT was seen after 48-72 hours. Decreased numbers of both mononucleated and multinucleated TRACP-positive osteoclast-like cells were seen by 48 hours in cultures treated with sCT. In contrast, hPTHrP[107-139] reduced the number of mononuclear TRACP-positive cells with only a late effect on multinucleated cells. Furthermore, the increased number of osteoclast-like cells seen in response to hPTH[1-34] was inhibited by carboxyl-terminal PTHrP. In summary, this study indicates that the extended bone resorption assay system is a complex one where both osteoclastic resorption and osteoclast maturation are evident. Using this system, we have shown that hPTHrP[107-139] acts as a potent long-term inhibitor of osteoclastic bone resorption, without evidence of escape from its effect. Its action to reduce the number of mononucleated osteoclast-like cells suggests that it affects several aspects of osteoclast activity.

Bone resorption by isolated chick osteoclasts in culture is stimulated by murine spleen cell supernatant fluids (osteoclast-activating factor) and inhibited by calcitonin and prostaglandin E2

The question of whether any of the agents known to activate bone resorption in vivo or in organ cultures acts directly on the osteoclast or via intermediate target cells that secondarily secrete locally paracrine factors is important for our understanding of bone remodeling. In an attempt to clarify this issue for some of the agents, we have taken advantage of the recent progress in obtaining and culturing relatively pure populations of osteoclasts. We performed an in vitro bone-resorbing assay in which isolated and partially purified chick osteoclasts were cultured on devitalized, paired and standardized bone disks prepared from rat calvaria prelabeled with both 45Ca and 3H-proline. Some of the isolated osteoclasts attached to the devitalized bone matrix, formed a ruffled border, and acidified the bone-resorbing compartment that they established with the matrix, thereby indicating that they resorbed bone in a physiologic manner. Salmon calcitonin added to these cultures (0.3 U/ml = 60 ng/ml) and prostaglandin E2 (PGE2) (10(-6) M) inhibited both basal and stimulated 45Ca and 3H-proline release. Neither parathyroid hormone (PTH) 1-34 (1 U/ml), 1,25-(OH)2-D3 (10(-8) and 10(-9) M), nor interleukin 1 (IL-1) (purified from P388D1 macrophage culture supernatant fluids or recombinant murine IL-1-alpha) (100 ng/ml) stimulated bone resorption in these cultures. In contrast, supernatant fluids from concanavalin A (Con-A)-activated murine spleen cell cultures (murine osteoclast-activating factor; OAF) consistently and significantly induced a 3- to 5-fold stimulation of bone resorption in this system.

Isolation of osteoclasts from Pagetic bone tissue: morphometry and cytochemistry on isolated cells

Giant osteoclasts and other cells were isolated from Pagetic bone tissue using 0.5 mM ethylene diamine tetraacetic acid on bone samples from 8 patients with Paget's disease. The cell suspension contained osteoclasts and osteoblasts as well as some mononuclear cells such as monocytes. The number of nuclei in isolated osteoclasts (33.85 +/- 20.92 nuclei/osteoclast) correlates fairly well (p less than 0.02) with the number of nuclei counted on histologic sections (15.88 +/- 11.80 nuclei/osteoclast) for samples from each patient. Enzyme histochemistry demonstrated acid phosphatase activity in isolated osteoclasts and in mononucleated cells, such as monocytes. Alkaline phosphatase was detected only in osteoblasts while succinate dehydrogenase was observed in osteoclasts, osteoblasts and monocytes. Esterases, such as nonspecific aliesterase and specific naphthol AS-D acetate esterase, were identified in osteoclasts and in macrophages. Inhibition of specific naphthol AS-D acetate esterase in osteoclasts by addition of sodium fluoride suggests that the enzyme could be of monocytic origin.

Cell separation: a review

This review summarizes currently available techniques for cell separation. Techniques that exploit differences in physical properties of cells are widely used but have a number of limitations. Those that are based on differences in surface properties may more readily permit reproducible separation of a functionally homogeneous population of cells. Unfortunately very few techniques achieve separation of cells on the basis of differences in their functional characteristics.

Differential serum dependence of cultured osteoclastic and osteoblastic bone cells

Sequential collagenase digestion of mice calvariae provides populations of bone cells that express either osteoclasts (OC) or osteoblastic (OB) activities after growth for 6 days in similar culture conditions consisting of minimal essential medium supplemented with 10% fetal calf serum (FCS). The OC characteristics (acid phosphatase activity and hyaluronate synthesis, and their stimulation by PTH) were recovered in the cell populations released early from calvariae, but these also contained OB cells and numerous spindle-shaped alkaline phosphatase positive cells that resembled fibroblasts. We have attempted to select for growth of OC cells in these early populations by exploiting differences in growth requirements of OC, OB, and fibroblastic cells. We find that after growth for 6 days in low serum (2% FCS), OC cell populations demonstrated a threefold increase in OC activity/cell, and cell yield was reduced to one-third of that obtained in 10% FCS. Spindle-shaped cells were absent in 2% FCS and OB marker activities (alkaline phosphatase and citrate decarboxylation) were reduced threefold. In contrast to OC cells, high serum (10% FCS) favored the growth and phenotypic expression of OB cells (late populations). Cell yield and OB marker activities/cell were twofold higher in OB cells grown in 10% FCS vs 2% FCS, whereas growth but not phenotypic expression was retained at 5% FCS. These data suggest that differential serum dependence of OC and OB cells may provide a basis for further enrichment for each cell type following sequential digestion.

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.

Carbonic anhydrase activity in isolated osteoclasts

Osteoclasts were isolated from the long bones of chicks by a nylon mesh filtering system. The cell purity, in terms of area of the slide occupied, was on the average 77.5% osteoclasts, 22% aggregated osteoblasts and matrix debris, and 1.5% individual blood and marrow cells. Viability, as measured by cytochalasin-blockable phagocytosis, of up to 99% was obtained. Electron microscopic examination revealed good retention of ultrastructural features. The presence of carbonic anhydrase and acid phosphatase in osteoclasts was verified by selective staining methods; the aggregates were positive for alkaline phosphatase. Carbonic anhydrase activity was 0.89 +/- 0.13 X 10(-4) micro Wilbur-Anderson units per osteoclast, and red blood cells had 0.12 +/- 0.03 X 10(-4) units/cell. Neither calcitonin nor parathyroid hormone influenced the activity of carbonic anhydrase. A review of other hormonal effects on carbonic anhydrase is provided.

Isolated osteoclasts and their presumed progenitor cells, the monocyte, in culture

Osteoclasts were isolated from the endosteal surface of day 19 embryonic chick tibias by mild trypsinization. Osteoclast enrichment was achieved by passing cell suspensions through Nitex screening of selective sizes, including the eventual selective retention of osteoclasts on 12 micrometers polycarbonate filters or by sequential sieving through Nitex screens and fractionation on Percoll gradients. The enrichment procedures produced osteoclast populations of 50-75% based on morphological criteria with the latter isolation method providing populations with less matrix debris. The results of light microscopy, transmission and scanning electron microscopic observations indicate that osteoclasts can be maintained in culture for up to 10 days with retention of osteoclast morphology. This morphology includes a specialized ruffled plasma membrane, large numbers of mitochondria, lysosomes, as well as a multinucleated cytoplasm. Furthermore, acid phosphatase and butyrate esterase histochemical measurements support these morphological observations. In addition, chick hatchling circulating monocytes were isolated and purified by Ficoll-hypaque gradient centrifugation with subsequent adhesion to glass petri dishes. With time in culture, these cells form multinucleated cells, but lack the ultrastructural complexity of the isolated osteoclasts. This report describes a unique culture system to study osteoclast function and illustrates the similarities and differences of this system to the monocyte-to-giant cell culture system.

Adhesion of osteoclasts and monocytes to developing bone

Osteoclast resorption of bone matrix during bone development is preceded by cell attachment to bone. An in vitro assay is described that measures adhesion of isolated osteoclasts and their presumed progenitor cell, the monocyte, to embryonic chick bone. Osteoclasts were isolated from day 19 chick tibia and enriched. Circulating chick hatchling monocytes were purified by Ficoll-hypaque sedimentation and attachment to plastic. Isolated cells were labeled with (32P)-KH2PO4 to quantitate cell attachment. In one series of experiments, labeled cells were inoculated into vessels containing day 19 tibias with either endosteal or periosteal bone surface exposed. Labeled cells were also inoculated into chambers containing day 6 (cartilage cores), day 12 (bone and cartilage cores), or day 19 embryonic tibias (bone). Cultures of stage 24 chick limb mesenchyme and embryonic chick skin fibroblasts served as controls. Results demonstrate that twice as many osteoclasts and monocytes adhere to bone as compared to fibroblasts and stage 24 limb mesenchymal cells. Furthermore, there does not appear to be selective adhesion to the endosteal as compared to periosteal bone surfaces. When the data is calculated on the basis of cell attachment per area of each substrate, the day 12 cores had slightly higher number of osteoclasts and monocytes attached compared to day 19 bones; day 6 cartilage cores bound few cells. These observations suggest that osteoclasts and monocytes have a high affinity to bone which seems to be influenced by the developmental age and composition of the substrate.

Isolated osteoclasts in primary culture: first observations on structure and survival in culture media

Osteoclasts were isolated mechanically from the medullary bone of laying hens kept 7 days on a low calcium diet. Osteoclast enrichment was achieved with 3-4 sedimentations of the cell suspension in test-tubes prepared by layering on the bottom with BSA 10% in MEM-HEPES or PBS, above which the cells were suspended in MEM-HEPES or PBS. The final suspension of osteoclasts was cultivated in MEM with 10% FCS for 3 weeks. The cultures were observed by phase-contrast and scanning electron microscopy (SEM). By the third day, the osteoclasts were completely spread onto the plastic dishes and a variety of morphologies and of intercellular contacts was established. Osteoclasts in culture do not lose their morphology; they survive for long periods and can be used in many experimental systems.

Isolation of osteoclasts from fetal rat long bones

Cell isolates containing multinucleate osteoclasts were obtained from longitudinally split fetal rat long bones by treatment with testicular hyaluronidase. The total yield of osteoclasts and the osteoclast enrichment of the isolate were increased if the intact bones were first cultured for 72 h. Even greater enhancement was obtained if the bones were treated with 1,25-dihydroxycholecalciferol [1,25(OH)2D3] during the culture period. This technique resulted in a cell population containing approximately 15% osteoclasts in yields greater than 50 osteoclasts per long bone. The yield of osteoclasts and the percentage of osteoclasts correlated well with the extent of bone resorption induced by 1,25(OH)2D3. The effectiveness of several isolation procedures was compared using the 1,25(OH)2D3-treated long bones. Conventional digestion with 1 mg/ml crude collagenase gave a much poorer yield of osteoclasts than simply agitating the split long bones. Hyaluronidase plus EDTA was not significantly different from EDTA alone. Even with milder procedures, however, the isolated osteoclasts were damaged as judged by their failure to exclude trypan blue. The osteoclasts are obviously very fragile cells. The isolation technique coupled with May-Grunwald-Giemsa staining permitted reliable determination of the median number of nuclei per osteoclast. This parameter was the same in uncultured bones or in bones cultured for 72 h in control media. Treatment with 1,25(OH)2D3 increased the nuclear number. At lower levels of bone resorption, nuclear number did not increase, but it was significantly greater in more highly resorbed bones.

A comparative study of osteoclasts: in situ versus smear specimens

Osteoclasts residing on rims and walls of bony concavities on remodeling proximal tibia from growing rats were examined by light microscopy following stripping of the periosteal connective tissue layer. Comparison of these cells in situ and after transfer to glass slides revealed the presence of numerous mononucleate osteoclasts, as well as typical multinucleate forms, all exhibiting a ruffled border and acid phosphatase and succinic dehydrogenase activity. Macrophage-like cells were situated adjacent to osteoclasts in situ. Osteoblasts were relatively inconspicuous. The possibility that the basic functional osteoclast unit is a mononucleate cell is discussed.

Characterization of subpopulations of OC and OB bone cells obtained by sedimentation at unit gravity

Immediately after isolation from calvaria, OC and OB bone cells populations 1-6 were individually characterized by sedimentation at unit gravity. This procedure was used to generate from each population 4 fractions that contained cells of different sizes. Sedimentation results suggested that freshly isolated OC cell populations consisted of cells that were generally smaller and demonstrated less size heterogeneity than OB cells. After sedimentation the cells in each fraction were cultured for 6 days and then characterized with regard to cell separation based on basal biochemical characteristics and hormonal responses to PTH and CT. The largest cells in the later released OC cells appeared to be a mixture of OC and OB cells (approx. 15% of populations 2 + 3). All OB cell fractions appeared to be free of OC cells. The highest basal OC activities and hormonal responses occurred in the larger cells of population 2, whereas in OB cells (populations 5 and 6), this occurred in the cells of small to intermediate size. Finally, although the absolute size of the cells in each fraction increased during culture, the size differential within the fractions and between OC and OB cells was maintained even after 6 days.

Characterization of osteoblast-like cells from fetal rat calvaria

An improved method is described for the isolation of osteoblast-like cells from fetal rat calvaria. The cells are mononuclear, 8.3 mu in diameter, alkaline phosphate positive, and possess rough endoplasmic reticulum, mitochondria, Golgi, and cytoplasmic processes. Isolated bone cells synthesize collagen and cholesterol, grow to confluence in primary culture, and respond to hormonal stimulation.

Bone cell cultures as an experimental model

The isolation and separation of bone cells with populations enriched for osteoclastic or osteoblastic phenotypes are described. Such systems offer the opportunity to compare and contrast the controls exerted by hormones, ions, and other agents on the functions of the individual bone cell types and may in the future provide explanation for the changes seen in bone tissue in various diseased states.

Phagocytosis and trypsin-resistant glass adhesion by osteoclasts in culture

Osteoclasts were cultured in vitro and tested for the functions normally associated with cells of the mononuclear phagocyte system. They were found to ingest glutaraldehyde-fixed red cells and latex, but not opsonised or complement-coated sheep red cells. Osteoclasts are glass-adherent and resist removal by trypsin. The implications of these findings for the identity of the osteoclast precursor cell and its mechanism of fusion are discussed.