Differential serum dependence of cultured osteoclastic and osteoblastic bone cells

The effect of methotrexate on mouse bone cells in culture

OBJECTIVE

We have recently shown that administration of long-term, low-dose methotrexate (MTX) causes severe osteopenia in female rats. This osteopenia is characterized both by decreased osteoblast function without a decrease in osteoblast numbers, and by increased bone resorption that is believed to represent a physiologic remodeling response by osteoclasts. The present study investigates the effects of varying doses of MTX on mouse bone cells in culture.

METHODS

Cells were obtained by sequential digestion of neonatal mouse calvariae, and cultured with fetal calf serum (10% for osteoblast-like cells and 2% for osteoclast-like cells). After 1 week, MTX was added to each culture in concentrations of 0.6 microM, 0.4 microM, 0.2 microM, 0.1 microM, 1 nM, and 0.5 nM. All experiments were done on 24 wells for each MTX concentration and for the controls. The effect on osteoblastic cells was assessed, at 7 days, by cell counts and by measurement of lysate alkaline phosphatase and supernatant osteocalcin levels, and, at 21 days, by analysis of the calcified matrix production, which was cultured with ascorbic acid and beta-glycerophosphate. For osteoclastic cells, cell count and lysate acid phosphatase levels were determined.

RESULTS

Levels of osteoblastic cells and lysate alkaline phosphatase were not changed by any of the concentrations of MTX. Matrix calcification and supernatant osteocalcin levels were diminished by MTX in a dose-responsive manner. Osteoclast-like cell numbers and acid phosphatase levels were not significantly affected by MTX.

CONCLUSION

These results suggest that diminished mouse osteoblastic cell function occurs with very low mean concentrations of MTX, in a dose-responsive manner. The mechanism seems to be inability of the cell to synthesize and calcify matrix, possibly through defective osteocalcin production. Thus, low-dose MTX may have an important impact on bone density by slowing osteoblastic matrix production.

Fluoride increases rat osteoblast function and population after in vivo administration but not after in vitro exposure

The effects of fluoride on bone tissue are now well documented by in vivo histological studies performed on both human and animal bone biopsies and demonstrating an increase in osteoblast (OB) population. In order to elucidate whether the mechanism of action of fluoride on osteoblasts was direct or indirect, 14 three-week-old Sprague-Dawley rats were selected. Seven animals received 100 ppm fluoride as sodium fluoride (NaF) in drinking water for one month. The other animals, which did not receive fluoride, were considered as controls. At the end of the experiment, femurs and vertebrae were excised and osteoblastic cells were obtained after collagenase digestion separately from each animal. The osteoblastic cells derived from control and NaF-treated rats were exposed in vitro to 10(-5) M NaF. Alkaline phosphatase (AP) activity was measured, and the cellular proliferation was assessed by 3H-thymidine incorporation. Thymidine incorporation and AP activity were significantly higher in osteoblastic cells derived from NaF-treated rats than in cells obtained from control rats (p = 0.05 and p < 0.01, respectively). In contrast, the osteoblast proliferation and activity were not modified after in vitro exposure to NaF in cells derived from control and NaF-treated rats. In conclusion, the function of osteoblasts was not modified after in vitro exposure to fluoride. In contrast, given in vivo to rats for one month, fluoride has a mitogenic effect on osteoblasts and stimulates their activity. These data emphasize the hypothesis that fluoride may act either on osteoprogenitor cells or through an indirect mechanism mediated by a cofactor.

Multinucleated giant cells in primary cultures derived from canine bone marrow--evidence for formation of putative osteoclasts

Mononucleated cells derived from canine bone marrow were maintained in vitro for up to 6 weeks. The culture characteristics and development of these cells were evaluated by histological, ultrastructural and histochemical methods. Within 1 week the cells had fused together to form flattened, multinucleated cells. Further fusing with one another and other mononucleated cells produced large (diameters more than 300 microns), multinucleated cells which frequently contained more than 50 nuclei per cell and exhibited ultrastructural and histochemical features that were strikingly similar to those displayed by osteoclasts. The confluent monolayer of mono- and multinucleated cells present at 4 weeks had, by the sixth week, become altered such that fibroblast overgrowth obliterated all other cells. During the development of the culture adipocytes became differentiated from mononuclear cells and frequently were located within spherical multicellular aggregates (spheroids). Functional assessments were employed to investigate whether the multinucleated cells generated in this way, represented osteoclast-like cells, or alternatively, were related to macrophage polykarya as found in foreign body granulomata in vivo. Neither resorption pits on sperm whale dentine slivers (diagnostic of osteoclasts), nor formation of granulomata in vitro, were observed. We believe that the present results indicate that the multinucleated cells generated from canine bone marrow mononuclear precursors in vitro, merit designation as osteoclast-like cells. Definitive characterisation however, must await further functional assessments of hormone responsiveness.

Impact of streptozotocin-induced diabetes on rat blood and alveolar bone affected by occlusal stress

Adult male and female rats were used as test animals. The experimental diabetes mellitus was provoked with one dose of i.v. injection of streptozotocin (65 mg/kg body weight), which interferes with the insulin release mechanism in pancreatic beta cells. After a follow-up period of 10 wk an average loss of 10% of body weight and an increase of 25% in the amount of blood obtained by decapitation was recorded in the test animals. The biochemical assays performed on the serum of the diabetic rats showed, for both sexes, a fourfold rise in the plasma glucose level, a threefold rise in plasma alkaline phosphatase activity and plasma alanine transferase activity as well as a 1.5-fold rise in plasma creatine value. The two latter values indicated systematic disorders reflected in the liver and the kidneys. An increase in serum total calcium and hydroxyproline values was also detected. The clinical studies of the gingiva showed diminished tissue resistance in diabetic rats. The histologic studies of alveolar bone revealed retarded formation of bone matrix and new bone in diabetic animals. However, the stimulated metabolism in alveolar bone, due to the artificially induced stress, increased marginal bone cell activity in both the diabetic and the control group, resulting in increased crestal resorption in the former group. The differences in tissue response observed among the diabetic animals affected and unaffected by stress originated from the disturbed recovering mechanism typically found in diabetic animals.

Mitogenic responses to and binding of insulin-like growth factor 1 and/or epidermal growth factor by bone cells

Populations of cells with different biochemical characteristics can be obtained following enzymatic digestion of newborn mouse calvaria. Previous studies had suggested an enrichment for periosteal fibroblasts in early released cells, based on their proliferative response to PGE2 and for osteoblasts in late released cells due to their high cAMP response to PTH. Intermediate cells were presumed to be a mixture of those two phenotypes. We have continued our characterization of these populations by studying their mitogenic responses to IGF-1 and EGF since these polypeptide growth factors have been reported to stimulate proliferation of immature bone cells in organ cultures of bone. [3H]Thymidine incorporation into acid insoluble radioactivity was stimulated by IGF-1 in all populations but the greatest increase was in intermediate populations. These latter cells were also the most highly responsive to EGF, which had a small growth stimulating effect on early released cells and no significant effect on late released cells. In bone cells both low and high affinity EGF binding sites were identified (Kd approximately 4 nM and 0.5 nM) and the total EGF binding sites/cell were approximately two-fold higher in intermediate than in early and late released populations. These data suggest that among sequentially released populations of bone cells the intermediate populations may be enriched for immature bone cells from the proliferative zone of bone.

Growth of osteoblasts on porous calcium phosphate ceramic: an in vitro model for biocompatibility study

Biomaterial implantation in animals is commonly used for biocompatibility studies as well as examination of long-term interaction between tissue and the test material. An in vitro cell culture model is proposed as an alternative which will save animal lives and reduce the pain and discomfort of animals used for such studies. In this study the biomaterial was matched to the cell types typical of the implant site of the particular material: porous calcium phosphate ceramic, used as dental and orthopaedic implants, with periosteal fibroblasts, osteoblasts and chondrocytes. All three cell types attached on to the ceramic and formed multicellular layers. Numbers of periosteal fibroblasts, osteoblasts and chondrocytes increased 29-, 23- and 17-fold, respectively, during the 10 wk period. Osteoblasts retained their phenotypic expression by producing only Type I collagen. Parathyroid hormone (PTH, 50 nM) suppressed the alkaline phosphatase activity of osteoblasts by over 50% and increased cAMP by more than 10-fold over control cultures.

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.

Production of and response to growth-stimulating activity in isolated bone cells

Autologous growth-promoting activity has been shown to be secreted by bone organ cultures. To identify the cellular source of these growth factors, we have studied the activity present in cell extracts prepared from isolated bone cells released early or late from mouse calvariae following collagenase digestion. Previous studies have established that early released cell populations reside on the bone surface and consist of a heterogeneous mixture of cells that are less osteoblastic than late released cells. We find that soluble extracts of the latter cells contain more growth-promoting activity/mg of cellular protein than those of the former. By this criterion late released cells appear to be the major source of endogenous growth factors. On the other hand, upon exposure to bone cell-derived cell extracts, early released cells express a greater-fold increase in [3H]thymidine incorporation into acid insoluble radioactivity than late released osteoblasts because of the high basal mitogenic activity of the latter which may be related to their production of autologous growth factor. These data suggest that both early and later released cells may be major targets for bone growth factors produced in situ by late released osteoblasts.

Isolation and characterization of highly serum-dependent cells released early from collagenase digested calvaria

A subclass of highly serum-dependent bone cells has been identified among the cells released early from calvaria following digestion in collagenase. Partial purification for these cells has been carried out based on the observation that they require serum for attachment to polystyrene culture flasks. This subclass of bone cells differs from adherent cells and late released osteoblasts, in that they express almost no cAMP response to PTH, require high levels of serum (10%) for initial growth and proliferation, and do not increase DNA synthesis in response to PTH. In common with adherent cells and late released osteoblasts, their proliferation is decreased by 1,25(OH)2D3 at doses above 0.2 ng/ml and they respond to PGE2 with increased DNA synthesis. These similarities suggest an ontogenic relationship with osteoblasts. Based on their differences, however, provisional identification of these cells as relatively undifferentiated mesenchymal cells is suggested.

Identification of osteoclasts by rhodamine-conjugated peanut agglutinin

Rhodamine-conjugated Arachis hypogaea (peanut agglutinin, PNA) lectin, which is specific for beta-D-galactosyl and beta-D-gal-[1,3]-D-galNAC residues, was used to identify osteoclasts in paraffin-embedded bone sections of fetal rat calvaria and a human bone-derived tumor, osteoclastoma. All multinucleated osteoclasts were positive for PNA-lectin. This was also confirmed by studying smears of isolated osteoclasts from rat and chicken long bones. In addition to multinuclear cells, some mononuclear cells on the endosteal surface of the rat calvaria and some bone marrow cells also revealed specific binding of PNA lectin. Isolated rat peripheral monocytes were also studied, and these showed specific binding of PNA lectin which was maintained unchanged for at least 3 days in culture. Different lectins could be useful as membrane markers for studying bone cell origin and maturation.

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

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

Osteoblast and osteoclast precursors in primary cultures of calvarial bone cells

Bone cells obtained by digestion of fetal mouse or chicken calvaria were tested for their ability to form or resorb bone in vitro. The isolated cells were precultured for 6 days and subsequently cocultured for 11 days with periosteum-free noninvaded fetal mouse long bone rudiments. Bone formation and resorption during coculture were evaluated by histology and 45Ca release from prelabeled bones. The calvarial origin of cells in cocultures was traced by labeling the cells with 3H-thymidine before coculture, followed by autoradiography. Many osteoblasts and osteoclasts as well as fibroblasts developed from mouse periosteal cells released late in the sequential digestion procedure and previously denoted as "osteoblastlike" (BL). No or few osteoblasts and osteoclasts but many fibroblasts developed from early released cell fractions that have previously been denoted as "osteoclastlike" (CL). Only osteoblasts and fibroblasts but not osteoclasts developed from chicken calvarial cell fractions. The osteoblasts developed primarily from cell fractions from the inner layer of the periosteum, previously denoted as "osteoblastlike" (OB). Cells obtained from the outer layer of the periosteum (PF) gave rise mainly to fibroblasts. These studies show that osteoblast and osteoclast precursor cells are maintained in monolayer cultures of periosteal cell fractions. However, sequential digestion of mouse calvaria does not lead to separation of the two types of bone cells. Rather, osteoclast and osteoblast precursors are released jointly, from the periosteal cell layers closest to the bone surface. In the chicken cell fractions osteoclast precursors are absent after preculture, resulting in a more homogeneous population of osteoblast and fibroblast but not osteoclast precursors.