The frequency of common progenitors for adipocytes and osteoblasts and of committed and restricted adipocyte and osteoblast progenitors in fetal rat calvaria cell populations

In fetal rat calvaria (RC) cell populations, adipocyte differentiation is stimulated by both dexamethasone (Dex) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], whereas osteoblast differentiation is stimulated by Dex but inhibited by 1,25(OH)2D3. We examined whether the osteoblastic and adipocytic colonies were derived from a common progenitor, from committed and restricted adipocyte and osteoblast progenitors, or from both and whether the adipocyte progenitors stimulated by 1,25(OH)2D3 constitute a population of progenitors that is different from that stimulated by Dex. RC cells were isolated by sequential enzyme digestion yielding five populations designated I-V. In population I the effect of Dex on adipocyte formation was greater than that of 1,25(OH)2D3, whereas the effect of 1,25(OH)2D3 was greater than that of Dex in populations III-V. We next applied replica plating techniques to further investigate the response characteristics of individual osteoprogenitors and adipocyte progenitors by looking at the fate of duplicate colonies derived from the same progenitor under different culture conditions. RC cells were plated at 1,000-1,500 cells/100 mm culture dish and a 17-microm mesh polyester membrane overlaid onto master dishes on day 4 or day 5 and removed on day 11 or day 12. Then, replicas and master dishes were cultured separately in medium containing either Dex, 1,25(OH)2D3, or Dex plus 1,25(OH)2D3 for a further 17-21 days and then fixed and stained with both Sudan IV and the von Kossa technique. Nine hundred twenty-seven matched colonies present on both master dishes and replica membranes were screened and colonies were classified as either adipocytic, osteoblastic (bone or osteoid), or fibroblastic. Results show convincingly that most of the osteoprogenitors present in fetal RC cells are committed and restricted to the osteoblastic cell lineage (95.29%); that the 1,25(OH)2D3-responsive adipocyte progenitors are different from the Dex-responsive adipocyte progenitors, but both are restricted to form adipocytes and finally; and that a common osteoblastladipocyte progenitor is present in a low frequency (4.71% of osteoprogenitors).

Resorptive state and cell size influence intracellular pH regulation in rabbit osteoclasts cultured on collagen-hydroxyapatite films

Diseases exhibiting excessive bone loss are often characterized by an increase in the size and number of osteoclasts in affected areas, suggesting that osteoclast size is associated with increased resorptive activity or efficiency. Because osteoclastic bone resorption depends on proton extrusion via a bafilomycin A1-sensitive vacuolar type H+ ATPase (V-ATPase), we investigated the relationship between osteoclast size and state of activity on the one hand, and proton-extruding mechanisms (bafilomycin A1-sensitive V-ATPase and amiloride-sensitive Na+/H+ exchange) on the other. In determining resorptive activities of individual osteoclasts, osteoclast-containing cell suspensions obtained from newborn rabbit long bones were cultured on apatite-collagen complex (ACC)-coated coverslips. Large osteoclasts resorbed 2.5 times more per cell than small osteoclasts, but the amount resorbed per nucleus was the same for the two categories. However, a much larger percentage of large osteoclasts was resorbing compared with small osteoclasts. To study pH regulatory mechanisms in individual large and small osteoclasts, the cells were loaded with the pH-sensitive indicator BCECF and analyzed by single-cell fluorescence. Small and large resorbing osteoclasts had significantly higher basal pH(i) than their nonresorbing counterparts. Also, small nonresorbing osteoclasts were insensitive to bafilomycin A1 addition or Na+ removal from the medium, large nonresorbing osteoclasts responded slightly, and all resorbing osteoclasts (small and large) responded strongly. Differences were also seen in the recovery from an acid load: both small and large nonresorbing osteoclasts were more sensitive to amiloride inhibition, while large resorbing cells were more sensitive to bafilomycin A1 inhibition. Small resorbing cells were inhibited equally by bafilomycin A1 and amiloride. These results clearly show that a greater proportion of large osteoclasts are active in resorption and that pH(i) regulation is associated with enhanced proton pump activity in actively resorbing osteoclasts. Thus, large and small osteoclasts differ in the proportion of cells that are resorbing, while pH regulatory mechanisms differ mainly between resorbing and nonresorbing cells.

Insulin-like growth factor-1 and -2 stimulate osteoprogenitor proliferation and differentiation and adipocyte formation in cell populations derived from adult rat bone

In the present study, we test the hypothesis that the stimulation of osteoprogenitor differentiation by the glucocorticoid dexamethasone (Dex) is mediated, at least in part, through components of the insulin-like growth factor (IGF) system. Because it has been suggested that osteoprogenitors and adipocyte progenitors originate from the same precursor cells, and that their differentiation in many systems is reciprocally regulated, the effects of Dex and IGF on adipocyte formation were also evaluated in the same cultures. In view of the presence of IGFs and their binding proteins in serum, we also evaluated to what degree the effects of IGF-1 and IGF-2 on differentiation of osteoblasts and adipocytes were affected by the serum concentration of the culture media. Bone cell populations were isolated from vertebrae of 3-month-old female Wistar rats using an explant culture technique. Osteoprogenitor differentiation was evaluated by a colony assay: Bone-forming osteoblastic colonies (bone nodules) derived from single osteoprogenitors were identified by alkaline phosphatase (ALP) staining and/or staining for mineralized matrix according to the von Kossa technique. Unmineralized nodules and osteoblastic colonies were subsequently identified by their distinctive color and morphology after methylene blue counterstaining. Differentiated adipocytes were identified by Sudan IV staining. IGF-1 and IGF-2 stimulated both osteoprogenitor and adipocyte progenitor differentiation in a dose-dependent pattern. The stimulation of osteoprogenitor differentiation by IGF was not dependent on Dex, but differentiation of adipocytes was. The stimulatory effects of IGF-1 and IGF-2 on osteoprogenitor differentiation were greater in media containing 2.5% fetal bovine serum (FBS) than in media containing 5% or 10% FBS, whereas stimulation of adipocyte formation was greater in media containing 10% FBS. Neutralizing antibody against the type 1 IGF receptor (IGF-1R) partially blocked IGF- and Dex-induced osteoprogenitor differentiation, but did not affect IGF-induced adipocyte formation. This suggests that IGF-stimulated osteoprogenitor differentiation is mediated through IGF-1R, that the stimulation of adipocyte formation by IGF is not, that the stimulatory effects of Dex on osteoprogenitor differentiation are partially mediated through IGF-1R, and that the effects on adipocyte differentiation appear to be mediated through signaling pathways other than the IGF-1R.

Differences in regulation of pH(i) in large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts

Osteoclasts are multinucleated cells that resorb bone by extrusion of protons and proteolytic enzymes. They display marked heterogeneity in cell size, shape, and resorptive activity. Because high resorptive activity in vivo is associated with an increase in the average size of osteoclasts in areas of greater resorption and because of the importance of proton extrusion in resorption, we investigated whether the activity of the bafilomycin A(1)-sensitive vacuolar-type H(+)-ATPase (V-ATPase) and amiloride-sensitive Na(+)/H(+) exchanger differed between large and small osteoclasts. Osteoclasts were obtained from newborn rabbit bones, cultured on glass coverslips, and loaded with the pH-sensitive indicator 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Intracellular pH (pH(i)) was recorded in single osteoclasts by monitoring fluorescence. Large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts differed in that large osteoclasts had a higher basal pH(i), their pH(i) was decreased by bafilomycin A(1) addition or removal of extracellular Na(+), and the realkalinization upon readdition of Na(+) was bafilomycin A(1) sensitive. After acid loading, a subpopulation of large osteoclasts (40%) recovered by V-ATPase activity alone, whereas all small osteoclasts recovered by Na(+)/H(+) exchanger activity. Interestingly, in 60% of the large osteoclasts, pH(i) recovery was mediated by both the Na(+)/H(+) exchanger and V-ATPase activity. Our results show a striking difference between pH(i) regulatory mechanisms of large and small osteoclasts that we hypothesize may be associated with differences in the potential resorptive activity of these cells.

Expression of mRNAs for type-I collagen, bone sialoprotein, osteocalcin, and osteopontin at different stages of osteoblastic differentiation and their regulation by 1,25 dihydroxyvitamin D3

We have used in situ hybridization to evaluate the effects of 1,25 dihydroxyvitamin D3 (1,25 (OH)2 D3) on the expression of mRNA for bone-matrix proteins and to determine whether mature osteoblasts respond differently to 1,25 (OH)2 D3 than younger, newly differentiated osteoblasts. Rat calvaria cells were cultured for 7, 12, 15, and 19 days to obtain a range of nodules from very young to very mature. At each time point, some cultures were treated with 10 nM 1,25 (OH)2 D3 for 24 h prior to fixation. In control cultures, type-I collagen mRNA was detectable in osteoblastic cells in very young nodules and increased with increasing maturity of the nodules and the osteoblasts lining them. The bone sialoprotein mRNA signal was weak in young osteoblasts, increased in older osteoblasts, and decreased in mature osteoblasts. Weak osteocalcin and osteopontin signals were seen only in osteoblasts of intermediate and mature nodules. 1,25 (OH)2 D3 treatment markedly upregulated osteocalcin and osteopontin mRNAs and downregulated mRNA levels of bone sialoprotein and, to a lesser extent, type-I collagen in both young and mature osteoblasts. However, a marked diversity of signal levels for bone sialoprotein, osteocalcin, and osteopontin existed between neighboring mature osteoblasts, particularly after 1,25 (OH)2 D3 treatment, which may therefore selectively affect mature osteoblasts, depending on their differentiation status or functional stage of activity.

Aluminum accelerates osteoblastic differentiation but is cytotoxic in long-term rat calvaria cell cultures

We have examined the effects of aluminum (Al) on osteoprogenitor proliferation and differentiation, cell survival, and bone formation in long-term rat calvaria (RC) cell cultures. RC cells were grown in alpha minimal essential medium containing 10% fetal bovine serum, 50 microg/ml ascorbic acid, and 10 mM beta-glycerophosphate with or without Al added to final concentrations of 1 microM-1 mM. Al caused a dose-dependent increase in the number of bone nodules present at early times (day 11) but had no significant effect on nodule numbers at later times (day 17). Time course experiments showed that Al increased nodule number beginning from day 7. Alkaline phosphatase activity, assessed at four stages during the differentiation sequence of RC cell cultures (from 4 to 13 days) was stimulated by Al at all times. However, Al decreased colony formation, inhibited cell growth in late log phase, and decreased saturation density of the treated cultures. Al concentrations of 30 microM and above resulted in degeneration of the cell layer and an increasing fibrillar appearance of the matrix present in between or adjacent to nodules when cultures were maintained for more than 15 days. The presence of Al significantly decreased the viability of cells obtained from 13-17 days cultures, as determined by plating efficiency and trypan blue exclusion. We frequently observed cellular toxicity (in 8 of 10 experiments) in cultures containing 300 microM Al, and by days 17-19, cells, nodules, and matrix were disintegrating in these cultures. We conclude that Al accelerates the rate of osteoprogenitor cell differentiation and the formation of bone nodules while concomitantly inhibiting nodule mineralization. However, concentrations that accelerate differentiation appear to be cytotoxic in long-term cultures.

Progesterone- and dexamethasone-dependent osteoprogenitors in bone cell populations derived from rat vertebrae are different and distinct

Previous experiments have demonstrated that bone cell populations derived from explants of lumbar vertebral bone of adult female rats contain osteoprogenitors that require dexamethasone (Dex) or progesterone (Prog) to proliferate and differentiate into fully differentiated bone-forming osteoblasts. We now show that the Prog-dependent population cannot be detected in male rats after sexual maturation, but is present in prepubertal rats of both sexes and can be induced in adult male-derived populations by culturing the explants in medium containing 17beta-estradiol (10(-9)-10(-8) M). This suggested that the Prog- and Dex-dependent osteoprogenitors in adult female-derived populations were probably distinct populations and that the survival of the Prog-dependent osteoprogenitors and/or their ability to proliferate are dependent on the presence of estrogen. We then proceeded to prove this by using replica plating. When one of the paired colonies duplicated was cultured in medium containing Dex (10(-8) M) and the other in medium containing Prog (10(-5) M), 5.0% of duplicates formed bone in Prog only, 11.1% formed bone in Dex only, and 3.4% formed bone in both Prog and Dex. In all cases the size of the bone-forming colonies in Dex-treated cultures was larger than that in Prog-treated cultures, indicating that the effects of Dex on osteoprogenitor proliferation are greater than those of Prog. The results demonstrate the existence of three classes ofosteoprogenitors in adult female rat-derived bone cell populations: a class responding to Dex only, a class responding to Prog only, and a class responding to both Dex and Prog. The results also indicate that the effects of Prog are not mediated by Prog binding to the glucocorticoid receptor and imply that Prog plays an important role in maintaining bone mass through regulating the class of osteoprogenitors responsive to Prog.

Macrophage colony stimulating factor increases bone resorption in dispersed osteoclast cultures by increasing osteoclast size

Several reports indicate that macrophage colony stimulating factor (MCSF) is one of the major factors required for osteoclast proliferation and differentiation. Paradoxically, it has also been reported that MCSF inhibits osteoclastic activity. We therefore decided to investigate in detail the effects of MCSF on resorption and osteoclast formation to try and clarify this issue. Osteoclast-containing cultures were obtained from rabbit long bones and cultured on plastic culture dishes or devitalized bovine bone slices. MCSF (4-400 ng/ml) stimulated osteoclastic bone resorption in a time-dependent manner and at all doses examined. After 48 h of culture in the presence of MCSF, we observed a 2-fold increase in the total area of bone resorbed, as well as a significant increase in the area of bone resorbed per osteoclast and the number of resorption pits per osteoclast. This effect was paralleled by an increase in the number of larger osteoclasts (as determined by the number of nuclei per cell) and an increase in the size and depth of the resorption pits. Since the total number of osteoclasts remained the same, the MCSF-induced increase in resorptive activity appeared to be related to an increase in the average size of the osteoclasts. When resorption was expressed as the amount of bone resorbed per osteoclast nucleus, larger osteoclasts resorbed more per nucleus, suggesting that large osteoclasts, as a population, are more effective resorbers than small osteoclasts. Interestingly, when osteoclasts were plated at one-fifth the standard density, the amount of bone resorbed per osteoclast decreased considerably, indicating that resorptive activity is also affected by cell density of osteoclasts and/or of other cells present. However, at this lower density MCSF still increased osteoclast size and resorption by the same fold increase over control, suggesting that the effect of MCSF was independent of factors related to cell density.

Osteoprogenitor cells in cell populations derived from mouse and rat calvaria differ in their response to corticosterone, cortisol, and cortisone

Osteoprogenitors present in cell populations derived from fetal or newborn rat and mouse calvaria differentiate in long term culture and form osteoblastic bone-forming colonies (bone nodules). Previous reports have indicated considerable differences between bone cell populations derived from these two species with regard to their proliferation in response to glucocorticoids. In the present investigation, we have focused on proliferation and differentiation of osteoprogenitor cells in these bone cell populations and evaluated the effect of corticosterone, the principal glucocorticoid of both mouse and rat. Cells were isolated by sequential collagenase digestion from calvaria of newborn (2-5 days) CD-1 mice [mouse calvariae (MC) cells] and term fetal Wistar rats [rat calvaria (RC) cells] and cultured for up to 25 days in alpha-minimal essential medium containing 10% fetal bovine serum (FBS), antibiotics, 50 microg/mL ascorbic acid, and 8-10 mmol/L beta-glycerophosphate. In agreement with previous observations by us and others, corticosterone increased cell growth in RC cell cultures, but inhibited cell growth in MC cultures. In RC cell cultures, corticosterone (1-1000 nmol/L) increased the nodule number in a dose-dependent manner (p < 0.001 for all concentrations above 3 nmol/L) with a maximal effect at 300 and 1000 nmol/L (threefold increase over control). In MC cells, on the other hand, corticosterone (0.3-1000 nmol/L) increased the nodule number only at 30 nmol/L (50%, p < 0.01) but inhibited nodule formation by 33% (p < 0.001) at 1000 nmol/L. In both RC and MC cultures a linear relationship was found between the number of cells plated and number of nodules formed. When cultures were treated with cortisol (30-300 nmol/L), similar effects were observed; the number of nodules dose dependently increased in RC cell cultures and dose dependently decreased in MC cell cultures. Significantly, however, the inactive glucocorticoid cortisone also increased bone nodule formation in RC cell cultures and decreased bone nodule formation in MC cell cultures. Carbenoxolone, which blocks 11 beta hydroxysteroid dehydrogenase and thus prevents conversion of cortisone to cortisol, partially inhibited the cortisone-induced effects on bone nodule formation in both RC and MC cell cultures, indicating that both RC and MC cells can convert inactive glucocorticoids to active metabolites. In conclusion, our results show that the glucocorticoids corticosterone and cortisol inhibit proliferation and differentiation of osteoprogenitors in MC cell cultures but stimulate these processes in rat-derived osteoprogenitors.

Retinoic acid responsiveness of cells and tissues in developing fetal limbs evaluated in a RAREhsplacZ transgenic mouse model

Limb morphogenesis is a complex phenomenon in which retinoids play an important role. Abnormal maternal retinoid levels from high oral doses cause fetal malformations, including abnormalities of the musculoskeletal system. Our purpose was to identify the retinoid-responsive cells in bone and cartilage during limb development by using a transgenic line of mice containing a reporter gene insert consisting of a retinoic acid response element linked to an Escherichia coli beta-galactosidase gene. Transgenic fetuses from day 11.5 after conception to birth (day 20) were analyzed histologically. Retinoid-responsive cells and tissues were first seen in the limb bud at 12.5 days in the webs between the forming digits. The webs stained maximally at 14.5 days, after which staining intensity subsided. Staining in the muscles was detectable at 13.5 days, at a stage coinciding with myoblast fusion. Specific regions of perichondrium and periosteum also stained at this stage. Occasional staining was observed in individual chondroblasts in all chondrogenic regions, including hypertrophic chondroblasts and certain articular surfaces of developing joints. Staining of these tissues decreased in intensity in subsequent stages. Osteoclasts started to express beta-galactosidase at 15.5 days and continued to stain into maturity. Our results indicate that specific subsets of cells respond to retinoids at specific stages in the course of normal limb development. In hypertrophic chondrocytes and cells in the webs and joints that display such a response, retinoid-induced effects may be linked to cell death that occurs in these regions. Staining in muscle, perichondrium, and periosteum may reflect retinoid-induced effects associated with cell differentiation and growth. These results suggest that retinoids play a role in a variety of tissues, including bone and cartilage, at specific stages during morphogenesis.

The decrease in bone mass associated with aging and menopause

The human skeleton accumulates bone up to approximately age 30, after which bone is gradually lost. Although estrogen replacement therapy prevents postmenopausal bone loss, it is not certain that estrogen deficiency alone is responsible for the decrease in bone mass. Progesterone deficiency could also be a factor, and progesterone replacement therapy has been shown to prevent postmenopausal bone loss associated with ovarian dysfunction. This article reviews what is known about bone remodeling and bone loss as a function of age and gender, discusses evidence from studies in rats that progesterone plays an important role in regulating bone formation, and suggests directions for future studies in predicting the success or failure of implant therapy based on the number and kinds of osteoprogenitor cells present.

Experimentally induced periodontitis in beagle dogs causes rapid increases in osteoclastic resorption of alveolar bone

This study was undertaken to observe osteoclast differentiation related to inflammatory progression in aggressive periodontitis induced in beagle dogs by ligature of the gingival sulcus. To monitor osteoclastic activity, we used histochemical methods (staining for tartrate-resistant acid phosphatase [TRAP]) to visualize osteoclasts and their TRAP-positive precursors and biochemical methods (ELISA assay of pyridinium crosslinks) to detect bone matrix degradation products in gingival crevicular fluid (GCF), serum, and urine. For histochemical study, tissue specimens were prepared from 3 adult female beagle dogs induced with experimental periodontitis by silk ligature placement below the gingival margin of mandibular molars ligated for 3, 7, and 21 days. For biochemical study for pyridinoline measurement, the 24 mandibular molars of 4 male beagle dogs were ligated. GCF, urine, and serum were collected at day 0 and at 3, 7, 14, and 21 days after ligation. In the early inflammatory phase of ligature-induced periodontitis (day 3), TRAP+ mononuclear and TRAP+ multinucleated cells were present in the gingival connective tissue, and active bone-resorbing cells were found in excavated lacunae at the alveolar crest, but osteoclasts were not infiltrating the periodontal ligament during this early phase. During later stages of the inflammatory process (7 and 21 days), osteoclasts appeared at both the gingival and ligament side of the alveolar bone. Osteoclastic bone resorption appeared to be more severe on the bone surface at the gingival side than on the bone surface of the periodontal ligament side. Measurement of pyridinoline significantly increased in GCF and urine 3 days after ligation. The results suggested that bone at the crest of the alveolar bone is rapidly resorbed within 3 days of inducing experimental periodontitis.

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.

Progesterone stimulates proliferation and differentiation of osteoprogenitor cells in bone cell populations derived from adult female but not from adult male rats

We examined the effects of the synthetic glucocorticoid dexamethasone (Dex) and of the sex steroids progesterone (Prog) and testosterone (Testo) on proliferation and differentiation of progenitors for osteoblasts, adipocytes, and macrophages in cell populations derived from lumbar vertebrae of adult male and female rats. To assay for these progenitors, we used a previously described colony assay, where progenitors are identified by the appearance of colonies of the differentiated phenotype in long term cultures of cell populations containing these progenitors. In cell populations derived from both males and females, Dex (10(-9)-10(-6) mol/L) induced a similar dose-dependent increase in the number of osteoblast colonies (bone nodules), colonies of alkaline phosphatase (AP)-positive cells, adipocyte colonies and macrophage colonies (ED2-positive cells). Prog (10(-8)-10(-5) mol/L), on the other hand, increased bone nodule formation in female-derived populations but not in male-derived populations. Maximal stimulation was seen at 10(-5) mol/L. 17 beta-Estradiol (E2) enhanced the Prog-induced increase in the number of bone nodules in a dose-related fashion. Maximal stimulation was seen at 10(-8) mol/L E2. E2 (10(-9)-10(-6) mol/L) had no effect on Dex-induced bone nodule formation, indicating that the effect is specific for Prog-induced stimulation of bone nodule formation. Prog also caused a dose-dependent increase in the number of colonies of AP-positive cells. Interestingly, the effect of Prog on the number of AP-positive colonies was the same in populations derived from both sexes. Prog-induced stimulation of adipocyte and macrophage development in female-derived populations was significantly greater than that in male-derived populations. Testo (10(-9)-10(-5) mol/L) had no effect on any of the parameters evaluated in populations derived from either males or females. These observations demonstrate that the effect of Prog on proliferation and differentiation of progenitors for osteoblasts, adipocytes, and macrophages in cell populations derived from lumbar vertebrae of adult male or female rats is sex-dependent and is seen either only (bone nodule formation) or more pronounced (adipocyte and macrophage development) in female-derived populations.

Progesterone-mediated stimulation of osteoprogenitor proliferation and differentiation in cell populations derived from adult or fetal rat bone tissue depends on the serum component of the culture media

We have shown previously that progesterone (Prog) and dexamethasone (Dex) stimulate osteoprogenitor proliferation and differentiation in cell populations derived from adult rat vertebrae and in primary cultures of fetal rat calvariae. In these two in vitro systems, osteoprogenitors can be identified by the appearance of colonies of differentiated osteoblasts producing bone (bone nodule formation). Culture conditions supporting proliferation and differentiation of osteoprogenitors include a requirement for the presence of serum in the culture media. Our major interest in the present study was to investigate whether Prog- and Dex-mediated osteoprogenitor proliferation and differentiation was observed to the same degree in different lots of fetal bovine serum (FBS). In addition, we wanted to investigate whether osteoprogenitors present in cell populations derived from fetal calvarial bone and those present in populations derived from adult vertebral bone would respond similarly under the different culture conditions. We found that, in populations derived from adult rat vertebrae, the effects of the serum component of the culture medium on the number of bone nodules induced by Prog and on the dose-dependency of the Prog effect were striking: in culture media containing the most effective serum the number of bone nodules was 22-fold higher than that in the least effective serum. In addition, Prog responses were detectable at 10(-5) M only in some sera but were significant at 10(-7) M in others. The effect of Dex in the adult rat vertebrae-derived populations was much less dependent on the serum used: the number of bone nodules in culture media containing the most effective serum was only 1.3 times greater than that in media containing the least effective serum. In cell populations derived from fetal calvariae, the serum dependence of the Prog response was less pronounced: a 4.3-fold increase over control was observed in the most effective serum, and a 2.4-fold increase in the least effective serum. No effects of the serum component of the culture medium on the Dex response were detectable. Thus, Prog-induced bone nodule formation appears to be strongly dependent on the particular type of FBS used for osteoprogenitors present in bone cell populations derived from adult rat vertebrae but much less so in populations obtained from fetal rat calvariae. Preliminary experiments suggest that the estrogen content of the culture media may be one of the determinants regulating Prog responsiveness of the osteoprogenitors. Dex-induced proliferation and differentiation of osteoprogenitors in bone cell populations derived from both adult rat vertebrae and fetal rat calvariae, on the other hand, did not appear to be strongly dependent on factor(s) present in the FBS component of the culture medium.

Analysis of chondroprogenitor frequency and cartilage differentiation in a novel family of clonal chondrogenic rat cell lines

We have isolated through sequential steps of subcloning a series of normal clonal cell lines enriched for chondroprogenitors that undergo differentiation in vitro from progenitors to mature chondroblasts and chondrocytes forming three-dimensional cartilage nodules. In the parental chondroblast clone RCJ 3.1C5 (C5), differentiation and cartilage formation occurred without added hormones or growth factors, but chondrogenesis could be stimulated markedly in the presence of the glucocorticoid steroid Dexamethasone (Dex). Limiting dilution analysis indicated that greater than one in ten C5 cells plated was a chondroprogenitor capable of differentiating and forming a cartilage nodule in low density cultures, but chondrogenesis was down-regulated in higher density cultures. Dex elicited a greater stimulatory effect on cartilage nodule formation when C5 cells were plated at higher rather than lower densities. Since Dex also maintained the chondrogenic potential of C5 cells passaged repeatedly, we subcloned C5 in the presence of Dex. Eight of eleven subclones were chondrogenic and the frequency of chondroprogenitors capable of cartilage formation in isolated subclones ranged from lower to much higher than in the parental C5 clone. Both Dex-independent as well as Dex-dependent clones were identified, although long-term maintenance of the chondrocyte phenotype in all subclones required Dex. These data suggest that there are Dex-dependent and Dex-independent chondroprogenitor cells, that cell-cell interactions and/or local factors can modulate cartilage nodule formation and that Dex-responsive steps are involved in long-term maintenance of chondroprogenitors in vitro. Thus, this unique family of non-transformed, clonal chondrogenic cell lines provides a quantifiable, readily manipulatable system in which cartilage differentiation and metabolism can be assessed.

Progesterone and dexamethasone stimulate proliferation and differentiation of osteoprogenitors and progenitors for adipocytes and macrophages in cell populations derived from adult rat vertebrae

We investigated the effects of the sex hormone progesterone (Prog) and the synthetic glucocorticoid dexamethasone (Dex) on proliferation and differentiation of progenitor cells of osteogenic, adipocytic, and hemopoietic lineages in cell populations derived from explants of adult female rat lumbar vertebrae. The cell populations were obtained by culturing bone explants in plasma clots immersed in alpha-minimum essential medium plus 10% fetal calf serum (standard medium) and then subculturing the outgrowth cells in standard medium plus 50 micrograms/ml of ascorbic acid, 5 mM beta-glycerophosphate, and with or without Prog or Dex. On day 6 of culture, these populations were analyzed for cAMP responses to parathyroid hormone (PTH), prostaglandin E2 (PGE2), and isoproterenol (IPT). Increases in intracellular cAMP were seen in response to PTH, PGE2, and IPT, and culturing in medium containing Prog increased these responses. At various time periods between days 4-27 of culture, the cultures were evaluated for the presence of bone nodules, alkaline phosphatase (AP)-positive colonies, adipocytes, monocytes, and macrophages. Prog and Dex increased the number of bone nodules and AP-positive colonies. The effect of Prog on bone nodule formation was smaller than that of Dex. In addition, the effect of Dex on bone nodule formation was evident after 10 days of culture, while the Prog-induced effects became significant at days 16-20 of culture. Both hormones also increased the number of Sudan IV-positive colonies (adipocytes), certain types of alpha-naphthyl butyrate esterase (alpha-NBE)-positive colonies (monocytes, macrophages, and T-lymphocytes), and ED2-positive colonies (macrophages). Prog-treated cultures contained more colonies of small spindle-shaped alpha-NBE-positive cells and fewer colonies of small round alpha-NBE-positive cells when compared with Dex-treated cultures. These data indicate that cell populations derived from adult rat lumbar vertebrae contain, among others, osteoprogenitors and progenitors for adipocytes and macrophages that are stimulated to proliferate and differentiate by Prog and Dex. The data also suggest that the effects of Prog and Dex differ qualitatively and quantitatively.

Aluminum inhibits both initiation and progression of mineralization of osteoid nodules formed in differentiating rat calvaria cell cultures

Osteoid nodules form in cultures of fetal rat calvaria (RC) cells grown in medium containing 10% fetal bovine serum (FBS) and 50 microns/ml of ascorbic acid. When 10 mM beta-glycerophosphate (beta-GP) is added, the nodules mineralize in two phases: an initiation phase that is dependent upon alkaline phosphatase activity for cleavage of beta-GP to inorganic phosphate (P(i)) and a progression phase that proceeds independently of the activity of alkaline phosphatase and does not require exogenous phosphate. We have used this system to investigate the effects of aluminum (Al3+)on mineralization. When AlCl3 was added to culture medium at concentrations of 0, 3, 10, 30, 100, and 300 muM, the total concentrations of aluminum were 0.98, 6.07, 16.82, 40.19, 88.45, and 284.52 muM, respectively. The corresponding free Al3+ concentrations, assessed after ultrafiltration, were found to be 1.11, 1.75, 3.40, 6.22, 5.38, and 12.11 muM. In cultures in which osteoid was formed and mineralization initiated in the presence of added Al+ (3-300 muM), a dose-dependent inhibition of mineralization occurred. Osteoid formed in the presence of added Al3+ mineralized normally when Al3+ was removed from cultures at the time of initiation of mineralization with beta-GP. In osteoid nodules grown in the absence of Al3+, addition of Al3+ (3-300 muM) at the start of the initiation phase of mineralization resulted in a dose-dependent inhibition of mineralization. Addition of Al3+ to cultures after mineralization had been initiated in the absence of Al3+ inhibited progression of mineralization at added Al3+ concentrations of 10 muM and above. Al3+ did not decrease the conversion of beta-GP to P(i) and caused a small but significant increase in alkaline phosphatase activity at added concentrations of 100 muM or greater. The data show that Al3+ inhibits both the initiation and progression phases of mineralization starting at added concentrations of 3-10 muM (approximately 1.7-3.4 muM free Al3+) and that mineralization of osteoid formed in the presence of Al3+ is unaffected if Al3+ is removed prior to the initiation of mineralization.

Tri-iodothyronine (T3) and dexamethasone interact to modulate osteoprogenitor cell differentiation in fetal rat calvaria cell cultures

We investigated the role of 3,5,3'-tri-iodothyronine (T3) in regulating differentiation of osteoprogenitor cells and also studied the effects of the glucocorticoid hormone dexamethasone (Dex) on the T3-induced effects on osteoprogenitor populations. This was done by determining the effects of either hormone alone, or of combinations of the two hormones, on the number of bone nodules formed in long-term cultures of rat calvaria cells. In this system, Dex has been shown to increase bone nodule formation, the maximal effective dose being 10 nM (Bellows et al. Endocrinology 121: 1985-1992; 1987). In standard culture medium containing 15% fetal bovine serum FBS), low concentrations of T3 (0.001-0.1 nM) had no effect on the number of bone nodules, while higher concentrations of 1-100 nM inhibited. However, in culture medium containing 10 nM Dex, the lower concentrations of T3 markedly increased the number of nodules. Short term pulse experiments with these low concentrations of T3 in the presence of Dex indicated that stimulation of nodule formation occurred only when T3 was present prior to confluency. Higher concentrations of T3 (1-100 nM) decreased nodule number whether or not Dex was added. We then cultured cells in medium containing FBS from which T3 and T4 were removed by treatment with AG-1 chi-10 resin. In both + or - Dex conditions, bone nodule formulation was increased 1.5 to 2-fold in T3, T4-depleted medium when compared with cultures maintained in standard culture medium.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Displacement and translocation of osteoblast-like cells by osteoclasts

Rabbit osteoclasts and rabbit osteoblast-like stroma cells (OB cells) were placed onto plastic surfaces and the migration patterns of individual osteoclasts and osteoclast-OB interactions were analyzed with time-lapse recording. To induce directed migration, the cultures were exposed to an electrical field of 0.01 or 0.1 V/mm. At 0.1 V/mm, osteoclasts moved directly toward the anode in some cases, clearing OB cells from their path of migration. In other cases, osteoclasts migrated toward the anode for part of the time but then changed direction and moved toward groups of OB cells. Observations were made on osteoclasts interacting with single OB cells or small colonies and on osteoclasts interacting with OB monolayers, at both field strengths; the results were independent of field strength. There were several characteristic behaviors. With single OB cells and small OB colonies, retraction of OB cells upon contact with the osteoclast was the predominant mechanism whereby these cells begin to move out of the path of the osteoclast. A pronounced ruffling or blebbing of the OB cell membrane often followed retraction. When osteoclasts displaced OB cells that were part of a monolayer, extension of an osteoclast lamellipodium underneath the edge of the OB cell layer generally preceded partial retraction of the OB cells involved. It sometimes appeared as if the detached or partially detached OB cells were "pushed" by the osteoclast, which in some cases resulted in OB cells being moved hundreds of microns in a period of a few hours, at rates comparable to the normal speed for osteoclast migration (congruent to 100 microns/h), much faster than the normal speed for OB cells (congruent to 10 microns/h).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of natural and synthetic retinoids on the differentiation of RCJ C5.18 chondrogenic cells

RCJ C 5.18 (C 5.18) is a chondrogenic clonal cell line which, under standard culture conditions, develops chondroblastic features including the production of a cartilagenous matrix. Retinoic acid (RA) is known to inhibit the chondrogenic differentiation of C 5.18 cells and this may parallel the teratogenic effects of retinoids in vivo; however, the question as to which of the 3 retinoic acid receptors (RAR alpha, beta, gamma) or the 3 retinoid X receptors (RXR alpha, beta, gamma) mediate this RA-induced inhibition remains unanswered. We tested several retinoids with different receptor binding characteristics. Cartilage formation in C 5.18 cultures was evaluated by counting the number of cartilage nodules formed, and by quantitating the glycosaminoglycan content of the cultures using alcian blue staining. All of the retinoids prevented cartilage formation in a dose-dependent manner. Treatment with the retinoids did not affect cell number, thereby ruling out any toxic effects. RA, which binds to all 3 RARs with similar affinity, produced a 50% inhibition (IC50) of cartilage formation at 4 x 10(-10) M. We also tested Ch55, which also binds to all 3 RARs, but with higher affinity than RA. This compound was approximately 10 times more potent than RA (IC50 2 x 10(-11) M). 9-cis RA, which binds to the 3 RARs with affinities similar to RA and also binds to the 3 RXRs, was less active (IC50 8 x 10(-9) M), suggesting that RXR binding interferes with the inhibitory effect of ligand-activated RARs. 9-cis retinal, for which the binding characteristics are unknown, had the same effect as 9-cis RA.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbonic anhydrase II mRNA expression in individual osteoclasts under "resorbing" and "nonresorbing" conditions

Rabbit osteoclasts can be transformed from a nonresorbing state to a resorbing state by transferring them from culture medium at pH 7.5 to one at pH 6.5. We evaluated whether expression of mRNA for carbonic anhydrase (CA-II) could be used as an indicator of the state of activity of individual osteoclasts. A cDNA probe to rabbit carbonic anhydrase II (CA-II) was prepared and used for in situ hybridization analysis of osteoclasts isolated from neonatal rabbit long bones. Quantitation by grain counting revealed heterogeneity within the osteoclast population: osteoclasts with a "compact" (rounded, less spread) morphology expressed higher levels of CA-II mRNA than "spread" osteoclasts with similar numbers of nuclei. When maintained at pH 6.5 for 6 h, the level of CA-II mRNA was increased significantly in osteoclasts of both morphologies compared with those in parallel cultures maintained at pH 7.5. These results were confirmed by quantitating CA-II mRNA using the polymerase chain reaction (PCR). Oligonucleotide primers specific for rabbit CA-II were synthesized and used to amplify CA-II cDNA transcribed from mRNA prepared from single or small numbers (one to eight cells) of osteoclasts that were collected with a micromanipulator. This generated a approximately 510 bp PCR product, corresponding to the predicted size of the CA-II fragment encompassed by the primers. For quantitation, CA-II mRNA levels were compared with the levels of a approximately 900 bp actin fragment that was coamplified in the same reaction mixture or amplified separately in a duplicate sample of the reaction mixture.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25-dihydroxyvitamin D3 stimulates adipocyte differentiation in cultures of fetal rat calvaria cells: comparison with the effects of dexamethasone

Progenitor cells for several mesenchymally derived cell types exist within freshly isolated fetal rat calvaria (RC) cell populations. We have characterized the effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on the differentiation of adipocytes from primary RC cells and compared these effects with those of dexamethasone (Dex). RC cells were plated at 3 x 10(4)/35-mm dish, and cultures were maintained for 14-19 days in alpha-Minimum Essential Medium containing 10% fetal bovine serum, 50 micrograms/ml ascorbic acid, 10 mM Na beta-glycerophosphate, and 0.1-100 nM 1,25-(OH)2D3 or 1-1000 nM Dex. Morphological (quantitation of adipocyte foci number and area after staining cultures with Sudan IV) and biochemical (glycerol-3-phosphate dehydrogenase activity) methods of assessing adipogenesis were used. In the presence of 1,25-(OH)2D3, adipocyte foci developed about 3 days after confluency as clusters of rounded or stellate cells. Stimulation of adipocyte foci development was dose dependent from 0.1-100 nM and was maximal with 10 nM 1,25-(OH)2D3; half-maximal stimulation occurred at about 1 nM. The presence of ascorbic acid and beta-glycerophosphate was not required for 1,25-(OH)2D3-induced stimulation of adipocytes, but both significantly increased the number of adipocyte foci in the presence of 1,25-(OH)2D3. The critical period for initiation of adipocyte differentiation with 1,25-(OH)2D3 was between 1-9 days, and once committed along the adipogenic pathway, adipocytes maintained their differentiated state in the absence of 1,25-(OH)2D3. Short term (48-h) pulses of 1,25-(OH)2D3 resulted in slight, but significant, increases in adipocyte formation. Other vitamin D3 metabolites were less effective than 1,25-(OH)2D3 in stimulating adipocyte differentiation. Dex (1-100 nM) also caused a dose-dependent increase in the differentiation of adipocyte foci in RC cell cultures. The adipocyte foci that developed in the presence of Dex frequently appeared earlier in culture, i.e. when cells reached confluency on days 6-7, and were more diffuse than those forming with 1,25-(OH)2D3. The stimulation of adipocyte differentiation by 1,25-(OH)2D3, however, was greater than that by Dex in mixed RC II-V cells. The combined effects of 1,25-(OH)2D3 and Dex were additive at low concentrations and synergistic at higher concentrations of either 1,25-(OH)2D3 or Dex. The data show that bone cell populations isolated from fetal RC contain adipocyte progenitors and that 1,25-(OH)2D3 as well as Dex are potent regulators of adipocyte differentiation within these bone cell populations.

Osteoclast differentiation in cocultures of a clonal chondrogenic cell line and mouse bone marrow cells

Previous reports have demonstrated that hemopoietic progenitor cells derived from mouse bone marrow can form osteoclast-like cells when cultured in the presence of stromal cells and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. We show here that in cocultures of mouse bone marrow cells and a clonal chondrogenic cell line (C5.18), a stimulation of the number of tartrate-resistant acid phosphatase-positive (TRAP+) colonies is seen with or without the addition of 1,25-(OH)2D3 to the cultures. A large proportion of the TRAP+ cells had calcitonin receptors. In addition resorption lacunae were observed on bone slices on which cocultures were maintained, thus these cells had the characteristics of osteoclasts. The number of osteoclast-containing colonies that formed in cocultures varied with the plating density of the C5.18 cells and the length of time the C5.18 cells were cultured before adding mouse bone marrow. These results suggested that osteoclast differentiation decreased with increasing cartilage differentiation. C5.18 cells treated with 1,25-(OH)2D3 before coculture stimulated TRAP+ osteoclast colony formation to a greater extent than untreated C5.18 cells, whereas C5.18 cells cultured in the presence of dexamethasone before coculture inhibited TRAP+ osteoclast colony formation relative to untreated C5.18 cells. Since 1,25-(OH)2D3 inhibits and dexamethasone stimulates cartilage differentiation in C5.18 cells, these results agree with the view that chondroprogenitor cells stimulate osteoclast colony formation, whereas cultures containing predominantly mature chondrocytes do not. Osteoclast-containing colonies were frequently associated with colonies of alkaline phosphatase-positive (AP+) cells. This raised the possibility that C5.18 cells stimulated osteoclast differentiation indirectly by increasing the numbers of AP+ stromal cells from the marrow population, which in turn could stimulate osteoclast differentiation from marrow hemopoietic progenitors. In cocultures in which the C5.18 cells were physically separated from the marrow cells, we also observed increased numbers of TRAP+ colonies growing in association with large colonies of AP+ cells, suggesting that C5.18 cells release a soluble factor that mediates these effects.

Differential effects of fluoride during initiation and progression of mineralization of osteoid nodules formed in vitro

Osteoid nodules form in cultures of fetal rat calvarial (RC) cells grown in medium containing 10% FBS and 50 micrograms/ml of ascorbic acid. When 10 mM beta-glycerophosphate (beta-GP) is added, osteoid nodules mineralize in two phases: an initiation phase, which is dependent upon alkaline phosphatase activity for conversion of beta-GP to P(i), and a progression phase that proceeds independently of alkaline phosphatase activity and does not require exogenous phosphate. We have now used this system to investigate the effects of fluoride (F-) on mineralization. In cultures in which osteoid was formed and mineralization initiated in the presence of F-, a dose-dependent inhibition of the initiation of mineralization occurred over a concentration range of 25-500 microM F- (p < 0.001 in all cases). The initiation of mineralization was not inhibited if F- was removed from the cultures at the time when mineralization was initiated with beta-GP. In osteoid nodules grown in the absence of F-, addition of F- resulted in a dose-dependent inhibition of the initiation of mineralization, with significant decreases in 45Ca uptake occurring at F- concentrations of 3 microM (p < 0.01) and higher. However, if F- was added to cultures after mineralization was initiated in the absence of F-, a stimulation of 45Ca uptake was observed at F- concentrations of 250 microM and above (p < 0.001). F- (1-1000 microM) did not affect the conversion of beta-GP to P(i) or alkaline phosphatase activity in the cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of retinoic acid on cartilage differentiation in a chondrogenic cell line

Previously we have isolated the monopotential chondrogenic cell line RCJ 3.1 C5.18 from the multipotential mesenchymal cell line RCJ 3.1 [Grigoriadis et al.: Endocrinology, 125:2103-2110, 1989]. When cultured for approximately 20 days under appropriate conditions, these cells from cartilage nodules. In the present investigation, we have used this cell line to study the effects of all-trans retinoic acid (RA) on chondroblast differentiation, cartilage formation, and cartilage degradation. Continuous exposure of cultures to RA (0.01-100 nM) inhibited chondroblast differentiation and glycosaminoglycan (GAG) accumulation in a dose-dependent manner, without comparable effects on cell growth. Pulse treatment with RA for various 4 day periods during a 17-24 day culture period established that RA inhibited differentiation of chondroprogenitors at all periods tested. These effects were reversible, except for part of the effect on early chondroprogenitors. Treatment with RA on days 13-17 in 17 day cultures not only resulted in cessation of cartilage formation, but also in disappearance of pre-existing cartilage nodules. We demonstrated that this was associated with RA-induced downregulation of GAG synthesis and increased degradation of cartilage proteoglycans. Hence, the inhibitory effects of RA on cartilage formation consist of inhibition of chondroblast differentiation, inhibition of GAG synthesis by differentiated chondroblasts, and stimulation of cartilage proteoglycan degradation by differentiated chondroblasts and/or chondrocytes. These results indicate that the clonal monopotential chondrogenic cell line RCJ 3.1 C5.18 forms a good model system to study the effects of retinoids on cartilage differentiation, formation, and degradation.

Effect of medium pH on osteoclast activity and osteoclast formation in cultures of dispersed rabbit osteoclasts

We investigated the effect of medium pH on activity of isolated osteoclasts and have also looked at the possibility that medium pH affects osteoclast numbers during culture. Osteoclast-containing cell suspensions prepared from neonatal rabbits were cultured on bovine bone slices at pH 6.5, 7.0, or 7.5. After 24 or 48 h of culture, the cells and bone slices were fixed and stained for tartrate-resistant acid phosphatase (TRAP). After counting the osteoclasts, the cells were removed and the resorption lacunae stained by immunostaining using anticollagen type I antibody and then quantitated. We found that the resorptive activity of isolated rabbit osteoclasts was sharply increased at pH 6.5-7. Osteoclast differentiation and proliferation, on the other hand, were optimal at pH 7.0-7.5 but decreased at pH 6.5. The results thus imply that pH regulation of the bone surface environment can dramatically alter both the number of osteoclasts and their resorptive activity.

Characterization of the 1,25-(OH)2D3-induced inhibition of bone nodule formation in long-term cultures of fetal rat calvaria cells

We investigated the effects of 1,25-dihydroxyvitamin D3[1,25-(OH)2D3], on osteoprogenitor cell differentiation and bone nodule formation at various stages of differentiation by evaluating the effects on long term cultures of fetal rat calvaria (RC) cells. RC cells were plated at 3 x 10(4) cells/35-mm dish in alpha-minimal essential medium containing 15% fetal bovine serum, ascorbic acid, and beta-glycerophosphate (beta-GP), conditions under which bone nodules form. 1,25-(OH)2D3 inhibited bone nodule formation in a dose-dependent manner with total inhibition occurring at 1-10 nM and half-maximal inhibition occurring at approximately 0.06 nM. 1,25-(OH)2D3 also significantly stimulated RC cell growth in a dose-dependent manner in both the presence and absence of ascorbic acid. Addition of 1 nM 1,25-(OH)2D3 at different times after the start of culture inhibited nodule formation when added before and up to the early multilayering stage (up to day 11 of culture), but had no effect on nodule number when added later. When 1,25-(OH)2D3 was added at the start of the culture period and removed at the early multilayering stage, nodule formation was also inhibited. Pulses of 48-h duration also inhibited nodule formation, with maximal effect occurring between days 3 and 11. Thus, 1,25-(OH)2D3 inhibited osteoprogenitor cell differentiation during the earlier stages of culture before visible bone nodule formation occurred and the effect was not reversible upon removal of 1,25-(OH)2D3. In cultures grown to the multilayering stage in medium without ascorbic acid and beta-GP and then changed to medium with ascorbic acid and beta-GP, 1,25-(OH)2D3 inhibited when present before, but not after, the addition of ascorbic acid and beta-GP. Two other vitamin D3 metabolites, 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] and 1,24,25-trihydroxyvitamin D3 [1,24,25-(OH)3D3] had inhibitory effects similar to 1,25-(OH)2D3. The effects were dose dependent for each metabolite tested and correlated with the biological effectiveness of these metabolites in other systems: i.e. 1,25-(OH)2D3 was more effective than 1,24,25-(OH)3D3 which in turn was more effective than 24,25-(OH)2D3. The data show that 1,25-(OH)2D3 inhibits osteoprogenitor cell differentiation at an early stage and at a time during which cell growth is stimulated.

The role of phospholipase A2 in interleukin-1 alpha-mediated inhibition of mineralization of the osteoid formed by fetal rat calvaria cells in vitro

Interleukin-1 (IL-1) may be an important mediator of bone remodeling, since it is a potent stimulator of bone resorption and has biphasic effects on bone formation. Continuous exposure of fetal rat calvaria (RC) cells to IL-1 alpha or IL-1 beta results in a dose-dependent inhibition of both bone nodule formation and mineralization of the organic matrix. In this study, the effects of recombinant human IL-1 alpha on the mineralization process were examined by the addition of IL-1 alpha late in the culture period, after osteoid nodules had formed and when they were induced to mineralize by the addition of organic phosphate. By means of a quantitative 45calcium radiolabeling assay, it was shown that short-duration exposures of fully-formed bone nodules to IL-1 alpha also inhibited mineralization, and that the duration of treatment directly correlated with the degree of inhibition. Because our earlier studies had demonstrated that IL-1 stimulated the release of PLA2 and PGE2 from RC cells, the effects of PLA2 and of inhibition of PGE2 synthesis on mineralization were investigated. Exogenous Naja naja group I PLA2 had little effect on the mineralization of bone nodules; however, Crotalus adamanteus group II PLA2 inhibited mineralization at concentrations similar to those found in the media from IL-1 alpha-treated cultures. Although PLA2 is thought to stimulate PGE2 synthesis by releasing arachidonic acid from membrane phospholipids, PGE2 release by RC cells accounted for only part of the IL-1 alpha-mediated inhibition, suggesting the presence of other mechanisms of exogenous PLA2 action in inhibiting mineralization.(ABSTRACT TRUNCATED AT 250 WORDS)

beta-Glycerophosphate-induced mineralization of osteoid does not alter expression of extracellular matrix components in fetal rat calvarial cell cultures

When fetal rat calvarial cells are cultured in medium containing vitamin C, osteoid nodules develop after approximately 15 days of culture. Upon addition of an organic phosphate (beta-glycerophosphate, beta GP), these nodules mineralize. We have now used this system to explore the suggestion made by others that a negative feedback may exist between matrix mineralization on the one hand and the synthesis of alkaline phosphatase and bone matrix collagen on the other by analyzing the synthesis of these proteins and the levels of their mRNAs in mineralizing and nonmineralizing cultures. Our results indicate that in the osteoid nodule-bone nodule system, matrix mineralization did not affect the mRNA levels for osteopontin, type I collagen, bone sialoprotein, or osteocalcin. Synthesis of total protein and collagen and the osteocalcin content of culture media were also not different in the mineralizing and nonmineralizing cultures. However, alkaline phosphatase mRNA was increased in early mineralizing cultures and alkaline phosphatase activity in the cell layer was also increased in mineralizing cultures. Thus, the hypothesis that a direct negative feedback exists between mineralization and matrix protein synthesis is not supported by our experiments.

Differential chemotactic responses of different populations of fetal rat calvaria cells to platelet-derived growth factor and transforming growth factor beta

We tested the chemotactic response to platelet-derived growth factor (PDGF) and transforming growth factor beta (TGF beta) of cells released enzymatically from fetal rat calvaria (RC). Both factors were chemotactic for RC cells, but the magnitude of the chemotactic response differed markedly between different populations and varied with time in culture of the cell populations. Cells released earlier from the calvaria showed a greater response than osteoblast-enriched populations released later. The optimal concentration of PDGF was the same for both alkaline phosphatase (AP)-positive and AP-negative cells within the populations. However AP-positive cells showed two peaks of response to TGF beta; one peak coincided with the TGF beta concentration also maximally affecting AP-negative cells, while the other occurred at a concentration 50-100 times higher. The results indicate that PDGF and TGF beta are chemotactic for both AP-positive and AP-negative cells in populations of cells derived from fetal calvariae, that chemotactic response declined with longer periods of time in culture, and that AP-positive osteoblast-like cells respond to a concentration of TGF beta that does not affect the AP-negative cells in the population.

Inorganic phosphate added exogenously or released from beta-glycerophosphate initiates mineralization of osteoid nodules in vitro

Rat calvaria (RC) cells grown in medium containing ascorbic acid form nodules of osteoid and cells. When 10 mM beta-Glycerophosphate (beta-GP) is added, the osteoid mineralizes in two phases: an initiation phase that is dependent upon alkaline phosphatase activity and a progression phase that proceeds independently of the activity of alkaline phosphatase and does not require added beta-GP (Bellows et al., Bone Miner 1991;14:27-40). The present experiments were performed to determine whether beta-GP is converted to inorganic phosphate (Pi) during the initiation phase of the mineralization process and whether increased Pi can replace beta-GP in the initiation phase. Measurements of Pi concentrations in the culture medium showed that during the first 8 h of the initiation phase of mineralization, 10 mM beta-GP was rapidly degraded resulting in Pi concentrations of 9-10 mM. The production rate of Pi from beta-GP was linear (r = 0.996) and the alkaline phosphatase activity in the same cultures indicated a potential for conversion of beta-GP to Pi that was greater than the actual conversion rate. The addition of 2-5 mM Pi in the absence of beta-GP also initiated mineralization. Mineralization initiated by either beta-GP or Pi progressed in the absence of added beta-GP or Pi. 100 microM Levamisole inhibited the initiation of beta-GP-induced mineralization and the conversion of beta-GP to Pi, but did not affect Pi-induced initiation of mineralization. The addition of 1-5 mM Pi to cultures in which mineralization had been initiated by 10 mM beta-GP had no significant effect on the progression phase of mineralization. Neither beta-BP nor Pi initiated 45Ca uptake in cultures without nodules (RC population I) and the histological appearance of the mineralized tissue in either phosphate source appeared identical. The present experiments show that beta-GP is rapidly and virtually completely degraded to Pi during the initiation phase of mineralization and that the addition of increased concentrations of Pi can replace beta-GP in the initiation phase of mineralization in the absence of non-specific 45Ca uptake or apparent cellular toxicity.

Interleukin-1 alpha stimulates the release of prostaglandin E2 and phospholipase A2 from fetal rat calvarial cells in vitro: relationship to bone nodule formation

We have shown previously that interleukin-1 (IL-1) has biphasic effects on the formation of bone nodules in long-term cultures of fetal rat calvarial (RC) cells (Ellies and Aubin, Cytokine 2:430-437, 1990). To determine the role of arachidonic acid metabolism in this process, we have examined the release of prostaglandin E2 (PGE2) and phospholipase A2 (PLA2) from RC cells under conditions that allowed concomitant analysis of the formation of bone nodules. Recombinant human IL-1 alpha (rhIL-1 alpha) stimulated PGE2 and PLA2 release in a time- and dose-dependent manner. PGE2 release was highest in preconfluent cultures (days 1-6) and was stimulated up to 8.5-fold in response to 50 U/ml of rhIL-1 alpha. In contrast, extracellular PLA2 activity was maximal in postconfluent cultures, with 50 U/ml of rhIL-1 alpha causing a 20-fold increase by day 15. PLA2 release by RC cells was not significantly affected by PGE2, the glucocorticoid dexamethasone, or the cyclooxygenase inhibitor indomethacin. Indomethacin partially blocked the inhibition of bone nodule formation caused by rhIL-1 alpha, and exogenous PGE2 reversed this effect. Addition of group I PLA2 from Naja naja venom to RC cells had no effect on bone nodule development; however, group II PLA2 from Crotalus adamanteus venom inhibited the formation of bone nodules in a dose range similar to that induced by rhIL-1 alpha. These results indicate that PGE2 release does not have a direct temporal correlation with increases in PLA2 activity. In addition, the data show that only part of the inhibition of bone formation seen with rhIL-1 alpha is mediated by PGE2 and suggest that extracellular PLA2 also accounts for part of the inhibition.

Initiation and progression of mineralization of bone nodules formed in vitro: the role of alkaline phosphatase and organic phosphate

Osteoid nodules form but do not mineralize in fetal rat calvaria cell cultures grown in alpha-minimal essential medium with 10% fetal bovine serum in the absence of Na beta-glycerophosphate (beta-GP). To study factors involved in the initiation and progression of mineralization, cultures were treated with beta-GP and radiolabelled with 0.1-0.2 microCi/ml 45Ca after nodules had formed (17-19 days in medium without beta-GP). Concentrations of beta-GP from 1 to 14 mM induced a dose-dependent increase in 45Ca uptake. 45Ca uptake was restricted to nodule-containing cultures and did not occur in cultures without nodules. Continuous labelling over 72 h compared with 2 h pulses over the same time period showed that little mineralization occurred over the first 8-12 h and that the rate of mineralization was maximal and constant after 24 h exposure to beta-GP. Calcium uptake from medium was slow during the first 12 h of beta-GP exposure but increased rapidly thereafter until the medium calcium concentration reached a steady state of between 0.5 and 0.6 mM. Measurement of calcium concentration in the medium after mineralization had been initiated (24 h after beta-GP exposure) showed a linear calcium uptake into nodules (r = 0.990) over a 7 h period at a rate of 9.2 micrograms calcium/h/culture. Initiation of mineralization was prevented by 100 microM levamisole, but not by 100 microM dexamisole. When 100 microM levamisole was added 24 h after mineralization had been initiated by the addition of beta-GP, the progression of mineralization was unaffected. Similarly, after mineralization had been initiated for 24 h by 10 mM beta-GP, mineralization continued independent of the presence of beta-GP. The data show that the initiation and progression of mineralization are separate phenomena and that organic phosphate and alkaline phosphatase play a crucial role in the initiation of mineralization but are not required for the continuation of mineralization of bone nodules.

Localization of bone sialoprotein (BSP) expression to sites of mineralized tissue formation in fetal rat tissues by in situ hybridization

Bone sialoprotein (BSP) is a major protein in the mineralized matrix of bone and dentine. To study the relationship between the expression of BSP and the formation of mineralized connective tissues, a cDNA probe to rat BSP was prepared for in situ hybridization analysis of developing fetal rat bones and teeth. When used for Northern hybridization analysis of rat bone marrow cells induced to differentiate into osteogenic cells by dexamethasone, the BSP cDNA revealed a specific induction of 1.6- and 2.0-kb mRNA species of BSP. In tissue sections a strong hybridization signal associated with osteoblasts was observed in areas of endochondral bone formation in the long bone metaphysis and condylar cartilage, and in the intramembranous bone of the calvaria and mandible. Hybridization reflecting a lower degree of expression was evident in cells of the transitional zone of mineralizing cartilage and in odontoblasts forming incisor dentine. Expression of BSP was also demonstrated in the hypertrophic cartilage cells in the long bone and condylar process. In contrast, expression of BSP could not be detected in the reserve or proliferative chondrocytes, fibroblasts and muscle cells. These studies demonstrate that the expression of BSP in bones and teeth is essentially restricted to cells directly involved in the formation of mineralizing connective tissue matrices, indicating that BSP has a specific role in biological mineralization and that it is a useful marker of bone formation.

Parathyroid hormone reversibly suppresses the differentiation of osteoprogenitor cells into functional osteoblasts

The effects of PTH on osteoprogenitor cell differentiation have been analyzed by quantifying its effects on bone nodule formation in an in vitro assay. Fetal rat calvaria cells were plated at 3 x 10(4) cells/35-mm dish, and cultures were maintained for 17-23 days in alpha-Minimal Essential Medium containing ascorbic acid, Na beta-glycerophosphate, and 10% fetal bovine serum. Continuous exposure to PTH at concentrations from 1 pM to 1 nM (2 x 10(-5) to 2 x 10(-2) IU/ml) caused a dose-dependent inhibition of bone nodule formation. Half-maximal inhibition occurred at 0.05 nM, and total inhibition at 1 nM, concentrations much lower than those required to elicit a significant cAMP response in rat calvaria cells. PTH at the concentrations used did not affect cell growth or saturation density. While continuous exposure to 1 nM PTH eliminated bone nodule formation, a single 48-h pulse administered at any time during the 17-day culture period had no effect. When 1 nM PTH was added on day 1 and removed at different times during the culture period, a time-related release from inhibition was observed. Cultures exposed to 1 nM PTH until nodules had developed in the corresponding control cultures and then switched to medium without added PTH rapidly formed clusters of differentiated osteoblasts and nodules within 3 days. PTH added at different times during the culture period and present continuously there-after suppressed formation of new nodules, the magnitude of the effect being a function of the duration of exposure. The results show that PTH at physiological concentrations is a potent suppressor of osteoblast differentiation and that its effect occurs at a late stage in the differentiation of osteoprogenitor cells, probably preventing differentiation of preosteoblasts into osteoblasts.

Continuously growing bipotential and monopotential myogenic, adipogenic, and chondrogenic subclones isolated from the multipotential RCJ 3.1 clonal cell line

The clonal multipotential RCJ 3.1 cell line, which gives rise to myotubes, adipocytes, chondrocytes, and osteoblasts, contains different progenitor subpopulations. By limiting dilution analysis, of 296 single colonies identified, approximately 20% contained a single recognizable cell type, approximately 10% contained two cell types, and approximately 1% contained three cell types. We recloned RCJ 3.1 and isolated continuously growing subclones, including four novel bipotential (adipocytes/chondrocytes; adipocytes/myotubes and chondrocytes/myotubes) cell populations, whose phenotypes bred true. In the bipotential subclones, single colony analyses confirmed the presence of single cells which could both self-renew the bipotential progenitors and give rise to their respective committed monopotential lineages. Eight subclones were restricted to a single cell lineage and were considered monopotential; one of these is a novel cell line differentiating into cartilage. Thus, we have isolated unique monopotential and bipotential progenitor cell lines which provide a valuable model for studying the mechanisms leading to lineage restriction in mesenchymal populations.

Determination of the capacity for proliferation and differentiation of osteoprogenitor cells in the presence and absence of dexamethasone

Osteoprogenitor cells present in single-cell suspensions prepared from fetal rat calvaria (RC) form discrete mineralized three-dimensional bone nodules when cultured long-term in the presence of ascorbic acid and beta-glycerophosphate. These cells (CFU-O) constitute less than 1% of the total cell population under standard culture conditions and their number is increased in the presence of dexamethasone. Using the formation of the bone nodule as a marker for CFU-O, we have now analyzed the proliferation and differentiation capacity of these CFU-O by redistribution and continuous subculture experiments in the presence and absence of dexamethasone. Cell redistribution experiments showed no increase in nodule number after one population doubling with either treatment. After 5.4 population doublings of the entire RC population, nodule number increased up to 2.0-fold in control cultures and 4.5-fold in cultures containing 10 nM dexamethasone. Continuous subculture experiments in which cultures were split 1:3 every 3 day for up to seven subcultures showed that nodule number decreased in parallel with the split ratio in the absence of dexamethasone, while with dexamethasone nodule number was elevated above the number present in primary cultures for 1 or 2 subcultures after which nodule number decreased with the split ratio. Bone nodules were present for up to 18 population doublings. Measurements of nodule area by automated image analysis showed that dexamethasone increased nodule size and that nodule size decreased from primary to 1st to 2nd subculture with or without dexamethasone. The data suggest that dexamethasone selectively stimulates the proliferation of osteoprogenitor cells and that these progenitor cells have a limited capacity for generating daughter cells capable of expressing the bone phenotype.

The effects of fluoride on osteoblast progenitors in vitro

The number of discrete, three-dimensional bone nodules formed in vitro from a class of osteoprogenitor cells present in fetal rat calvaria cell populations (RC cells) is linearly related to the number of cells plated, implying that this system functions as a colony assay for the expression of osteoprogenitor cells. To determine the effect of fluoride on the expression of these cells, primary RC cells were grown for periods of up to 21 days in alpha-MEM (minimal essential medium) containing 5-15% heat-inactivated fetal bovine serum (FBS), 50 micrograms/ml ascorbic acid, 10 mM Na beta-glycerophosphate, and NaF at concentrations from 10 microM to 5 mM. The continuous presence of NaF resulted in an increase in the number of bone nodules with maximal response occurring at 500 microM (p less than 0.001). A similar response at 500 microM NaF was observed also with regard to alkaline phosphatase activity. NaF levels up to 500 microM did not affect the growth of the mixed RC cell population, however, higher concentrations (1 mM) significantly reduced cell numbers (p less than 0.001) suggestive of cytotoxicity. Plating efficiency tests for colony formation in the presence of 0.5 to 2 mM NaF showed that the decreases in nodule formation observed at concentrations above 500 microM correlated with cytotoxicity. NaC1 at 1 mM had no effect on nodule formation, alkaline phosphatase activity, or cell growth. The results show that NaF stimulates osteoprogenitor cell number in vitro and that the maximal effect occurs at concentrations close to toxic levels.

The effects of sodium fluoride on the resorptive activity of isolated osteoclasts

It is not clear to what extent the increased bone mass observed in vertebral trabecular bone of fluoride-treated mammals is a consequence of effects of fluoride on the number and activity of osteoclasts or of osteoblasts. In the present communication, we have analyzed the effects of NaF on the activity of isolated rabbit osteoclasts cultured on thin slices of devitalized compact bovine bone. Osteoclastic resorption was quantitated by counting the number of resorption lacunae and measuring their surface area and their depth using scanning electron microscopy. Our results show that NaF in concentrations of 0.5-1.0 mM decreased the number of resorption lacunae made by individual osteoclasts and decreased the resorbed area per osteoclast. We argue that the concentration of fluoride in these experiments may be within the range "seen" by osteoclasts in mammals treated for prolonged periods with approximately 1 mg of NaF/kg body weight (bw) per day.

A band of F-actin containing podosomes is involved in bone resorption by osteoclasts

Isolated rabbit osteoclasts cultured on devitalized thin bone slices excavate resorption lacunae that can be visualized with brightfield or phase-contrast microscopy. Superimposition of the brightfield images of such resorption lacunae and the fluorescence images of the corresponding osteoclasts after fixation and staining with rhodamine-conjugated phalloidin revealed that a bright fluorescent band of F-actin-containing podosomes precisely outlined the resorption lacunae in stationary osteoclasts. When the resorption lacunae were being extended laterally, the clearly delineated band of podosomes corresponded to the advancing edge of the resorbing osteoclast and the most recently excavated part of the lacunae. Reshaping and reorganization of the bright bands preceded development of the lateral boundary of the lacunae. Podosomes forming these bands were highly dynamic, changed in size and location, and appeared and disappeared continuously. Their lifespan varied between 2 and 12 min. Similar bands were also seen in vivo in bone-resorbing osteoclasts on the endocranial surface of growing calvariae. Podosomes disappeared in osteoclasts treated with calcitonin, resulting in the disruption of the fluorescent bands. Our results suggest that podosomes are an essential part of the resorption apparatus of osteoclasts.

Forskolin has biphasic effects on osteoprogenitor cell differentiation in vitro

Cells isolated from fetal rat calvaria (RC) and maintained in vitro in medium containing ascorbic acid and B-glycerophosphate form three-dimensional, mineralized nodules having the histological, immunohistological, and ultrastructural characteristics of woven bone. We have studied the effects of forskolin (FSK), a diterpene that activates adenylate cyclase, in this system. While 10(-7)-10(-5) M FSK significantly stimulated cAMP levels in RC cells, lower concentrations did not. cAMP levels with 10(-5) M FSK reached a maximum by 30 min at 37 degrees C and returned to basal level in 2-3 hr. Changes in cAMP levels correlated with changes in cellular shape: cells treated with 10(-5) M FSK assumed a stellate morphology, lost microfilament bundles, and reduced their substrate adhesiveness, while cells treated with 10(-9) M were not affected. Exponential growth and saturation densities of FSK-treated cultures were similar to untreated cultures, indicating that FSK was neither toxic nor stimulatory to the population. The effect on bone nodule formation of FSK present continuously depended on concentration: 10(-5) M FSK significantly inhibited the number of nodules formed, while 10(-9) M FSK significantly stimulated bone nodule formation. Single short treatments with either 10(-5) M or 10(-9) M FSK had no effect on nodule formation, but repeated short duration treatments (1 hr every 2 days for 21 days) gave results similar to continuous exposure. These results indicate that intermittent elevations in intracellular cAMP have an inhibitory effect on bone formation. In addition, our work indicates that low concentrations of FSK stimulate differentiation of osteoprogenitor cells possibly through a non-cAMP-dependent process.

Prostaglandin E2 initially inhibits and then stimulates bone resorption in isolated rabbit osteoclast cultures

Osteoclasts were isolated from the long bones of neonatal rabbits and cultured on devitalized bovine bone slices for 8, 24, 48 and 72 h with and without prostaglandin E2 (PGE2) (10(-6) M). The number of osteoclasts present at the end of the culture periods was counted after staining the cells for tartrate resistant acid phosphatase (TRAP). After removal of the cells, the resorption lacunae excavated by the osteoclasts were observed by scanning electron microscopy (SEM) and their size and depth calculated by computer-assisted morphometric and stereomorphometric techniques. PGE2 had no effect on the number of TRAP positive multinucleated osteoclasts, but decreased the number of TRAP positive mononuclear cells. The total area of the excavated pits and the area excavated per osteoclast in PGE2-treated cultures were decreased by 62 and 58% respectively after 8 h in culture. After 24 h in culture, the total excavated area and the excavated area per osteoclast were still 44 and 38% lower in the PGE2-treated cultures than in the corresponding control cultures. However, after 48 h of culture, resorptive activity in PGE2-treated cultures was consistently greater than in control cultures. In the course of a 48 h culture period, the PGE2 concentration decreased from 1.0 x 10(-6) to 0.3 x 10(-6) M. Thus, despite the continuous presence of PGE2, the resorptive activity of osteoclasts not only recovered from the transient inhibitory effect of PGE2, but was actually greater than in the control cultures. This confirms that the effects of PGE2 in isolated osteoclast preparations are inhibitory in short term cultures, but shows that the effects of PGE2 in such preparations are stimulatory in longer term cultures. Proliferating stromal cells with osteoblast-like characteristics comprised approximately 45% of the 'osteoclast' cultures at the start of the cultures, but their number increased to 93% of the total cell population at 48 h and to 98% at 72 h. Our results suggest that the PGE2-induced stimulation of osteoclastic activity represents an indirect effect mediated by stromal cells derived from bone marrow. Our results also indicate that the increased resorptive activity in PGE2 treated cultures can be accounted for by an increase in the size of the resorption lacunae and is not caused by an increase in osteoclast number.

Effects of dexamethasone and vitamin D3 on cartilage differentiation in a clonal chondrogenic cell population

We have investigated the regulation of chondroblast/chondrocyte differentiation using a unique clonal cell population, designated RCJ 3.1C5 (C5), which differentiates into discrete three-dimensional cartilage nodules when grown in the presence of 15% fetal calf serum. Histologically, the nodules resembled hyaline cartilage; they contained large rounded chondrocytes surrounded by a refractile matrix which stained intensely with Alcian blue, exhibited metachromasia after Toluidine blue staining, and stained with an antibody against type II collagen. The cartilage nodules that formed did not mineralize, despite the presence of organic phosphate in the culture medium. The synthetic glucocorticoid dexamethasone (DEX) increased the number of cartilage nodules formed in a dose-dependent manner (ED50, approximately 10(-9) M), with a maximal stimulatory dose of 10(-8) M. DEX had no effect on the population doubling time and saturation density. The effects of DEX on the number of cartilage nodules were similar whether it was added from the beginning of the culture period (starting during exponential growth) or at confluence. In contrast, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] inhibited cartilage nodule formation in a dose-dependent manner (IC50, approximately 5 x 10(-10) M), with maximum inhibition at 10(-7) M. In addition, 1,25-(OH)2D3 decreased cell proliferation and saturation density. Equimolar doses of the vitamin D3 metabolites 24,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 had no effect. C5 cells treated with 1,25-(OH)2D3 in the absence of DEX during the exponential growth phase exhibited a reduced capacity to form cartilage nodules upon subsequent exposure to DEX. At confluence, before cartilage nodules had formed, C5 cells responded to PTH and prostaglandin-E2 with increases in intracellular cAMP of about 10- and 95-fold respectively. After cartilage nodules were present, prostaglandin-E2 responsiveness decreased to about 25-fold, whereas there was no significant change in PTH responsiveness. DEX decreased the population alkaline phosphatase levels at all times measured, whereas 1,25-(OH)2D3 had a biphasic effect: an increase at 5 days in culture, followed by a decrease at later times in culture. These data indicate that the clonal cell line RCJ 3.1C5 is a useful model system in which to investigate cartilage differentiation.

Expression of calcitonin receptors during osteoclast differentiation in mouse metatarsals

Metatarsal bones of 15-day-old mouse embryos contain proliferative tartrate-resistant acid phosphatase (TRAP) negative (-) osteoclast progenitors that progressively differentiate into multinucleated TRAP positive (+) osteoclasts. Using histochemical and autoradiographic techniques, we have examined the expression of calcitonin receptors during osteoclast differentiation in mouse metatarsals. Fresh mouse metatarsals from embryos aged 14-17 days and metatarsals from 15-day-old embryos cultured for 1, 2, 3, and 6 days were stained for TRAP. Calcitonin binding to osteoclasts and their precursors was studied by incubating metatarsals with [125I]salmon calcitonin (sCT) and quantitating grain counts from autoradiographs of tissue sections. Calcitonin receptors first appear on nonproliferating osteoclast precursors, most often just after or simultaneously with the development of TRAP activity. The effect of sCT on the development of TRAP+ mononuclear preosteoclasts was examined by culturing 15-day-old metatarsals in the continuous presence of 5 mU sCT for periods of up to 3 days and quantitating the number of TRAP+ mononuclear preosteoclasts that develop. Calcitonin did not affect the differentiation of osteoclasts up to the stage of the TRAP+ mononuclear preosteoclast.

Effects of dexamethasone on expression and maintenance of cartilage in serum-containing cultures of calvaria cells

The effects of dexamethasone on the ability of cells enzymatically isolated from 21-day fetal rat calvaria to produce cartilage in vitro has been investigated. Primary cultures of single-cell suspensions of rat calvaria were grown for up to 28 days in vitro in alpha-minimal essential medium containing 15% fetal bovine serum, 50 micrograms/ml ascorbic acid, 10 mM Na beta-glycerophosphate and dexamethasone at concentrations of 1 microM to 1 nM. Two types of nodules were present in dexamethasone-containing cultures. One has been characterized previously as bone (Bellows et al. 1986). The second morphologically resembled hyaline cartilage, possessed a strong Alcian blue-positive matrix and contained type-II, but not type-I, collagen. Both bone and cartilaginous nodules were spatially distinct and developed in isolation from each other. Cartilaginous nodules were found in the highest number at a dexamethasone concentration of 100 nM. Time-course experiments revealed that while the number of bone nodules increased continuously at least to day 28, the number of cartilaginous nodules remained constant after cultures had reached confluency. When cells were isolated separately from frontal and parietal bones and sutural regions, the greatest number of cartilaginous nodules developed from parietal bones. Since 21-day fetal rat calvaria contains 2 distinct patches of cartilage at the periphery of the parietal bones, it seems likely that this cartilaginous tissue is the origin of the cartilage cells. The results demonstrate that cultures of rat calvaria cells contain chondrocytes and possibly chondroprogenitor cells that are distinct from osteoprogenitors.(ABSTRACT TRUNCATED AT 250 WORDS)