Growth and differentiation of isolated calvarium cells in a serum-free medium

Osteoclast progenitor cells present in significant amounts in mouse calvarial osteoblast isolations and osteoclastogenesis increased by BMP-2

Enzymatically released cells from neonatal mouse calvarial bones are frequently used as primary mouse osteoblasts for in vitro studies. We, here, report that although these cells lack mRNA expression of the osteoclastic genes Calcr, Acp5 and Mmp-9 at early time points after their isolation, these transcripts are gradually upregulated when the calvarial osteoblast cultures are differentiated to more mature osteoblasts in long term cultures. Similarly, Calcr, Acp5, Mmp-9, as well as Rank and Nfatc1 mRNA expressions are robustly increased when the osteoblast cultures were induced to differentiate by treatment with bone morphogenetic protein (BMP-2). The increased Calcr mRNA resulted in functionally active calcitonin receptors. Enhanced osteoblastic differentiation was associated with increased Rankl mRNA expression and decreased Opg and Cfs1 mRNA expression. Treatment of the osteoblastic cells with BMP-2 or RANKL, either on plastic dishes or bone slices, resulted in the formation of multinucleated tartrate-resistant acid phosphatase positive cells, which were able to excavate resorption pits and release CTX from the bones. In contrast, increased osteoblastic differentiation induced by BMP-2 in the mouse calvarial osteoblastic cell line MC3T3-E1 was not associated with increased mRNA expression of Calcr, Acp5, Rank, c-Fms or Oscar. Interestingly, Ctsk mRNA was increased during osteoblastic differentiation in both mouse calvarial osteoblast cultures and in MC3T3-E1 cultures. Also osteoblasts isolated from mouse long bones by outgrowth from explant cultures were contaminated with osteoclast progenitors able to differentiate into bone resorbing osteoclasts. These data indicate that mouse calvarial osteoblast cultures contain osteoclast progenitor cells, which will be differentiated along the osteoclastic lineage during osteoblastic differentiation. Moreover, the data show that BMP-2 not only stimulates osteoblastic differentiation but can also induce osteoclastogenesis through increased RANKL.

Growth of rat osteoblast-like cells in a lipid-enriched culture medium and regulation of function by parathyroid hormone and 1,25-dihydroxyvitamin D

To examine the role of lipid metabolism in the growth and function of osteoblast-like cells, we studied ROS 17/2.8 osteosarcoma cells and primary cultures of rat calvarial osteoblasts during growth in a serum-free medium supplemented by purified human lipoproteins or by liposomes. Increase in ROS cell number was measured in sparse (1-5 X 10(3)/cm2) cultures over 6-8 days. Liposomes (0-300 micrograms/ml) and high (HDL), low (LDL), and very low density (VLDL) lipoprotein fractions (0-300 micrograms apoprotein) markedly stimulated cell growth. Cells plated at 5 X 10(3)/cm2 achieved growth rates in the presence of LDL or HDL comparable to 10% fetal bovine serum. Serum-free culture with exogenous lipid maintained the response of cell cyclic AMP accumulation to parathyroid hormone. Cyclic AMP response to parathyroid hormone was enhanced by glucocorticosteroid, and was attenuated by 1,25-dihydroxyvitamin D (1,25(OH)2D) with an EC50 (10(-10) M) comparable to that previously observed in serum-cultured cells (J. Biol. Chem. 258:736, 1985). 1,25(OH)2D also increased the alkaline phosphatase activity in ROS cells cultured in lipid-supplemented serum-free culture. Lipoproteins or liposomes also markedly enhanced the proliferative response of sparse cultures of normal rat osteoblasts to polypeptide mitogens.

Osteoblastlike cells in a serum-free methylcellulose medium form colonies: effects of insulin and insulinlike growth factor I

Osteoblastlike (OB) cells obtained from a heterogeneous primary cell population by enzymatic cell digestion of calvaria of newborn rats are grown in a serum-free viscous alpha-MEM/F-12 medium containing 0.8% methylcellulose. In contrast to cell monolayers in conventional tissue cultures OB cells proliferate into colonies of rounded-up cells to form morulalike spherical cell clusters containing up to 100 cells. These colonies, with different cell numbers, are clearly not fibroblastlike since fibroblasts from the same rats always grow as a cell monolayer. Alkaline phosphatase activity and cAMP responsivness to PTH are expressed more markedly (70% and 250% respectively) by OB cells in the described culture system than in conventional tissue cultures. Rounded-up OB cells sediment and colonies stick to the dish; proliferation of OB cells is favored and starts 3-4 days after inoculation. Increasing concentrations of insulinlike growth factor (IGF) I (0.4-35 nM) and insulin (20-660 nM), as well as increasing initial cell density, enhances mitogenic activity of these cells in a dose-dependent way. On a molar ratio IGF I (physiological concentrations) is 10 times as potent as insulin (pharmacological concentrations) with respect to proliferation. If less than 10(5) cells/ml are inoculated, there exists an apparent relationship between initial cell density and major onset of replication, indicating the presence and accumulation of local growth factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural differences between bone formed intramuscularly following the transplantation of isolated calvarial bone cells or chondrocytes

Bone formed in intramuscular transplants of isolated syngeneic calvarial bone cells in mice, was compared with endochondral bone induced by cartilage produced by analogous transplants of isolated epiphyseal chondrocytes, as well as with parietal bones forming the bulk of the calvaria. Transplanted calvarial cells produced islands of bone, some of which contained intraosseous cavities. Osteoclasts inside these cavities were observed only in 14-day-old transplants and bone marrow cells in 28-day and older transplants. On the contrary, bone marrow appeared soon after formation of bone trabeculae in endochondral bone. The percentage area occupied by bone marrow in these specimens was about twentyfold larger than in the bone formed by transplanted bone cells. On the other hand, the bone marrow area in the latter type of bone was somewhat smaller but of similar order as in parietal bones. Moreover, both in parietal bones and in bone formed by isolated bone cells, the bone marrow was devoid of fat cells which were numerous in bone arising by endochondral ossification. It appears, therefore, that the ratio of bone marrow to the bone tissue area in parietal bones depends more on the intrinsic properties of osteoblasts than on the local factors in the environment of the developing bone. In the case of bone induced by cartilage, the bone marrow/bone tissue area could be determined both by the extent of cartilage resorption by vascularized tissue and by the properties of osteoblasts.

Bone formation by isolated calvarial osteoblasts in syngeneic and allogeneic transplants: light microscopic observations

Taking advantage of recently developed methods for osteoblast isolation, we used these cells to study bone morphogenesis in syngeneic and allogeneic intramuscular transplants. Syngeneic osteoblasts from fetal rat calvaria produced small islands of bone by the third day after transplantation. These islands increased in size and began to fuse after about 14 days. At the surface of the woven bone laid down first, lamellar bone developed. The amount of this bone increased, and in 56-day-old transplants solid blocks of bone were present. Osteoclasts were scarce, and the woven bone remained unresorbed. Bone marrow was absent. The structure of bone in transplants differed from that of mature calvarial bones in which only remnants of woven bone remained and bone marrow was well developed. The scarcity of osteoclasts in transplants could be caused by their relative paucity among the injected cells, since these cells responded strongly to added parathyroid hormone by increased production of cyclic adenosine monophosphate (cAMP) but only weakly to calcitonin. Osteoblasts isolated from the surface of calvarial lamellar bone of 28-day-old rats formed woven bone similar to the bone formed by fetal cells. This suggests that the type of bone produced does not depend on the intrinsic properties of the osteoblasts. Bone formed in an allogeneic system was surrounded by infiltrations containing lymphocytes, macrophages, and osteoclastlike cells in 14-day-old transplants. Osteoblasts at the bone periphery were destroyed and bone matrix was resorbed by infiltrating cells. Numerous bone lacunae were enlarged, suggesting the occurrence of osteocytic osteolysis. Isolated osteoblasts cultured for three population doublings or longer did not form bone after transplantation, although they retained some reactivity toward parathyroid hormone.

Calcification of cartilage matrix in chondrocyte cultures derived from rachitic rat growth plate cartilage

Chondrocytes obtained from collagenase-digested epiphyseal growth plate cartilage of rachitic rats were grown in multilayer cultures. The cultured chondrocytes produced a metachromatic matrix and further electron microscopic examination revealed typical features of cartilage matrix collagen fibrils and matrix vesicles. The alkaline phosphatase activity in cultures was high during the entire 3-week culture period. Acid phosphatase showed a marked increase in activity during the first week of culture. The appearance of apatite crystals in the synthesized matrix was monitored by electron microscopy over a 3-week period. First crystals were consistently found to be associated with matrix vesicles, and in the older cultures calcification spread into the surrounding matrix. No collagen fibrils associated with mineralization were observed during the early culture period. This study clearly demonstrates that in chondrocyte cultures the first mineral crystals were found within or in close association with matrix vesicles. This gives further support to the hypothesis that matrix vesicles are the primary site of mineralization in cartilage. In addition to calcification studies it is suggested that this model is suitable for studying the effects of hormones or other agents on rachitic chondrocytes in vitro.

Selective emergence of differentiated chondrocytes during serum-free culture of cells derived from fetal rat calvaria

Cells dispersed from the chondrocranial portions of fetal rat calvaria proliferated and performed specialized functions during primary culture in a chemically defined medium. Mature cultures were typified by multilayered clusters of redifferentiating cartilage cells. Flattened cells that lacked distinguishing features occupied areas between the clusters. Alkaline phosphate-enriched, ultrastructurally typical chondrocytes within the clusters were encased in a dense extracellular matrix that stained prominently for chondroitin sulfate proteoglycans. This matrix contained fibrils measuring 19 nm in diameter, which were associated with proteoglycan granules that preferentially bound ruthenium red. A progressive increase in the number of cells indicated the proliferation of certain elements in the primary culture. The cells in primary culture were biochemically as well as morphologically heterogeneous since they were found to synthesize type I and type II collagens. Homogeneous populations of redifferentiated chondrocytes were recovered as floating cells and were shown to express the chondrocyte phenotype in secondary culture. Subcultured cells synthesized type II collagen and its precursors almost exclusively and incorporated 35SO4 into proteoglycan monomer and aggregates to a greater degree than the cells in primary culture. The pattern of proteoglycan monomer and aggregate labeling resembled that of intact cartilage segments and bovine articular chondrocytes. Skin fibroblasts harvested from the same rat fetuses failed to proliferate when maintained under identical conditions. Hence, exogenous hormones, growth factors, and protein are not required for chondrocyte growth and maturation.

Calcium transport and cellular distribution in quiescent and serum-stimulated primary cultures of bone cells and skin fibroblasts

Primary cultures of bone cells and skin fibroblasts were examined for their Ca++ content, intracellular distribution and Ca++ fluxes. Kinetic analysis of 45Ca++ efflux curves indicated the presence of three exchangeable Ca++ compartments which turned over at different rates: a "very fast turnover" (S1), a "fast turnover" (S2, and a "slow turnover" Ca++ pool (S3). S1 was taken to represent extracellular membrane-bound Ca++, S2 represented cytosolic Ca++, and S3 was taken to represent Ca++ sequestered in some intracellular organelles, probably the mitochondria. Bone cells contained about twice the amount of Ca++ as compared with cultured fibroblasts. Most of this extra Ca++ was localized in the "slow turnover" intracellular Ca++ pool (S3). Serum activation caused the following changes in the amount, distribution, and fluxes of Ca++: (1) In both types of cells serum caused an increase in the amount of Ca++ in the "very fast turnover" Ca++ pool, and an increase in the rate constant of 45Ca++ efflux from this pool, indicating a decrease in the strength of Ca++ binding to ligands on cell membranes. (2) In fibroblasts, serum activation also caused a marked decrease in the content of Ca++ in the "slow turnover" Ca++ pool (S3), an increase in the rates of Ca++ efflux from the cells to the medium, and from S3 to S2, as well as a decrease in the rate of influx into S3. (3) In bone cells the amount of Ca++ in S3 remained high in "serum activated" cells, the rate of efflux from S3 to S2 increased, and the rate of influx into S3 also increased. The rate of efflux from the cells to the medium did not change. The results suggest specific properties of bone cells with regard to cell Ca++ presumably connected with their differentiation. Following serum activation we investigated the time course of changes in the amount of exchangeable Ca++ in bone cells and fibroblasts, in parallel with measurements of 3H-thymidine incorporation and cell numbers. Serum activation caused a rapid decrease in the content of cell Ca++ which was followed by a biphasic increase lasting until cell division.