In vitro induction of cartilage-specific macromolecules by a bone extract

Advanced Bioink for 3D Bioprinting of Complex Free-Standing Structures with High Stiffness

One of the challenges in 3D-bioprinting is the realization of complex, volumetrically defined structures, that are also anatomically accurate and relevant. Towards this end, in this study we report the development and validation of a carboxylated agarose (CA)-based bioink that is amenable to 3D printing of free-standing structures with high stiffness at physiological temperature using microextrusion printing without the need for a fugitive phase or post-processing or support material (FRESH). By blending CA with negligible amounts of native agarose (NA) a bioink formulation (CANA) which is suitable for printing with nozzles of varying internal diameters under ideal pneumatic pressure was developed. The ability of the CANA ink to exhibit reproducible sol-gel transition at physiological temperature of 37 °C was established through rigorous characterization of the thermal behavior, and rheological properties. Using a customized bioprinter equipped with temperature-controlled nozzle and print bed, high-aspect ratio objects possessing anatomically-relevant curvature and architecture have been printed with high print reproducibility and dimension fidelity. Objects printed with CANA bioink were found to be structurally stable over a wide temperature range of 4 °C to 37 °C, and exhibited robust layer-to-layer bonding and integration, with evenly stratified structures, and a porous interior that is conducive to fluid transport. This exceptional layer-to-layer fusion (bonding) afforded by the CANA bioink during the print obviated the need for post-processing to stabilize printed structures. As a result, this novel CANA bioink is capable of yielding large (5-10 mm tall) free-standing objects ranging from simple tall cylinders, hemispheres, bifurcated 'Y'-shaped and 'S'-shaped hollow tubes, and cylinders with compartments without the need for support and/or a fugitive phase. Studies with human nasal chondrocytes showed that the CANA bioink is amenable to the incorporation of high density of cells (30 million/mL) without impact on printability. Furthermore, printed cells showed high viability and underwent mitosis which is necessary for promoting remodeling processes. The ability to print complex structures with high cell densities, combined with excellent cell and tissue biocompatibility of CA bodes well for the exploitation of CANA bioinks as a versatile 3D-bioprinting platform for the clinical translation of regenerative paradigms.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

[Osteoinduction and -reparation]

Traumata, diseases, developmental deformities, and tumor resections frequently cause bone defects and atrophies. In general, three different mechanisms exist by which bone restoration can be achieved: (1) osteogenesis initiated by vital, osteoblastic cells of autografts; (2) osteoconduction (or creeping substitution); and (3) osteoinduction. The latter mechanism means the differentiation of pluripotent, mesenchymal-type cells (located in a recipient bed with strong regenerative capacity) into cartilage- and bone-forming progenitor cells under the influence of inductive bone morphogenetic proteins (BMPs). Some BMPs are physiologically included in low concentrations as organic components in bone tissue. They can diffuse from demineralized bone implants into the recipient bed and induce a differentiation into new bone tissue. Nine different BMPs have been isolated, characterized, and cloned. Some of these possess inductive properties and can initiate new bone formation in muscle tissue or in bone defects. In the future recombinant BMPs will be available in unlimited quantities. This will lead to completely new therapeutic concepts in reconstructive bone surgery.

Establishment of bone morphogenetic protein 2 responsive chondrogenic cell line

A clonal cell line named RMD-1 was established from the skeletal muscle of a 20-day fetal rat. RMD-1 represents a morphologically homogeneous population of undifferentiated mesenchymal cells, expressing alpha-smooth muscle actin and type I collagen, but no cartilage-associated genes. When cultured in agarose gel containing 100 ng/ml of recombinant human bone morphogenetic protein 2 (rhBMP-2; BMP-2), RMD-1 cells formed colonies and showed chondrocyte-like features as assessed by their ultrastructure, metachromatic staining with toluidine blue, and the production of large hydrodynamic-size proteoglycans. RMD-1 cells also differentiated into chondrocytes when the cells were plated at high density (over 2.5 x 10(5) cells/cm2) on type I collagen and incubated in medium containing 0.5% fetal bovine serum and 100 ng/ml of BMP-2. This chondrogenic differentiation was evidenced by a distinct morphological change into spherical cells, an increase in the levels of sulfated glycosaminoglycans, a decrease in type I collagen mRNA and the expression of cartilage-associated genes, including type II collagen, type IX collagen, aggrecan and alkaline phosphatase. In the presence of ascorbic acid and 10% serum, RMD-1 cells increased in size and expressed type X collagen as well as high alkaline phosphatase activity, then induced matrix mineralization. Thus, RMD-1 is a unique cell line that can differentiate from undifferentiated mesenchymal cells into hypertrophic chondrocytes.

Ectopic mineralized cartilage formation in human undifferentiated pancreatic adenocarcinoma explants grown in nude mice

Mineralized as well as nonmineralized cartilage-like structures enclosing cells resembling chondrocytes were found in human-derived undifferentiated but not in poorly differentiated pancreatic adenocarcinoma explants grown in nude mice. The structures reacted with anti-mouse IgG but not with antibodies against human cytokeratin 19, indicating that the newly formed tissue was of mouse origin. High activity of alkaline phosphatase was found in cell layers surrounding the structures and in cells embedded in the matrix. The extracellular matrix was strongly positive after Sirius red staining, reacted with anti-collagen type II antibodies, and the presence of proteoglycans was demonstrated with Alcian blue staining and by metachromasia after Giemsa staining. Electron microscopic inspection revealed the presence of bundles of both thick collagenous fibrils with low levels of fine filamentous material and thin collagenous fibrils with high concentrations of filamentous components. The majority of both types of matrices was found to be partially or completely calcified. The mean area density of the cartilage-like structures in the undifferentiated tumors was 0.31%. The frequent formation of the cartilage-like structures in the rapidly growing undifferentiated explants and its absence in the slowly growing, more differentiated explants suggest that low oxygen tensions in combination with altered levels of growth factors, such as members of the transforming growth factor beta superfamily, create conditions that induce differentiation of fibroblasts to chondrocytes. It is concluded that these human tumors grown in nude mice can be used as an in vivo model to study ectopic formation of mineralized cartilage.

Chondrogenesis of neural crest cells: effect of poly-L-lysine and bone extract

The mechanisms of chondrogenic differentiation are generally studied in vitro by analyzing the action of agents that promote or affect chondrogenesis in embryonic mesenchyme, such as cells of the embryonic limb bud. However, it is not known whether progenitor cells of the craniofacial skeleton, which are of a different embryonic origin and derived from the neural crest, are similarly responsive to such agents. To gain insight into the regulation of chondrogenic differentiation in cells derived from neural crest, we have treated chick embryonic neural crest explants in vitro with poly-L-lysine (PL, M(r) 380 kDa) or bovine bone extract (BBE), two agents known to enhance chondrogenesis of limb mesenchymal cells. Both cephalic (normally chondrogenic) and trunk (normally nonchondrogenic) neural crest cells were analyzed. Chondrogenic differentiation was determined by histological, immunohistochemical and autoradiographic methods. Our results indicate that both PL (380 kDa) and BBE significantly enhance chondrogenesis of cephalic neural crest cells, suggesting that the mechanism of chondrogenesis of these ectodermally derived cells is similar to that of mesodermally derived limb mesenchymal cells. However, trunk neural crest cells did not undergo chondrogenesis in response to PL or BBE. These data show that chondrogenesis can be enhanced in cranial ectodermal neural crest cells in a manner similar to that in the limb mesenchyme. However, since nonchondrogenic trunk neural crest cells are not responsive, an inherent potential for cartilaginous differentiation is necessary for exogenous stimulation of chondrogenesis.

Effect of demineralized bone powder on osteoblast-like cells in culture. A potential rapid quality control assay

Demineralized bone powder (DBP) has been shown to induce osteogenesis in a variety of bone defects and extra-osseous sites. Previous investigations have been carried out in animal models which are time-consuming and expensive. We studied the effect of DBP on well-established populations of osteoblast and non-osteoblast-like cells in culture to establish an inexpensive, efficient and reliable assay for bone induction. DBP and BP (non-demineralized powder), of particle size 38-53 microns, were prepared from rat long bones. ROS (rat osteosarcoma) 17/2.8 and ROS 24/1 cell lines were subcultured weekly. For both 17/2.8 (well differentiated) and 24/1 (poorly differentiated) cells, proliferation, i.e. cell count, was significantly greater in DBP enriched medium when compared with control or medium with BP. Cell counts for wells with BP were no different from controls. The increased cell count in DBP-enriched medium was significant on days 2-5 (peak effect 2-3 days). Alkaline phosphatase production reached peak levels after day 3 when proliferation was beginning to taper off. In this study a consistent increase in osteoblast proliferation and alkaline phosphatase production under the influence of DBP was demonstrated. The tissue culture assay for proliferation must now be correlated with bone induction in vivo. In future, the method may be useful for investigating the mechanism of bone induction.

Chondrocyte-like colony formation of mesenchymal cells by dentin extracts in agarose gel culture

In this study, the effects of guanidine extracts from demineralized bovine dentin matrix on rat mesenchymal cells were investigated by use of an agarose gel culture. The dentin extracts were divided into water-soluble and -insoluble fractions. Rat mesenchymal cells obtained from the cultivation of skeletal muscle tissue and embedded in agarose gel were treated with these two fractions. After three weeks of cultivation, the treated cells formed colonies that were stained metachromatically with toluidine blue in a dose-dependent manner. The activity necessary to form chondrocyte-like colonies by the water-insoluble fraction was significantly higher than that by the water-soluble fraction. Each chromatographic fraction of the water-insoluble part of dentin extracts on tandem Sephacryl S-200 High-resolution columns was also investigated. Chondrocyte-like colony-forming activity was concentrated in a single fraction. However, the electrophoretic pattern of this fraction revealed that there were still some bands of molecular weight between 18 and 30 kDa. According to the Western blot analysis of this fraction, there was a band corresponding to purified transforming growth factor beta (TGF-beta) under the non-reducing condition. After reduction, this band disappeared and we found a band corresponding to a component of 13 kDa as well as TGF-beta. These findings suggest that TGF-beta is present not only in bone but also in the dentin matrix.

Characterization of bone-derived chondrogenesis-stimulating activity on embryonic limb mesenchymal cells in vitro

Demineralized bone matrix contains factors which stimulate chondrogenesis and osteogenesis in vivo. A water-soluble extract of bone has been shown to stimulate chondrogenesis in vitro in embryonic limb mesenchymal cells (Syftestad, Lucas & Caplan, 1985). The aim of this study was to analyse the cellular mechanism of the bone-derived chondrogenesis-stimulating activity, with particular attention on how normal requirements for chondrogenesis may be altered. The effects of bovine bone extract (BBE) on chondrogenesis in vitro were studied using micromass cultures of chick limb bud mesenchyme isolated from embryos at Hamburger-Hamilton (HH) stage 23/24, an experimental system which is capable of undergoing chondrogenic differentiation. Bovine diaphyseal long bones were demineralized and extracted with guanidine-HCl to prepare BBE (Syftestad & Caplan, 1984). High-density mesenchyme cultures (30 x 10(6) cells/ml) were exposed to different doses of BBE (0.01-1.0 mg ml-1) and chondrogenesis was quantified based on cartilage nodule number and [35S]sulphate incorporation. BBE was tested on micromass cultures of varying plating densities (2-30 x 10(6) cells/ml), on cultures of 'young' limb bud cells (HH stage 17/18), and on cultures enriched with chondroprogenitor cells obtained from subridge mesoderm. Since poly-L-lysine (PL) has recently been shown (San Antonio & Tuan, 1986) to promote chondrogensis, PL and BBE were introduced together in different doses, in the culture medium, to determine if their actions were synergistic. Our results show that BBE stimulates chondrogenesis in a dose-dependent manner and by a specific, direct action on the chondroprogenitor cells but not in normally non-chondrogenic, low density or 'young' limb bud cell cultures. The effects of PL and BBE are additive and these agents appear to act by separate mechanisms to stimulate chondrogenesis; PL primarily enhances nodule formation, and BBE appears to promote nodule growth.

Matrix vesicles as a marker of endochondral ossification

Endochondral ossification in bone development and repair, and in induced bone formation in mesenchymal tissues, involves recruitment of mesenchymal cells, their differentiation into chondrocytes, and calcification of the cartilagenous matrix. Stimulation of proteoglycan synthesis is used as a biochemical marker of chondrogenesis, however it does not distinguish among chondrogenic phenotypes. Chondrocytes derived from the resting zone and adjacent growth zone cartilage of the costochondral junction of young rats, produce matrix vesicles in culture which are enriched in alkaline phosphatase specific activity with respect to the plasma membrane. Matrix vesicles isolated from cultures of neonatal rat muscle mesenchymal cells are not enriched in this enzyme activity. Alkaline phosphatase in matrix vesicles produced by growth zone chondrocytes is stimulated by 1,25(OH)2D3; enzyme in matrix vesicles produced by resting zone chondrocytes is stimulated by 24,25(OH)2D3; enzyme in matrix vesicles isolated from mesenchymal cell cultures is responsive to neither metabolite. Matrix vesicle phospholipase A2 is stimulated by 1,25(OH)2D3 in growth zone chondrocytes cultures; inhibited by 24,25(OH)2D3 in resting zone chondrocyte cultures; and is unaffected by either metabolite in mesenchymal cell cultures. These observations suggest that matrix vesicle production, as defined by alkaline phosphatase enrichment, and responsiveness of matrix vesicle enzymes to vitamin D metabolites, can be used as markers of phenotypic maturation during chondrogenesis in vivo and in vitro.

Bovine tooth-derived bone morphogenetic protein

Eight groups of dental tissues were mechanically dissected from the mandibles of one-year-old steers; they were then defatted and decalcified in HCl. The noncollagenous proteins were extracted with various solvents from collections of tissue and bio-assayed for osteo-inductive activity. Collectively, the hard tissue (dentin, enamel, and cementum) noncollagenous proteins were fractionated by molecular sieve chromatography, hydroxyapatite affinity chromatography, and ion exchange chromatography. Osteo-inductive activity of each protein fraction was determined by implantation in the quadriceps muscle pouch of mice. The quantity of bone was measured by computerized image analysis. From 71% to 83% of 41 implants of dental hard tissues induced bone formation. The quantity of bone was greater from unerupted than from erupted teeth. Dental soft tissues that had no osteo-inductive activity were rich in a 14-kDa protein, presumably matrix gamma-carboxyglutamic acid-rich proteins. Proteins with Mr of from 15 to 28 kDa were associated with osteo-inductive activity. Components with Mr greater than 28 kDa had no activity. These observations suggest that bovine teeth have a selection of osteo-inductive proteins that is comparable in range of MW to bovine bone morphogenetic protein.

Ectopic induction of cartilage and bone by water-soluble proteins from bovine bone using a collagenous delivery vehicle

A controlled-release delivery vehicle for water-soluble osteogenic proteins from demineralized bone matrix was constructed using purified type I collagen. The water-soluble proteins were isolated from a 4 M GdnHCl extract of bone matrix. Although the water-soluble proteins were capable of inducing cartilage formation in vitro, they were incapable of inducing cartilage or bone in vivo when implanted intramuscularly into mice in the absence of an appropriate delivery vehicle. The collagen-based delivery vehicle alone was also incapable of inducing osteogenesis in vivo. However, when the water-soluble proteins were incorporated into the delivery vehicle, the combination was capable of inducing cartilage and bone 76% of the time. These results demonstrate that it is possible to formulate a controlled-release delivery vehicles for soluble bioactive factors which upon release interact with local responsive cells.

Transforming growth factor-beta s are equipotent growth inhibitors of interleukin-1-induced thymocyte proliferation

The effects of two forms of transforming growth factor-beta, TGF-beta 1 and TGF-beta 2, upon the proliferative response of murine thymocytes were investigated in this study. TGF-beta 1 and TGF-beta 2 were found to be equipotent growth inhibitors of interleukin-1 (IL-1)- and phytohemagglutinin (PHA)-stimulated thymocytes when added at the initiation of the cultures. These factors suppressed the proliferative response in a dose-dependent fashion between 0.4 and 100 pM. The proliferative response was maximally inhibited (90% inhibition) at 100 pM. The half-maximal inhibitory dose (ID50) was 6 and 4 pM for TGF-beta 1 and TGF-beta 2, respectively. These factors were less effective or ineffective at suppressing the proliferation of thymocytes which had been prestimulated for 24 to 48 hr by IL-1 and PHA. Neither factor inhibited interleukin-2 (IL-2)-dependent thymocyte proliferation or the proliferation of an IL-2-dependent cytotoxic T cell line (CTL-L), suggesting that the anti-proliferative actions of these factors was by inhibition of cellular events triggered by IL-1. Furthermore, anti-TGF-beta 1 antibodies did neutralize the biological actions of TGF-beta 1 and these antibodies did block the binding of 125I-labeled TGF-beta 1 to cell surface receptors showing that the inhibitory action is mediated through specific receptors for TGF-beta 1 on thymocytes. These antibodies, however, did not neutralize the anti-proliferative action of TGF-beta 2. Although TGF-beta 1 and TGF-beta 2 exhibit very similar biological activities, these molecules are antigenically different and, therefore, have different tertiary structures.

A water-soluble fraction from adult bone stimulates the differentiation of cartilage in explants of embryonic muscle

A water-soluble fraction of a 4 M guanidine HCl extract of demineralized adult bovine bone stimulated the differentiation of cartilage in explants of minced skeletal muscle from embryonic chick legs; cartilage was also induced by a semipurified protein preparation. Cartilage could be identified in treated cultures at 1 week with muscle from day-9 embryos, not before 2 weeks with muscle from day-12 embryos, and not before 3 weeks with muscle from day-19 embryos. The ability to respond to this water-soluble fraction by exhibiting cartilage differentiation was dose-dependent, but not confined to any particular muscle region of the day-12 embryonic leg. These observations indicate that bone-derived soluble chondroinductive agents act on cells in minced embryonic muscle preparations. The induction of cartilage is dependent upon the accessibility of the responding cells to the agents, on the concentration of inductive agents, and on the developmental age of the responsive tissue.

Osteogenesis in rats with an inductive bovine composite

Subcutaneous (S.C.) implantation of allogeneic demineralized bone matrix in rats results in endochondral bone formation. In contrast, implants of bovine demineralized bone matrix in rat S.C. tissue show inconsistent cartilage and bone formation, presumably due to an intense inflammatory reaction at the implant site. To overcome this response, a partially purified bone inducing extract was prepared from bovine bone by a series of steps that included demineralization, guanidine/HCl extraction, gel filtration, and cation exchange chromatography. To develop a carrier, the inactive guanidine/HCl-extracted matrix was then trypsinized to remove the inflammatory and immunogenic components, thus yielding a predominantly collagenous matrix. Bovine composites were prepared by combining different amounts of the bone inducing extract with a carrier that consisted of the trypsinized bone matrix and purified soluble bovine dermal collagen. Subcutaneous implantation of the composite preparation resulted in dose-dependent endochondral bone formation in rats. The inductive activity and the low-level inflammatory response were comparable to allogeneic implants.

Chondrogenesis and osteogenesis of bone marrow-derived cells by bone-inductive factor

Rat bone marrow cells were intraperitoneally implanted within a diffusion chamber with a decalcified bone matrix or a 4 M guanidine hydrochloride extracted matrix (G-res) as control. The chamber was harvested after 28 days and soft X-ray photography, histological examination, determination of alkaline phosphatase activity and calcium content were performed. With the decalcified bone matrix, cartilage and bone formation was observed and both alkaline phosphatase activity and calcium content were significantly higher than those in control chambers. Each chromatographic fraction on Sephacryl S-200 of the 4 M guanidine hydrochloride extract (G-ext) from the decalcified bone matrix was reconstituted with G-res and implanted either subcutaneously or intraperitoneally within a diffusion chamber with marrow cells. Intrachamber or subcutaneous cartilage and bone formation was detected by only one chromatographic fraction. When marrow-derived fibroblast-like cells were implanted intraperitoneally within a diffusion chamber with a decalcified bone matrix, cartilage and bone formation was detected, which was not the case with G-res. These results suggest that a certain factor, probably bone morphogenetic protein, which induces ectopic bone formation, allows marrow cells to differentiate into bone and cartilage tissues and there may exist so-called "inducible osteoprogenitor cells" in the marrow-derived fibroblast-like cell preparation.

Chemotactic response of embryonic limb bud mesenchymal cells and muscle-derived fibroblasts to transforming growth factor-beta

Transforming growth factor beta (TGF-beta) was tested for its ability to stimulate a chemotactic response in Stage 24 embryonic chick limb bud mesenchymal cells and muscle-derived fibroblasts. TGF-beta stimulated dose-dependent chemotaxis in both cell populations. Maximal chemotaxis was achieved with a concentration of 5 ng/ml for limb bud cells and as low as 15 pg/ml for muscle-derived fibroblasts. TGF-beta was not chemokinetic at these levels. Several other proteins found in bone, namely fibronectin, type I collagen, and osteonectin, were not chemotactic. However, both Bone Gla-protein and basic-FGF were found to be chemotactic but less effective than TGF-beta. Comparison with extracts of adult bone indicates that while TGF-beta is a potent chemoattractant, it does not account for all the chemotactic activity found in adult bone.

Initiation and enhancement of bone formation. A review

Knowledge of some of the fundamental biochemical factors that may influence the initiation and continued growth of bone-forming cell lines is presented. The discussion is limited to those factors shown experimentally to be present locally in bone tissue and synthesized in the environment of bone-forming cells. The current state of knowledge of basic research findings on osteogenic factors is given in detail. Cooperative actions of these locally produced and systemic factors are the primary stimuli that result in increased bone growth and volume.

Packaging and delivery of bone induction factors in a collagenous implant

Extracts of demineralized rat bones which contained factors stimulating bone induction were reconstituted with highly purified human type I collagen to provide a suitable and easily manipulated delivery system for surgical implantation. When implanted subcutaneously in rats, the implants governed and delineated the dimensions of the resulting bony tissue. It is proposed that this implant system has clinical application in the filling of osseous defects within the scope of orthopaedic and oral and maxillofacial surgery. It is presented here as a potential improvement over conventional implant materials without osteoinductive properties.

Enhancement by extracts of mineralized tissues of protein production by human gingival fibroblasts in vitro

Non-confluent cell cultures were exposed to both guanidine and guanidine-EDTA extracts of cementum, dentine and alveolar bone, at concentrations from 2 to 50 micrograms/ml for 48 h. The cells were radioactively labelled during the last 24 h. Total protein production was measured via incorporation of radioactive proline; collagen production was estimated by digestion of the radioactive protein mixture with bacterial collagenase. All guanidine-EDTA extracts elicited statistically-significant increases in total protein production when compared to controls. At 50 micrograms/ml of extract, the increase in protein production was 340, 143 and 338 per cent for bone, cementum and dentine, respectively. Similar results were obtained for collagen production. Guanidine-EDTA extracts also stimulated an increase in the production of specific proteins, as ascertained by gel electrophoresis. In contrast, the guanidine extracts had no effect on either protein or collagen production. Thus the functions of gingival fibroblasts can be altered by proteins from associated mineralized tissues. Identification of such proteins and their biological functions would enhance knowledge of the mechanisms that regulate connective-tissue regeneration.

Modulation of the rabbit chondrocyte phenotype by retinoic acid terminates type II collagen synthesis without inducing type I collagen: the modulated phenotype differs from that produced by subculture

The differentiated phenotype of rabbit articular chondrocytes can be characterized by the synthesis of high levels of cartilage specific proteoglycan and collagen (type II). Treatment of these cells in primary monolayer culture for periods of up to 18 days with 0.03 to 3.0 micrograms/ml retinoic acid (RA) resulted in suppression of colony formation, altered morphology, and decreased (eightfold) proteoglycan and collagen synthesis. With the exception of collagen synthesis, these changes were complete with all doses after 4 days of treatment. Collagen synthesis declined more slowly; it was dose dependent after 4 days and maximally inhibited by all doses by 9 days. Detailed analysis of the collagen phenotype was performed using SDS-PAGE of intact chains and 2-D CNBr peptide analysis. RA caused cessation of type II synthesis, and transient stimulation of type III and type I trimer collagen synthesis, without induction of type I collagen. Essentially identical results were obtained with retinol. The resultant collagen phenotype differed significantly from the type I-containing phenotype induced by subculture. Thus, suppression of this differentiated program did not elicit a common modulated phenotype. The results are discussed in the context of direct and indirect mechanisms of RA-dependent modulation of chondrocyte gene expression.

Differentiation of rat mesenchymal cells by cartilage-inducing factor. Enhanced phenotypic expression by dihydrocytochalasin B

The role of cell shape in chondrogenesis was studied by using rat mesenchymal cells cultured with cartilage-inducing factor (CIF). Here we report that enhanced expression of chondroblastic markers by induced cells was attained by culturing cells in monolayer in the presence of dihydrocytochalasin B (DHCB). This effect was optimal at 3 microM DHCB and was apparent after 3 days in culture. Mesenchymal cells cultured with DHCB alone exhibited no detectable increase in cartilage proteoglycan synthesis, whereas cells cultured with 3 microM DHCB and 0.1 nM CIF showed a 4-5 fold increase in proteoglycan synthesis. When cells were cultured with CIF alone on plastic, only small increases in proteoglycan synthesis were observed. Cells cultured with CIF in monolayer and then transferred to a permissive environment (either agarose or cultured with DHCB) showed enhanced synthesis of chondroblastic proteins. These results suggest that expression, but not induction, of a chondroblastic phenotype by CIF is inhibited by growth in monolayer. The altering of cell shape with DHCB releases that inhibition.

Induction of chondrogenesis in muscle, skin, bone marrow, and periodontal ligament by demineralized dentin and bone matrix in vivo and in vitro

Induction of chondrogenesis in vivo by rolls of demineralized dentin implanted in muscle, subcutaneous connective tissue of skin, medullary cavity of femur, and periodontal ligament of rat was investigated. Specimens were examined at various times up to 21 days after implantation, using light microscopy and morphometric analysis. Induction of cartilage occurred most quickly in muscle, followed by subcutaneous connective tissue of skin and medullary cavity of femur, and most slowly in periodontal ligament. Significantly more cartilage was found in muscle than in subcutaneous connective tissue of skin and medullary cavity of femur at the times examined, and least of all in periodontal ligament. Outgrowth of cells from rat muscle, dermis and subcutaneous tissue, bone marrow and periodontal ligament cultured in vitro on demineralized bone matrix for up to 35 days produced similar results.

Partial isolation and characterization of a chemotactic factor from adult bovine bone for mesenchymal cells

Demineralized adult bone matrix has the capacity to initiate de novo ectopic endochondral bone formation 2-3 weeks following intramuscular implantation into suitable hosts. An early step in this process is the migration of mesenchymal cells to the implant site; these cells later differentiate into cartilage and bone. Adult bone has been shown to contain a number of bioactive factors, such as chemotactic factors for various cell types, including osteoblasts. We have used embryonic chick limb bud mesenchymal cells to construct an in vitro assay for testing chemotactic activity derived from bone matrix extracts. With a modified Boyden chamber, water-soluble components from a 4 M guanidinium chloride extract of demineralized adult bovine bone matrix were found to stimulate the directional migration of these chick embryonic limb bud mesenchymal cells as well as embryonic muscle-derived fibroblasts and cells derived from embryonic skin. The chemotactic activity was destroyed by treatment with heat (100 degrees C) or trypsin. Partial purification by molecular sieve chromatography suggested that the chemotactic factor(s) has a molecular weight of between 50,000 and 90,000. This factor can be separated from bone matrix-derived chondrogenic stimulating activity by either ion exchange or molecular sieve chromatography. These observations confirm that bone matrix contains a chemoattractant for mesenchymal cells that may be important for in vivo recruitment of cells as part of the process of ectopic bone formation or in cases of bone repair.

The in vitro chondrogenic response of limb-bud mesenchyme to a water-soluble fraction prepared from demineralized bone matrix

Demineralized adult bone matrix initiates de novo ectopic endochondral ossification 2-3 weeks following its intramuscular implantation into adult animals. This phenomenon appears to be similar, in some ways, to inductive cell-matrix interactions which regulate cartilage and bone formation during development. In the present study, we used embryonic chick limb-bud mesenchymal-cell cultures to bioassay extracts of demineralized bone matrix for chondrogenic activity. Guanidinium-chloride (4 M) extracts of demineralized bovine bone were dialyzed against buffers of decreasing ionic strength and then cold water. The cold-water-soluble fraction was found to stimulate chondrogenesis in intermediate-density limb-bud cell cultures (2.2 X 10(6) cells per 35-mm dish), as revealed by visual inspection with phase optics, toluidine-blue staining of fixed plates, and [35S] sulfate incorporation in the cell layer. Further fractionation of this material by anion-exchange, carbohydrate-affinity, and molecular-sieve chromotography produced a semipurified preparation possessing chondrogenic-stimulating activity at doses ranging from 3 to 10 micrograms/ml. The in vitro chondrogenic response of limb-bud mesenchymal cells was dose-dependent, required a minimal initial plating density of 2.08 X 10(5) cells/mm2 of culture dish, and developed gradually over 8-10 days. At an optimal dose of extract, a continuous exposure period of at least 2-3 days was necessary to produce detectable chondrogenic stimulation. In addition, the amount of cartilage formed following an 8-day exposure was markedly influenced by the culture 'age' of the mesenchymal cells (i.e., the time between plating and the start of treatment).(ABSTRACT TRUNCATED AT 250 WORDS)

Chondrogenesis in agarose gel culture. A model for chondrogenic induction, proliferation and differentiation

An in vitro model has been developed to study chondrogenic induction, proliferation and differentiation. Embryonic rat mesenchymal cells isolated from muscle and embedded in agarose were treated with a partially purified extract from bovine demineralized bone powder. Treated cells proliferated and synthesized matrix similar to differentiated chondrogenic cells in a dose-dependent manner. By employing an enzyme-linked immunosorbent assay (ELISA), cartilage-specific proteoglycan and type II collagen synthesis were quantitated. Of the cells tested, only embryonic mesenchymal cells from muscle responded to bone extract. Proteoglycan synthesis was sensitive to type of medium and cell density.

Purification and characterization of two cartilage-inducing factors from bovine demineralized bone

Two naturally occurring peptides that induce chondrogenesis in culture have been purified to apparent homogeneity. These cartilage-inducing factors (CIF-A and CIF-B) were isolated from bovine demineralized bone by dissociative extraction, gel filtration, cation-exchange chromatography, and reversed-phase HPLC. CIF-A and CIF-B at concentrations of 1-10 ng/ml each induce embryonic rat mesenchymal cells in culture to assume a cartilage morphology and synthesize cartilage-specific proteoglycan and type II collagen. The amino acid compositions of CIF-A and CIF-B are similar but not identical. Both factors have an apparent Mr of 26,000, as determined by NaDodSO4/PAGE. In the presence of 2-mercaptoethanol, both are converted to species of about one-half that Mr, indicating that they are dimers of identical or very similar chains.

Osteoarthritis. Emerging evidence for cell interactions in the breakdown and remodeling of cartilage

Interactions among the cells of joint components appear to contribute to the dynamic events associated with breakdown of the cartilage matrix and remodeling of the articular surface in osteoarthritis. The mediator interleukin-1 may be one of the stimuli that induce release of hydrolytic factors from synovial membrane cells and chondrocytes, which contribute to the degradation of collagen and proteoglycans in the cartilage matrix. The remodeling process is associated with chondrocyte proliferation and vascular and cellular ingrowth from subchondral bone with osteophyte formation. It is postulated that growth factors isolated from cartilage and bone may contribute to repair and remodeling of the articular end of the joint.

An electron microscopic demonstration of induction of chondrogenesis in neonatal rat muscle outgrowth cells in monolayer cultures

Second passage fibroblast-like cells grown from explants of neonatal rat muscle continue to demonstrate fibroblast-like properties for many days when cultured on plastic surfaces. Such cells can be induced to change to a chondrocyte-like mode of expression by the addition of effector materials prepared from bovine cortical bone decalcified with 0.6 N HCl. Other studies show that similar demineralized bone particles and extracts from them have, in vivo, osteoinductive properties. Optimum conditions for this differentiation in monolayer culture were found in the use of 2% fetal calf serum with Dulbecco's modified Eagles medium. At 10% fetal calf serum the chondrogenic changes could not be detected. Light microscopy showed a sequence of morphological changes, after 36 h in culture, which resembled those seen at the beginning of osteogenesis in vivo. Induced cultures showed abundant extracellular proteoglycan production. Isotope incorporation studies showed stimulation of glycosaminoglycan synthesis in response to effector materials in soluble form. Type II collagen could be detected after three days. Electron microscopic analysis of induced and control cultures showed unequivocal evidence for marked production of an extensive extracellular matrix in the region of effector particles. The cells themselves change shape and develop an abundant system of lysosome-like vesicles and a very active, highly engorged endoplasmic reticulum and Golgi apparatus. After nine days in culture, evidence for the formation of a ruthenium red stained structure on the surface of the cells in contact with inductive particles, was observed. The simple monolayer culture system described provides a direct means by which the presence of active chondrogenic fractions may be assessed, and in which the mechanism of action of the effectors can be studied.

An osteo-inductive bone matrix extract stimulates the in vitro conversion of mesenchyme into chondrocytes

Urea and guanidine extracts of demineralized beef and rabbit bone matrix were assayed both in vivo and in vitro. One month following intramuscular implantation into mouse thighs, these extracts induced ectopic cartilage and bone. Seven days following continuous in vitro exposure to the same extracts, stage 24 chick limb bud mesenchymal cells in cultures had differentiated into greater numbers of chondrocytes than controls. These results suggest the feasibility of using limb bud mesenchymal cell cultures as an in vitro assay for bone matrix derived, extractable bioactive factors which effect the conversion of mesenchymal cells into chondrocytes as a requisite step in in vivo osteogenesis.

In vitro transformation of mesenchymal cells derived from embryonic muscle into cartilage in response to extracellular matrix components of bone

Subcutaneous implantation of demineralized diaphyseal bone matrix into rats induces cartilage and bone formation in vivo. When minced skeletal muscle is cultured on hemicylinders of demineralized bone in vitro, mesenchymal cells are transformed into chondrocytes. In the present investigation, the potential of extracellular matrix components of bone to trigger cartilage differentiation in vitro was examined. Extraction of bone hemicylinders with 6 M guanidine X HCl resulted in the absence of chondrogenesis in vitro and endochondral bone formation in vivo. Biologically inactive hemicylinders of bone were then reconstituted with the guanidine extract and also with partially purified components extracted from bone matrix and bioassayed. Reconstitution completely restored the ability to elicit chondrogenesis in vitro and endochondral bone differentiation in vivo. Reconstitution of the whole guanidine extract on Millipore filters coated with gels of tendon collagen (type I) and subsequent culture with minced skeletal muscle also resulted in cartilage induction in vitro. These observations show that the extracellular matrix of bone is a repository of factors that govern local cartilage and bone differentiation.