Effects of cell density on the expression of differentiation by chick embryo chondrocytes

Chondrogenic potential of mesenchymal stem cells from horses using a magnetic 3D cell culture system

BACKGROUND

Mesenchymal stem cells (MSCs) represent a promising therapy for the treatment of equine joint diseases, studied due to their possible immunomodulatory characteristics and regenerative capacity. However, the source of most suitable MSCs for producing cartilage for regenerative processes in conjunction with biomaterials for an enhanced function is yet to be established.

AIM

To compare the chondrogenicity of MSCs derived from synovial fluid, bone marrow, and adipose tissue of horses, using the aggrecan synthesis.

METHODS

MSCs from ten horses were cultured, phenotypic characterization was done with antibodies CD90, CD44 and CD34 and were differentiated into chondrocytes. The 3D cell culture system in which biocompatible nanoparticles consisting of gold, iron oxide, and poly-L-lysine were added to the cells, and they were forced by magnets to form one microspheroid. The microspheroids were exposed to a commercial culture medium for 4 d, 7 d, 14 d, and 21 d. Proteoglycan extraction was performed, and aggrecan was quantified by enzyme-linked immunosorbent assay. Keratan sulfate and aggrecan in the microspheroids were identified and localized by immunofluorescence.

RESULTS

All cultured cells showed fibroblast-like appearance, the ability to adhere to the plastic surface, and were positive for CD44 and CD90, thus confirming the characteristics and morphology of MSCs. The soluble protein concentrations were higher in the microspheroids derived from adipose tissue. The aggrecan concentration and the ratio of aggrecan to soluble proteins were higher in microspheroids derived from synovial fluid than in those derived from bone marrow, thereby showing chondrogenic superiority. Microspheroids from all sources expressed aggrecan and keratan sulfate when observed using confocal immunofluorescence microscopy. All sources of MSCs can synthesize aggrecan, however, MSCs from synovial fluid and adipose tissue have demonstrated better biocompatibility in a 3D environment, thus suggesting chondrogenic superiority.

CONCLUSION

All sources of MSCs produce hyaline cartilage; however, the use of synovial liquid or adipose tissue should be recommended when it is intended for use with biomaterials or scaffolds.

Treatment with Y-27632, a ROCK Inhibitor, Increases the Proinvasive Nature of SW620 Cells on 3D Collagen Type 1 Matrix

The concept of using tissue density as a mechanism to diagnose a tumor has been around for centuries. However, this concept has not been sufficiently explored in a laboratory setting. Therefore, in this paper, we observed the effects of cell density and extracellular matrix (ECM) density on colon cancer invasion and proliferation using SW620 cells. We also attempted to inhibit ROCK-I to determine its effect on cell invasion and proliferation using standard molecular biology techniques and advanced imaging. Increasing cell seeding density resulted in a 2-fold increase in cell invasion as well as cell proliferation independent of treatment with Y-27632. Increasing collagen I scaffold density resulted in a 2.5-fold increase in cell proliferation while treatment with Y-27632 attenuated this effect although 1.5 fold increase in cell invasion was observed in ROCK inhibited samples. Intriguingly, ROCK inhibition also resulted in a 3.5-fold increase in cell invasion within 3D collagen scaffolds for cells seeded at lower densities. We show in this paper that ROCK-I inhibition leads to increased invasion within 3D collagen I microenvironments. This data suggests that although ROCK inhibitors have been used clinically to treat several medical conditions, its effect largely depends on the surrounding microenvironment.

Polymer scaffolds fabricated with pore-size gradients as a model for studying the zonal organization within tissue-engineered cartilage constructs

The zonal organization of cells and extracellular matrix (ECM) constituents within articular cartilage is important for its biomechanical function in diarthroidal joints. Tissue-engineering strategies adopting porous three-dimensional (3D) scaffolds offer significant promise for the repair of articular cartilage defects, yet few approaches have accounted for the zonal structural organization as in native articular cartilage. In this study, the ability of anisotropic pore architectures to influence the zonal organization of chondrocytes and ECM components was investigated. Using a novel 3D fiber deposition (3DF) technique, we designed and produced 100% interconnecting scaffolds containing either homogeneously spaced pores (fiber spacing, 1 mm; pore size, about 680 microm in diameter) or pore-size gradients (fiber spacing, 0.5-2.0 mm; pore size range, about 200-1650 microm in diameter), but with similar overall porosity (about 80%) and volume fraction available for cell attachment and ECM formation. In vitro cell seeding showed that pore-size gradients promoted anisotropic cell distribution like that in the superficial, middle, and lower zones of immature bovine articular cartilage, irrespective of dynamic or static seeding methods. There was a direct correlation between zonal scaffold volume fraction and both DNA and glycosaminoglycan (GAG) content. Prolonged tissue culture in vitro showed similar inhomogeneous distributions of zonal GAG and collagen type II accumulation but not of GAG:DNA content, and levels were an order of magnitude less than in native cartilage. In this model system, we illustrated how scaffold design and novel processing techniques can be used to develop anisotropic pore architectures for instructing zonal cell and tissue distribution in tissue-engineered cartilage constructs.

Phenotypic analysis of bovine chondrocytes cultured in 3D collagen sponges: effect of serum substitutes

Repair of damaged cartilage usually requires replacement tissue or substitute material. Tissue engineering is a promising means to produce replacement cartilage from autologous or allogeneic cell sources. Scaffolds provide a three-dimensional (3D) structure that is essential for chondrocyte function and synthesis of cartilage-specific matrix proteins (collagen type II, aggrecan) and sulfated proteoglycans. In this study, we assessed porous, 3D collagen sponges for in vitro engineering of cartilage in both standard and serum-free culture conditions. Bovine articular chondrocytes (bACs) cultured in 3D sponges accumulated and maintained cartilage matrix over 4 weeks, as assessed by quantitative measures of matrix content, synthesis, and gene expression. Chondrogenesis by bACs cultured with Nutridoma as a serum replacement was equivalent or better than control cultures in serum. In contrast, chondrogenesis in insulin-transferrin-selenium (ITS(+3)) serum replacement cultures was poor, apparently due to decreased cell survival. These data indicate that porous 3D collagen sponges maintain chondrocyte viability, shape, and synthetic activity by providing an environment favorable for high-density chondrogenesis. With quantitative assays for cartilage-specific gene expression and biochemical measures of chondrogenesis in these studies, we conclude that the collagen sponges have potential as a scaffold for cartilage tissue engineering.

Effects of cell density on proliferation and matrix synthesis of chondrocytes embedded in atelocollagen gel

The effects of cell density on the proliferation and chondroitin sulfate synthesis of chondrocytes embedded in Atelocollagen gel were examined. Chondrocytes of 21 10-week-old Japanese white rabbits isolated by collagenase digestion were embedded in Atelocollagen gel and cultured in Dulbecco's modified Eagles medium at cell densities of 2 x 105 cells/ml (105 group), 2 x 106 cells/ml (106 group), and 2 x 107 cells/ml (107 group) for 4 weeks. Chondrocytes in the 105 group gradually proliferated more than the other two groups. In contrast, most chondrocytes in the 107 group showed increased capability to produce chondroitin 6-sulfate. Cartilage-like tissue was produced from high-density cultures (107 cells/ml), although a decrease in cell number was seen. Even in three-dimensional cultures, the proliferation and chondroitin sulfate synthesis of chondrocytes were influenced by the cell density. These results are informative for the clinical application of chondrocyte transplantation in three-dimensional cultures for cartilage repair.

Chondrogenic potential of skeletal cell populations: selective growth of chondrocytes and their morphogenesis and development in vitro

Most vertebrate embryonic and post-embryonic skeletal tissue formation occurs through the endochondral process in which cartilage serves a transitory role as the anlage for the bone structure. The differentiation of chondrocytes during this process in vivo is characterized by progressive morphological changes associated with the hypertrophy of these cells and is defined by biochemical changes that result in the mineralization of the extracellular matrix. The mechanisms, which, like those in vivo, promote both chondrogenesis in presumptive skeletal cell populations and endochondral progression of chondrogenic cells, may be examined in vitro. The work presented here describes mechanisms by which cells within presumptive skeletal cell populations become restricted to a chondrogenic lineage as studied within cell populations derived from 12-day-old chicken embryo calvarial tissue. It is found that a major factor associated with selection of chondrogenic cells is the elimination of growth within serum-containing medium. Chondrogenesis within these cell populations appears to be the result of permissive conditions which select for chondrogenic proliferation over osteogenic cell proliferation. Data suggest that chondrocyte cultures produce autocrine factors that promote their own survival or proliferation. The conditions for promoting cell growth, hypertrophy, and extracellular matrix mineralization of embryonic chicken chondrocytes in vitro include ascorbic acid supplementation and the presence of an organic phosphate source. The differentiation of hypertrophic chondrocytes in vitro is associated with a 10-15-fold increase in alkaline phosphatase enzyme activity and deposition of mineral within the extracellular matrix. Temporal studies of the biochemical changes coincident with development of hypertrophy in vitro demonstrate that proteoglycan synthesis decreases 4-fold whereas type X collagen synthesis increases 10-fold within the same period. Ultrastructural examination reveals cellular and extracellular morphology similar to that of hypertrophic cells in vivo with chondrocytes embedded in a well formed extracellular matrix of randomly distributed collagen fibrils and proteoglycan. Mineral deposition is seen in the interterritorial regions of the matrix between the cells and is apatitic in nature. These characteristics of chondrogenic growth and development are very similar in vivo and in vitro and they suggest that studies of chondrogenesis in vitro may provide a valuable model for the process in vivo.

Compressive force promotes sox9, type II collagen and aggrecan and inhibits IL-1beta expression resulting in chondrogenesis in mouse embryonic limb bud mesenchymal cells

The initial modeling and subsequent development of the skeleton is controlled by complex gene-environment interactions. Biomechanical forces may be one of the major epigenetic factors that determine the form and differentiation of skeletal tissues. In order to test the hypothesis that static compressive forces are transduced into molecular signals during early chondrogenesis, we have developed a unique three-dimensional collagen gel cell culture system which is permissive for the proliferation and differentiation of chondrocytes. Mouse embryonic day 10 (E10) limb buds were microdissected and dissociated into cells which were then cultured within a collagen gel matrix and maintained for up to 10 days. Static compressive forces were exerted onto these cultures. The time course for expression pattern and level for cartilage specific markers, type II collagen and aggrecan, and regulators of chondrogenesis, Sox9 and IL-1beta, were analyzed and compared with non-compressed control cultures. Under compressive conditions, histological evaluation showed an apparent acceleration in the rate and extent of chondrogenesis. Quantitatively, there was a significant 2- to 3-fold increase in type II collagen and aggrecan expression beginning at day 5 of culture and the difference was maintained through 10 days of cultures. Compressive force also causes an elevated level of Sox9, a transcriptional activator of type II collagen. In contrast, the expression and accumulation of IL-1beta, a transcriptional repressor of type II collagen was down-regulated. We conclude that static compressive forces promote chondrogenesis in embryonic limb bud mesenchyme, and propose that the signal transduction from a biomechanical stimuli can be mediated by a combination of positive and negative effectors of cartilage specific extracellular matrix macromolecules.

Developmental restriction of embryonic calvarial cell populations as characterized by their in vitro potential for chondrogenic differentiation

The mechanism(s) by which the cells within the calvaria tissue are restricted into the osteogenic versus the chondrogenic lineage during intramembranous bone formation were examined. Cells were obtained from 12-day chicken embryo calvariae after tissue condensation, but before extensive osteogenic differentiation, and from 17-day embryo calvariae when osteogenesis is well progressed. Only cell populations from the younger embryos showed chondrogenic differentiation as characterized by the expression of collagen type II. The chondrocytes underwent a temporal progression of maturation and endochondral development, demonstrated by the expression of collagen type II B transcript and expression of collagen type X mRNA. Cell populations from both ages of embryos showed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated that when the younger embryonic cultures were grown in conditions that were permissive for chondrogenesis, the number of chondrogenic cells increased from approximately 15 to approximately 50% of the population, while the number of osteogenic cells remained almost constant at approximately 35-40%. Pulse labeling of the cultures with BrdU showed selective labeling of the chondrogenic cells in comparison with the osteogenic cells. These data indicate that the developmental restriction of skeletal cells of the calvaria is not a result of positive selection for osteogenic differentiation but a negative selection against the progressive growth of chondrogenic cells in the absence of a permissive or inductive environment. These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chondrogenic potential. Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the progressive limitation of the reparative capacity of the cranial bones.

Polyionic regulation of cartilage development: promotion of chondrogenesis in vitro by polylysine is associated with altered glycosaminoglycan biosynthesis and distribution

The development of cartilage nodules in cultures of chick limb bud mesenchyme (Hamburger-Hamilton stages 23/24) is significantly promoted when the culture medium is supplemented with (poly-L-lysine (PL) (M(r) greater than or equal to 14K) (San Antonio and Tuan, 1986. Dev. Biol. 115: 313). Here we present findings consistent with the hypothesis that PL may promote chondrogenesis by interacting electrostatically with sulfated glycosaminoglycans (GAGs): (1) poly-L-ornithine, poly-L-histidine, poly-D,L-lysine, and lysine-containing heteropolypeptides stimulate chondrogenesis in proportion to their contents of cationic residues; (2) the effects of PL are diminished when limb mesenchyme cultures are supplemented with exogenous GAGs, including heparin, dermatan sulfate, and chondroitin sulfate; (3) in high density cultures of limb bud mesenchyme, the release of sulfated macromolecules, but not of proteins in general, into the culture medium was significantly inhibited by PL (398K M(r)) treatment, and a net increase in total GAG content of the PL-treated cultures was observed; and (4) in monolayer cultures of cells derived from other chick embryonic tissues, including liver, skeletal muscle, and calvaria, PL treatment promoted the cell layer-associated retention of sulfated GAG. These effects were not observed using the nonstimulatory, low M(r) PL (4K). Based on the above findings and those from previous studies, it is proposed that PL may promote chondrogenesis by interacting electrostatically with cartilage GAGs, thus trapping the extracellular matrix around the newly emerging cartilage nodules and thereby stabilizing their growth and differentiation.

Studies of mineralization in tissue culture: optimal conditions for cartilage calcification

The optimal conditions for obtaining a calcified cartilage matrix approximating that which exists in situ were established in a differentiating chick limb bud mesenchymal cell culture system. Using cells from stage 21-24 embryos in a micro-mass culture, at an optimal density of 0.5 million cells/20 microliters spot, the deposition of small crystals of hydroxyapatite on a collagenous matrix and matrix vesicles was detected by day 21 using X-ray diffraction, FT-IR microscopy, and electron microscopy. Optimal media, containing 1.1 mM Ca, 4 mM P, 25 micrograms/ml vitamin C, 0.3 mg/ml glutamine, no Hepes buffer, and 10% fetal bovine serum, produced matrix resembling the calcifying cartilage matrix of fetal chick long bones. Interestingly, higher concentrations of fetal bovine serum had an inhibitory effect on calcification. The cartilage phenotype was confirmed based on the cellular expression of cartilage collagen and proteoglycan mRNAs, the presence of type II and type X collagen, and cartilage type proteoglycan at the light microscopic level, and the presence of chondrocytes and matrix vesicles at the EM level. The system is proposed as a model for evaluating the events in cell mediated cartilage calcification.

Differential modulation of growth and phenotypic expression of chondrocytes in sparse and confluent cultures by growth factors in cartilage

The growth-promoting actions of cartilage extracts (CE) on rabbit cultured chondrocytes were studied to assess the role of local acting growth factors in the generation and expansion of highly differentiated cells. In the present study, DNA synthesis and proteoglycan synthesis in the cultured chondrocytes were monitored by flow cytofluorometry and double-isotope autoradiography by using [3H]thymidine and [35S]sulfate. We report here that actions of the same set of growth factors extracted from cartilage evokes differential cellular responses depending upon cell density. Growth factors in the optimal dose of CE (2 micrograms/ml) or epidermal growth factor (EGF, 40 ng/ml) did not reveal such a cell density-dependent effect on cellular proliferation. However, growth factors in CE induced proteoglycan synthesis selectively in nonproliferating and expressing cells in confluent culture.

Comparative morphological and biochemical analysis of hypertrophic, non-hypertrophic and 1,25(OH)2D3 treated non-hypertrophic chondrocytes

A comparative study of Type X collagen expression, chondrocyte morphology, and the expression of two genes controlling chondrocyte morphology (beta-actin and fibronectin) was carried out on chondrocytes derived from a tissue that remains as permanent hyaline cartilage in vivo (embryonic chicken caudal sterna), from a tissue that undergoes endochondral replacement (embryonic chicken ventral vertebrae) and caudal sternal chondrocytes treated with 1,25(OH)2D3. Under identical in vitro growth conditions and times, sternal chondrocytes grew as rounded non-adherent cells, and vertebral chondrocytes grew as adherent polygonal cells. Upon treatment with 10(-8) M 1,25(OH)2D3 over a twelve day period the sternal chondrocytes showed complete adherence and took on an identical appearance as the vertebral chondrocytes. Cellular adherence of both vertebral and 1,25(OH)2D3 treated sternal chondrocytes was associated with 10 X increased beta-actin, fibronectin and their corresponding mRNA's. Changes in connective tissue expression were observed with altered cellular morphology. Total collagen synthesis was 35-50% lower in both hormone treated and vertebral chondrocytes. Type II collagen was the major collagen type produced by all chondrocyte cultures; however, in both vertebral and 1,25(OH)2D3 treated sternal chondrocytes, a 60 kD collagenous protein was identified. This short chain collagen was determined to be Type X collagen based on its molecular weight and its CNBr peptide maps. Analysis of Type X mRNA levels using a 33 base pair anti sense oligonucleotide sequence to Type X, demonstrated that vertebral cells showed six to seven times more mRNA than sternal chondrocytes. However, the low mRNA levels of type X mRNA in sternal chondrocytes were increased two to three times by 1,25(OH)2D3 treatment. These studies demonstrate that the steroid hormone 1,25(OH)2 vitamin D3 induced morphological, biochemical and molecular changes indicative of chondrocyte maturation from a hyaline to a more hypertrophic phenotype.

Induction and prevention of chondrocyte hypertrophy in culture

Primary chondrocytes from whole chick embryo sterna can be maintained in suspension culture stabilized with agarose for extended periods of time. In the absence of FBS, the cells remain viable only when seeded at high densities. They do not proliferate at a high rate but they deposit extracellular matrix with fibrils resembling those of authentic embryonic cartilage in their appearance and collagen composition. The cells exhibit many morphological and biochemical characteristics of resting chondrocytes and they do not produce collagen X, a marker for hypertrophic cartilage undergoing endochondral ossification. At low density, cells survive in culture without FBS when the media are conditioned by chondrocytes grown at high density. Thus, resting cartilage cells in agarose cultures can produce factors required for their own viability. Addition of FBS to the culture media leads to profound changes in the phenotype of chondrocytes seeded at low density. Cells form colonies at a high rate and assume properties of hypertrophic cells, including the synthesis of collagen X. They extensively deposit extracellular matrix resembling more closely that of adult rather than embryonic cartilage.

Expression of differentiated function by mineralizing cultures of chicken osteoblasts

This report documents osteoblast differentiation in vitro, as demonstrated by the 50-100X increase of proteins which are known markers of the osteoblast phenotype. Collagen type I and osteocalcin synthesis and accumulation, alkaline phosphatase activity, and matrix calcification show similar temporal relationships that are analogous to those seen during in vivo bone development. Chicken embryonic osteoblast progenitor cells were selected by initial growth at low densities in minimal medium. Upon subcultivation into nutrient-enriched medium at higher cell densities, near homogeneous populations of osteoblasts were obtained as demonstrated by the greater than 80% enrichment of cells positive for alkaline phosphatase activity. A comparison was made between cells grown in the presence or absence of 10 mM beta-glycerolphosphate (beta-GPO4), a chemical stimulant of matrix calcification, as a function of time. Cultures treated with beta-GPO4 showed visible calcification at Day 12 when culture monolayers became confluent. By Day 30, numerous large foci of calcification were visible and a 20-fold increase in calcium (Ca) content was observed. In contrast, untreated cultures had only a 3-fold increase in Ca content with many smaller diffuse areas of calcification. DNA, RNA, and total protein levels were nearly identical between the two cultures, indicating that beta-GPO4 had no marked effect on either cell proliferation or transcriptional activity. The major collagen type produced by either culture was type I, with no detectable type III as determined by CNBr peptide mapping and delayed reduction analysis. Alkaline phosphatase activity showed a rapid approximately 50-fold induction by Day 18 and remained elevated in control cultures. However, cultures treated with beta-GPO4 demonstrated a rapid 80% decline of enzyme activity after 18 days. In contrast, total osteocalcin levels showed a 100-fold induction by Day 18 and remained elevated in both control and beta-GPO4-treated cultures throughout the time period examined. While the overall levels of osteocalcin were the same in beta-GPO4-treated and untreated cultures, 2- to 5-fold more osteocalcin was associated with the more mineralized matrices of the beta-GPO4-treated cultures. In order to confirm the association of osteocalcin with areas of mineralization, co-localization of mineral to osteocalcin and collagen was carried out by combining vital labeling with tetracycline and immunofluorescent staining with anti-osteocalcin and anti-collagen antibodies. Both collagen and osteocalcin showed strong localization with areas of mineralization.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of parathyroid hormone (1-34) on cyclic AMP level, ornithine decarboxylase activity, and glycosaminoglycan synthesis of chondrocytes from mandibular condylar cartilage, nasal septal cartilage, and spheno-occipital synchondrosis in culture

Previously, we reported methods for isolating chondrocytes from the craniofacial complex and their culture in vitro. The response of these chondrocyte cultures to bovine parathyroid hormone (1-34) (PTH) has now been investigated. PTH stimulated glycosaminoglycan (GAG) synthesis, a characteristic of the cartilage phenotype in cultured chondrocytes isolated from mandibular condylar cartilage (MCC), nasal septal cartilage (NSC), and spheno-occipital synchondrosis (SOS). These stimulations of GAG synthesis by PTH were dose-dependent. PTH also increased accumulation of cyclic AMP (cAMP) and the activity of ornithine decarboxylase (ODC), a rate-limiting enzyme in polyamine biosynthesis. However, PTH did not stimulate DNA synthesis. The increases in the cAMP level, ODC activity, and GAG synthesis after addition of PTH (10(-7) mol/L) were greatest in MCC-chondrocytes and least in NSC-chondrocytes. The difference in the responses to PTH of these three types of chondrocytes may reflect differences of the characteristics of these cells in vivo.

Stimulation of DNA synthesis in quiescent rabbit chondrocytes in culture by limited exposure to somatomedin-like growth factors

Cartilage-derived factor (CDF), extracted from fetal bovine cartilage, and multiplication-stimulating activity (MSA) stimulated DNA synthesis in quiescent rabbit costal chondrocytes in culture under serum-free conditions. As described previously, when added in the presence of fibroblast growth factor (FGF) or epidermal growth factor (EGF) a somatomedin-like growth factor, CDF or MSA, synergistically stimulated DNA synthesis in the cultured chondrocytes. The present study showed that exposure of the cells to MSA or CDF for only the initial 5 h was sufficient for transmission of their full stimulatory effect. Furthermore, the limited exposure did not alter the time course of stimulation of DNA synthesis: [3H]thymidine incorporation into DNA began to increase after 16 h and reached a maximum after 24 h. In contrast to the somatomedin-like growth factors, FGF and EGF were required continuously in the culture medium during traverse of the entire G1 phase for stimulation of DNA synthesis, and the mitogenic effects of FGF and EGF in cultured chondrocytes were stronger than those of CDF and MSA. Synergistic stimulation of DNA synthesis by CDF or MSA in the presence of FGF or EGF could be observed as long as FGF or EGF was continuously present, even when CDF or MSA was withdrawn after the first 5 h of culture. These findings suggest that, in contrast to FGF and EGF, somatomedin-like growth factors affect an early distinct stage in the G1 phase of chondrocytes.

Differential effects of parathyroid hormone and somatomedin-like growth factors on the sizes of proteoglycan monomers and their synthesis in rabbit costal chondrocytes in culture

In the proteoglycans extracted from rabbit costal chondrocytes in culture, two populations of proteoglycans were distinguished by density gradient centrifugation under dissociative conditions. The major component was the faster sedimenting population (proteoglycan I), the putative 'cartilage-specific' proteoglycans, and the minor component was the slower sedimenting population (proteoglycan II). The monomeric size of proteoglycan I was closely related to the differentiation-state of chondrocytes and was a good marker of the differentiated chondrocytes. Treatment of the cultures with parathyroid hormone (PTH) induced an increase in the monomeric size of proteoglycan I. This increase was ascribed to an increase in the molecular size of the glycosaminoglycan chain in proteoglycan I. On the other hand, somatomedin-like growth factors, such as multiplication-stimulating activity (MSA) and cartilage-derived factor (CDF), did not affect the size of proteoglycan I, while they markedly stimulated the synthesis of proteoglycan I. In contrast, treatment with nonsomatomedin growth factors, such as fibroblast growth factor (FGF) and epidermal growth factor (EGF), resulted in not only a decrease in glycosaminoglycan synthesis but also a slight decrease in size of proteoglycan I. However, synthesis and size of proteoglycan II were little affected by these agents. Thus, the present study clearly shows that PTH and somatomedin-like growth factors have differential functions in bringing about the expression of the differentiated phenotype of chondrocytes: PTH influences chain elongation and termination of glycosaminoglycans in proteoglycan I, while somatomedin-like growth factors affect primarily the synthesis and secretion of proteoglycan I.

Pericellular coat of chick embryo chondrocytes: structural role of hyaluronate

Chondrocytes produce large pericellular coats in vitro that can be visualized by the exclusion of particles, e.g., fixed erythrocytes, and that are removed by treatment with Streptomyces hyaluronidase, which is specific for hyaluronate. In this study, we examined the kinetics of formation of these coats and the relationship of hyaluronate and proteoglycan to coat structure. Chondrocytes were isolated from chick tibia cartilage by collagenase-trypsin digestion and were characterized by their morphology and by their synthesis of both type II collagen and high molecular weight proteoglycans. The degree of spreading of the chondrocytes and the size of the coats were quantitated at various times subsequent to seeding by tracing phase-contrast photomicrographs of the cultures. After seeding, the chondrocytes attached themselves to the tissue culture dish and exhibited coats within 4 h. The coats reached a maximum size after 3-4 d and subsequently decreased over the next 2-3 d. Subcultured chondrocytes produced a large coat only if passaged before 4 d. Both primary and first passage cells, with or without coats, produced type II collagen but not type I collagen as determined by enzyme-linked immunosorbent assay. Treatment with Streptomyces hyaluronidase (1.0 mU/ml, 15 min), which completely removed the coat, released 58% of the chondroitin sulfate but only 9% of the proteins associated with the cell surface. The proteins released by hyaluronidase were not digestible by bacterial collagenase. Monensin and cycloheximide (0.01-10 microM, 48 h) caused a dose-dependent decrease in coat size that was linearly correlated to synthesis of cell surface hyaluronate (r = 0.98) but not chondroitin sulfate (r = 0.2). We conclude that the coat surrounding chondrocytes is dependent on hyaluronate for its structure and that hyaluronate retains a large proportion of the proteoglycan in the coat.

Effect of low density lipoprotein on glycosaminoglycan secretion by cultured human smooth muscle cells and fibroblasts. Influence of serum concentration and cell proliferation rate

Glycosaminoglycan (GAG) secretion was studied in cultures of human fibroblasts and arterial smooth muscle cells. Supplementation of culture medium with whole human serum increased the secretion of GAG but this effect disappeared as cell density increased. Lipoprotein-free serum (LFS) supported cell growth but led to a decrease in GAG secretion and in cell cholesterol. Addition of human low density lipoprotein (LDL) to the medium containing 10% LFS produced increases in GAG secretion (200%) and cell cholesterol (300%) and a decrease (60%) in cell population. The effects of LDL were considerably smaller in medium containing 5% LFS; this was related to the lower rate of proliferation in this medium, since there was a close relationship between rate of proliferation and stimulation of GAG secretion by LDL independent of serum concentration. In addition, fetal smooth muscle cells showed a qualitatively different response to LDL in 5% LFS, with a biphasic dose-response of GAG secretion and cell number. It is concluded that: (1) whole human serum stimulates GAG secretion by sparse cell cultures, (2) LFS can support cell growth but not GAG secretion, (3) LDL stimulates GAG secretion but has a cytotoxic effect, (4) the degree of GAG stimulation by LDL is dependent on the proliferative state of cells, (5) at low serum concentrations fibroblasts and fetal smooth muscle cells show differences in response to LDL which are not evident at higher serum concentration.

Synthesis of proteoglycans, collagen, and elastin by cultures of rabbit auricular chondrocytes--relation to age of the donor

Chondrocytes were isolated from the auricular cartilage of rabbits, aged 1 week to 30 months, and grown in short-term cell culture. The cells from the 1-week animals were small, polygonal, and mononucleated, while the chondrocytes from the older animals were larger, rounded, and frequently binucleated. The synthesis of proteoglycans, collagen, and elastin was determined by isotope incubation. Chemical characterization of the proteoglycans was also performed. The production of the matrix macromolecules showed a clear age dependence with peak synthesis occurring at different ages. Proteoglycans were actively synthesized by chondrocytes from all age groups with a broad maximum between 2 weeks and 5 months followed by a sharp decline to about 50% of the 1-week level at 12-30 months. Collagen synthesis peaked at 2 weeks, declining progressively thereafter to about 60% of the 1-week level at 30 months. Elastin synthesis was highest in the 1-week cultures and thereafter fell quickly to very low levels. In all age groups the chondrocytes synthesized predominantly cartilage-typic proteoglycans, i.e., large aggregate forming molecules containing chondroitin sulfate. Monomers and aggregates showed a size maximum at 2-8 weeks. The degree of sulfation of the chondroitin sulfate and the proportion of 6-sulfate increased with age. These findings support the concept of "age programs" for the biosynthesis and turnover of different matrix macromolecules.

Cell shape and cartilage differentiation of early chick limb bud cells in culture

The onset of chondrogenesis in te embryonic chick is preceded by a prechondrogenic condensation of the prospective cartilage cells. Similarly, in culture, the chondrogenic phenotype is only expressed by limb mesoderm cells plated at densities above confluence. This has led to the proposal that chondrogenic differentiation requires immediate histogenic interactions prior to overt chondrogenesis (Solursh et al., 1978, In Vitro 14, 51-61; and, 1980, Dev. Biol. 78, 141-150). It is noticeable that in both the in vitro situations cells secreting a cartilaginous matrix are rounded in shape. Recently, Glowacki, Trepman and Folkman (personal communication) have shown a dependence of phenotypic expression by mature chondrocytes on cell shape. Cells maintained in a rounded configuration by culturing on a semi-adhesive substratum (poly(HEMA)) synthesised more sulphur-containing extracellular matrix than cells allowed to flatten on normal tissue culture plastic. We have investigated whether there is a similar shape-dependent relationship in the differentiation of chick embryonic mesoblasts into actively secreting chondroblasts. Our results show that a rounded cell shape is conductive to the synthesis of a sulphated matrix.

In vitro conditions affecting the synthesis of sulfated proteoglycans by normal and rheumatoid synovial cells in culture

In vitro conditions affecting synthesis of sulfated proteoglycans by cell suspensions derived from monolayer cell cultures of normal and rheumatoid synovial tissue were examined. The capacity of cells to synthesize proteoglycans was estimated by the incorporation of 35S--sulfate into cetylpyridinium chloride--precipitable material. Synthesis of sulfated proteoglycans was maximal during log phase, and after 2--3 hours of recovery from disaggregation. Normal synovial cells appeared to be more sensitive to changes in serum concentration than were rheumatoid synovial cells, but rheumatoid synovial cells were more sensitive to changes in cell density. The proportion of newly synthesized extracellular proteoglycans increased with the duration of incubation in 35S--sulfate.

Extracellular matrix formation by chondrocytes in monolayer culture

In previous studies were have reported on the secretion and extracellular deposition of type II collagen and fibronectin (Dessau et al., 1978, J. Cell Biol., 79:342-355) and chondroitin sulfate proteoglycan (CSPG) (Vertel and Dorfman, 1979, Proc. Natl. Acad. Sci. U. S. A. 76:1261-1264) in chondrocyte cultures. This study describes a combined effort to compare sequence and pattern of secretion and deposition of all three macromolecules in the same chondrocyte culture experiment. By immunofluorescence labeling experiments, we demonstrate that type II collagen, fibronectin, and CSPG reappear on the cell surface after enzymatic release of chondrocytes from embryonic chick cartilage but develop different patterns in the pericellular matrix. When chondrocytes spread on the culture dish, CSPG is deposited in the extracellular space as an amorphous mass and fibronectin forms fine, intercellular strands, whereas type II collagen disappears from the chondrocyte surface and remains absent from the extracellular space in early cultures. Only after cells in the center of chondrocyte colonies shape reassume spherical shape does the immunofluorescence reveal type II collagen in the refractile matrix characteristic of differentiated cartilage. By immunofluorescence double staining of the newly formed cartilage matrix, we demonstrate that CSPG spreads farther out into the extracellular space that type II collagen. Fibronectin finally disappears from the cartilage matrix.

The synthesis of cartilage collagen by rabbit and human chondrocytes in primary cell culture

This report describes a method for preparing primary cell cultures of differentiated rabbit sternal and human vertebral cartilage cells. These cell cultures were shown to synthesize primarily alpha1 chains, which is taken to mean that at least 82% of the collagen produced is cartilage specific collagen (type II).

Lineages, quantal cell cycles, and the generation of cell diversity

Most theories of determination or differentiation assume that embryonic cells differ from mature cells. Embryonic cells are thought to have metastable control mechanisms. These labile controls are believed to become progressively more stabilized as the cells differentiate. Zygote, blastula, neural plate, limb bud, somite, or ‘stem’ cells are conceived of as undifferentiated, totipotent, or multipotential cells. As such, these cells supposedly have available for activation a larger repertoire of phenotypic programmes than their progeny. A necessary corollary to this view is that the activation of one particular phenotypic programme out of the many available is a function of instructive exogenous inducing molecules.

Incorporation of fucose and glucosamine into cell bound and medium released macromolecules

(1) Determinations were carried out on the incorporation of fucose-6-(3H) and glucosamine-6-(3H) into trichloracetic acid insoluble macromolecules which remained bound to the cells or were released into the medium of chick embryo muscle cell cultures. The radioactivity determined in the medium was corrected for unspecific binding of label to components of the medium. (2) During an incorporation period of six hours the incorporation per microgram DNA with fucose as label into cell bound macromolecules is about twice as high as the incorporation into macromolecules released into medium. With glucosamine about twice as much is incorporated into medium released into the cell bound macromolecules. (3) The incorporation per microgram DNA increased during a culture period of three days but the increase ceases at different times during this culture period when determined with fucose or glucosamine or for cell bound and medium released material. (4) An increase in cell density increases the incorporation per DNA of fucose and to a much slighter extent that of glucosamine. Reduction of cell density by addition of cytosine arabinoside to the medium does not increase the incorporation per microgram DNA. (5) The effect of changes of fibroblast/myoblast ratios on the incorporation of fucose and glucosamine were examined. No significant effect was observed for a ratio of 10-30% fibroblasts when control cultures or cultures after cell sedimentation were maintained in complete medium. Marked changes were observed after culture in medium without protein components. Under these conditions an increase in the fibroblast/myoblast ratios were observed as well as an increase in the incorporation of label into medium released and a decrease into cell bound macromolecules.

An effect of accumulated matrix on sulfation among cells in a cartilage colony: an autoradiographic study

In this report an autoradiographic approach is used to compare synthetic activities of cells within differentiated cartilage colonies. While amino acid incorporation is umiform throughout the colony, H-3-uridine is incorporated more actively by cells having little matrix, cells which are typically in the peripheral regions of a colony. On the other hand S-35-O4 is incorporated most actively by cells in the colony centers. This difference in sulfation appears to occur independently of the mitotic state of the cells, since it is apparent in both growing and near-stationary cultures. Instead, there is a correlation between the accumulation of extracellular matrix and more active levels of sulfation. In support of the idea that matrix creates a microenvironment more favorable to chondrogenesis is the observation that a brief treatment with hyaluronidase, which removes about 60% of the S-35-O4 from prelabeled cultures, depresses isolation of labeled glycosaminoglycans. The possible role of extracellular matrices in altering the expression of differentiated functions by creating a more favorable microenvironment is considered.