Chondrogenesis in chick limb mesenchyme in vitro derived from distal limb bud tips: changes in cyclic AMP and in prostaglandin responsiveness

Harnessing cAMP signaling in musculoskeletal regenerative engineering

This paper reviews the most recent findings in the search for small molecule cyclic AMP analogues regarding their potential use in musculoskeletal regenerative engineering.

Lead induces chondrogenesis and alters transforming growth factor-beta and bone morphogenetic protein signaling in mesenchymal cell populations

BACKGROUND

It has been established that skeletal growth is stunted in lead-exposed children. Because chondrogenesis is a seminal step during skeletal development, elucidating the impact of Pb on this process is the first step toward understanding the mechanism of Pb toxicity in the skeleton.

OBJECTIVES

The aim of this study was to test the hypothesis that Pb alters chondrogenic commitment of mesenchymal cells and to assess the effects of Pb on various signaling pathways.

METHODS

We assessed the influence of Pb on chondrogenesis in murine limb bud mesenchymal cells (MSCs) using nodule formation assays and gene analyses. The effects of Pb on transforming growth factor-beta (TGF-beta) and bone morphogenetic protein (BMP) signaling was studied using luciferase-based reporters and Western analyses, and luciferase-based assays were used to study cyclic adenosine monophosphate response element binding protein (CREB), beta-catenin, AP-1, and nuclear factor-kappa B (NF-kappaB) signaling. We also used an ectopic bone formation assay to determine how Pb affects chondrogenesis in vivo.

RESULTS

Pb-exposed MSCs showed enhanced basal and TGF-beta/BMP induction of chondrogenesis, evidenced by enhanced nodule formation and up-regulation of Sox-9, type 2 collagen, and aggrecan, all key markers of chondrogenesis. We observed enhanced chondrogenesis during ectopic bone formation in mice preexposed to Pb via drinking water. In MSCs, Pb enhanced TGF-beta but inhibited BMP-2 signaling, as measured by luciferase reporter assays and Western analyses of Smad phosphorylation. Although Pb had no effect on basal CREB or Wnt/beta-catenin pathway activity, it induced NFkappaB signaling and inhibited AP-1 signaling.

CONCLUSIONS

The in vitro and in vivo induction of chondrogenesis by Pb likely involves modulation and integration of multiple signaling pathways including TGF-beta, BMP, AP-1, and NFkappaB.

Differential regulation of EP receptor isoforms during chondrogenesis and chondrocyte maturation

Regulation of chondrogenesis and chondrocyte maturation by prostaglandins has been a topic of interest during recent years. Particular focus on this area derives from the realization that inhibition of prostaglandin synthesis with non-steroidal anti-inflammatory drugs could impact these cartilage-related processes which are important in skeletal development and are recapitulated during bone healing either post-trauma or post-surgery. In addition to reviewing the relevant literature focused on prostaglandin synthesis and signaling through the G-protein coupled EP receptors, we present novel findings that establish the expression profile of EP receptors in chondroprogenitors and chondrocytes. Further, we begin to examine the signaling that may be involved with the transduction of PGE2 effects in these cells. Our findings suggest that EP2 and EP4 receptor activation of cAMP metabolism may represent a central axis of events that facilitate the impact of PGE2 on the processes of mesenchymal stem cell commitment to chondrogenesis and ultimate chondrocyte maturation.

PGE2 signal through EP2 promotes the growth of articular chondrocytes

EP2 was identified as the major PGE2 receptor expressed in articular cartilage. An EP2 agonist increased intracellular cAMP in articular chondrocytes, stimulating DNA synthesis in both monolayer and 3D cultures. Hence, the EP2 agonist may be a potent therapeutic agent for degenerative cartilage diseases.

INTRODUCTION

Prostaglandin E2 (PGE2) exhibits pleiotropic effects in various types of tissue through four types of receptors, EP1-4. We examined the expression of EPs and effects of agonists for each EP on articular chondrocytes.

MATERIALS AND METHODS

The expression of each EP in articular chondrocytes was examined by immunohistochemistry and RT-PCR. A chondrocyte cell line, MMA2, was established from articular cartilage of p53(-/-) mice and used to analyze the effects of agonists for each EP. A search for molecules downstream of the PGE2 signal through the EP2 agonist was made by cDNA microarray analysis. The growth-promoting effect of the EP2 agonist on chondrocytes surrounded by cartilage matrix was examined in an organ culture of rat femora.

RESULTS AND CONCLUSION

EP2 was identified as the major EP expressed in articular cartilage. Treatment of MMA2 cells with specific agonists for each EP showed that only the EP2 agonist significantly increased intracellular cAMP levels in a dose-dependent manner. Gene expression profiling of MMA2 revealed a set of genes upregulated by the EP2 agonist, including several growth-promoting and apoptosis-protecting genes such as the cyclin D1, fibronectin, integrin alpha5, AP2alpha, and 14-3-3gamma genes. The upregulation of these genes by the EP2 agonist was confirmed in human articular chondrocytes by quantitative mRNA analysis. On treatment with the EP2 agonist, human articular chondrocytes showed an increase in the incorporation of 5-bromo-2-deoxyuracil (BrdU), and the organ culture of rat femora showed an increase of proliferating cell nuclear antigen (PCNA) staining in articular chondrocytes surrounded by cartilage matrix, suggesting growth-promoting effects of the PGE2 signal through EP2 in articular cartilage. These results suggested that the PGE2 signal through EP2 enhances the growth of articular chondrocytes, and the EP2 agonist is a candidate for a new therapeutic compound for the treatment of degenerative cartilage diseases.

Protein kinase A regulates chondrogenesis of mesenchymal cells at the post-precartilage condensation stage via protein kinase C-alpha signaling

Chondrogenesis of mesenchymal cells during in vitro micromass culture requires the generation of cyclic adenosine monophosphate (cAMP) and subsequent activation of cAMP-dependent protein kinase A (PKA). In this study, we investigated the regulatory activity of PKA during chondrogenesis of chick limb bud mesenchymal cells. PKA activity was high in 1-day and 2-day cultures, which was followed by a slight decrease in 4-day and 5-day old cultures. Inhibition of PKA blocked chondrogenesis. It did not affect precartilage condensation, but it blocked the progression from the precartilage condensation stage to cartilage nodule formation. The PKA inhibition-induced blockage of chondrogenesis was accompanied by an altered expression of N-cadherin. Although expression of N-cadherin was detected during the early period of chondrogenesis, it became reduced as chondrogenesis proceeded. Still, inhibition of PKA maintained expression of N-cadherin throughout the micromass culture period. The inhibition of PKA did not affect expression of protein kinase C-alpha (PKCalpha), PKCepsilon, PKCdelta, and PKClambda/iota, which are the isoforms expressed in differentiating mesenchymal cells. However, PKA inhibition completely blocked activation of PKCalpha. Because PKC activity regulates N-cadherin expression and chondrogenesis, the PKA-mediated regulation of PKCalpha appears to be responsible for the PKA regulation of N-cadherin expression and chondrogenesis. Taken together, our results suggest that PKA regulates chondrogenesis by activating PKCalpha at the stage of post-precartilage condensation.

Effect of compressive loading on chondrocyte differentiation in agarose cultures of chick limb-bud cells

It is well established that mechanical loading is important to homeostasis of cartilage tissue, and growing evidence suggests that it influences cartilage differentiation as well. Whereas the effect of mechanical forces on chondrocyte biosynthesis and gene expression has been vigorously investigated, the effect of the mechanical environment on chondrocyte differentiation has received little attention. The long-term objective of this research is to investigate the regulatory role of mechanical loading in cell differentiation. The goal of this study was to determine if mechanical compression could modulate chondrocyte differentiation in vitro. Stage 23/24 chick limb-bud cells, embedded in agarose gel, were subjected to either static (constant 4.5-kPa stress) or cyclic (9.0-kPa peak stress at 0.33 Hz) loading in unconfined compression during the initial phase of commitment to a phenotypic lineage. Compared with nonloaded controls, cyclic compressive loading roughly doubled the number of cartilage nodules and the amount of sulfate incorporation on day 8, whereas static compression had little effect on these two measures. Neither compression protocol significantly affected overall cell viability or the proliferation of cells within nodules. Since limb-bud mesenchymal cells were seeded directly into agarose, an assessment of cartilage nodules in the agarose reflects the proportion of the original cells that had given rise to chondrocytes. Thus, the results indicate that about twice as many mesenchymal cells were induced to enter the chondrogenic pathway by cyclic mechanical compression. The coincidence of the increase in sulfate incorporation and nodule density indicates that the primary effect of mechanical compression on mesenchymal cells was on cellular differentiation and not on their subsequent metabolism. Further studies are needed to identify the primary chondrogenic signal associated with cyclic compressive loading and to determine the mechanism by which it influences commitment to or progression through the chondrogenic lineage, or both.

Prostaglandins mediate the effects of 1,25-(OH)2D3 and 24,25-(OH)2D3 on growth plate chondrocytes in a metabolite-specific and cell maturation-dependent manner

Prior studies have shown that 1,25-(OH)2D3 stimulates alkaline phosphatase, phospholipase A2 (PLA2), and protein kinase C (PKC)-specific activities, and production of prostaglandin E2 (PGE2) in growth zone chondrocytes. In contrast, 24,25-(OH)2D3 stimulates alkaline phosphatase and PKC-specific activities but inhibits PLA2-specific activity and PGE2 production in resting zone cells. This indicates that different mechanisms are involved in the action of 1,25-(OH)2D3 and 24,25-(OH)2D3 on their respective target cells. In this study, we examined the hypothesis that differential regulation of prostaglandin production modulates the activity of PKC and alkaline phosphatase. To do this, we examined the effect of the cyclooxygenase inhibitor indomethacin (Indo) on alkaline phosphatase, PLA2, and PKC-specific activities in growth plate chondrocytes treated with these two vitamin D metabolites. In addition, we examined whether inhibition of PKC altered PGE2 production. In growth zone cells, Indo inhibited basal alkaline phosphatase and blocked the 1,25-(OH)2D3-dependent increase in alkaline phosphatase. This effect was due to inhibition of both plasma membrane and matrix vesicle alkaline phosphatase. In resting zone cells, Indo increased basal alkaline phosphatase activity in a dose-dependent manner, but it did not further enhance the 24,25-(OH)2D3-dependent stimulation of this enzyme. The effect of Indo was found in both plasma membranes and matrix vesicles. These data indicate that 1,25-(OH)2D3-dependent increases in alkaline phosphatase-specific activity in growth zone cells are mediated through increased prostaglandin production, whereas 24,25-(OH)2D3-mediated changes in enzyme activity in resting zone cells are mediated through decreased prostaglandin production. Regulation of PLA2 by either 1,25-(OH)2D3 or 24,25-(OH)2D3 in their target cells was unaffected by Indo, indicating that the effect of the vitamin D metabolites on this enzyme is not dependent on changes in PGE2 production. The rapid increase in 1,25-(OH)2D3-dependent PKC-specific activity in growth zone cells was inhibited by Indo, whereas there was a potentiation of the effect of 24,25-(OH)2D3 on PKC activity in resting zone cells. In addition, inhibition of PKC blocked the 1,25-(OH)2D3-dependent increase in PGE2 production in growth zone cells and the 24,25-(OH)2D3-dependent decrease in PGE2 production by resting zone cells. These data indicate that prostaglandins are involved in mediating the rapid effects of 1,25-(OH)2D3 on growth zone cells, and contribute to the effects of 24,25-(OH)2D3 on resting zone cells; in both instances, the vitamin D metabolites exert their effects on PKC through changes in arachidonic acid via the action of PLA2. In addition, PKC by itself may mediate the production of PGE2.

Rapid, fluorometric DNA determination for chick limb-bud mesenchymal-cell microcultures

Micromass cultures of chick and mouse limb-bud mesenchymal cells are commonly used for in vitro studies of cellular differentiation. Previously, adaptation of these cultures to 96-well plates facilitated analyses of various aspects of cellular behavior and the effects of different media components in these cultures. These adjustments allowed development of a serum-free medium for chick limb-bud mesenchymal cells and substantially decreased costs associated with media and reagents. Here we report a further development for this model system; a Hoechst 33342-based in situ DNA assay that provides reliable data much more quickly and with considerably less effort than had been feasible in the past. Because it allows quantitation of products of cellular differentiation and DNA in the same cultures, the number of cultures needed to provide the same data is essentially halved and the accuracy of normalized values for quantitative estimates of markers of differentiation is improved. Studies of the effects of retinoic acid on chick limb-bud mesenchymal cells were performed to document the usefulness of this method.

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

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

Dexamethasone promotes von kossa-positive nodule formation and increased alkaline phosphatase activity in costochondral chondrocyte cultures

This study examined the effect of dexamethasone on von Kossa-positive nodule formation and alkaline phosphate specific activity of costochondral chondrocytes at two distinct stages of maturation. The nodules formed by the more mature growth zone chondrocyte cultures contained von Kossa-positive deposits in the extracellular matrix that had a punctate morphology. The nodules formed by the less mature resting zone cells also contained von Kossa-positive deposits, but differentiation was delayed by three-to-five days compared to the growth zone cell cultures. Dexamethasone stimulated the number of nodules formed and shortened the length of time required for von Kossa-positive nodule formation in both types of cultures. During the first 48 h of exposure to dexamethasone, alkaline phosphatase specific activity in the cell layer of both resting zone and growth zone cultures was increased in a dose-dependent manner. At 12 days post-confluence and thereafter, enzyme activity was inhibited in the dexamethasone-treated cultures. Changes in matrix vesicle alkaline phosphatase specific activity reflected those changes seen in the cell layer after dexamethasone treatment, but with higher magnitude, suggesting that one effect of dexamethasone might be to regulate matrix vesicle function. With the exception of one culture, the chondrocytes did not synthesize type X collagen under any of the experimental conditions used. Fourier transform infrared spectroscopy (FTIR) failed to detect the presence of calcium phosphates in any of the cultures exposed to dexamethasone except one. These results demonstrate that dexamethasone promotes early differentiation events, including nodule formation and increased alkaline phosphatase activity, in costochondral chondrocyte cultures. The failure to detect type X collagen synthesis and mineralization in both dexamamethasone-treated and control cultures suggests that these cultures lack the factors necessary for terminal differentiation and mineralization.

Stage- and region-dependent responses of chick wing-bud mesenchymal cells to retinoic acid in serum-free microcultures

Retinoic acid (RA) has been shown to affect skeletal patterning in vivo in both developing and regenerating limbs. Regional differences in RA concentrations alone cannot account for the region-specific cell behaviors involved in limb-skeletal morphogenesis. The present study explores a role for regional differences in signal interpretation in RA's effects along the anteroposterior and proximodistal axes of stage 21-22 and 23-24 chick wing-buds. Mesenchymal cells isolated from specific limb regions were grown in chemically defined medium and exposed to 5 or 50 ng/ml of RA for 4 days in high-density microtiter cultures. Previous studies showed that RA's effects on chondrogenesis and growth in such cultures differed depending on the position along the limb's proximodistal axis from which the cells were isolated. The present study is the first to show that such differences in RA-responsiveness also exist along the limb's anteroposterior axis, especially in the distal subridge mesenchyme. The region-dependent relationships between RA's effects on growth and chondrogenesis suggest that RA affects these two behaviors through different mechanisms. The regional differences in the responsiveness of these cells to exogenous RA are discussed with respect to their correspondence to the in vivo patterns of expression of RA-binding proteins, RA-receptors, and other patterning-related genes.

Stage- and region-dependent chondrogenesis and growth of chick wing-bud mesenchyme in serum-containing and defined tissue culture media

During development, limb-bud mesenchymal cells carry out complex spatiotemporal-patterns of growth and differentiation. Tissue and organ culture facilitate analysis of environmental influences on these cell behaviors, allowing their partial dissection into exogenous and endogenous components. Two factors that complicate such in vitro analyses are the heterogeneity of the cultured cells and imprecise knowledge of culture medium composition. Limb mesenchyme comprises a heterogenous cell population with important regional differences in cell type. Dividing the limb into subregions helps limit the cellular heterogeneity and using chemically defined, serum-free medium alloys concerns about medium composition. In the present study, mesenchyme from different regions along the anteroposterior and proximodistal axes of stage 21-22 and stage 23-24 chick wing buds was grown in high-density microtiter cultures in chemically defined and in serum-containing medium. Four-day cultures of the various regions were compared in terms of culture morphology and the accumulation of Alcian blue-positive cartilage matrix and DNA. The results demonstrate stage- and region-dependent differences in the in vitro growth, differentiation, and responsiveness of these cells. For example, mesenchyme from the distal anterior region of the wing bud exhibited lower intrinsic chondrogenic capacity and greater responsiveness to serum than other regions. Patterns of in vitro chondrogenesis also suggest that, at the stages examined, distal wing-bud mesenchyme may be less homogeneous than has been believed. A case is made for the suitability of serum-free medium for future in vitro studies of chick limb-bud mesenchyme. The results are considered in relation to the process of limb development and regional expression of pattern-related genes.

Effects of a putative prostaglandin E2 antagonist, AH6809, on chondrogenesis in serum-free cultures of chick limb mesenchyme

In the present study, we have examined the effects of a putative antagonist of prostaglandin E2 (PGE2), AH6809, on chondrogenesis in serum-free cultures of mesenchyme from distal tips of stage 25 chick limb buds in order to test the hypothesis that endogenous PGE2, through receptor-linked adenylate cyclase (AC), initiates differentiation of cartilage in limb mesenchyme. Daily addition of 10(-4) M concentrations of AH6809 produced marked inhibition of chondrogenesis over a 5-day period of cell culture as evaluated by Alcian green binding to cartilage matrix components. Inhibition of chondrogenesis by this compound was further shown to be reversible and treatment of cells with the antagonist limited to periods when chondrocytes had differentiated and were actively secreting cartilage-specific matrix components had little effect. Preincubation of control cells in 10(-4) M concentrations of AH6809 inhibited PGE2-induced activation of AC by greater than 80% without significant (P greater than .05) inhibition of basal activity by the antagonist. Responses to parathyroid hormone, which increased AC activity by 7-fold, and forskolin which increased AC activity by 23-fold in control cells, were also uninhibited by preincubation in AH6809. The results demonstrate that blockade of PGE2-AC linked receptors in prechondrogenic limb mesenchyme inhibits chondrogenesis supporting the hypothesis that endogenous PGE2 concentrations in undifferentiated limb mesenchyme play an initiating role in the differentiation of cartilage.

Development of PTH-responsive adenylate cyclase activity during chondrogenesis in cultured mesenchyme from chick limb buds

The present study investigated the development of parathyroid hormone (PTH)-responsive adenylate cyclase (AC) activity in chondrogenic cells differentiating from chick limb mesenchyme in culture. Mesenchyme from stage 25 chick embryos was removed from the distal tip (0.3 mm) of limb buds and cultured for a 6 day period in high density micromass cultures. Under these conditions, initial appearance of cartilage matrix and chondroblasts occurred on day 3 of culture and rapidly progressed over the next 3 days to produce, by day 6, a highly confluent and homogeneous layer of cartilage matrix and chondrocytes. Cells initially dissociated from limb mesenchyme on day 0 were essentially unresponsive to PTH, but development of AC-coupled, PTH receptors occurred rapidly during the initial 24 hours of culture. Based on data from dose-response experiments, prechondrogenic cells on day 1 of culture had synthesized their full complement of these receptors relative to fully differentiated chondrocytes in cultures at day 6. Inhibition of chondrocyte differentiation by retinoic acid did not significantly affect the initial development of AC-coupled, PTH receptors but it almost completely prevented synthesis of cartilage matrix. The results indicate that development of AC-coupled PTH receptors during chondrogenesis precedes, by at least 48 hours, overt differentiation of chondrocytes and the accumulation of cartilage-specific extracellular matrix and appears to represent one of the earliest reported events in chondrocyte differentiation. The lack of effect of retinoids on development of these receptors indicates that the inhibitory effects of retinoids on differentiating cartilage are at least somewhat specific for genes regulating synthesis of extracellular matrix molecules.

Gap junctional communication during limb cartilage differentiation

The onset of cartilage differentiation in the developing limb bud is characterized by a transient cellular condensation process in which prechondrogenic mesenchymal cells become closely apposed to one another prior to initiating cartilage matrix deposition. During this condensation process intimate cell-cell interactions occur which are necessary to trigger chondrogenic differentiation. In the present study, we demonstrate that extensive cell-cell communication via gap junctions as assayed by the intercellular transfer of lucifer yellow dye occurs during condensation and the onset of overt chondrogenesis in high density micromass cultures prepared from the homogeneous population of chondrogenic precursor cells comprising the distal subridge region of stage 25 embryonic chick wing buds. Furthermore, in heterogeneous micromass cultures prepared from the mesodermal cells of whole stage 23/24 limb buds, extensive gap junctional communication is limited to differentiating cartilage cells, while the nonchondrogenic cells of the cultures that are differentiating into the connective tissue lineage exhibit little or no intercellular communication via gap junctions. These results provide a strong incentive for considering and further investigating the possible involvement of cell-cell communication via gap junctions in the regulation of limb cartilage differentiation.

Responsiveness of adenylate cyclase to PGE2 and forskolin in isolated cells from micromass cultures of chick limb mesenchyme during chondrogenesis

Exogenous PGE2 stimulation of adenylate cyclase (AC) in intact and enzymatically dissociated micromass cultures of mesenchymal cells derived from the distal tip of stage 25 chick limb buds was examined over a six day period of culture. Responsiveness to PGE2 was measured in both dissociated and intact cell layers in an effort to determine if an inhibitory interaction occurred between PGE2 receptors and the extracellular matrix synthesized by differentiating chondrocytes. PGE2 responsiveness was maximal in both dissociated and intact prechondrogenic mesenchyme after 24 hours in culture and declined significantly as chondrocyte differentiation occurred on days 3 and 6. Equivalent activation of AC activity by PGE2 at each time point examined was noted in both cell groups. In contrast to the decreased responsiveness of differentiating chondrocytes to PGE2, stimulation of AC by forskolin resulted in increased levels of activity in differentiating chondrocytes of both cell groups between days 3-6. The results of the present study demonstrate that the decline in PGE2 responsiveness of differentiating chondrocytes most likely involves specific changes in the PGE2 receptor complex and not in either the interaction of the receptor with extracellular matrix components or a reduction in the available pool of AC present.

Stimulation of limb cartilage differentiation by cyclic AMP is dependent on cell density

Cyclic AMP (cAMP) has been implicated in the regulation of limb cartilage differentiation. This study represents an attempt to clarify potential mechanisms by which cAMP might regulate chondrogenesis. We have found that the ability of cAMP to stimulate limb cartilage differentiation in vitro is dependent on cell density. Dibutyryl cAMP (dbcAMP) elicits a striking increase in the accumulation of Alcian blue, pH 1.0-positive cartilage matrix, and a corresponding three- to fourfold increase in the accumulation of 35S-labeled glycosaminoglycans (GAG) by limb mesenchymal cells cultured in low serum medium at densities greater than confluence (i.e. micromass cultures established with 1-2 x 10(5) cells in 10 microliters of medium). Moreover, dbcAMP causes a striking (two- to fourfold) increase in the steady-state cytoplasmic levels of mRNAs for cartilage-characteristic type II collagen and the core protein of cartilage-specific sulfated proteoglycan in these high density, supraconfluent cultures. In contrast, cAMP does not promote the chondrogenesis of limb mesenchymal cells cultured at subconfluent densities (i.e. cultures initiated with 2.5-5 x 10(4) cells in 10 microliters of medium). In these low density cultures, dbcAMP does not promote the formation of cartilage matrix, sulfated GAG accumulation or the accumulation of cartilage-specific mRNAs. These observations suggest that cAMP may exert its regulatory effect in part by facilitating cell-cell communication during the critical condensation phase of chondrogenesis.