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

Bone Morphogenetic Protein-9 Is a Potent Chondrogenic and Morphogenic Factor for Articular Cartilage Chondroprogenitors

Articular cartilage contains a subpopulation of tissue-specific progenitors that are an ideal cell type for cell therapies and generating neocartilage for tissue engineering applications. However, it is unclear whether the standard chondrogenic medium using transforming growth factor beta (TGFβ) isoforms is optimal to differentiate these cells. We therefore used pellet culture to screen progenitors from immature bovine articular cartilage with a number of chondrogenic factors and discovered that bone morphogenetic protein-9 (BMP9) precociously induces their differentiation. This difference was apparent with toluidine blue staining and confirmed by biochemical and transcriptional analyses with BMP9-treated progenitors exhibiting 11-fold and 5-fold greater aggrecan and collagen type II (COL2A1) gene expression than TGFβ1-treated progenitors. Quantitative gene expression analysis over 14 days highlighted the rapid and phased nature of BMP9-induced chondrogenesis with sequential activation of aggrecan then collagen type II, and negligible collagen type X gene expression. The extracellular matrix of TGFβ1-treated progenitors analyzed using atomic force microscopy was fibrillar and stiff whist BMP9-induced matrix of cells more compliant and correspondingly less fibrillar. Polarized light microscopy revealed an annular pattern of collagen fibril deposition typified by TGFβ1-treated pellets, whereas BMP9-treated pellets displayed a birefringence pattern that was more anisotropic. Remarkably, differentiated immature chondrocytes incubated as high-density cultures in vitro with BMP9 generated a pronounced anisotropic organization of collagen fibrils indistinguishable from mature adult articular cartilage, with cells in deeper zones arranged in columnar manner. This contrasted with cells grown with TGFβ1, where a concentric pattern of collagen fibrils was visualized within tissue pellets. In summary, BMP9 is a potent chondrogenic factor for articular cartilage progenitors and is also capable of inducing morphogenesis of adult-like cartilage, a highly desirable attribute for in vitro tissue-engineered cartilage.

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.

Physical Stimuli-Induced Chondrogenic Differentiation of Mesenchymal Stem Cells Using Magnetic Nanoparticles

Chondrogenic commitments of mesenchymal stem cells (MSCs) require 3D cellular organization. Furthermore, recent progresses in bioreactor technology have contributed to the development of various biophysical stimulation platforms for efficient cartilage tissue formation. Here, an approach is reported to drive 3D cellular organization and enhance chondrogenic commitment of bone-marrow-derived human mesenchymal stem cells (BM-hMSCs) via magnetic nanoparticle (MNP)-mediated physical stimuli. MNPs isolated from Magnetospirillum sp. AMB-1 are endocytosed by the BM-hMSCs in a highly efficient manner. MNPs-incorporated BM-hMSCs are pelleted and then subjected to static magnetic field and/or magnet-derived shear stress. Magnetic-based stimuli enhance level of sulfated glycosaminoglycan (sGAG) and collagen synthesis, and facilitate the chondrogenic differentiation of BM-hMSCs. In addition, both static magnetic field and magnet-derived shear stress applied for the chondrogenic differentiation of BM-hMSCs do not show increament of hypertrophic differentiation. This MNP-mediated physical stimulation platform demonstrates a promising strategy for efficient cartilage tissue engineering.

Hyaluronan in limb morphogenesis

Hyaluronan (HA) is a large glycosaminoglycan that is not only a structural component of extracellular matrices, but also interacts with cell surface receptors to promote cell proliferation, migration, and intracellular signaling. HA is a major component of the extracellular matrix of the distal subapical mesenchymal cells of the developing limb bud that are undergoing proliferation, directed migration, and patterning in response to the apical ectodermal ridge (AER), and has the functional potential to be involved in these processes. Here we show that the HA synthase Has2 is abundantly expressed by the distal subridge mesodermal cells of the chick limb bud and also by the AER itself. Has2 expression and HA production are downregulated in the proximal central core of the limb bud during the formation of the precartilage condensations of the skeletal elements, suggesting that downregulation of HA may be necessary for the close juxtaposition of cells and the resulting cell-cell interactions that trigger cartilage differentiation during condensation. Overexpression of Has2 in the mesoderm of the chick limb bud in vivo results in the formation of shortened and severely malformed limbs that lack one or more skeletal elements. Skeletal elements that do form in limbs overexpressing Has2 are reduced in length, exhibit abnormal morphology, and are positioned inappropriately. We also demonstrate that sustained HA production in micromass cultures of limb mesenchymal cells inhibits formation of precartilage condensations and subsequent chondrogenesis, indicating that downregulation of HA is indeed necessary for formation of the precartilage condensations that trigger cartilage differentiation. Taken together these results suggest involvement of HA in various aspects of limb morphogenesis.

Chondrogenic Potential of Multipotential Cells from Human Adipose Tissue

The use of stem cells for cell-based tissue-engineering strategies represents a promising alternative for the repair of cartilaginous defects. The multilineage potential of a population of putative mesodermal stem cells obtained from human lipoaspirates, termed processed lipoaspirate cells, was previously characterized. The chondrogenic potential of those cells was confirmed with a combination of histological and molecular approaches. Processed lipoaspirate cells under high-density micromass culture conditions, supplemented with transforming growth factor-beta1, insulin, transferrin, and ascorbic acid, formed well-defined nodules within 48 hours of induction and expressed the cartilaginous markers collagen type II, chondroitin-4-sulfate, and keratan sulfate. Reverse transcription polymerase chain reaction analysis confirmed the expression of collagen type II and the cartilage-specific proteoglycan aggrecan. In summary, human adipose tissue may represent a novel plentiful source of multipotential stem cells capable of undergoing chondrogenesis in vitro.

Distinct functions of BMP4 and GDF5 in the regulation of chondrogenesis

Bone morphogenetic protein 4 (BMP4) and growth/differentiation factor 5 (GDF5) are closely related protein family members and regulate early cartilage patterning and differentiation. In this study, we compared the functional outcome of their actions systematically at various stages of chondrogenesis in mouse embryonic limb bud mesenchyme grown in micromass cultures. Overall, both growth factors enhanced cartilage growth and differentiation in these cultures. Uniquely, BMP4 not only accelerated the formation and maturation of cartilaginous nodules, but also induced internodular mesenchymal cells to express cartilage differentiation markers. On the other hand, GDF5 increased the number of prechondrogenic mesenchymal cell condensation and cartilaginous nodules, without altering the overall pattern of differentiation. In addition, GDF5 caused a more sustained elevated expression level of Sox9 relative to that associated with BMP4. BMP4 accelerated chondrocyte maturation throughout the cultures and sustained an elevated level of Col10 expression, whereas GDF5 caused a transient increase in Col10 expression. Taken together, we conclude that BMP4 is instructive to chondrogenesis and induces mesenchymal cells toward the chondrogenic lineage. Furthermore, BMP4 accelerates the progression of cartilage differentiation to maturation. GDF5 enhances cartilage formation by promoting chondroprogenitor cell aggregation, and amplifying the responses of cartilage differentiation markers. These differences may serve to fine-tune the normal cartilage differentiation program, and can be exploited for the molecular manipulation in biomimetics.

Phosphorylation of SOX9 by cyclic AMP-dependent protein kinase A enhances SOX9's ability to transactivate a Col2a1 chondrocyte-specific enhancer

Sox9 is a high-mobility-group domain-containing transcription factor required for chondrocyte differentiation and cartilage formation. We used a yeast two-hybrid method based on Son of Sevenless (SOS) recruitment to screen a chondrocyte cDNA library and found that the catalytic subunit of cyclic AMP (cAMP)-dependent protein kinase A (PKA-Calpha) interacted specifically with SOX9. Next we found that two consensus PKA phosphorylation sites within SOX9 could be phosphorylated by PKA in vitro and that SOX9 could be phosphorylated by PKA-Calpha in vivo. In COS-7 cells cotransfected with PKA-Calpha and SOX9 expression plasmids, PKA enhanced the phosphorylation of wild-type SOX9 but did not affect phosphorylation of a SOX9 protein in which the two PKA phosphorylation sites (S(64) and S(211)) were mutated. Using a phosphospecific antibody that specifically recognized SOX9 phosphorylated at serine 211, one of the two PKA phosphorylation sites, we demonstrated that addition of cAMP to chondrocytes strongly increased the phosphorylation of endogenous Sox9. In addition, immunohistochemistry of mouse embryo hind legs showed that Sox9 phosphorylated at serine 211 was principally localized in the prehypertrophic zone of the growth plate, corresponding to the major site of expression of the parathyroid hormone-related peptide (PTHrP) receptor. Since cAMP has previously been shown to effectively increase the mRNA levels of Col2a1 and other specific markers of chondrocyte differentiation in culture, we then asked whether PKA phosphorylation could modulate the activity of SOX9. Addition of 8-bromo-cAMP to chondrocytes in culture increased the activity of a transiently transfected SOX9-dependent 48-bp Col2a1 chondrocyte-specific enhancer; similarly, cotransfection of PKA-Calpha increased the activity of this enhancer. Mutations of the two PKA phosphorylation consensus sites of SOX9 markedly decreased the PKA-Calpha activation of this enhancer by SOX9. PKA phosphorylation and the mutations in the consensus PKA phosphorylation sites of SOX9 did not alter its nuclear localization. In vitro phosphorylation of SOX9 by PKA resulted in more efficient DNA binding. We conclude that SOX9 is a target of cAMP signaling and that phosphorylation of SOX9 by PKA enhances its transcriptional and DNA-binding activity. Because PTHrP signaling is mediated by cAMP, our results support the hypothesis that Sox9 is a target of PTHrP signaling in the growth plate and that the increased activity of Sox9 might mediate the effect of PTHrP in maintaining the cells as nonhypertrophic chondrocytes.

Syndecan-3 in limb skeletal development

Syndecan-3 is a member of a family of heparan sulfate proteoglycans that function as extracellular matrix receptors and as co-receptors for growth factors and signalling molecules. A variety of studies indicate that syndecan-3 is involved in several aspects of limb morphogenesis and skeletal development. Syndecan-3 participates in limb outgrowth and proliferation in response to the apical ectodermal ridge; mediates cell-matrix and/or cell-cell interactions involved in regulating the onset of chondrogenesis; may be involved in regulating the onset of osteogenesis and joint formation and, plays a role in regulating the proliferation of epiphyseal chondrocytes during endochondral ossification.

Activation of protein kinase A is a pivotal step involved in both BMP-2- and cyclic AMP-induced chondrogenesis

We studied the roles of protein kinase A (PKA) activation and cyclic AMP response element binding protein (CREB) phosphorylation in chondrogenesis using serum-free chicken limb bud micromass cultures as a model system. We showed the following points: 1) in micromass cultures, activation of PKA enhances chondrogenesis and increases the phosphorylation of CREB; 2) BMP-2, a chondrogenic stimulator, increases PKA activity and the level of phosphorylated CREB (P-CREB); 3) H8, a PKA inhibitor, inhibits chondrogenesis; 4) the chondrogenic activities of BMP-2 and cAMP are suppressed by H8; and 5) long-term TPA treatment (a protein kinase C (PKC) modulator) inhibits chondrogenesis and decreases the levels of CREB and P-CREB. These results suggest that activation of PKA is a physiological event during chondrogenesis that is involved in the chondrogenic effects of both BMP-2 and cyclic AMP (cAMP)-dependent pathways.

Chondrogenesis of a non-collagen-based cartilage in the sea lamprey,Petromyzon marinus

Chondrogenesis of the trabeculae, non-collagen-based cartilages in prolarval stages of the sea lamprey, Petromyzon marinus, was examined by light and electron microscopy. Chondrogenesis of the trabecular cartilages in prolarval lampreys commenced with the formation of mesenchymal condensations. Two peaks in mesenchymal cell density occurred, one prior to condensation formation and a second immediately before cartilage differentiation. The possibility of inductive influences by epithelio-mesenchymal interactions on the initiation of chondrogenesis is discussed. Bilateral condensations first appeared by day 17 post fertilization ventromedial to the eyes in a band of tightly packed yolk-laden mesenchymal cells that represent neural crest derived tissue. Cartilage differentiation occurred by day 19 post fertilization and was indicated by the presence of matrix-synthesizing organelles and the first ultrastructural appearance in the extracellular matrix of lamprin, a structural protein unique to lamprey cartilage. Lamprin was initially deposited as discrete 15- to 40-nm globules. Subsequently, lamprin appeared as fibrils aggregated into branching and parallel arrays arranged in pericellular, territorial, and interterritorial zones. Lengthening of the trabecular cartilages was primarily by appositional growth at the rostral end. The timing of the appearance of trabecular cartilages in prolarval stages likely reflects the functional importance of these structures for supporting the brain as the lamprey initiates burrowing behaviour.

Inhibition of in vitro limb cartilage differentiation by syndecan-3 antibodies

The transmembrane heparan sulfate proteoglycan syndecan-3 is transiently expressed in high amounts during the cellular condensation process that characterizes the onset of limb cartilage differentiation. During condensation, limb mesenchymal cells become closely juxtaposed and undergo cell-cell and cell-matrix interactions that are necessary to trigger cartilage differentiation and cartilage-specific gene expression. To test directly the possible involvement of syndecan-3 in regulating the onset of limb chondrogenesis, we examined the effect of polyclonal antibodies against a syndecan-3 fusion protein on the chondrogenic differentiation of chick limb mesenchymal cells in micromass culture. Syndecan-3 antiserum elicits a dose-dependent inhibition of the accumulation of Alcian blue-stainable cartilage matrix by high density limb mesenchymal cell micromass cultures (2 x 10(5) cells/10 microliters) and a corresponding reduction in steady-state levels of mRNAs for cartilage-characteristic type II collagen and the core protein of the cartilage proteoglycan aggrecan. In preimmune serum-treated control cultures proliferating cells are limited to the periphery of areas of cartilage matrix deposition, whereas large numbers of proliferating cells are uniformly distributed throughout the undifferentiated cultures supplemented with syndecan-3 antiserum. Limb mesenchymal cells cultured at lower densities (1 x 10(5) cells/10 microliters) in the presence of preimmune serum form extensive precartilage condensations characterized by the close juxtaposition of rounded cells by day 2 of culture. In contrast, in the presence of syndecan-3 antiserum, the cells fail to aggregate but rather remain flattened and spatially separated from one another, suggeting that syndecan-3 antibodies impair the formation of precartilage condensations. These results indicate that syndecan-3 plays an important role in regulating the onset of limb chondrogenesis, perhaps by mediating the cell-cell and cell-matrix interactions required for condensation and subsequent cartilage differentiation.

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.

Growth regulatory properties of endothelins

Endothelins are produced by endothelial and epithelial cells, macrophages, fibroblasts, and many other types of cells. Their receptors are present in numerous cells, including smooth muscle cells, myocytes, and fibroblasts. Evidence now suggests that the three isoforms of endothelins (ET-1 and the other two related isopeptides, ET-2 and ET-3) regulate growth in several of these cells. Endothelin-1 influences DNA synthesis, the expression of protooncogenes, cell proliferation, and hypertrophy. The participation of ET in mitogenesis involves activation of multiple transduction pathways, such as the production of second messengers, the release of intracellular pools of calcium, and influx of extracellular calcium. Moreover, ET-1 acts in synergism with various factors, such as EGF, PDGF, bFGF, TGFs, insulin, etc., to potentiate cellular transformation or replication. Several of these factors may in turn stimulate the synthesis and/or the release of endothelins. The production and release of endothelins are also increased in acute and chronic pathological processes, e.g., atherosclerosis, postangioplastic restenosis, hypertension, and carcinogenesis. It is postulated that endothelins act in a paracrine/autocrine manner in growth regulation and play an important role mediating vascular remodeling in some cardiovascular diseases. The present review analyses the implication of endothelins (ET-1, -2, and -3) in physiopathology related to their growth regulatory properties.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

Syndecan 3: a member of the syndecan family of membrane-intercalated proteoglycans that is expressed in high amounts at the onset of chicken limb cartilage differentiation

A partial cDNA that encodes a newly discovered member of the syndecan family of integral membrane proteoglycans, which we have termed syndecan 3, has been isolated from an embryonic chicken limb bud cDNA library. Syndecan 3 is distinct from but structurally related to syndecan and fibroglycan, two previously characterized members of this family of membrane-intercalated proteoglycans. Syndecan 3 contains a cytoplasmic domain potentially associated with the cytoskeleton that is 85% identical in amino acid sequence to the cytoplasmic domain of syndecan. Syndecan 3 also possesses a hydrophobic transmembrane domain and an extracellular domain containing several clustered potential glycosaminoglycan attachment sites. Like syndecan, the ectodomain of syndecan 3 has a single dibasic protease-susceptible site adjacent to the transmembrane domain, which might be involved in shedding the ectodomain from the cell surface. A striking feature of syndecan 3 is an extensive (182 amino acid) threonine, serine, and proline (T+S+P)-rich domain that closely resembles T+S+P-rich regions in several mucin-like proteins in which O-linked oligosaccharides are bound to the threonine and serine residues. Syndecan 3 is expressed in high amounts during a critical phase of chicken limb chondrogenesis in which limb mesenchymal cells condense, round up, and interact with one another before depositing a cartilage matrix. The multiple functional domains of syndecan 3 provide potential sites for mediating the adhesive cell-matrix interactions and cytoskeletal reorganization involved in this critical condensation process.

Type IX collagen gene expression during limb cartilage differentiation

Changes in the steady-state levels of mRNAs for the alpha 1(IX) and alpha 2(IX) polypeptide chains of cartilage-characteristic type IX collagen were examined during the course of chick limb chondrogenesis in vitro and in vivo. Cytoplasmic type IX collagen mRNAs begin to accumulate at the onset of overt chondrogenesis in high density micromass culture coincident with the crucial condensation phase of the process, in which prechondrogenic mesenchymal cells become closely juxtaposed prior to depositing a cartilage matrix. The initiation of type IX collagen mRNA accumulation at condensation coincides with the initiation of accumulation of cartilage proteoglycan core protein mRNA and with a striking increase in type II collagen mRNA accumulation. Following condensation in vitro, there is a concomitant progressive increase in cytoplasmic type IX collagen, core protein, and type II collagen mRNA levels which parallels the progressive accumulation of cartilage matrix. Type IX collagen mRNAs also begin to accumulate at the initiation of overt chondrogenesis in vivo in the chondrogenic central core of the developing limb bud. In contrast, little, or no type IX collagen mRNAs are detectable in the nonchondrogenic peripheral regions of the developing limb bud.

Lack of chondrogenic expression in mouse limb bud micromass cultures exposed to exogenous beta galactosidase or N-acetyl-beta-glucosaminidase

The effect of two exoglycosidases, beta-galactosidase and N-acetyl-beta-glucosaminidase (GlcNAc-ase) on chondrogenic expression of stage 19 mouse limb bud micromass cultures was investigated. Chondrogenic expression was monitored by Alcian blue staining and immunofluorescent localization of cartilage-specific proteoglycan and type II collagen. Chondrogenesis was inhibited by exposure to 0.1 U/ml beta-galactosidase or 0.025 U/ml GlcNAc-ase for 24 h or longer in culture. The effect of both enzymes was concentration and time dependent. Exoglycosidic hydrolysis of galactose or N-acetylglucosamine was substantiated by treatment with HRP-conjugated peanut agglutinin and succinylated wheat germ agglutinin, respectively. Cells treated with beta-galactosidase appeared to be flattened with a stellate morphology, whereas GlcNAc-ase-treated cells were bipolar forming ridge-like mounds that had a directional orientation. The antichondrogenic effect was not alleviated when the cells were induced to assume a spherical shape upon treatment with cytochalasin D. DNA measurements indicated that the lack of chondrogenic expression was not related to cell attachment or cell proliferation. These data support the hypothesis that the expression of specific terminal sugars on cell surface glycoconjugates of limb bud cells represents an important component of the chondrogenic process.

Promotion of embryonic limb cartilage differentiation in vitro by staurosporine, a protein kinase C inhibitor

Phorbol 12-myristate 13-acetate (PMA), a protein kinase C-activating phorbol ester, is known to inhibit chondrogenic differentiation by embryonic limb mesenchyme cells in vitro. The present study demonstrates that staurosporine, a potent inhibitor of protein kinase C, conversely stimulates cartilage differentiation in cultures of limb mesenchyme cells isolated from whole wing buds of stage 23/24 chick embryos or from the distal subridge region of stage 25 wing buds. In high density micromass cultures, in which limb mesenchyme cells undergo extensive spontaneous cartilage differentiation, exposure to 5-20 nM staurosporine promotes an accelerated accumulation of type II collagen and cartilage proteoglycan mRNA transcripts and a 2- to 3-fold increase in matrix glycosaminoglycan deposition. Even in low density, monolayer cultures in which the mesenchymal cells do not normally form cartilage, treatment with 5 nM staurosporine induces extensive Alcian blue-positive matrix production, a striking 4- to 18-fold rise in sulfated glycosaminoglycan accumulation, and a dramatic elevation of cartilage-characteristic gene transcript expression. Moreover, concurrent treatment with staurosporine overcomes the inhibitory effects of PMA on in vitro limb cartilage differentiation. The results suggest the hypothesis that protein kinase C might function as a negative modulator of chondrogenic differentiation during embryonic limb development.

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.