Increase in levels of cyclic AMP during avian limb chondrogenesis in vitro

Glucose enhances aggrecan expression in chondrocytes via the PKCα/p38-miR141-3p signaling pathway

Aggrecan is a high molecular weight proteoglycan that plays a critical role in cartilage structure and the function of joints, providing intervertebral disc and cartilage with the ability to resist compressive loads. Aggrecan degradation in articular cartilage is a significant event in early-stage osteoarthritis (OA). Currently, no effective treatment exists for OA other than pain relief. Dextrose (D-glucose) prolotherapy has shown promising activity in the treatment of different musculoskeletal disorders, including OA. However, little is known about the molecular mechanism of the glucose effect in OA and on the regulation of chondrogenesis. We show for the first time that glucose upregulates aggrecan expression and subsequent chondrogenesis in ATDC5 cells. Moreover, we found that glucose-induced aggrecan expression is mediated through the protein kinase Cα (PKCα)- and p38-dependent pathway. As demonstrated by microRNA (miR) and luciferase analyses, the glucose-induced PKCα/p38 signaling axis is responsible for downregulating miR141-3p which targets to the 3'untranslated region of aggrecan. In summary, we show that glucose enhances chondrogenesis by upregulating aggrecan expression via the PKCα-p38-miR141-3p signaling pathway. This result provides new insights into the mechanism of glucose on chondrogenesis.

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.

One-day treatment of small molecule 8-bromo-cyclic AMP analogue induces cell-based VEGF production for in vitro angiogenesis and osteoblastic differentiation

Small molecule-based regenerative engineering is emerging as a promising strategy for regenerating bone tissue. Small molecule cAMP analogues have been proposed as novel biofactors for bone repair and regeneration and, while promising, the effect that these small molecules have on angiogenesis, a critical requirement for successful bone regeneration, is still unclear. Our previous research demonstrated that the small molecule cAMP analogue 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP) was able to promote initial osteoblast adhesion on a polymeric scaffold via cAMP signalling cascades. Here, we report that 8-Br-cAMP is capable of inducing in vitro cell-based VEGF production for angiogenesis promotion. We first demonstrated that treating osteoblast-like MC3T3-E1 cells with 8-Br-cAMP for 1 day significantly increased VEGF production and secretion. We then demonstrated that 8-Br-cAMP-induced cell-secreted VEGF is biologically active and may promote angiogenesis, as evidenced by increased human umbilical vein endothelial cells (HUVECs) migration and tubule formation. In addition, treatment of MC3T3-E1 cells with 8-Br-cAMP for as short as a single day resulted in enhanced ALP activity as well as matrix mineralization, demonstrating in vitro osteoblastic differentiation. A short-term 8-Br-cAMP treatment also addresses the concern of non-specific cytotoxicity, as our data indicate that a 1-day 8-Br-cAMP treatment scheme supports cellular proliferation of MC3T3-E1 cells as well as HUVECs. While the major concern associated with small molecule drugs is the risk of non-specific cytotoxicity, the short exposure treatment outlined in this paper provides a very promising strategy to mitigate the risk associated with small molecules. Copyright © 2013 John Wiley & Sons, Ltd.

Small molecules and their controlled release that induce the osteogenic/chondrogenic commitment of stem cells

Stem cell-based tissue engineering plays a significant role in skeletal system repair and regenerative therapies. However, stem cells must be differentiated into specific mature cells prior to implantation (direct implantation may lead to tumour formation). Natural or chemically synthesised small molecules provide an efficient, accurate, reversible, and cost-effective way to differentiate stem cells compared with bioactive growth factors and gene-related methods. Thus, investigating the influences of small molecules on the differentiation of stem cells is of great significance. Here, we review a series of small molecules that can induce or/and promote the osteogenic/chondrogenic commitment of stem cells. The controlled release of these small molecules from various vehicles for stem cell-based therapies and tissue engineering applications is also discussed. The extensive studies in this field represent significant contributions to stem cell-based tissue engineering research and regenerative medicine.

Regulation of chondrogenesis by protein kinase C: Emerging new roles in calcium signalling

During chondrogenesis, complex intracellular signalling pathways regulate an intricate series of events including condensation of chondroprogenitor cells and nodule formation followed by chondrogenic differentiation. Reversible phosphorylation of key target proteins is of particular importance during this process. Among protein kinases known to be involved in these pathways, protein kinase C (PKC) subtypes play pivotal roles. However, the precise function of PKC isoenzymes during chondrogenesis and in mature articular chondrocytes is still largely unclear. In this review, we provide a historical overview of how the concept of PKC-mediated chondrogenesis has evolved, starting from the first discoveries of PKC isoform expression and activity. Signalling components upstream and downstream of PKC, leading to the stimulation of chondrogenic differentiation, are also discussed. Although it is evident that we are only at the beginning to understand what roles are assigned to PKC subtypes during chondrogenesis and how they are regulated, there are many yet unexplored aspects in this area. There is evidence that calcium signalling is a central regulator in differentiating chondroprogenitors; still, clear links between intracellular calcium signalling and prototypical calcium-dependent PKC subtypes such as PKCalpha have not been established. Exploiting putative connections and shedding more light on how exactly PKC signalling pathways influence cartilage formation should open new perspectives for a better understanding of healthy as well as pathological differentiation processes of chondrocytes, and may also lead to the development of novel therapeutic approaches.

Comparative analysis of osteogenic/chondrogenic differentiation potential in primary limb bud-derived and C3H10T1/2 cell line-based mouse micromass cultures

Murine micromass models have been extensively applied to study chondrogenesis and osteogenesis to elucidate pathways of endochondral bone formation. Here we provide a detailed comparative analysis of the differentiation potential of micromass cultures established from either BMP-2 overexpressing C3H10T1/2 cells or mouse embryonic limb bud-derived chondroprogenitor cells, using micromass cultures from untransfected C3H10T1/2 cells as controls. Although the BMP-2 overexpressing C3H10T1/2 cells failed to form chondrogenic nodules, cells of both models expressed mRNA transcripts for major cartilage-specific marker genes including Sox9, Acan, Col2a1, Snorc, and Hapln1 at similar temporal sequence, while notable lubricin expression was only detected in primary cultures. Furthermore, mRNA transcripts for markers of osteogenic differentiation including Runx2, Osterix, alkaline phosphatase, osteopontin and osteocalcin were detected in both models, along with matrix calcification. Although the adipogenic lineage-specific marker gene FABP4 was also expressed in micromass cultures, Oil Red O-positive cells along with PPARγ2 transcripts were only detected in C3H10T1/2-derived micromass cultures. Apart from lineage-specific marker genes, pluripotency factors (Nanog and Sox2) were also expressed in these models, reflecting on the presence of various mesenchymal lineages as well as undifferentiated cells. This cellular heterogeneity has to be taken into consideration for the interpretation of data obtained by using these models.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

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.

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.

On the importance of cAMP and Ca++ in mandibular condylar growth and adaptation

The origin of the mandibular condylar cartilage is not periosteal, like that of the other secondary cartilages; this cartilage originates in a cellular blastema of its own. Despite the fact that the development of secondary cartilages, in general, is dependent on mechanical irritation, that of the condylar cartilage is not. The low level of function experienced postnatally seems to favor growth, but because the proliferation cells of the condylar cartilage are multipotential, they switch their differentiation pathway in the direction of osteoblasts in the absence of function, and growth of the cartilage ceases. This regulation of differentiation is mediated by maturation of the cartilage cells. If function is not present, maturation advances rapidly, and the mature cartilage induces bone formation instead of cartilage. Cyclic AMP and Ca are important mediators in this process, because they affect the advancement of maturation.

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.

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.

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.

The mechanism of precartilage mesenchymal condensation: a major role for interaction of the cell surface with the amino-terminal heparin-binding domain of fibronectin

Using low magnification Hoffman Modulation Contrast microscopy to rapidly identify precartilage mesenchymal condensations in chick limb bud cultures, we have determined the effect on condensation number of treatments disruptive of the interaction of cell surface components with endogenously produced fibronectin. A monoclonal antibody directed against the amino-terminal heparin-binding domain of fibronectin reduced the number of condensations by more than 50%, as did the oligopeptide gly-arg-gly, which is a repeated motif in that fibronectin domain. In contrast, monoclonal antibodies directed against the collagen- and integrin-binding domains of fibronectin, or oligopeptides containing the fibronectin integrin-recognition sequence arg-gly-asp-ser, had no significant effect on condensation number. Addition of Flavobacterium heparinase to cultures also reduced condensation number by more than 50%. Alcian blue staining of sulfated proteoglycan was greatly reduced in differentiated cultures that had been exposed to treatments that reduced condensation number. Taken together with the accompanying study, which directly demonstrates an adhesive interaction between the amino-terminal domain of extracellular fibronectin and heparin-like molecules on the surfaces of latex bead probes, the data presented here strongly indicate a major role for the corresponding cell-matrix interaction in mediating precartilage condensation in limb mesenchyme.

Exogenous glycosaminoglycans modulate chondrogenesis, cyclic AMP level and cell growth in limb bud mesenchyme cultures

Effects of hyaluronate, heparin and chondroitin-6-sulfate were studied on micromass cultures of chick limb bud mesenchyme (Hamburger and Hamilton stages 23-24). Histochemical, electron microscopical, biochemical and radiochemical investigations of day 4 cultures revealed dose-dependent inhibitory effects of these glycosaminoglycans on chondrogenesis, cyclic AMP level and growth of cells. In addition, hyaluronate with 100 micrograms/ml dose caused a displacement of newly formed proteoglycan from cultures into the medium. It is supposed that exogenous glycosaminoglycans influence ionic equilibrium in the immediate vicinity of cells and disturb the organization of the prechondrogenic extracellular matrix resulting in alterations of cell membrane--cytoskeleton associations. These alterations may provoke a reduction in cyclic AMP level and DNA synthesis. It is suggested that a reduction in cyclic AMP level preceding the expression of cartilage phenotype results in the inhibition of chondrogenesis.

Inhibition of chondrogenesis by retinoic acid in limb mesenchymal cells in vitro: effects on PGE2 and cyclic AMP concentrations

Effects of retinoic acid (RA) on prostaglandin E2 (PGE2) and cyclic AMP (cAMP) concentrations were investigated in high density, micromass cultures of mesenchymal cells derived from chick limb buds. Exposure of cells during the initial 24 h of culture to RA concentrations between 0.05-1.0 micrograms/ml inhibited chondrogenesis in a dose-dependent manner with 1.0 micrograms/ml totally inhibiting cartilage formation. Concentrations of PGE2 and cAMP increased during the prechondrogenic period in control cells in a closely related way and remained elevated throughout the six-day period examined. Addition of RA (0.05 and 0.5 micrograms/ml) did not significantly alter cAMP concentrations at any time point, but significantly elevated PGE2 levels relative to control cells in six-day cultures in a concentration-dependent manner. Addition of dibutyryl cAMP enhanced chondrogenesis in control cells between days 3 and 4, but failed to alter the inhibitory effect of RA on chondrogenesis. The results indicate that while PGE2 and cAMP are important signals in cartilage differentiation, the inhibitory effects of RA on this process are mediated through some other mechanism.

Effects of molecular oxygen on chick limb bud chondrogenesis

Chondrogenesis is an important process in the development of the embryonic chick limb. If limb buds are dispersed just prior to the initiation of chondrogenic differentiation and their cells seeded densely in culture as three-dimensional "micromasses," some of the cells differentiate to form chondrogenic nodules. These nodules characteristically produce sulfated proteoglycans and type II collagen. Two conditions within the early avian limb core have been linked causatively to the initiation of chondrogenesis: a limitation in the availability of molecular oxygen and a low NAD content of the tissue. The O2 limitation is thought to be responsible for the low NAD level. We examined the effects of molecular oxygen on the NAD content of chick limb-bud cells in micromass culture, the formation of chondrocytic nodules, and the production of type II collagen and sulfated proteoglycans. The NAD content of the cells in the micromasses and the production of type II collagen did not vary greatly as a function of oxygen availability. The development of the nodules was modified, but not eliminated, by high oxygen partial pressure (0.95). It was eliminated by anoxia. Proteoglycan synthesis was decreased significantly by high oxygen tension and its sulfation was also decreased, more so in the wing-bud than the leg-bud cells. The results suggest that in culture, high oxygen tension is compatible with some, but not all, aspects of chondrogenic differentiation of cells from embryonic chick limbs.

Lung organoid culture

An organoid culture system for lung cells is described in which morphogenesis of lung histotypic structures and differentiation of both pneumocytes type II and mesenchyme occur. The principle of this technique is the culture of mouse fetal lung cells at high density on a membrane filter at the medium/air interface. In the course of cultivation, cell sorting-out, epithelial cell aggregation, formation of an alveolar-like lumen in the organoids and formation of a basal lamina occur. Epithelial differentiation culminates in the production of lamellar bodies, and the mesenchyme develops into mature connective tissue. Morphogenesis and differentiation depend on the stage of fetal development from which the lung cells were derived but appear independent of the formation of a basal lamina. Various drugs have been tested for their effects on morphogenesis and differentiation in this lung organoid culture: some of them inhibit differentiation or damage the mesenchyme, others stimulate surfactant production. Due to the quite complex morphogenetic and cellular events occurring in lung organoid culture, it may be an applicable tool for alternative in vitro screening methods.

Nuclear events during early chondrogenesis: phosphorylation of the precartilage 35.5-kDa domain-specific chromatin protein and its regulation by cyclic AMP

During chondrogenesis in vivo and in vitro, a family of nonhistone proteins (Mr 35,500), designated PCP 35.5, is lost from the nuclei of precartilage mesenchyme cells. A basic subcomponent of this family, designated PCP 35.5b, is phosphorylated during the first few hours of chondrogenesis in vitro by a phosphorylating system whose activity is enhanced 12- to 15-fold by exposure of differentiating precartilage cells to dibutyryl cyclic AMP. This phosphorylating system is present in isolated precartilage cell nuclei, where it retains its dependence on cyclic AMP and its specificity for PCP 35.5b. Assays for nuclear cyclic AMP inhibitable protein phosphatase activity capable of dephosphorylating PCP 35.5b were negative, indicating that the system responsible for phosphorylating this protein is a cyclic AMP-dependent protein kinase. Chromatin fractionation studies indicate that PCP 35.5b is localized at sites previously shown to be closely associated with DNase I-sensitive domains of precartilage cell chromatin. These studies define PCP 35.5b as a strategically located component of precartilage cell chromatin which is the major or sole chromatin target of cyclic AMP-dependent phosphorylation during chondrogenesis. This chromatin modification occurs prior to overt cartilage differentiation and may therefore play a regulatory role in the acquisition of the cartilage cell phenotype.

Chondrogenesis in mouse limb buds in vitro: effects of dibutyryl cyclic AMP treatment

We studied the effects of dibutyryl cyclic AMP (dbcAMP) on mouse limb-bud chondrogenesis at three stages of embryonic development. After 24 h of culture, limb buds with or without a covering of ectoderm were treated with 1 mM dbcAMP for 48 h and were then compared with untreated cultured limb buds. Treatment with dbcAMP enhanced cartilaginous differentiation in organ cultures of stage-17 and -19 (according to Theiler's) limb buds, although the presence of ectoderm reduced the level of dbcAMP stimulation. By stage 20, treatment with dbcAMP irreversibly inhibited cartilaginous differentiation. These results suggest that the responsiveness of mesenchymal limb-bud cells to dbcAMP is stage related. The results of histological studies as well as of analyses of DNA content and sulphated glycosaminoglycan accumulation supported the hypothesis that dbcAMP treatment induces recruitment of initially non-chondrogenic cells whose commitment explains the enhancement of cartilaginous differentiation. Limb-bud competence for chondrogenesis throughout the three developmental stages studied is also discussed.

Caffeine effects on cyclic AMP levels in the mouse embryonic limb and palate in vitro

Caffeine is a teratogen that causes limb and palate malformations in rodents. Since the ability to raise cyclic nucleotide levels is a known biological action of caffeine, cyclic AMP levels were measured in CD-1 mouse embryonic forelimb from whole embryo culture and embryonic limb and palate cells grown in primary culture following treatment with various concentrations of caffeine (0, 1, 3, or 10 mM). In forelimb buds from whole embryo culture, a dose-dependent response was observed. Caffeine at 1 mM concentration stimulated cyclic AMP levels to 151% of control value at 60 min. Even greater stimulation of cyclic AMP occurred at higher caffeine concentrations. A dose-dependent response was seen in both limb and palate cell culture. In limb cell culture, all caffeine concentrations significantly stimulated cyclic AMP after 10 min compared to control. In palate cell culture, there was a twofold increase in cyclic AMP at the 1-mM caffeine concentration. At higher caffeine concentrations, cyclic AMP was significantly increased after 60 min. In addition, stimulation of cyclic AMP in cultured limb and palate cells by isoproterenol, a beta-adrenergic agonist, was used as a positive control. Isoproterenol stimulated a 2.5-fold greater response in the palate cells than in the limb bud cells at isoproterenol levels of 10(-5) or 10(-4) M. The increase of cyclic AMP may be influential in the process of abnormal limb or palate development.

Collagen gene expression during limb cartilage differentiation

As limb mesenchymal cells differentiate into chondrocytes, they initiate the synthesis of type II collagen and cease synthesizing type I collagen. Changes in the cytoplasmic levels of type I and type II collagen mRNAs during the course of limb chondrogenesis in vivo and in vitro were examined using cloned cDNA probes. A striking increase in cytoplasmic type II collagen mRNA occurs coincident with the crucial condensation stage of chondrogenesis in vitro, in which prechondrogenic mesenchymal cells become closely juxtaposed before depositing a cartilage matrix. Thereafter, a continuous and progressive increase in the accumulation of cytoplasmic type II collagen mRNA occurs which parallels the progressive accumulation of cartilage matrix by cells. The onset of overt chondrogenesis, however, does not involve activation of the transcription of the type II collagen gene. Low levels of type II collagen mRNA are present in the cytoplasm of prechondrogenic mesenchymal cells at the earliest stages of limb development, well before the accumulation of detectable levels of type II collagen. Type I collagen gene expression during chondrogenesis is regulated, at least in part, at the translational level. Type I collagen mRNAs are present in the cytoplasm of differentiated chondrocytes, which have ceased synthesizing detectable amounts of type I collagen.

Epithelial effects on limb chondrogenesis involve extracellular matrix and cell shape

Collagen gel cultures of limb bud mesenchymal cells are normally permissive for chondrogenesis but become inhibitory for chondrogenesis when they are preconditioned by limb ectoderm. This inhibition is specific for cartilage differentiation, inasmuch as myoblast differentiation is unaffected and flattened, fibroblastic cells are more numerous on conditioned gels. The antichondrogenic effect of ectoderm-conditioned gels is not blocked by agents that elevate intracellular cyclic AMP levels and that promote chondrogenesis under other conditions. In contrast, the inhibitory effect of the ectoderm is alleviated when cultures are treated with cytochalasin D, a cytoskeleton-disrupting agent that causes the cells to remain spherical. These results suggest that ectoderm-conditioned collagen gels inhibit chondrogenesis through an effect on cell shape.

The effect of prostaglandins on the cyclic AMP content of limb mesenchymal cells

We have been investigating the hypothesis that prostaglandins including prostaglandin E2 (PGE2) produced during the critical condensation phase of limb chondrogenesis are involved in the regulation of cartilage differentiation by acting as local modulators of cyclic AMP (cAMP) accumulation. The purpose of the present study was to determine directly whether PGE2 and other prostanoids which had previously been shown to stimulate in vitro chondrogenic differentiation do indeed elevate the cAMP content of limb mesenchymal cells, and to determine whether the ability of various prostanoids to increase cAMP production by these cells directly reflects the potencies of these same molecules in stimulating chondrogenesis. We have found that PGE2 does indeed elicit a striking elevation in the cAMP content of subridge mesenchymal cells, indicating that the cells possess adenylate cyclase-coupled receptors for this molecule. The effect of PGE2 on cAMP accumulation is potentiated by a phosphodiesterase inhibitor, thus paralleling the potentiating effect phosphodiesterase inhibitors have on PGE2-stimulated in vitro chondrogenesis. The effect of PGE2 on cAMP content is dose-dependent with a 3-fold increase seen at 10(-8)M, which is the lowest concentration at which PGE2 effectively stimulates chondrogenesis. PGE1, which is just as effective as PGE2 in stimulating chondrogenesis, is just as effective as PGE2 in stimulating cAMP accumulation. PGA1, which is a much less effective stimulator of chondrogenesis than PGE2 or PGE1, is less than half as potent as these molecules in elevating cAMP levels. PGF1 alpha, 6-keto PGF1 alpha, and thromboxane B2, which have little or no effect on chondrogenesis, have little or no effect on cAMP content.(ABSTRACT TRUNCATED AT 250 WORDS)

Somite chondrogenesis: alterations in cyclic AMP levels and proteoglycan synthesis

Cyclic AMP (cAMP) levels have been shown to have a positive influence on chondrogenesis in limb buds and pelvic cartilage. In the present study the level of cAMP was measured during somite chondrogenesis in vitro and found to decrease from 1.38 pmol/micrograms DNA on day 0 to 0.9 pmol/micrograms DNA on day 6. Inclusion of notochord with somites caused a marked reduction, with levels decreasing from 1.41 pmol/micrograms DNA on day 0 to 0.36 pmol/micrograms DNA on day 6. Concurrently, the incorporation of radioactive sulfate into sulfated glycosaminoglycans increased from day 3 to day 6 by 38% in somite and 77% in somite-notochord explants. The aggregation of proteoglycans was analyzed by gel chromatography and found to increase with a corresponding decrease in cAMP levels. The results indicate that a decrease in cAMP levels may be necessary for chondrogenic expression in somites.

Effects of cyclic AMP on limb bud chondrogenesis in low cell density culture

In a previous paper, it was shown that the limb bud mesodermal cells differentiated into cartilage even at low cell density by lowering the serum content in the culture medium (Hattori & Ide, Exp cell res 150 (1984) 338) [20]. The present paper describes the effects of cAMP on limb bud chondrogenesis at low cell density. cAMP promoted chondrogenesis at low cell density in cultures with various concentrations of serum. The limb bud cells differentiated into cartilage cells without forming aggregates. cAMP inhibited the loss of chondrogenic capability in serum-rich medium. The relationship between cAMP level and serum content is also discussed.

The vasculature and limb development

The developing vascular pattern of the embryonic chick limb results from a combination of two properties: the intrinsic self-assembly and branching properties of the vascular cells and the extrinsic information associated with the expanding mitotic population of mesenchymal cells; and the inhibitory factors which restrict the entrance of vessels into particular domains and/or decrease the branching frequency of such vessels. It is hypothesized that an important component of limb pattern formation is the interplay between the dividing population of mesenchymal cells and the intrinsic properties of the vascular cells. It is further asserted that the presence of particular vascular elements may, indeed, be 'positional information'. Two examples are cited involving aspects of limb duplication to support this possibility; it is suggested that vascular vessel size of a host limb may dictate the polarity of duplication events. The presented hypothesis emphasizes that the interplay between the intrinsic properties of self-assembly into tissues and extrinsic factors which establish boundaries and morphologies is involved in both vascular and limb pattern formation.

Elevated rate of DNA synthesis and its correlation to cAMP-phosphodiesterase activity during induction of polydactyly in mouse embryos heterozygous for Hemimelia-extra toe (Hmx)

The induction of polydactyly in mouse embryos heterozygous for Hemimelia-extra toe (Hmx) is associated with aberrant outgrowth of the developing autopod on day 12 of gestation. We have quantitated the rate of DNA synthesis and the activity of cAMP-phosphodiesterase (PDE) that is characteristic of the prospective polydactylous region. Mid-stage 18 hind-limb buds were labeled with [3H]dThd either in situ using whole embryo culture, or as isolated preaxial autopod fragments cultured on a membrane substratum. The mean specific activities of incorporation were compared for normal (+/+) and mutant (Hmx/+) genotypes. A significant (P less than or equal to 0.01) 19% increase, peculiar to the prospective polydactylous region, was measured after 4 hours in embryo culture. The same increment was detected after 4 hours in organ culture, but was amplified linearly to 55% when incubation was extended to 20 hours. During this period, continuous exposure to 1.0 mM IBMX (3-isobutyl-1-methyl xanthine), an inhibitor of cAMP-PDE activity, "slowed down" the rate of DNA synthesis to untreated +/+ proportions. When cAMP-PDE activity was assayed in uncultured autopods, a significant (P less than or equal to 0.01) 18% increase was detected within the prospective polydactylous region specifically on stage 18 of gestation. This is the developmental phase during which polydactylous outgrowth is induced in situ. Thus, uncontrolled cAMP-PDE activity may, in part, provoke the enhanced rate of cell proliferation.

Cyclic nucleotides and growth regulation of the mandibular condylar cartilage of the rat in vitro

By exogenous interference in the intracellular level of cAMP and cGMP during growth in vitro without and with compression, an indication was obtained for the mediatorial involvement of these cyclic nucleotides in the major growth-processes in the mandibular condylar cartilage of 4-day-old rats. Raising the intracellular level of cAMP reduced proliferative activity in the prechondroblast zone, did not affect matrix synthesis by the functional chondroblasts and stimulated the process of hypertrophy. Intracellular elevation of cGMP had an antagonistic effect; it stimulated proliferation as well as matrix synthesis, but had no effect on hypertrophy. In this specific type of cartilage, cGMP can be considered as a major secondary intracellular messenger for proliferation-stimulating continuous biomechanical stimuli.

Hormonal stimulation of avian embryonic cartilage growth in vitro: histologic and ultrastructural features

We studied the histologic and ultrastructural features of embryonic chick cartilage after the cartilage had been incubated in serum-free medium that contained hormones and growth factors known to stimulate in vitro cartilage growth. Pelvic cartilages from 9 d chick embryos were incubated in BGJb ( Fitton -Jackson modification) medium alone (control) or medium containing one of the following: N6 monobutyryl cyclic AMP 0.5 mM, forskolin 100 microM, triiodothyronine (T3) 10 nM, insulin 45 nM, or somatomedin C 0.67 nM. At the end of 3 d of incubation the cartilages were fixed in buffered formalin. Significant growth (increases in size, wet and dry weight) was seen with each treatment group. N6-Monobutyryl cAMP treated cartilage had an increased number of flattened immature chondrocytes with large nuclei and prominent nucleoli. The histologic and ultrastructural features of forskolin treated cartilage were indistinguishable from N6-monobutyryl cAMP treatment. The T3 treated cartilage contained large hypertrophic chondrocytes with prominent lacunar typical of mature cartilage. T3 treated cartilage had considerable vacuole formation and dilated endoplasmic reticulum. Insulin and somatomedin treated cartilage had histologic appearance similar to control cartilage. Thus, the effects of various hormones on embryonic cartilage growth in vitro can be separated as to whether growth is the result of chondrocytic hyperplasia (cyclic AMP mediated), chondrocytic hypertrophy with maturation (T3), or a combination of both hyperplasia and hypertrophy (insulin and somatomedin-C).

Arachidonate metabolism during chondrogenesis in vitro

Chick embryo limb bud mesenchyme cells undergoing chondrogenesis in vitro were labeled with [3H] arachidonic acid and [14C] palmitic acid, and stimulated by mechanical means to convert a portion of their incorporated [3H] to radiolabeled compounds which co-chromatographed with authentic prostaglandins in the appropriate thin layer chromatography system. Chondrogenesis was (1) inhibited by concentrations of indomethacin or eicosa-5,8,11,14-tetraynoic acid which inhibited conversion of [3H] to prostaglandinlike compounds; and (2) stimulated by prostaglandin E2. We interpret these data to mean that (1) cells undergoing chondrogenesis in vitro are able to metabolize endogenous arachidonic acid to prostaglandins, and (2) synthesis of prostaglandinlike compounds is requisite to chondrogenesis in vitro.

The morphology and hormonal responsiveness of developing skeletal elements in chick limb buds

While parathyroid hormone (PTH), calcitonin (CT), and certain prostaglandins (PGs) are known to regulate the metabolism of both osteogenic and osteolytic cells of the adult skeleton through an adenosine 3', 5'-monophosphate-dependent mechanism, little is known about the development of this hormonally mediated response in embryonic skeletal tissues. In the present study, the responsiveness of embryonic skeletal elements to PTH and PGE2 was examined during various stages of development utilizing cAMP concentrations as an indicator of hormone-receptor interaction. The cytology of the limb skeletal system was examined also at each stage tested in order to compare the differentiated cellular phenotypes with their hormonal responsiveness. Prior to differentiation of cartilaginous elements in developing limb buds (stage 20-21), cells were responsive to PGE2 and epinephrine (EPI) but not to PTH. The first consistent response to PTH occurred coincident with the initial differentiation of the cartilage phenotype in limb buds (stage 24-25). A responsiveness to both PTH and PGE2 was progressively increased as maturation of cartilaginous and osteogenic elements occurred (stage 26-35). The initial response to CT was detected within cartilage rods in which osteogenic cells had differentiated (stage 33-35). The results of this study indicate that PGE2-sensitive cells exist within the developing limb prior to cytodifferentiation. The development of PTH responsiveness within embryonic chick limb buds is correlated with the onset of both chondrogenesis and osteogenesis in vivo.

Cyclic nucleotides affect growth and differentiation of cultured rat embryonic shields

The modified organ culture of rat embryonic shields provides favorable conditions during 2 weeks for the differentiation of main tissue types. Since the terminal differentiation in explants is inferior to that obtained in the homografts of the same shields under the kidney capsule, we tried to improve the culture medium by adding some known regulatory molecules: db-cAMP, db-cGMP, ATP, AMP, and butyric acid. These agents were added to the liquid medium in the concentration of 1 mM. In the first part of the study the explants were fixed and weighed after 8 or 14 days in vitro culture, and histological sections were examined. When the explants were treated with db-cAMP during the second week of culture, the skeletal muscle appeared more frequently in the treated series than in controls, and the weight of the treated explants was sometimes increased when compared with the control series. The db-cGMP had no effect on differentiation, but stimulated the growth of the explants when applied during the first week of culture. On the contrary, the db-cAMP when added during the first week, severely impeded the growth of explants. Other agents seem to be ineffective. In the second part, the content of cAMP and cGMP was measured in normal explants. The radioimmunoassay showed the same content of cAMP and cGMP during the entire culture period. In the third part of our study the incorporation of tritiated uridine and tritiated thymidine was measured during the second week of culture after the addition of db-cAMP. During the first days of treatment with db-cAMP the uptake of tritiated uridine and thymidine was inhibited, whereas on the seventh day the uptake was similar to that of the control. We can conclude that both cyclic nucleotides have a visible effect on growth whereas only cAMP has a positive impact on the differentiation of myotubes in cultured rat embryonic shields.

Teratogenic effects of cholinergic insecticides in chick embryos--IV. The role of tryptophan in protecting against limb deformities

The mechanism by which organophosphate (OP) insecticides cause micromelia in embryonic chick limbs was examined using a tissue culture approach. Limb bud cells in micromass culture were assayed for their proliferative and chondrogenic activities, [3H]thymidine and 35SO4 = incorporation, respectively, into the trichloroacetic acid-insoluble constituents of the cell masses and/or the accumulation of 35S-labeled soluble macromolecular products in the culture medium. There was no obvious correlation between either the teratogenicity or toxicity of the insecticide in ovo and the inhibition of proliferation and chondrogenesis in vitro. In addition, nicotinamide, which prevents insecticide-induced micromelia in ovo, did not improve the proliferative and chondrogenic performance of insecticide-treated cells in culture. On the other hand, 2-pyridinealdoxime methochloride, which offers little or no protection against micromelia in ovo, did protect both the proliferative and chondrogenic activities of the limb bud cells in micromass culture. These observations suggest that the actions of the insecticides on the cells in culture are not the same as those that produce micromelia in ovo. L-Tryptophan antagonized OP insecticide-induced micromelia in the embryo. In micromass culture, a much greater concentration of tryptophan was needed to support the chondrogenic than the proliferative activities of the limb bud cells. Moreover, a greater concentration of tryptophan was needed to support the chondrogenic activities of the leg bud than the wing bud cells. These in vitro responses of the limb bud cells to tryptophan deprivation are analogous to the in ovo response of the limbs to the teratogenic OP insecticides. A possible explanation of the roles of tryptophan and nicotinamide in preventing the limb deformities is offered.

The effect of prostaglandins on in vitro limb cartilage differentiation

A variety of studies indicate that a key event in limb chondrogenic differentiation is a cellular condensation process during which an intimate cell-cell interaction occurs that triggers cartilage differentiation by elevating cAMP levels. It has recently been demonstrated that when limb mesenchymal cells are subjected to high density monolayer culture under conditions conducive to chondrogenesis, they synthesize several prostaglandins, including PGE2 and prostacyclin, which are important local modulators of cAMP formation in a number of cells and tissues. In the present study, we demonstrate that exogenous PGE2 stimulates the in vitro chondrogenic differentiation of the subridge mesoderm of the embryonic chick limb bud. The stimulatory effect of PGE2 is greatly potentiated by the phosphodiesterase inhibitor, theophylline, suggesting its influence on chondrogenesis is mediated by its ability to increase cAMP levels. The stimulatory effect of PGE2 is dose-dependent and can be detected at a concentration as low as 10(-8)M. PGE1 is just as effective as PGE2 in stimulating in vitro chondrogenesis, whereas PGA1 and PGF1 alpha are less than half as effective. Thromboxane B2 has no effect on chondrogenesis. On the basis of our results, the possibility that endogenous prostaglandins might regulate limb cartilage differentiation by acting as local regulators of cAMP content is discussed.

Effects of prostaglandins on cyclic AMP levels in isolated cells from developing chick limbs

Effects of prostaglandins (PGs) on accumulation of cyclic AMP (cAMP) in the presence of a phosphodiesterase inhibitor were investigated in cells isolated from avian limb buds at various stages of development. Cells were responsive to PGE2 at the earliest stage investigated (stage 20-21) which was well in advance of specific cytodifferentiation of limb tissues. At three later stages (24-25; 26-28; 30-32), the responsiveness of cells isolated from the developing skeletal anlagen of the limb progressively increased coincident with the differentiation and maturation of the cartilage phenotype. Cells isolated from stage 26-28 cartilage rods were responsive also to prostacyclin (PGI2); however, the response produced was only about 50% of the response to an equivalent concentration of PGE2. Cells were not responsive to either PGF2 alpha or 6-keto PGF1 alpha, at concentrations of 30-33 micrograms/ml demonstrating a degree of specificity for PGE2 and PGI2. In the absence of the phosphodiesterase inhibitor, PGE2 increased cAMP accumulation two-fold over the controls and produced a concentration-dependent response between 0.3-30 micrograms/ml. The results demonstrate that PGs are capable of modulating cAMP levels of undifferentiated limb mesenchymal cells as well as embryonic cartilage cells and suggest a role for these compounds in limb chondrogenesis.

Analysis of cartilage differentiation from skeletal muscle grown on bone matrix. III. Environmental regulation of glycosaminoglycan and proteoglycan synthesis

The ability of numerous nutritional and topographic factors to influence differentiation of embryonic mesenchyme has given rise to several theories which attempt to explain the development of muscle and cartilage from these similar-appearing cells. Some theories are challenged by the observation that a substratum of demineralized bone is capable of supporting the transformation of skeletal muscle into cartilage in vitro and that the potential to form cartilage still resides within cloned myoblasts and fibroblasts of skeletal muscle. In the present study, culture media CMRL-1066, minimal essential medium (MEM), and F-12 provide varied nutritional environments and are tested for their ability to support the morphological and biochemical transformation of skeletal muscle into cartilage. Morphologically, CMRL-1066 reproducibly supports hyaline cartilage formation, whereas MEM does so in only one out of three explants onto demineralized bone, and F-12 is incapable of supporting formation of a hyaline matrix. Biochemically, each medium is sufficient to elicit synthesis of cartilage-like patterns of sulfated glycosaminoglycans and proteoglycan monomer. Synthesis of hyaluronic acid (HA) initially increases in explants grown in CMRL-1066, but decreases prior to chondrogenesis. MEM elicits a similar increase in HA synthesis, but the subsequent decrease is not as rapid. In F-12, synthesis remains depressed throughout the experiment. The data show that increases in HA synthesis occur concurrent with the appearance of fibroblast-like cells, which normally precede chondroblasts. Decreases in HA synthesis correlate well with the onset of chondrogenesis. Explants grown in CMRL-1066 reproducibly from cartilage and synthesize the greatest amounts of proteoglycan aggregate. Those grown in MEM form cartilage infrequently, synthesize reduced amounts of proteoglycan aggregate-like material, and contain greater amounts of HA, of low molecular weight. The data demonstrate that chondrogenesis can be subtly regulated by environmental factors, and such factors regulate both the morphological and biochemical expression of the phenotype through HA synthesis.

Cyclic nucleotides during chondrogenesis: concentration and distribution in vivo and in vitro

This study correlates endogenous levels of cAMP and cGMP with their immunohistochemical localization during chondrogenic differentiation of C57B1/6J mouse limb mesenchyme in vivo and in vitro. A transient decrease in cGMP but not cAMP was found from days 12 to 13 in vivo correlating with early stages of chondrogenesis in the developing limb. Intracellular levels of both cAMP and cGMP in high density limb mesenchyme cultures increased 25% after 24 hr in culture when aggregate and nodule formation was detectable. When cells were seeded at different initial plating densities to delay the onset of aggregate and nodule formation, increased levels of intracellular cAMP correlated temporally with the appearance of nodules. Both cyclic AMP and cGMP were immunohistochemically localized in perichondrial cells and chondrocytes in vivo and in vitro. Therefore, (1) cAMP levels correlated temporally with the appearance of chondrogenic cells and (2) cAMP and cGMP were immunohistochemically localized to chondrogenic cells. These data indicate that fluctuations of both cAMP and cGMP levels may be involved in limb cartilage differentiation. Although increases in both nucleotides were found to correlate with the onset of chondrogenesis in vitro, in vivo data suggest that the amount of cAMP relative to cGMP rather than the absolute amount of an individual cyclic nucleotide may be more significant in modulating differentiation.

Adenosine 3',5'-monophosphate: a modulator of embryonic chick cartilage growth

We tested the hypothesis that cyclic AMP plays a significant role in modulating the growth of embryonic chick cartilage by determining whether cyclic AMP levels change in growing embryonic cartilage and whether cyclic AMP could stimulate embryonic cartilage growth in a long term in vitro organ culture. Cyclic AMP levels were low (0.1 pmol/mg wet wt) in 8-d chick embryo pelvic cartilage, and increased progressively through the 11th d of embryonic development at which time they reached a maximum (1.8 pmol/mg wet weight) and thereafter remained constant. We developed an in vitro organ culture system to determine whether cyclic AMP, a factor known to stimulate radiolabeled precursor incorporation into macromolecules in short-term studies does, in fact, stimulate growth of cartilage. Individual pelvic cartilages were isolated from 9-d chick embryos, placed in serum-free medium (BGJb-FJ modification) and incubated for 3 to 5 d during which time they increased in size (39 and 60% in length, respectively), wet weight (90 and 141%, respectively), and content of total soluble protein (30 and 48%, respectively). N6-monobutyryl cyclic AMP (BtcAMP) added to the medium caused a dose-dependent (0.05 to 1.0 mM) stimulation of growth. After 3 d of incubation, 1.0 mM BtcAMP increased wet weight (125%), [14C]leucine incorporation into protein (75%), and [3H]thymidine incorporation into DNA (48%) compared with control cartilages incubated in medium alone. 1-methyl-3-isobutyl xanthine, a phosphodiesterase inhibitor, also increased cartilage growth above control while sodium butyrate, AMP, and ATP had no effect. Histological examination of cartilage grown in medium was similar to that of cartilage developing in ovo, whereas, cartilage grown in medium containing BtcAMP showed marked hypercellularity with many immature chondrocytes. Our observations are compatible with the hypothesis that cyclic AMP can significantly modulate the growth of embryonic cartilage.

Immunohistochemical localization of cyclic AMP during normal and abnormal chick and mouse limb development

This paper describes the immunohistochemical localization of cAMP during limb chondrogenesis in talpid3 chick, brachypod mouse, and normal embryos. Comparisons were made between chick wing buds at Stages 22, 25, and 30, and mouse hind limb buds at Days 11, 12.5 and 14. At Stage 22, the normal mesenchyme in the chick displayed areas of bright fluorescence compared to a lesser intense and more evenly distributed fluorescence in talpid3. Sections of the central region from normal Stage 25 limb buds exhibited an intense fluorescence that was uniformly distributed, whereas, in talpid3 staining was more mosaic with some areas fluorescing brightly and others showing little fluorescence. At Stage 30 the staining pattern was similar between normal and talpid3, with the fluorescence being brighter in the cartilage tissue than in the surrounding soft tissue. Difference in the staining patterns of normal and brachypod limb tissue were not detectable. At Days 11 and 12.5, tissue from both genotypes displayed a very bright, uniform fluorescence. In the 14-day hind limb buds, the staining patterns were comparable to those observed in Stage 30 chick wing buds. However, under in vitro conditions conducive for the expression of the chondrogenic phenotype, differences in the intensity and extensiveness of fluorescent staining were detectable in cultures derived from 12-day normal and brachypod hind limb mesenchyme. Compared to the control, the uneven distribution of immunofluorescence in the talpid3 limb buds and the differences in intensity and extensiveness of fluorescence in the brachypod cultures support the hypothesis that cAMP is involved in limb cartilage differentiation.