Thyroid hormone, insulin, and glucocorticoids are sufficient to support chondrocyte differentiation to hypertrophy: a serum-free analysis

Expression of thyroid hormone receptor isoforms in rat growth plate cartilage in vivo

Although thyroid hormone has been known for many years to be a potent regulator of skeletal maturation in vivo, it has not definitively been determined whether this effect is a result of a direct or indirect action of the hormone. Previous in vivo studies have suggested that thyroid hormone may stimulate longitudinal bone growth by increasing the secretion of growth hormone; however, growth hormone alone is unable to stimulate cartilage maturation. There are also indications that thyroid hormone is able to act directly on growth plate chondrocytes through growth hormone-independent mechanisms. In this study, we demonstrate that rat growth plate chondrocytes in vivo express genes encoding three of the four isoforms of the thyroid hormone receptors described to date, but the corresponding protein can only be detected for the TRalpha1 and TRbeta1 isoforms of the receptor. As has been noted in other tissues, there is generally poor correlation between the mRNA levels for each isoform and the relative amount of corresponding protein as measured by immunoblotting, suggesting the possibility that receptor expression may be regulated by post-transcriptional mechanisms.

Chondrogenic differentiation of cultured human mesenchymal stem cells from marrow

In the adult human, mesenchymal stem cells (MSCs) resident in bone marrow retain the capacity to proliferate and differentiate along multiple connective tissue lineages, including cartilage. In this study, culture-expanded human MSCs (hMSCs) of 60 human donors were induced to express the morphology and gene products of chondrocytes. Chondrogenesis was induced by culturing hMSCs in micromass pellets in the presence of a defined medium that included 100 nM dexamethasone and 10 ng/ml transforming growth factor-beta(3) (TGF-beta(3)). Within 14 days, cells secreted an extracellular matrix incorporating type II collagen, aggrecan, and anionic proteoglycans. hMSCs could be further differentiated to the hypertrophic state by the addition of 50 nM thyroxine, the withdrawal of TGF-beta(3), and the reduction of dexamethasone concentration to 1 nM. Increased understanding of the induction of chondrogenic differentiation should lead to further progress in defining the mechanisms responsible for the generation of cartilaginous tissues, their maintenance, and their regeneration.

Cartilage-derived morphogenetic proteins and cartilage morphogenesis

Cartilage morphogenesis is a prerequisite for skeletal development and maintenance. The morphogenesis of cartilage determines the shape of bones, and joints including articular cartilage, ligaments, and tendon. This article reviews the recent advances in cartilage-derived morphogenetic proteins (CDMPs) and related bone morphogenetic proteins (BMPs). Cartilage-derived morphogenetic proteins (CDMPs) are related to BMPs and are critical for cartilage and joint morphogenesis. Cartilage morphogenesis is a multistep cascade that includes factors for initiation, promotion, and maintenance of cartilage phenotype. The extracellular matrix of cartilage consists of a constellation of macromolecules such as collagens, proteoglycans, and glycoproteins. Morphogens bind to extracellular matrix components and assemble a morphogenetic scaffold. Recent advances in CDMPs may aid in articular cartilage repair and regeneration.

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

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

Thyroid hormone inhibits growth and stimulates terminal differentiation of epiphyseal growth plate chondrocytes

As a continuation of our studies on mineralization in epiphyseal growth plate (GP) chondrocyte cultures, the effects of tri-iodothyronine (T3) in both beta-glycerophosphate-containing, serum-free (HL-1) and beta-glycerophosphate-free, serum-containing medium (DATP5) were studied. The GP cells responded to T3 in a serum-, stage-, and dosage-dependent manner. Added at graded levels (0.1-10.0 nM) to preconfluent cultures (from day 7) in both HL-1 and DATP5, T3 caused progressive decreases in protein, collagen, and DNA synthesis but increased mineral deposition. In postconfluent cultures, these effects of T3 were generally muted. In preconfluent cultures, proteoglycan (PG) levels were not significantly affected in DATP5, although in HL-1 they were decreased by approximately 50%. In postconfluent cultures, T3 increased PG levels in DATP5 but had no effect in HL-1. In HL-1, alkaline phosphatase (ALP) activity was progressively increased by 200-500% in both pre- and postconfluent cultures. In DATP5 in preconfluent cultures, T3 initially stimulated but later suppressed ALP; in postconfluent cultures, T3 also transiently increased ALP but did not suppress activity upon longer exposure. The inhibitory effects of T3 on protein, PG, and DNA levels of GP chondrocytes suggest that in vivo its effects on bone growth must occur primarily after cellular proliferation. Apparently by binding to the 50 kDa thyroxine-binding globulin, which cannot penetrate the PG barrier, accessibility of T3 to GP chondrocytes is limited until the time of vascular penetration when its stimulatory effects on ALP and mineral deposition become critical for continued bone development.

Modulation of chondrocyte proliferation by ascorbic acid and BMP-2

Chondrocytes show an unusual ability to thrive under serum-free conditions as long as insulin, thyroxine, and cysteine are present. Studies with sternal chondrocytes from chick embryos indicate that thymidine incorporation in chondrocytes cultured under serum-free conditions is 30-50% of that seen with fetal bovine serum (FBS). In contrast, skin fibroblast proliferation in serum-free culture is <5% of that seen with serum. Addition of 30-50 microM ascorbic acid to serum-free medium stimulates chondrocyte proliferation 4-5x, resulting in levels of thymidine incorporation higher than that seen with 10% serum. Three to five hours of ascorbate exposure is sufficient to stimulate proliferation, with maximal stimulation seen after 12-15 h. Bromo-deoxyuridine (BrdU) labelling indicated that approximately 25% of chondrocytes transit S phase during a 4-h period (16-20 h after ascorbate). Once maximal stimulation is reached, the proliferation rate remains fairly constant over at least 40 h. Ascorbate therefore increases the steady-state level of chondrocytes in the cycle. Because the stimulation of chondrocyte proliferation was greater than the net increase in cell numbers, we examined the level of apoptosis. Nuclear morphology, terminal uridine nucleotide end-labelling (TUNEL) assay, and 7-AAD/Hoechst dye FACS analyses all indicated that approximately 15% of the ascorbate-treated chondrocytes were undergoing apoptosis, while only 5% of the control chondrocytes were apoptotic. When prehypertrophic chondrocytes from the cephalic region of embryonic sternae were stimulated to undergo hypertrophy with rhBMP-2 + ascorbate, levels of apoptosis were similar to that seen with ascorbate alone. In contrast, treatment of caudal chondrocytes with BMP plus ascorbate does not induce hypertrophy, and the proportion of apoptotic cells was less than that seen with ascorbate alone. These results imply that in chondrocytes the transition to hypertrophy is associated with a decreased number of proliferating cells and a relatively high level of apoptosis.

Role of the subchondral vascular system in endochondral ossification: endothelial cells specifically derepress late differentiation in resting chondrocytes in vitro

Endochondral ossification in growth plates proceeds through several consecutive steps of late cartilage differentiation leading to chondrocyte hypertrophy, vascular invasion, and, eventually, to replacement of the tissue by bone. It is well established that the subchondral vascular system is pivotal in the regulation of this process. Cells of subchondral blood vessels act as a source of vascular invasion and, in addition, release factors influencing growth and differentiation of chondrocytes in the avascular growth plate. To elucidate the paracrine contribution of endothelial cells we studied the hypertrophic development of resting chondrocytes from the caudal third of chick embryo sterna in co-culture with endothelial cells. The design of the experiments prevented cell-to-cell contact but allowed paracrine communication between endothelial cells and chondrocytes. Under these conditions, chondrocytes rapidly became hypertrophied in vitro and expressed the stage-specific markers collagen X and alkaline phosphatase. This development also required signaling by thyroid hormone in synergy. Conditioned media could replace the endothelial cells, indicating that diffusible factors mediated this process. By contrast, smooth muscle cells, fibroblasts, or hypertrophic chondrocytes did not secrete this activity, suggesting that the factors were specific for endothelial cells. We conclude that endochondral ossification is under the control of a mutual communication between chondrocytes and endothelial cells. A finely tuned balance between chondrocyte-derived signals repressing cartilage maturation and endothelial signals promoting late differentiation of chondrocytes is essential for normal endochondral ossification during development, growth, and repair of bone. A dysregulation of this balance in permanent joint cartilage also may be responsible for the initiation of pathological cartilage degeneration in joint diseases.

In vitro chondrogenesis of bone marrow-derived mesenchymal progenitor cells

A culture system that facilitates the chondrogenic differentiation of rabbit bone marrow-derived mesenchymal progenitor cells has been developed. Cells obtained in bone marrow aspirates were first isolated by monolayer culture and then transferred into tubes and allowed to form three-dimensional aggregates in a chemically defined medium. The inclusion of 10(-7) M dexamethasone in the medium induced chondrogenic differentiation of cells within the aggregate as evidenced by the appearance of toluidine blue metachromasia and the immunohistochemical detection of type II collagen as early as 7 days after beginning three-dimensional culture. After 21 days, the matrix of the entire aggregate contained type II collagen. By 14 days of culture, there was also evidence for type X collagen present in the matrix and the cells morphologically resembled hypertrophic chondrocytes. However, chondrogenic differentiation was achieved in only approximately 25% of the marrow cell preparations used. In contrast, with the addition of transforming growth factor-beta 1 (TGF-beta 1), chondrogenesis was induced in all marrow cell preparations, with or without the presence of 10(-7) M dexamethasone. The induction of chondrogenesis was accompanied by an increase in the alkaline phosphatase activity of the aggregated cells. The results of RT-PCR experiments indicated that both type IIA and IIB collagen mRNAs were detected by 7 days postaggregation as was mRNA for type X collagen. Conversely, the expression of the type I collagen mRNA was detected in the preaggregate cells but was no longer detectable at 7 days after aggregation. These results provide histological, immunohistochemical, and molecular evidence for the in vitro chondrogenic differentiation of adult mammalian progenitor cells derived from bone marrow.

Induction and characterization of metallothionein in chicken epiphyseal growth plate cartilage chondrocytes

Following exposure to cadmium or zinc, chickens were sacrificed and the liver, kidney, and bone epiphyseal growth plates harvested. When cytosolic extracts of the growth plate cartilage were fractionated by gel filtration chromatography, a protein with high metal-binding capacity and low ultraviolet (UV) absorbance eluted in the same position as liver metallothionein (MT) and a MT standard. Cd or Zn treatment resulted in a 25-fold or 5-fold induction in growth plate MT, respectively. In liver the greatest level of MT induction was seen with short-term Cd exposures. In contrast, MT levels in the growth plate increased as the duration of Cd exposure increased. Induction of MT in growth plate chondrocyte cell cultures was observed for media Cd concentrations of > or = 0.1 microM and Zn concentrations of > or = 100 microM. Basal and inducible levels of MT declined through the culture period and were lowest in the terminally differentiated mineralized late stages of the culture. Alkaline phosphatase activity was also lowest in the late-stage cultures, while total cellular protein increased throughout the culture period. Treatment of chondrocytes with Zn prior to Cd exposure resulted in a protective induction of MT. Pre-treatment of chondrocytes with dexamethasone resulted in suppressed synthesis of MT upon Cd exposure and greater Cd toxicity. Both Cd and Zn resulted in significantly increased levels of MT mRNA in chondrocyte cell cultures. Dexamethasone treatment resulted in an approximate 2- to 3-fold increase in MT mRNA. This is contrary to the finding that MT protein levels were decreased by dexamethasone. The findings suggest that an increased rate of MT degradation in dexamethasone-treated and late-stage chondrocyte cultures may be associated with the terminally differentiated phenotype.

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

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

Chondrocyte survival and differentiation in situ are integrin mediated

Chondrocytes in specific areas of the chick sternum have different developmental fates. Cephalic chondrocytes become hypertrophic and secrete type X collagen into the extracellular matrix prior to bone deposition. Middle and caudal chondrocytes remain cartilaginous throughout development and continue to secrete collagen types II, IX, and XI. The interaction of integrin receptors with extracellular matrix molecules has been shown to affect cytoskeleton organization, proliferation, differentiation, and gene expression in other cell types. We hypothesized that chondrocyte survival and differentiation including the deposition into interstitial matrix of type X collagen may be integrin receptor mediated. To test this hypothesis, a serum-free organ culture sternal model that recapitulates normal development and maintains the three-dimensional relationships of the tissue was developed. We examined chondrocyte differentiation by five parameters: type X collagen deposition into interstitial matrix, sternal growth, actin distribution, cell shape, and cell diameter changes. Additional sterna were analyzed for apoptosis using a fragmented DNA assay. Sterna were organ cultured with blocking antibodies specific for integrin subunits (alpha2, alpha3, or beta1). In the presence of anti-beta1 integrin (25 microg/ml, clone W1B10), type X collagen deposition into interstitial matrix and sternal growth were significantly inhibited. In addition, all chondrocytes were significantly smaller, the actin was disrupted, and there was a significant increase in apoptosis throughout the specimens. Addition of anti-alpha2 (10 microg/ml, clone P1E6) or anti-alpha3 (10 microg/ml, clone P1B5) integrin partially inhibited type X collagen deposition into interstitial matrix; however, sternal growth and cell size were significantly decreased. These data are the first obtained from intact tissue and demonstrate that the interaction of chondrocytes with extracellular matrix is required for chondrocyte survival and differentiation.

Rapid chondrocyte maturation by serum-free culture with BMP-2 and ascorbic acid

In serum-containing medium, ascorbic acid induces maturation of prehypertrophic chick embryo sternal chondrocytes. Recently, cultured chondrocytes have also been reported to undergo maturation in the presence of bone morphogenetic proteins or in serum-free medium supplemented with thyroxine. In the present study, we have examined the combined effect of ascorbic acid, BMP-2, and serum-free conditions on the induction of alkaline phosphatase and type X collagen in chick sternal chondrocytes. Addition of either ascorbate or rhBMP-2 to nonconfluent cephalic sternal chondrocytes produced elevated alkaline phosphatase levels within 24-72 h, and simultaneous exposure to both ascorbate and BMP yielded enzyme levels at least threefold those of either inducer alone. The effects of ascorbate and BMP were markedly potentiated by culture in serum-free medium, and alkaline phosphatase levels of preconfluent serum-free cultures treated for 48 h with BMP+ascorbate were equivalent to those reached in serum-containing medium only after confluence. While ascorbate addition was required for maximal alkaline phosphatase activity, it did not induce a rapid increase in type X collagen mRNA. In contrast, BMP added to serum-free medium induced a three- to fourfold increase in type X collagen mRNA within 24 h even in the presence of cyclohexamide, indicating that new protein synthesis was not required. Addition of thyroid hormone to serum-free medium was required for maximal ascorbate effects but not for BMP stimulation. Neither ascorbate nor BMP induced alkaline phosphatase activity in caudal sternal chondrocytes, which do not undergo hypertrophy during embryonic development. These results indicate that ascorbate+BMP in serum-free culture induces rapid chondrocyte maturation of prehypertrophic chondrocytes. The mechanisms for ascorbate and BMP action appear to be distinct, while BMP and thyroid hormone may share a similar mechanism for induction.

Apoptosis of terminally differentiated chondrocytes in culture

During the process of endochondral ossification chondrocytes progress through stages of terminal differentiation culminating in apoptotic death. We have developed a serum-free suspension culture that allows terminal differentiation and facilitates the investigation of factors affecting chondrocyte apoptosis. We have found that chondrocytes not committed to terminal differentiation, i.e., those from the caudal region of chick embryo sterna, a region that remains cartilaginous for some months after the chick hatches, maintained high viability in serum-free suspension culture. A strong dependence of viability on culture density and sensitivity to induction of apoptosis with the protein kinase inhibitor, staurosporine, was consistent with the proposal that these chondrocytes, like nearly all cells, require intercellular communication for survival. Chondrocytes that were committed to terminal differentiation, i.e., those from the cephalic region of chick embryo sterna, a region that is replaced by bone before the chick hatches, expressed the hypertrophic phenotype but maintained their viability in culture for only approximately 6 days. Subsequent cell death was very consistent between cultures and shown to occur by an apoptotic process by analysis of DNA fragmentation and cell morphology. Short-term viability of hypertrophic chondrocytes was independent of culture density and relatively resistant to treatment with staurosporine. Induction of the hypertrophic phenotype in immature chondrocytes committed them to cell death and prevention of expression of the hypertrophic phenotype prevented cell death. We conclude that commitment of chondrocytes to terminal differentiation is associated with a commitment to apoptosis and apoptosis of hypertrophic chondrocytes in growth cartilage does not require initiation by external signals.

Transferrin promotes endothelial cell migration and invasion: implication in cartilage neovascularization

During endochondral bone formation, avascular cartilage differentiates to hypertrophic cartilage that then undergoes erosion and vascularization leading to bone deposition. Resting cartilage produces inhibitors of angiogenesis, shifting to production of angiogenic stimulators in hypertrophic cartilage. A major protein synthesized by hypertrophic cartilage both in vivo and in vitro is transferrin. Here we show that transferrin is a major angiogenic molecule released by hypertrophic cartilage. Endothelial cell migration and invasion is stimulated by transferrins from a number of different sources, including hypertrophic cartilage. Checkerboard analysis demonstrates that transferrin is a chemotactic and chemokinetic molecule. Chondrocyte-conditioned media show similar properties. Polyclonal anti-transferrin antibodies completely block endothelial cell migration and invasion induced by purified transferrin and inhibit the activity produced by hypertrophic chondrocytes by 50-70% as compared with controls. Function-blocking mAbs directed against the transferrin receptor similarly reduce the endothelial migratory response. Chondrocytes differentiating in the presence of serum produce transferrin, whereas those that differentiate in the absence of serum do not. Conditioned media from differentiated chondrocytes not producing transferrin have only 30% of the endothelial cell migratory activity of parallel cultures that synthesize transferrin. The angiogenic activity of transferrins was confirmed by in vivo assays on chicken egg chorioallantoic membrane, showing promotion of neovascularization by transferrins purified from different sources including conditioned culture medium. Based on the above results, we suggest that transferrin is a major angiogenic molecule produced by hypertrophic chondrocytes during endochondral bone formation.

Effects of concanavalin A on chondrocyte hypertrophy and matrix calcification

Resting chondrocytes do not usually undergo differentiation to the hypertrophic stage and calcification. However, incubating these cells with concanavalin A resulted in 10-100-fold increases in alkaline phosphatase activity, binding of 1,25(OH)2-vitamin D3, type X collagen synthesis, 45Ca incorporation into insoluble material, and calcium content. On the other hand, other lectins tested (including wheat germ agglutinin, lentil lectin, pea lectin, phytohemagglutinin-L, and phytohemagglutinin-E) marginally affected alkaline phosphatase activity, although they activate lymphocytes. Methylmannoside reversed the effect of concanavalin A on alkaline phosphatase within 48 h. Concanavalin A did not increase alkaline phosphatase activity in articular chondrocyte cultures. In resting chondrocyte cultures, succinyl concanavalin A was as potent as concanavalin A in increasing alkaline phosphatase activity, the incorporation of [35S]sulfate, D-[3H]glucosamine, and [3H]serine into proteoglycans, and the incorporation of [3H]serine into protein, although concanavalin A, but not succinyl concanavalin A, induced a rapid change in the shape of the cells from flat to spherical. These findings suggest that concanavalin A induces a switch from the resting, to the growth-plate stage, and that this action of concanavalin A is not secondary to changes in the cytoskeleton. Chondrocytes exposed to concanavalin A may be useful as a novel model of endochondral bone formation.

Constitutive expression of parathyroid hormone-related peptide (PTHrP) stimulates growth and inhibits differentiation of CFK2 chondrocytes

We have examined the effects of constitutive expression of PTHrP on the growth and differentiation of populations of cells derived from a clonal chondrocytic cell line, CFK2. Cells were stably transfected with cDNA encoding either full-length, secretory PTHrP (CFK2P) or nonsecretory PTHrP (CFK2P-SS). In cultures of cells plated at low density, secretory PTHrP acted as a potent mitogen compared with nonsecretory PTHrP or exogenous PTHrP-(1-34), both of which stimulated only a minor increase in proliferation. In populations of control cells maintained postconfluent for several weeks, there was a dramatic increase in expression of mRNA for type II collagen, aggrecan, and link protein. Addition of exogenous PTHrP-(1-34) at a concentration of 10(-8) M to these cultures was ineffective in inhibiting this time-dependent increase in expression of matrix proteins. In contrast, populations of cells producing either secretory or nonsecretory forms of PTHrP, maintained over the same time period, demonstrated an almost complete inhibition of mRNA expression for matrix proteins. These observations demonstrate that PTHrP acts as a bifunctional modulator of chondrogenesis and that some of its biological activity is exerted via a mechanism distinct from the recognised signal transduction pathways linked to the PTH/PTHrP receptor.

In serum-free culture thyroid hormones can induce full expression of chondrocyte hypertrophy leading to matrix calcification

A serum-free culture system has been developed to examine the biologic factors involved in the regulation of cellular maturation, extracellular matrix assembly, and calcification in the physis of the bovine fetal growth plate. Isolated prehypertrophic chondrocytes in high density culture undergo a process of cellular maturation whereby full expression of the hypertrophic phenotype is characterized first by type X collagen synthesis followed by matrix calcification. Using this culture system, we compared the capacity of tri-iodothyronine (T3) with thyroxine (T4) to stimulate expression of the hypertrophic phenotype and matrix calcification in three (B, C, and D) maturationally distinct prehypertrophic chondrocyte subpopulations. The B cell subpopulation was the most mature followed by C and D subpopulations in order of decreasing maturity. Comparisons were made to cultures in fetal calf serum (FCS). In Dulbecco's modified Eagle's medium supplemented with insulin, transferrin, and selenium, both hormones (T3/T4) separately induced, in a dose-dependent manner, chondrocyte maturation to the hypertrophic phenotype characterized by increased type X collagen mRNA and induction of protein synthesis of this molecule, together with increased alkaline phosphatase activity, and eventually calcification of the extracellular matrix. Such cellular maturation to the hypertrophic phenotype was not observed in the absence of T3 or T4 with subpopulations C and D. Only in older fetuses (> 210 days) was this observed and then only in the B subpopulation. Furthermore, T3 was at least 50-fold more potent than T4. The effects of T3 were most pronounced with the most immature cells (subpopulations C and D) where, in the case of the subpopulation C, in contrast to 0.5 nM T3 50 nM T4 was unable to induce expression of the hypertrophic phenotype. Alkaline phosphatase activity was also increased in the C cell subpopulation treated with 1 nM T3 (35.5 U/micrograms of DNA) over that supplemented with 50 nM T4 (7.8 U/micrograms of DNA). Furthermore, matrix calcification, measured by the incorporation of 45Ca2+ into the cell layer, always occurred earlier in cells cultured with T3 compared with T4. Cellular maturation to the hypertrophic phenotype was not accompanied by significant changes in DNA content; this ordinarily increases during culture in the presence of serum. Compared with cells cultured in the presence of serum, either thyroid hormone more potently induced cellular maturation. This study demonstrates that the most immature chondrocytes at the prehypertrophic stage are direct targets for T3 and T4 and, to a much a lesser degree, that either hormone is able to induce full chondrocyte hypertrophy from an early maturational stage leading to matrix calcification. But T3 is much more potent than T4. These studies also offer a new serum-free chemically defined medium containing T3 or T4 for the culture of defined prehypertrophic chondrocytes that supports matrix assembly, hypertrophic expression, followed by matrix calcification.

Intermittent compression stimulates cartilage mineralization

The effects of intermittent hydrostatic compressive force (ICF; 13 kPa applied at 0.3 Hz frequency), as a substitute for moderate loading in vivo, on ossifying bone organ cultures, were evaluated by means of (histo)-morphometry. In earlier studies, biochemical tests have shown an increased 45Ca intake and an increased alkaline phosphatase activity in bone organ cultures that received ICF, suggesting that ICF promoted matrix mineralization. The purpose of this study was to examine whether an effect of ICF on mineralization can be described by means of histomorphometrical analysis. Fetal mouse metatarsal bone rudiments were cultured for 5 days in serum-free medium, with (experimental) or without (control) ICF. Linear measurements taken during culture demonstrated that the dark zone in the center of the rudiment, representing mineralized hypertrophic cartilage, became significantly longer in the group that received ICF when compared with the control group. This finding was in conformation with the former studies. Histological sections of the rudiments, stained with Goldner's trichrome method were used to study changes at the cellular level and to describe the position and relative amount of mineralizing cartilage matrix (defined as Goldner-positive matrix [GPM]). Histomorphometry demonstrated that ICF treatment significantly increased the length of the hypertrophic cartilaginous zone and enhanced the amount of GPM between the mineralizing hypertrophic chondrocytes. However, the total length of the zone containing GPM was not increased, nor was the future bone collar, consisting of a thin osteoid seam, lengthened by ICF. These data indicate that the cellular processes involved in chrondrocyte hypertrophy were accelerated by ICF, as well as the extracellular processes leading to matrix mineralization. The study supports the earlier conclusion that embryonic bone rudiments are sensitive to mechanical stimulation and that moderate loading promotes their ossification in vitro.

Cartilage morphogenesis: role of bone and cartilage morphogenetic proteins, homeobox genes and extracellular matrix

Cartilage morphogenesis is one of the central topics in skeletal development. Cartilage geometry determines the future architecture of bones, joints and associated ligaments and tendons. Recent progress in this area has come from purification, cloning and expression of genes encoding bone and cartilage morphogenetic proteins (BMPs and CDMPs). BMPs initiate de novo cartilage and bone differentiation. BMPs are a family of pleiotropic signals for progenitor cell migration by chemotaxis, proliferation, and differentiation. Very recently another class of related morphogenetic proteins, CDMPs have been isolated and cloned. CDMPs may be critical for mesenchymal condensation prior to overt cartilage differentiation, the first step in morphogenesis of both cartilage and bone. The cartilage morphogenetic cascade is a cellular and molecular continuum driven by regulatory signalling molecules such as BMPs and CDMPs and their receptors, homeobox genes, transcription factors, and finally the synthesis and supramolecular assembly of structural macromolecules of the extracellular matrix. BMPs and CDMPs bind to heparin, heparan sulfates, and collagens I and IV. Thus there is a symbiosis of regulatory and structural macromolecules in the morphogenesis of cartilage. An avalanche of recent advances from seemingly disparate areas bodes well for the complete elucidation of the molecular basis of morphogenesis of cartilage, the architectural blue-print for the skeleton.

Calbindin-D28K and -D9K and 1,25(OH)2 vitamin D3 receptor immunolocalization and mineralization induction in long-term primary cultures of rat epiphyseal chondrocytes

Rat epiphyseal plat chondrocytes were grown on glass slides, as nonadhering monolayer cultures for up to 6 weeks. Chondrocyte growth, differentiation and maturation, matrix formation and mineralization, and the temporospatial distribution of the vitamin D-dependent calcium-binding proteins, calbindin-D9K and -D28K, and the 1,25(OH)2D3 receptor (VDR), were all monitored. Chondrocytes became confluent in 2.5 weeks, differentiated to acquire a chondrocyte (polygonal) morphology, produced extracellular matrix, and finally formed a true monolayer mineralizing cartilaginous tissue, with all the stages of chondrocyte development within a single culture. beta-Glycerophosphate promoted initial matrix mineralization in 4 weeks and accelerated cell differentiation. High nominal calcium and ascorbic acid were needed for abundant matrix formation. VDR occurred at all differentiation stages, in the nuclei and nucleoli and in the cytoplasm. Calbindin-D28K and -D9K were not coexpressed. Calbindin-D28K was found in prechondroblasts, chondroblasts, and in newly differentiated chondrocytes. It was cytoplasmic in prechondroblasts and subsequently also in the perinuclear region and in nuclei, suggesting migration to the nuclear chromatin. Calbindin-D28K was nuclear only in newly differentiated chondrocytes in vitro and was not found in mature chondrocytes. In contrast, calbindin-D9K was present in the cytoplasm of mature and hypertrophic chondrocytes only. It was first in the cell body and eventually migrated within and to the far end of long cell processes with a decreasing cytoplasmic concentration showed by decreased immunostaining intensity, and ultimately hypertrophy of chondrocytes in culture. These in vitro patterns of calbindins-D and VDR accurately reflect their in vivo distributions. The genomic action of vitamin D, in vitro, resulted in the synthesis of nuclear VDR and calbindins-D.(ABSTRACT TRUNCATED AT 250 WORDS)

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Osteogenic protein-1 promotes growth and maturation of chick sternal chondrocytes in serum-free cultures

We examined the effect of recombinant human osteogenic protein-1 (OP-1, or bone morphogenetic protein-7), a member of the bone morphogenetic protein family, on growth and maturation of day 11, 15 and 17 chick sternal chondrocytes in high density monolayers, suspension and agarose cultures for up to 5 weeks. OP-1 dose-dependently (10-50 ng/ml) promoted chondrocyte maturation associated with enhanced alkaline phosphatase activity, and increased mRNA levels and protein synthesis of type X collagen in both the presence and absence of serum. In serum-free conditions, OP-1 promoted cell proliferation and chondrocyte maturation, without requiring either thyroid hormone or insulin, agents known to support chick chondrocyte differentiation in vitro. When grown in agarose under the same conditions, TGF-beta 1 and retinoic acid neither initiated nor promoted chondrocyte differentiation. The results demonstrate that OP-1, as the sole medium supplement, supports the maturation of embryonic chick sternal chondrocytes in vitro.

Thyroxine is the serum factor that regulates morphogenesis of columnar cartilage from isolated chondrocytes in chemically defined medium

Epiphyseal chondrocytes cultured in a medium containing 10% serum may be maintained as three dimensional aggregates and differentiate terminally into hypertrophic cells. There is an attendant expression of genes encoding type X collagen and high levels of alkaline phosphatase activity. Manipulation of the serum concentration to optimal levels of 0.1 or 0.01% in this chondrocyte pellet culture system results in formation of features of developing cartilage architecture which have been observed exclusively in growth cartilage in vivo. Cells are arranged in columns radiating out from the center of the tissue, and can be divided into distinct zones corresponding to the recognized stages of chondrocyte differentiation. Elimination of the optimal serum concentration in a chemically defined medium containing insulin eliminates the events of terminal differentiation of defined cartilage architecture. Chondrocytes continue to enlarge into hypertrophic cells and synthesize type X collagen mRNA and protein, but in the absence of the optimal serum concentration, alkaline phosphatase activity does not increase and the cells retain a random orientation. Addition of thyroxine to the chemically defined medium containing insulin and growth hormone results in dose-dependent increases in both type X collagen synthesis and alkaline phosphatase activity, and reproduces the optimal serum-induced morphogenesis of chondrocytes into a columnar pattern. These experiments demonstrate the critical role of thyroxine in cartilage morphogenesis.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Autocrine signals enable chondrocytes to survive in culture

We recently proposed that most mammalian cells other than blastomeres may be programmed to kill themselves unless continuously signaled by other cells not to. Many observations indicate that some mammalian cells are programmed in this way, but is it the case for most mammalian cells? As it is impractical to test all of the hundreds of types of mammalian cells, we have focused on two tissues--lens and cartilage--which each contain only a single cell type: if there are cells that do not require signals from other cells to avoid programmed cell death (PCD), lens epithelial cells and cartilage cells (chondrocytes) might be expected to be among them. We have previously shown that rat lens epithelial cells can survive in serum-free culture without signals from other cell types but seem to require signals from other lens epithelial cells to survive: without such signals they undergo PCD. We show here that the same is true for rat (and chick) chondrocytes. They can survive for weeks in culture at high cell density in the absence of other cell types, serum, or exogenous proteins or signaling molecules, but they die with the morphological features of apoptosis in these conditions at low cell density. Medium from high density cultures, FCS, or a combination of known growth factors, all support prolonged chondrocyte survival in low density cultures, as long as antioxidants are also present. Moreover, medium from high density chondrocyte cultures promotes the survival of lens epithelial cells in low density cultures and vice versa. Chondrocytes isolated from adult rats behave similarly to those isolated from developing rats. These findings support the hypothesis that most mammalian cells require signals from other cells to avoid PCD, although the signals can sometimes be provided by cells of the same type, at least in tissues that contain only one cell type.

Differential effects of insulin-like growth factor I and growth hormone on developmental stages of rat growth plate chondrocytes in vivo

Skeletal growth depends upon enchondral ossification in growth plate cartilage, within which chondrocytes undergo well defined stages of maturation. We infused IGF-I or growth hormone (GH), two key regulators of skeletal growth, into hypophysectomized rats and compared their effects on growth plate chondrocyte differentiation using qualitative and quantitative autoradiography, stereology, and incident light fluorescence microscopy. Stem cell cycle time was shortened from 50 to 15 and 8 d after treatment with IGF-I and GH, respectively. Proliferating cell cycle time decreased from 11 to 4.5 and 3 d, and duration of the hypertrophic phase decreased from 6 to 4 and 2.8 d. Average matrix volume per cell at each differentiation stage was similar for normal, hormone-treated, and untreated hypophysectomized groups. Mean cell volume and cell height were significantly reduced by hypophysectomy at the proliferative and hypertrophic stages, but were restored to physiological values by IGF-I and GH. In contrast, cell productivity, i.e., increases in cell volume, height, and matrix production per unit of time, did not reach normal values with either IGF-I or GH, and this parameter was inversely proportional to cell cycle time or phase duration. IGF-I and GH are thus capable of stimulating growth plate chondrocytes at all stages of differentiation, albeit to variable degrees with respect to individual cell activities. Although it is generally accepted that GH acts at both the stem and proliferating phases of chondrocyte differentiation, our data represent the first evidence in vivo that IGF-I is also capable of stimulating stem cells.