Environmental regulation of type X collagen production by cultures of limb mesenchyme, mesectoderm, and sternal chondrocytes

Effect of geographic variation on the proteome of sea cucumber (Stichopus japonicus)

Sea cucumber is a sensitive organism that is easily challenged by environmental change. The aim of this study was to characterize the proteome of sea cucumbers from 5 main Chinese origins, including Xiamen (XM), Dalian (DL), Weihai (WH), Yantai (YT) and Qingdao (QD). In this work, a tandem mass tag (TMT) labeling proteomic approach was applied to identify and quantify the proteome of sea cucumber. A total of 5051 proteins were identified in the body wall; among those proteins, 1594 proteins (31.6%) were identified as enzyme proteins, and 33 proteins belonged to collagen. In addition, the 10 most highly abundant proteins were further discussed. Among all quantified proteins, 2266 were significantly differentially expressed proteins (SDEPs) across the 5 origins. These SDEPs were related to pigmentation (5 proteins), antioxidant activity (13 proteins), and immune system processes (29 proteins). Based on SDEPs, DL differed the most from QD and XM, as well as WH and YT, as shown in principal component analysis (PCA) and hierarchical clustering. In conclusion, one-fourth of the significantly different proteins found in the sea cucumber body wall among the 5 main Chinese locations indicated the sensitivity of sea cucumber to variations in temperature, environment, and feeding.

Chondrogenic phenotype in responses to poly(ɛ-caprolactone) scaffolds catalyzed by bioenzymes: effects of surface topography and chemistry

Biodegradable poly(ε-caprolactone) (PCL) has been increasingly investigated as a promising scaffolding material for articular cartilage tissue repair. However, its use can be limited due to its surface hydrophobicity and topography. In this study, 3D porous PCL scaffolds fabricated by a fused deposition modeling (FDM) machine were enzymatically hydrolyzed using two different biocatalysts, namely Novozyme®435 and Amano lipase PS, at varied treatment conditions in a pH 8.0 phosphate buffer solution. The improved surface topography and chemistry of the PCL scaffolds were anticipated to ultimately boost the growth of porcine articular chondrocytes and promote the chondrogenic phenotype during cell culture. Alterations in surface roughness, wettability, and chemistry of the PCL scaffolds after enzymatic treatment were thoroughly investigated using several techniques, e.g., SEM, AFM, contact angle and surface energy measurement, and XPS. With increasing enzyme content, incubation time, and incubation temperature, the surfaces of the PCL scaffolds became rougher and more hydrophilic. In addition, Novozyme®435 was found to have a higher enzyme activity than Amano lipase PS when both were used in the same enzymatic treatment condition. Interestingly, the enzymatic degradation process rarely induced the deterioration of compressive strength of the bulk porous PCL material and slightly reduced the molecular weight of the material at the filament surface. After 28 days of culture, both porous PCL scaffolds catalyzed by Novozyme®435 and Amano lipase PS could facilitate the chondrocytes to not only proliferate properly, but also function more effectively, compared with the non-modified porous PCL scaffold. Furthermore, the enzymatic treatments with 50 mg of Novozyme®435 at 25 °C from 10 min to 60 min were evidently proven to provide the optimally enhanced surface roughness and hydrophilicity most significantly favorable for induction of chondrogenic phenotype, indicated by the greatest expression level of cartilage-specific gene and the largest production of total glycosaminoglycans.

Mesenchymal Stem Cells: Time to Change the Name!

Mesenchymal stem cells (MSCs) were officially named more than 25 years ago to represent a class of cells from human and mammalian bone marrow and periosteum that could be isolated and expanded in culture while maintaining their in vitro capacity to be induced to form a variety of mesodermal phenotypes and tissues. The in vitro capacity to form bone, cartilage, fat, etc., became an assay for identifying this class of multipotent cells and around which several companies were formed in the 1990s to medically exploit the regenerative capabilities of MSCs. Today, there are hundreds of clinics and hundreds of clinical trials using human MSCs with very few, if any, focusing on the in vitro multipotential capacities of these cells. Unfortunately, the fact that MSCs are called “stem cells” is being used to infer that patients will receive direct medical benefit, because they imagine that these cells will differentiate into regenerating tissue‐producing cells. Such a stem cell treatment will presumably cure the patient of their medically relevant difficulties ranging from osteoarthritic (bone‐on‐bone) knees to various neurological maladies including dementia. I now urge that we change the name of MSCs to Medicinal Signaling Cells to more accurately reflect the fact that these cells home in on sites of injury or disease and secrete bioactive factors that are immunomodulatory and trophic (regenerative) meaning that these cells make therapeutic drugs in situ that are medicinal. It is, indeed, the patient's own site‐specific and tissue‐specific resident stem cells that construct the new tissue as stimulated by the bioactive factors secreted by the exogenously supplied MSCs. Stem Cells Translational Medicine2017;6:1445–1451

Enhanced BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage in mice

Meckel's cartilage is a transient supporting tissue of the embryonic mandible in mammals, and disappears by taking different ultimate cell fate along the distal-proximal axis, with the majority (middle portion) undergoing degeneration and chondroclastic resorption. While a number of factors have been implicated in the degeneration and resorption processes, signaling pathways that trigger this degradation are currently unknown. BMP signaling has been implicated in almost every step of chondrogenesis. In this study, we used Noggin mutant mice as a model for gain-of-BMP signaling function to investigate the function of BMP signaling in Meckel's cartilage development, with a focus on the middle portion. We showed that Bmp2 and Bmp7 are expressed in early developing Meckels' cartilage, but their expression disappears thereafter. In contrast, Noggin is expressed constantly in Meckel's cartilage throughout the entire gestation period. In the absence of Noggin, Meckel's cartilage is significantly thickened attributing to dramatically elevated cell proliferation rate associated with enhanced phosphorylated Smad1/5/8 expression. Interestingly, instead of taking a degeneration fate, the middle portion of Meckel's cartilage in Noggin mutants undergoes chondrogenic differentiation and endochondral ossification contributing to the forming mandible. Chondrocyte-specific expression of a constitutively active form of BMPRIa but not BMPRIb leads to enlargement of Meckel's cartilage, phenocopying the consequence of Noggin deficiency. Our results demonstrate that elevated BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage, and support the idea that withdrawal of BMP signaling is required for normal Meckel's cartilage development and ultimate cell fate.

Comprehension of terminal differentiation and dedifferentiation of chondrocytes during passage cultures

A high density collagen type I coated substrate (CL substrate) was used to evaluate the chondrocyte phenotypes in passaged cultures. With increasing age of cell population (population doubling (PD)=0-14.5), the frequency of non-dividing spindle shaped cells without ALP activity increased, accompanied with an increase in gene expression of collagen type I, meaning the senescence of dedifferentiated cells. At the middle age of cell population (PD=5.1 and 6.6), the high frequency of polygonal shaped cells with ALP activity existed on the CL substrate together with up-regulated expressions of collagen types II and X, indicating the terminal differentiation of chondrocytes. When the chondrocytes passaged up to the middle age were embedded in collagen gel, the high frequency of single hypertrophic cells with collagen type II formation was recognized, which supports the thought that the high gene expression of collagen type II was attributed to terminal differentiation rather than redifferentiation. These results show that the CL substrate can draw out the potential of terminal differentiation in chondrocytes, which is unattainable by a polystyrene surface, and that the CL substrate can be a tool to evaluate cell quality in three-dimensional culture with the collagen gel.

Structure and function of the notochord: an essential organ for chordate development

The notochord is the defining structure of the chordates, and has essential roles in vertebrate development. It serves as a source of midline signals that pattern surrounding tissues and as a major skeletal element of the developing embryo. Genetic and embryological studies over the past decade have informed us about the development and function of the notochord. In this review, I discuss the embryonic origin, signalling roles and ultimate fate of the notochord, with an emphasis on structural aspects of notochord biology.

Mechanical tension in distraction osteogenesis regulates chondrocytic differentiation

Differentiation of chondrocytes to cells of osteoblastic phenotype occurs during an interim period of bone development, fracture repair and distraction osteogenesis. To study the relationship between tension-stress and chondrogenesis, uniaxial strains (0 microstrains, 2000 microstrains, 20000 microstrains, 200000 microstrains, 300000 microstrains) were applied in a rabbit model of mandibular distraction osteogenesis. The results demonstrated that cell differentiation, apoptosis and tissue development in the newly formed gap tissue showed a correlation to the applied strain magnitudes. Only strains of 20000 microstrains resulted in a statistically significant (P<0.05) formation of cartilage struts with embedded chondrocyte-like cells. However, chondrocyte-like cells were rarely detected in samples distracted at lower or higher strain magnitudes. Osteoblasts appeared to replace cartilaginous matrix by mineralized bone matrix. The phenotypic change from chondrocytes to osteoblasts was accompanied by a decreased proteoglycan synthesis. a change in the expression from type II collagen towards type I and involved asymmetric cell divisions and apoptotic cell death. Therefore, we suggest that mechanical strain is an external stimulus responsible for phenotypic cell alterations.

Maintenance of regional histodifferentiation patterns and a spatially restricted expression of type X collagen in rat Meckel's cartilage explants in vitro

The major, central portion of Meckel's cartilage undergoes fibrous transformation and contributes to the sphenomandibular ligament, whereas its distal end undergoes endochondral ossification ultimately giving rise to inner-ear ossicles. This regional histodifferentiation of Meckel's cartilage is known to be associated with the spatially restricted expression of type X collagen. The objective of this study was to determine if this unique histodifferentiation is regulated by local environmental factors or by a preprogrammed genetic mechanism. Meckel's cartilage, and condylar cartilage used for comparison, were isolated from 17-day-old rat embryos and from newborn rats, respectively. The cartilage explants were maintained in vitro for 50 days with or without supplementation with 10% fetal bovine serum. When the explants were cultured under serum-free conditions, well-regulated cartilage development was observed. Expression of type X collagen, a differentiation marker for hypertrophic cartilage, was restricted to the distal end of Meckel's cartilage, whereas type II and IX collagens were found uniformly along the entire explant. Matrix calcification was examined histochemically using alizarin red S staining and found to be restricted to the distal end of Meckel's cartilage. Both Meckel's and condylar cartilage cultured with 10% fetal bovine serum developed unregulated dysmorphogenesis. These data suggest that, although Meckel's cartilage has an intrinsic potential to differentiate to its terminal stage, external regulatory factors can significantly influence its normal development at the molecular level.

Type X collagen and other up-regulated components of the avian hypertrophic cartilage program

Elucidating the cellular and molecular processes involved in growth and remodeling of skeletal elements is important for our understanding of congenital limb deformities. These processes can be advantageously studied in the epiphyseal growth zone, the region in which all of the increase in length of a developing long bone is achieved. Here, young chondrocytes divide, mature, become hypertrophic, and ultimately are removed. During cartilage hypertrophy, a number of changes occur, including the acquisition of synthesis of new components, the most studied being type X collagen. In this review, which is based largely on our own work, we will first examine the structure and properties of the type X collagen molecule. We then will describe the supramolecular forms into which the molecule becomes assembled within tissues, and how this changes its physical properties, such as thermal stability. Certain of these studies involve a novel, immunohistochemical approach that utilizes an antitype X collagen monoclonal antibody that detects the native conformation of the molecule. We describe the developmental acquisition of the molecule, and its transcriptional regulation as deduced by in vivo footprinting, transient transfection, and gel-shift assays. We provide evidence that the molecule has unique diffusion and regulatory properties that combine to alter the hypertrophic cartilage matrix. These conclusions are derived from an in vitro system in which exogenously added type X collagen moves rapidly through the cartilage matrix and subsequently produces certain changes mimicking ones that have been shown normally to occur in vivo. These include altering the cartilage collagen fibrils and effecting changes in proteoglycans. Last, we describe the subtractive hybridization, isolation, and characterization of other genes up-regulated during cartilage hypertrophy, with specific emphasis on one of these--transglutaminase.

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.

Doxycycline inhibits type X collagen synthesis in avian hypertrophic chondrocyte cultures

Doxycycline, a member of the tetracycline family, has been shown to reduce a type X collagen epitope as detected by immunohistochemistry with a monoclonal antibody in an avian explant culture system (). It was also shown to decrease collagenase and gelatinase activities and thus matrix degradation. This study investigates the effect of doxycycline on type X collagen synthesis in monolayer cultures of hypertrophic chondrocytes. Protein synthesis was evaluated by radioisotopic labeling during doxycycline, tetracycline, or minocycline treatment. Radiolabeled proteins were analyzed by gel electrophoresis, and total collagen was quantitated by hydroxyproline analysis. Additionally, the synthesis of type X collagen was measured by immunoprecipitation. Doxycycline was found to inhibit type X production more effectively than either of the other tetracyclines at comparable dose levels. Furthermore, type X collagen was inhibited more than other collagens, non-collagenous proteins and proteoglycans, with maximal inhibition at 80 microg/ml and an IC50 of 7 microg/ml. This inhibition by doxycycline was specific for type X collagen at 10 microg/ml, and the pattern was distinct from cycloheximide, a recognized inhibitor of protein translation. This suppression of type X collagen could not be overcome by excess extracellular calcium, conditions that have been demonstrated to induce synthesis of this protein (2).

Monoclonal antibodies against human chondrocytes

Cell-specific antigens are mainly found in cells or membrane surfaces rather than in the surrounding matrix. However, until now it was not possible to produce antibodies specific for cellular structures of chondrocytes. In 1989, Lance (Immunol. Lett. 21:63-73; 1989) first established specific monoclonal antibodies for human articular chondrocytes tested only by immunofluorescence. Studies describing the specificity of these five antibodies (HUMC 1-5) and their relevance for immunohistological analysis of cartilage tissue were not available until now. Therefore, the aim of the following study was to investigate the distribution of HUMC 1, 2, 3, 4, and 5 in mesenchymal cells in vivo and in vitro immunohistochemically. Further investigations concentrate on the localization of chondrocyte specific antigens using immunoelectron microscopy. Immunohistological studies showed positive immunostainings with all five antibodies in human chondrocytes in vivo and in vitro. A cross-reaction with human fibroblasts and osteoblasts for the antibodies HUMC 2 and HUMC 5 was observed. Furthermore, a parallel loss of immunoreactivity for HUMC 1, HUMC 3, and HUMC 4 was observed in cultured chondrocytes indicating that the specific antigens vanish during differentiation observed in vitro. Subsequent immunoblot analysis employing collagens as antigens did not show any reactivity. Using immunoelectron microscopy, gold particle labeling was observed in intracytoplasmatic vesicles of isolated chondrocytes. Our results indicate that HUMC 1, HUMC 3, and HUMC 4 are specific for cartilage cells and might be suitable for immunohistological analysis of different cartilage tissues and pathologically altered chondrocytes.

TGF-beta and basement membrane matrigel stimulate the chondrogenic phenotype in osteoblastic cells derived from fetal rat calvaria

Primary cultures of fetal rat calvarial cells contain a spectrum of osteogenic phenotypes including undifferentiated mesenchymal cells, osteoprogenitor cells, and osteoblasts. We recently demonstrated that rat calvarial osteoblast-like cells grown on basement membrane undergo profound morphological changes resembling a canalicular network in bone. In the present study, we examined the effect of reconstituted basement membrane Matrigel on chondroblastic versus osteoblastic differentiation of different cell subpopulations obtained by five consecutive enzymatic digestions of rat calvarial cell populations. We found that the appearance of canalicular cell processes decreased with the later digests. When cells from the fourth and fifth digest were grown on top of Matrigel for 7 days, the majority of the cell aggregates displayed chondrocytic characteristics but none of the cells became hypertrophic. When individual chondroblastic cell aggregates were subsequently transferred from Matrigel to plastic, they started expressing types I and X collagens, alkaline phosphatase, and osteocalcin. Within the next 7 days (days 8-14 of the experiment), the majority of cells increased in size, and at day 17 on plastic (day 24 of the experiment) mineralized bone nodules formed. The chondroblastic differentiation of calvarial cells grown on Matrigel could be inhibited by a specific transforming growth factor-beta 1 (TGF-beta 1) but not by a TGF-beta 2 antibody. Addition of recombinant TGF-beta 1 to similar cultures promoted the appearance of chondroblastic cell aggregates. The cartilage phenotype could not, on the contrary, be promoted by growing the cells on other extracellular matrices such as a collagen I gel. We suggest that TGF-beta 1 in concert with the basement membrane extracellular matrix induces chondroblastic differentiation of rat calvarial osteoprogenitor cells.

Adenine nucleotide metabolism by chondrocytes in vitro: role of ATP in chondrocyte maturation and matrix mineralization

The objective of the investigation was to explore the notion that chondrocytes in the growth plate secrete nucleotides and that these compounds are used to regulate cell maturation and matrix mineralization. Chondrocytes were isolated from the cephalic region of chick embryo sterna and maintained in culture until confluent. To promote expression of the mature phenotype, cultures were then treated with retinoic acid. During the culture period, medium was removed and analyzed for nucleotides using a modified reverse-phase high-performance liquid chromatography (HPLC) procedure. We found that culture medium, conditioned by the chondrocytes, contained significant quantities of nucleotides. Moreover, the nucleotide concentrations were similar in magnitude to levels reported for media conditioned by other cell types. In terms of species, adenosine diphosphate (ADP) was the major nucleotide present in the conditioned medium; adenosine monophosphate (AMP) was present, but at a lower concentration than ADP. To examine the possibility that adenosine triphosphate (ATP) was released by the cultured chondrocytes, but was rapidly degraded into ADP and AMP, we examined the kinetics of ATP breakdown by chondrocytes. We found that chondrocytes degraded over 70% of exogenous ATP within 15 minutes. Similar experiments performed with ADP and AMP indicated that these nucleotides were also degraded by the cells, but at a slower rate than ATP. To determine whether the extracellular nucleotides modulate cartilage development, we examined the effect of exogenous ATP on four major determinants of chondrocyte function: alkaline phosphatase activity, cell proliferation rate, anaerobic metabolism, and mineral deposition. We found that ATP caused only minimum alterations in cell number and alkaline phosphatase activity; however, it increased the lactate content of the medium probably by stimulating anaerobic glycolysis. We noted that ATP had a significant effect on the amount and type of mineral deposited into chondrocyte cultures. Compared with untreated controls, ATP stimulated formation of a small amount of poorly crystallized calcium phosphate. The results of the study show for the first time that chondrocytes release nucleotides into the extracellular milieu. Although they are rapidly degraded, they serve to regulate both mineral formation and energy metabolism.

Biosynthesis and characterization of type X collagen in human fetal epiphyseal growth plate cartilage

We examined in vitro collagen biosynthesis by organ cultures from human fetal epiphyseal growth plate cartilage. The biosynthetic products were characterized by NaCl fractional precipitation, limited proteolytic digestion, and sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Organ cultures of human fetal epiphyseal growth plate cartilage synthesized large amounts of type X collagen in addition to type II, type IX, and type XI collagens. The individual polypeptide chains of human type X collagen migrated with an apparent M(r) of 45 kDa after proteolytic digestion with pepsin. The migration pattern of these molecules did not change when examined under reducing and nonreducing conditions, indicating that they did not contain intrahelical disfulfide bonds. Comparison of the rates at type X collagen biosynthesis at weeks 20 and 24 of human fetal development showed a marked increase of 24 weeks. Northern hybridization analysis of total RNA from freshly isolated epiphyseal growth plate chondrocytes with a cDNA corresponding to the carboxyl terminus of human type X collagen indicated that the developmental increase of type X collagen production is determined by pre-translational mechanisms.

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.

Chondrogenic potential of chick embryonic calvaria: I. Low calcium permits cartilage differentiation

Calvaria from day-14 calcium-deficient chick embryos produced by long-term maintenance in shell-less culture, exhibit a cartilage-like phenotype (Jacenko and Tuan [1986] Dev. Biol. 115:215-232), which is restored to an osteogenic phenotype upon calcium repletion to the embryo. The expression of cartilage markers in a typically osteogenic tissue under calcium deficiency implies the presence of chondrogenic cells, and questions the conditions associated with calcium deficiency which may cause their divergent pathway of differentiation. In the present study, by explanting normal and shell-less embryonic calvarial pairs in organ culture in vitro and experimentally regulating their calcium supply, the calcium status of the calvaria was modulated as a function of medium calcium. Histological and immunoblotting analyses demonstrated for the first time that calvaria possess cells which can form genuine cartilage. This chondrogenic potential is expressed only in a low-calcium environment, where cartilage forms in both normal and shell-less calvarial pairs; their calcium-supplemented counterparts, however, develop as fully osteogenic tissues. Furthermore, chondrogenesis in both normal and shell-less calvaria indicates that the chondrogenic cells must be endogenous constituents of the calvaria, rather than being derived elsewhere in response to systemic calcium deficiency. Finally, the correlation between matrix under-calcification and cartilage expression in the embryonic calvarium suggests that calcium, perhaps in the form of matrix mineral, may modulate cell differentiation during skeletogenesis.

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.

Chondrogenic potential of chick embryonic calvaria: II. Matrix calcium may repress cartilage differentiation

Chick embryos cultured in the absence of their eggshell are rendered severely calcium-deficient, and develop a cartilage-like phenotype in the calvarium, a normally osteogenic tissue. In the preceding paper (Jacenko and Tuan [1995] Dev. Dyn. 202:13-26), experiments using organ cultured calvaria from day-12 normal and shell-less embryos showed that depletion of calcium alone may be responsible in promoting chondrogenic differentiation in calvaria. Here these findings were extended using an in vivo calvarial grafting technique, such that the extent of calvarial matrix calcification was a function of the calcium status of both the graft and the host. In these calvarial grafts, undermineralized regions again were shown to support chondrogenesis. To identify possible mechanisms which promote chondrogenesis in the calvaria, cells were enzymatically dissociated from the calvaria and cultured in media with varied levels of soluble calcium, under conditions which should modulate cell-to-cell interactions, including monolayer, micromass, agarose gels, and suspension cultures. Soluble calcium had no effect on calvarial cell differentiation, whereas conditions which enhanced cell-cell interactions, e.g., suspension culture, elicited cartilage expression. Based on these findings, we propose that the calcified matrix of the calvarium is repressive to chondrogenesis during normal development, but that the lack of mineral in a calcium-deficient calvarium creates a microenvironment permissive for cell-to-cell interactions which lead to chondrogenic differentiation.

Co-expression of collagen types II and X mRNAs in newly formed hypertrophic chondrocytes of the embryonic chick vertebral body demonstrated by double-fluorescence in situ hybridization

Collagen types II and X mRNAs have been demonstrated simultaneously in newly formed hypertrophic chondrocytes of embryonic chick vertebral cartilage using a double-fluorescence in situ hybridization technique. Digoxigenin- and biotin-labelled type-specific collagen II and X cDNA probes were used. In the embryonic chick vertebra at stage 45, two different fluorescence signals (Fluorescein isothiocyanate and Rhodamine)--one for collagen type II mRNA, the other for type X mRNA--showed differential distribution of the two collagen mRNAs in the proliferating and hypertrophic chondrocyte zones. Several layers of newly formed hypertrophic chondrocytes expressing both collagen types II and X genes were identified in the same section as two different fluorescent colour signals. Low levels of fluorescent signals for collagen type II mRNA were also detected in the hypertrophic chondrocyte zone. Cytological identification of maturing chondrocyte phenotypes, expressing collagen mRNAs, is easier in sections processed by non-radioactive in situ hybridization than in those subjected to radioactive in situ hybridization using 3H-labelled cDNA probes. This study demonstrates that double-fluorescence in situ hybridization is a useful tool for simultaneously detecting the expression of two collagen genes in the same chondrocyte population.

Proteoglycan synthesis by cultured human chondrocytes

Iliac crest biopsies are important in the detection of human skeletal dysplasias. Therefore, culture of these cells may serve as a valuable method for studying proteoglycan metabolism in chondrocytes of individuals with skeletal abnormalities. Morphological and biochemical studies were performed on human iliac crest chondrocytes grown in monolayer and in agarose gels. Two proteoglycan populations of different hydrodynamic size and glycosaminoglycan composition were synthesized by cells grown in monolayer. Chondrocytes cultured in an agarose gel for 2 weeks synthesized proteoglycans identical to those of the native tissue with respect to hydrodynamic size and glycosaminoglycan chain length. However, the ratio of chondroitin-6-sulfate to chondroitin-4-sulfate was higher than in the native tissue. This ratio was not influenced by different sulfate concentrations in the medium. Moreover, treatment with ascorbic acid did not influence proteoglycan synthesis; however, there was a pericellular accumulation of proteoglycans.

Retinoic acid induces a shift in the energetic state of hypertrophic chondrocytes

In the epiphyseal growth plate, chondrocyte maturation is accompanied by dramatic alterations in energy metabolism. To explore the relationship between these two events, we used retinoic acid (RA) to promote chondrocyte maturation in culture. The specific question that was addressed was, does RA treatment of cultured chondrocytes in vitro induce a change in energy status similar to that seen in hypertrophic chondrocytes in vivo. Maturing chondrocytes isolated from the cephalic region of day 18 chick embryo sterna were allowed to grow for 7-14 days in monolayer until confluent and then treated with 10-300 nM RA. Immature chondrocytes from the caudal region of sternum were grown in parallel and served as control cells for the study. We found that in maturing cephalic cell cultures, RA had a rapid and profound effect on oxidative metabolism. The retinoid caused a reduction in the energy charge ratio (ECR) and the ATP/ADP ratio and a sharp decrease in cell ATP levels. Maximum inhibition was observed when the RA concentration was 10-35 nM. Compared with the adenine nucleotides, creatine phosphate levels were decreased to a lesser extent by RA, although there was substantial inhibition of creatine kinase activity. We expected to find a compensatory elevation in glycolytic activities; however, the lactate levels in the medium of the treated cells indicated that anaerobic glycolysis was depressed. In contrast to the cephalic chondrocytes, when caudal cell cultures were treated with RA, lactate formation was stimulated and there were minimal effects on oxidative metabolism. To determine the mechanism of inhibition of glycolysis, we measured the activity of pyruvate kinase in RA-treated cephalic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron microscopy of calcification during high-density suspension culture of chondrocytes

Chondrocyte cultures grown in centrifuge tubes with intermittent centrifugation differentiate into hypertrophic chondrocytes and form calcification. We examined chondrocytes cultured in this system electron microscopically. Rat growth-plate chondrocytes were seeded in a plastic centrifuge tube and cultured in the presence of Eagle's minimum essential medium supplemented with 10% fetal bovine serum and 50 micrograms of ascorbic acid per ml. Specimens were examined by using electron microscopy and selected-area electron-diffraction techniques. In the early stage of culture, a few chondrocytes were scattered and extracellular matrices were not observed. In the middle stage of the cultures, the chondrocytes resembled proliferative cells. Matrix vesicles appeared to be budding from the cell surfaces of chondrocytes and were observed sparsely in the extracellular matrices, which were well formed around the chondrocytes. Matrix vesicles increased substantially during the following cultures. In the mature stage of the cultures, crystal formation related to matrix vesicles was observed. In the 33-day cultures, several masses of calcified matrix were formed and it was confirmed to be apatite by selected-area electron diffraction analysis. The chondrocytes appeared hypertrophic during this same stage. The 56-day culture was similar to the 33-day culture. It was concluded that this culture system provides an extracellular-matrix mineralization which is produced by chondrocytes per se.

Multiple negative elements in a gene that codes for an extracellular matrix protein, collagen X, restrict expression to hypertrophic chondrocytes

During skeletal development, chondrocytes go through several stages of differentiation. The last stage, chondrocyte hypertrophy, occurs in areas of endochondral ossification. Mature hypertrophic chondrocytes differ from immature chondrocytes in that they become postmitotic, increase their cellular volume up to eightfold, and synthesize a unique set of matrix molecules. One such molecule is a short collagenous protein, collagen X. Previous studies have shown that collagen X is not expressed by other cell types and that its specific expression in hypertrophic chondrocytes is controlled by transcriptional mechanisms. To define these mechanisms, plasmid constructs containing the chicken collagen X gene promoter and 5' flanking regions fused to a reporter gene (chloramphenicol acetyl transferase, CAT) were transfected into primary cultures of collagen X-expressing and nonexpressing cells. A construct containing a short (558 bp) promoter exhibited high levels of CAT activity in all cell types (fibroblasts, immature, and hypertrophic chondrocytes). Adding a 4.2-kb fragment of 5' flanking DNA to this construct resulted in a dramatic reduction of CAT activity in fibroblasts and immature chondrocytes, but had no effect in hypertrophic chondrocytes. Addition of three subfragments of the 4.2-kb fragment to the initial construct, either individually or in various combinations, showed that all subfragments reduced CAT activity somewhat in non-collagen X-expressing cells, and that their effects were additive. Unrelated DNA had no effect on CAT activity. The results suggest that multiple, diffuse upstream negative regulatory elements act in an additive manner to restrict transcription of the collagen X gene to hypertrophic chondrocytes.

Type X collagen is transcriptionally activated and specifically localized during sternal cartilage maturation

Type X collagen is an extracellular matrix protein which is synthesized by chondrocytes when they undergo hypertrophy. We present evidence here that the expression of type X collagen in the developing chick sternum is controlled primarily by transcriptional mechanisms. Using chondrocyte nuclei isolated from 15-, 16-, 17- and 18-day chick embryonic sterna, nuclear run-off assays demonstrate that type X collagen gene transcription begins at day 16 in chondrocytes isolated from the cephalic portion. This occurs two days prior to mineralization of this tissue as observed by alizarin red staining. The rate of type X transcription increases dramatically through days 17 and 18. Western blot analyses of extracts of freshly isolated sternal chondrocytes from the same stages show that intracellular levels of the type X protein follow the same time course. Immunostaining with a monoclonal antibody specific for type X collagen demonstrates that the initial appearances of hypertrophic cells and pericellular type X collagen occur at embryonic day 16 in the cephalic portion of sterna. Observation of immunostained cephalic sternal sections from day 18 embryos by confocal microscopy reveals that type X collagen is localized in a capsule-like configuration around each hypertrophic chondrocyte.

Hypertrophic chondrocytes undergo further differentiation in culture

Conditions have been defined for promoting growth and differentiation of hypertrophic chondrocytes obtained in culture starting from chick embryo tibiae. Hypertrophic chondrocytes, grown in suspension culture as described (Castagnola P., G. Moro, F. Descalzi Cancedda, and R. Cancedda. 1986. J. Cell Biol. 102:2310-2317), when they reached the stage of single cells, were transferred to substrate-dependent culture conditions in the presence of ascorbic acid. Cells showed a change in morphology, became more elongated and flattened, expressed alkaline phosphatase, and eventually mineralized. Type II and X collagen synthesis was halted and replaced by type I collagen synthesis. In addition the cells started to produce and to secrete in large amount a protein with an apparent molecular mass of 82 KD in reducing conditions and 63 KD in unreducing conditions. This protein is soluble in acidic solutions, does not contain collagenous domains, and is glycosylated. The Ch21 protein, a marker of hypertrophic chondrocytes and bone cells, was synthesized throughout the culture. We have defined this additional differentiation stage as an osteoblast-like stage. Calcium deposition in the extracellular matrix occurred regardless of the addition of beta glycerophosphate to the culture medium. Comparable results were obtained both when the cells were plated at low density and when they were already at confluence and maintained in culture without passaging up to 50 d. When retinoic acid was added to the hypertrophic chondrocyte culture between day 1 and day 5 the maturation of the cells to the osteoblast-like stage was highly accelerated. The switch in the collagen secretion was already observed after 2 d and the production of the 63-kD protein after 3 d. Mineralization was observed after 15-20 d.

The influence of bone and marrow on cartilage hypertrophy and degradation during 30-day serum-free culture of the embryonic chick tibia

In this study, an organ culture system is defined which demonstrates complete loss of cartilage matrix from embryonic chick tibiae. Efficient loss of the cartilage matrix occurs within 30 days of serum-free culture only when the intact tibiae containing bone, marrow, and cartilage tissue are cultured. During organ culture nonhypertrophic chondrocytes become hypertrophic and stain positively for type X collagen and alkaline phosphatase. The cartilage loses Safranin O staining, and finally all cartilage matrix disappears leaving the bony collar and marrow cells. If the tibial cartilage is separated from the bony collar and cultured alone in serum-free medium, the nonhypertrophic chondrocytes also hypertrophy; the matrix loses Safranin O staining; however, some components of the matrix including type X collagen still remain after 30 days. In the presence of serum, the chondrocytes will hypertrophy but cartilage degradation is not evident. The results of this study support the conclusions that 1) hypertrophy is inherently programmed in the chondrocyte and 2) while Safranin O staining of cartilage cultured alone is diminished in serum-free organ culture, the degradation of cartilage is complete only when bone and marrow are also present.

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

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

In situ hybridization studies on the expression of type X collagen in fetal human cartilage

Type X collagen is a short, non-fibril-forming collagen restricted to the hypertrophic, calcifying zone of growth plate cartilage. It is developmentally regulated and found exclusively in hypertrophic cartilage. Here we report on the structure and distribution of human type X collagen based on the cloning of a PCR fragment covering 292 bp of the carboxy-terminal, non-triple-helical domain. Seventy-five percent of the sequence are identical to that of chicken type X collagen at nucleic acid level and 84% at amino acid level. This probe was used for in situ hybridization analyses of type X collagen expression in a human growth plate. Human fetal cartilage, which is different from the avian cartilage-bone transition zone, showed strong type X collagen expression confined to the lower hypertrophic zone of the growth plate. The upper zone of hypertrophic chondrocytes did not contain alpha 1(X) transcripts, indicating that type X collagen expression follows cellular hypertrophy. The distribution of type X collagen mRNA has been previously unreported in chondrocytes from zones of secondary ossification and in chondrocytes associated with endochondral bone trabecules containing calcified cartilage. In situ hybridization analyses with probes for type I and II collagen on consecutive sections indicated a spatial gradient in chondrocyte differentiation in the human epiphysis. Chondrocytes of low type II collagen expression in the resting zone are followed by proliferating columnar chondrocytes with strong type II collagen expression and a zone of hypertrophic chondrocytes synthesizing type X and type II collagen. In contrast to findings in avian growth cartilage in some of our samples of human epiphyseal cartilage hypertrophic chondrocytes continued to strongly express type II collagen down to the chondro-osseous junction. Transcripts of the alpha 2(I) collagen gene, however, were detected only in perichondrium, vascular cavities, and bone, but not in hypertrophic or any other chondrocytes. The above observations demonstrate that the isolation of the human type X collagen DNA will contribute to studies of pathways of chondrocyte differentiation in the mammalian growth plate.

Elevated extracellular calcium concentrations induce type X collagen synthesis in chondrocyte cultures

Chondrocytes at different stages of cellular differentiation were isolated from the tarsal element (immature chondrocytes) and zones 2 and 3 (mature chondrocytes) of 12-d chick embryo tibiotarsus. The chondrocytes from the two sources differed in their cell morphologies, growth rate and production of type X collagen. In 24 h, zone 2 and 3 chondrocytes synthesized 800 times more type X collagen than tarsal chondrocytes. The effect of exogenous CaCl2 (5 and 10 mM) on the synthesis of type X collagen by both mature and immature chondrocytes was tested. After a 72-h incubation of zone 2 and 3 chondrocytes with CaCl2 type X collagen increased 8-fold with 5 mM and 10-fold with 10 mM Ca2+. [3H]Proline incorporation into culture medium and matrix macromolecules increased 11 and 32% with 5 and 10 mM CaCl2, respectively. Type II collagen synthesis was not affected by elevated extracellular Ca2+ during this 72-h period. Similar studies with tarsal chondrocytes demonstrated a time- and dose-dependent response to CaCl2 with type X collagen levels reaching a 4-fold and 15-fold increase over controls with 5 and 10 mM Ca2+, respectively, at 48 h. Elevated extracellular Ca2+ had no effect on cell proliferation. These observations offer the first direct evidence of the induction of type X collagen synthesis with elevated extracellular Ca2+.

Distribution of type X collagen in tibiotarsi of broiler chickens with vitamin D deficiency

Type X collagen is a significant component of the extracellular matrix of the hypertrophic zone of physeal cartilage, but its precise role in endochondral ossification has not been determined. The concentration of type X collagen increases in physeal cartilage in chicks with vitamin D deficiency. The purpose of our study was to determine whether defective endochondral ossification due to vitamin D deficiency was associated with abnormalities in the distribution of type X collagen in the proximal tibiotarsus of chicks. To accomplish this, we induced vitamin D deficiency in broiler chicks and sequentially evaluated the pattern of type X collagen immunoreactivity in the proximal tibiotarsus using a monoclonal antibody specific for chicken type X collagen. Type X collagen immunoreactivity was present in the matrix of the prehypertrophic zone, hypertrophic zone, cartilage cores of the primary spongiosa, and within the chondrocytes of the prehypertrophic and early hypertrophic zones in vitamin D-deficient and D-replete chicks. However, rachitic chicks exhibited two consistent differences in type X collagen immunoreactivity: hypertrophic chondrocytes in the late hypertrophic zone and primary spongiosa contained intracellular type X collagen; and type X collagen was concentrated into laminated aggregates in the pericellular and territorial matrices in the late hypertrophic zone and primary spongiosa.(ABSTRACT TRUNCATED AT 250 WORDS)

Human and sheep growth-plate cartilage type X collagen synthesis and the influence of tissue storage

Direct comparison of type X collagen synthesized by human, sheep and chick growth-plate cartilage has shown that the human type X collagen is similar to the chick in both its molecular mass, containing component alpha-chains of 59 kDa with helical regions of 45 kDa, and apparent absence of disulphide-stabilized aggregates, whereas the sheep type X collagen has slightly larger alpha-chains (63 kDa) accounted for by a longer helical region (49 kDa) that contains cystine residues essential for the formation of the high-molecular-mass aggregates found with this species. Type X collagen from all three species showed heterogeneity in primary collagen structure as revealed by Staphylococcus aureus V8 proteinase-generated peptide maps. Collagen synthesis by growth-plate cartilage in culture, particularly synthesis of type IX and X collagen, was shown to be very sensitive to prior storage and suggests caution in the interpretation of changes detected when examining collagen synthesis by growth plates in culture.

Localization of type X collagen in canine growth plate and adult canine articular cartilage

Type X collagen was extracted from ends of canine growth plates by pepsin digestion after 4 M guanidine hydrochloride extraction, purified by stepwise salt precipitation (2.0 M NaCl in 0.5 M acetic acid), and chromatographed on a Bio-Gel A1.5 M column in 1.0 M CaCl2. Without reduction on sodium dodecyl sulfate (SDS) polyacrylamide gels, the preparation yielded a single, high-molecular-weight (mol wt) band; after reduction, a single band of relative mol wt 5.0 x 10(4) was found. Polyclonal sera were raised against the purified collagen and used in the immunolocalization of canine type X collagen. As expected, indirect immunoperoxidase (IP) or indirect immunofluorescent staining with the polyclonal sera demonstrated that most of the immunoreactivity was localized in the zone of provisional calcification of the growth plate and in cartilage remnants in the metaphyseal region of the physis. A progressive decrease in staining toward the diaphysis of the fetal canine long bone was apparent as the trabecular structures were remodeled to bone. Unexpectedly, type X collagen was also detected in the zone of calcified, mature articular cartilage. It was concentrated in the pericellular matrix of the chondrocytes, appeared at or just above the tidemark, and was expressed immediately before mineralization. Identification of type X collagen in both the canine growth plate and the zone of calcified articular cartilage suggests that cells in the deep layer of cartilage and in the zone of calcified cartilage in the adult animal retain some characteristics of a growth plate and may be involved in regulation of mineralization at this critical interface. The expression of growth plate-like properties would allow the deep chondrocytes of mature articular cartilage to play a role in remodeling of the joint with age and in the pathogenesis of osteoarthritis.

Formation of cartilage tissue in vitro

Articular cartilage is notoriously defective in its capacity for self-repair, making joints particularly sensitive to degenerative processes. However, methods are now available for the preparation of large numbers of differentiated chondrocytes from a small biopsy sample from any patient. The cells are amplified by proliferation as fibroblast-like cells that will re-express the cartilage phenotype when placed in suspension or gel culture. The chondrocytes can be collected from gel cultures after agarase treatment and reconstituted into cartilage tissue in pellet cultures. In addition, these chondrocytes can be suspended in an appropriate delivery vehicle and implanted into defect sites with a high reparative success rate in an animal model. Appropriate procedures can now be tested in appropriate patient populations.

Gene expression and extracellular matrix ultrastructure of a mineralizing chondrocyte cell culture system

Conditions were defined for promoting cell growth, hypertrophy, and extracellular matrix mineralization of a culture system derived from embryonic chick vertebral chondrocytes. Ascorbic acid supplementation by itself led to the hypertrophic phenotype as assessed by respective 10- and 15-fold increases in alkaline phosphatase enzyme activity and type X synthesis. Maximal extracellular matrix mineralization was obtained, however, when cultures were grown in a nutrient-enriched medium supplemented with both ascorbic acid and 20 mM beta-glycerophosphate. Temporal studies over a 3-wk period showed a 3-4-fold increase in DNA accompanied by a nearly constant DNA to protein ratio. In this period, total collagen increased from 3 to 20% of the cell layer protein; total calcium and phosphorus contents increased 15-20-fold. Proteoglycan synthesis was maximal until day 12 but thereafter showed a fourfold decrease. In contrast, total collagen synthesis showed a greater than 10-fold increase until day 18, a result suggesting that collagen synthesis was replacing proteoglycan synthesis during cellular hypertrophy. Separate analysis of individual collagen types demonstrated a low level of type I collagen synthesis throughout the 21-d time course. Collagen types II and X synthesis increased during the first 2 wk of culture; thereafter, collagen type II synthesis decreased while collagen type X synthesis continued to rise. Type IX synthesis remained at undetectable levels throughout the time course. The levels of collagen types I, II, IX, and X mRNA and the large proteoglycan core protein mRNA paralleled their levels of synthesis, data indicating pretranslational control of synthesis. Ultrastructural examination revealed cellular and extracellular morphology similar to that for a developing hypertrophic phenotype in vivo. Chondrocytes in lacunae were surrounded by a well-formed extracellular matrix of randomly distributed collagen type II fibrils (approximately 20-nm diam) and extensive proteoglycan. Numerous vesicular structures could be detected. Cultures mineralized reproducibly and crystals were located in extracellular matrices, principally associated with collagen fibrils. There was no clear evidence of mineral association with extracellular vesicles. The mineral was composed of calcium and phosphorus on electron probe microanalysis and was identified as a very poorly crystalline hydroxyapatite on electron diffraction. In summary, these data suggest that this culture system consists of chondrocytes which undergo differentiation in vitro as assessed by their elevated levels of alkaline phosphatase and type X collagen and their ultrastructural appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell hypertrophy and type X collagen synthesis in cultured articular chondrocytes

Articular cartilage is a permanent tissue whose cells do not normally take part in the endochondral ossification process. To determine whether articular chondrocytes possess the potential to express traits associated with this process such as cell hypertrophy and type X collagen, chondrocytes were isolated from adult chicken tibial articular cartilage and maintained in long-term suspension cultures. As a positive control in these experiments, we used parallel cultures of chondrocytes from the caudal portion of chick embryo sternum. Both articular and sternal chondrocytes readily proliferated and progressively increased in size with time in culture. Many had undergone hypertrophy by 4-5 weeks. Analysis of medium-released collagenous proteins revealed that both articular and sternal chondrocytes initiated type X collagen synthesis between 3 and 4 weeks of culture; synthesis of this macromolecule increased with further growth. Immunofluorescence analysis of 5-week-old cultures showed that about 15% of articular chondrocytes and 30% of sternal chondrocytes produced type X collagen; strikingly, there appeared to be no obvious relationship between type X collagen production and cell size. The results of this study show that articular chondrocytes from adult chicken tibia possess the ability to express traits associated with endochondral ossification when exposed to a permissive environment. They suggest also that the process of cell hypertrophy and initiation of type X collagen synthesis are independently regulated both in articular and sternal chondrocytes.

Collagens of the chicken eggshell membranes

An immunohistochemical analysis of the eggshell membranes shows the occurrence of type X collagen while type I collagen was not detected by using an appropriate monoclonal antibody with untreated shell membranes. A positive immuno-reaction for type I collagen was obtained after digestion of the shell membranes with pepsin. These observations indicate the possibility that type I collagen epitope was masked by type X collagen and that type X collagen may serve as an inhibitory boundary for biomineralization.

The effects of long term monolayer culture on the proteoglycan phenotype of a clonal population of mature human malignant chondrocytes

Articular cartilage chondrocytes maintain biosynthetic heterogeneity in cell culture, but undergo irreversible dedifferentiation of their proteoglycan phenotype, as defined by keratan sulfate content. A recently described cell line of malignant human chondrocytes, 105KC, was the first to maintain a differentiated keratan sulfate-proteoglycan phenotype in long-term culture. A clone of 105KC, labeled KC2H3, is currently described and represents a distinct and metabolically more homogeneous population of mature chondrocytes than 105KC. KC2H3 cells universally express keratan sulfate biosynthesis, as defined by indirect immunofluorescence. In addition, KC2H3 expresses a more mature proteoglycan phenotype than 105KC, as demonstrated by the keratan sulfate content: 24% of glycosaminoglycan content of the aggregating proteoglycans of KC2H3 versus 13% for 105KC. Further reported are the effects of long term monolayer culture on the proteoglycan phenotype expressed by KC2H3. After more than 16 months in continuous monolayer, KC2H3 cells remained morphologically indistinguishable from those maintained in suspension alternating with monolayer. In addition, the proteoglycan phenotype remained mature, without a tendency towards dedifferentiation. The flattened morphology adopted by chondrocytes while in monolayer has been considered a stimulus of dedifferentiation; the present study is the first to examine the direct effects of physical state on a homogeneous and stable population of chondrocytes.

Formation of chondrous and osseous tissues in micromass cultures of rat frontonasal and mandibular ectomesenchyme

Rat frontonasal and mandibular mesenchyme was isolated from day-12 1/2 (stage-22) rat embryos and cultured at high density for up to 12 days. The stage chosen was based on the observation that mandibular mesenchyme at this stage became independent of its epithelium with respect to the production of both cartilage and bone. Frontonasal cultures developed aggregates of anastomosing columns of cells within 2 days. These grew as the cells enlarged, laying down an Alcian-blue-positive matrix by day 3 of culture. Significant mineral was detected by von Kossa staining by day 5 at which time the aggregates covered a large portion of the culture, eventually covering the entire micromass by day 10-12. Mandibular cultures developed centrally located nodular aggregates by 3 days of culture. These nodules increased in number, spreading outwards as the cells enlarged, laying down an Alcian-blue-positive matrix by day 4 and mineral by days 6-7. At this time the nodules began to elongate and coalesce, but never covered the entire culture over the 12-day period. Antibody staining revealed that in both cultures the cells were initially positive for type I collagen. Subsequently, the aggregates began expressing type II collagen, followed by type X, which coincided with the onset of mineralization. At this time some cells were negative for these cartilage markers, but positive for osteoblast markers, bone sialoprotein II, osteocalcin and type I collagen. In addition osteonectin and alkaline phosphatase were demonstrable in all of the aggregate cells late in the culture period. This provided clear evidence that chondroblast and osteoblast differentiation was proceeding within these cultures. The culture of rat facial mesenchyme should prove very useful, not only for the analysis of bone and cartilage induction and lineage relationships, but also in furthering our knowledge of craniofacial differentiation, growth and pattern formation by extending our analysis to a mammalian system.

Immunohistochemical localization of type X collagen in the proximal tibiotarsi of broiler chickens and turkeys

Type X collagen is a prominent component of the extracellular matrix in cartilage destined to mineralize during endochondral ossification, yet its role is only now being determined. As a prelude to determining what, if any, alterations occur in the distribution of type X collagen in growth plates of poultry with rickets or tibial dyschondroplasia, our objective in the current study was to determine the distribution of type X collagen in the proximal tibiotarsi of broiler chickens and turkeys from 1 day of age through physeal closure. Proximal tibiotarsi from five male broiler chickens, five female broiler chickens and five male turkeys were collected at 1, 7, 14, 28, 56, and 98 days of age and processed for immunohistochemistry; a monoclonal antibody for type X collagen was used to demonstrate type X collagen distribution. Our findings indicate that type X collagen is produced in the prehypertrophic and early hypertrophic zones of the avian growth plate and is incorporated into the extracellular matrix in these zones. Furthermore, intracellular type X collagen is markedly decreased in more mature areas of the growth plate, although type X collagen remains a prominent component of the extracellular matrix until the matrix is completely resorbed. In addition, the distribution of type X collagen is similar in the proximal tibiotarsi of broiler chickens and turkeys at comparable stages of endochondral ossification and distribution of type X collagen in the secondary center of ossification parallels that in the physis.

Changes in osteonectin distribution and levels are associated with mineralization of the chicken tibial growth cartilage

Osteonectin is a calcium-binding matrix protein thought to play a role in regulating calcium distribution in a variety of biologic processes. To examine its role in endochondral bone formation, we examined the distribution of the protein during mineralization of the chicken tibial growth cartilage, using immunohistochemistry and immunoelectron microscopy. The synthesis of osteonectin was also determined in chondrocyte populations isolated from premineralizing and mineralizing regions of growth cartilage and assayed in short-term culture. The results show that a very low level of osteonectin is detectable in the resting, proliferating, and early hypertrophic zones of growth cartilage; in these zones, osteonectin is largely cell-associated. In contrast, a large amount of osteonectin is present in the mineralizing zone where it is associated with the matrix. Biosynthetic data from short-term culture experiments indicate, however, that osteonectin is synthesized and secreted by chondrocytes from both premineralizing and mineralizing zones. As indicated by immunoprecipitation, Northern hybridization, in vitro translation of hybrid-selected messenger RNA (mRNA), and electrophoretic analysis, osteonectin synthesized by chondrocytes of the premineralizing zones is not obviously different in structure from that synthesized by chondrocytes of the mineralizing zone. We conclude that osteonectin is a product of chondrocytes in each zone of growth cartilage but accumulates only in the mineralizing zone. The high affinity of the protein for calcium could favor its retention in calcifying matrix.

In vitro development of hypertrophic chondrocytes starting from selected clones of dedifferentiated cells

Single cells from enzymatically dissociated chick embryo tibiae have been cloned and expanded in fresh or conditioned culture media. A cloning efficiency of approximately 13% was obtained using medium conditioned by dedifferentiated chondrocytes. A cloning efficiency of only 1.4% was obtained when conditioned medium from hypertrophic chondrocytes was used, and efficiencies of essentially 0 were found with fresh medium or medium conditioned by J2-3T3 mouse fibroblasts. Cell clones were selected by morphological criteria and clones showing a dedifferentiated phenotype (fibroblast-like) were further characterized. Out of 38 clones analyzed, 17 were able to differentiate to the hypertrophic chondrocyte stage and reconstitute hypertrophic cartilage when placed in the appropriate culture conditions. Cells from these clones expressed the typical markers of chondrocyte differentiation, i.e., type II and type X collagens. Clones not undergoing differentiation continued to express only type I collagen. Hypertrophic chondrocytes from differentiating clones were analyzed at the single cell level by immunofluorescence; all the cells were positive for type X collagen, while approximately 50% of them showed positivity for type II collagen.

Immunoelectron microscopy of type X collagen: supramolecular forms within embryonic chick cartilage

To determine the supramolecular forms in which avian type X collagen molecules assemble within the matrix of hypertrophic cartilage, we performed immunoelectron microscopy with colloidal gold-labeled monoclonal antibodies. In addition double-labeled analyses were performed for the molecule and type II collagen, employing two monoclonal antibodies attached to different size gold particles. Both in situ limb cartilages and the extracellular matrix of chondrocyte cultures were examined. We observed in both systems that the type X collagen is present in two forms. One is as fine filaments (less than 5 nm in diameter) within mats which are found predominantly in the pericellular matrix of the hypertrophic chondrocytes. The second form is in association with the fibrils (10-20 nm in diameter) which also react with the antibody for type II collagen. It seems that the filamentous mats represent a form in which the type X collagen is initially secreted from the cell. The type X associated with the striated fibrils most likely represents a secondary association of the molecule with preexisting type II/IX/XI fibrils. The data are consistent with our previously proposed hypothesis that type X collagen is involved in, and perhaps even "targets," certain matrix components for degradation and removal.

Hypertrophy and calcification of rabbit permanent chondrocytes in pelleted cultures: synthesis of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor

Cartilage calcification at specific sites is a key event that leads to skeletal development and growth. To obtain insights into the control of cartilage calcification, we examined whether cells distributed in permanent cartilage regions might have the ability to express the calcification-related phenotype in a permissive environment. Chondrocytes were isolated from the permanent and growth plate cartilages of 4-week-old rabbit ribs. They were seeded as a pelleted mass in a centrifuge tube and cultured in Eagle's minimum essential medium supplemented with 10% fetal bovine serum. These cells proliferated for several generations, and then synthesized large amounts of proteoglycans, yielding a cartilage-like tissue in 16 days. Cultures from the permanent and growth plate cartilages showed similar time courses for increases in DNA synthesis and proteoglycan production that reached similar maximal levels. Thereafter, they initiated the syntheses of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induced matrix calcification without additional phosphate. The increases in alkaline phosphatase, 1,25-dihydroxycholecalciferol receptor, and calcium contents in cultures from the permanent cartilage were consistently delayed for 4-7 days relative to the growth plate-derived cells, but caught up by Day 28. The maximal levels of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor in the cultures from the permanent cartilage were 40- to 100-fold higher than that of the in vivo permanent cartilage. These results provide evidence that permanent cartilage cells in postnatal young rabbit ribs have the capacity to express alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induce calcification in a permissive environment, although they never express these calcification-related phenotypes in vivo.

Induction and prevention of chondrocyte hypertrophy in culture

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

Extracellular matrix and cell surface as determinants of connective tissue differentiation

This paper reviews in vitro studies, largely from the author's laboratory, concerning the conditions that are permissive for the differentiation of limb bud mesenchymal cells into chondrocytes. In high-density cell culture, even in a defined medium, the same normal sequence of events that is found in vivo in developing cartilage is also observed. This system can be used to study heritable disorders in model systems such as in mutant mouse embryos. In addition, single mesenchymal cells can differentiate into hypertrophic chondrocytes in hydrated collagen gel or agarose cultures. A rounded cell shape promotes chondrogenesis, while a flattened cell shape promotes fibroblast differentiation. The actin cytoskeleton is shown to play a central role in regulating connective tissue cell differentiation. By use of such cell culture manipulations, it is now possible to grow large numbers of fibroblastic cells from human biopsy material for storage and to carry out experimental studies after re-expression of chondrogenesis in gel cultures. It is suggested that cytoskeletal-extracellular matrix interactions play a fundamental role in connective tissue differentiation. Matrix receptors might be developmentally regulated and modify epithelial effects on mesenchymal cells. In this way mesenchymal cells differentiate in a highly organized manner in spatial and temporal terms.

Expression of the human chondrocyte phenotype in vitro

We report a culture scheme in which human epiphyseal chondrocytes lose their differentiated phenotype in monolayer and subsequently reexpress the phenotype in an agarose gel. The scheme is based on a method using rabbit chondrocytes. Culture in monolayer allowed small quantities of cells to be amplified and provided a starting point to study expression of the differentiated human chondrocyte phenotype. The cells cultured in monolayer produced type I procollagen, fibronectin, and small noncartilaginous proteoglycans. Subsequent culture in agarose was associated with the acquisition of typical chondrocyte ultrastructural features and the synthesis of type II collagen and cartilage-specific proteoglycans. The switch from the nonchondrocyte to the differentiated chondrocyte phenotype occurred under these conditions between 1 and 2 wk of agarose culture and was not necessarily homogeneous throughout a culture. This culture technique will facilitate direct investigation of human disorders of cartilage that have been addressed in the past by alternative approaches.