Localisation of collagen types and fibronectin in cartilage by immunofluorescence

Properties of the Nasal Cartilage, from Development to Adulthood: A Scoping Review

OBJECTIVE

Nasal septum cartilage is a hyaline cartilage that provides structural support to the nasal cavity and midface. Currently, information on its cellular and mechanical properties is widely dispersed and has often been inferred from studies conducted on other cartilage types such as the knee. A detailed understanding of nasal cartilage properties is important for several biological, clinical, and engineering disciplines. The objectives of this scoping review are to (1) consolidate actual existing knowledge on nasal cartilage properties and (2) identify gaps of knowledge and research questions requiring future investigations.

DESIGN

This scoping review incorporated articles identified using PROSPERO, Cochrane Library (CDSR and Central), WOS BIOSIS, WOS Core Collection, and ProQuest Dissertations and Theses Global databases. Following the screening process, 86 articles were considered. Articles were categorized into three groups: growth, extracellular matrix, and mechanical properties.

RESULTS

Most articles investigated growth properties followed by extracellular matrix and mechanical properties. NSC cartilage is not uniform. Nasal cartilage growth varies with age and location. Similarly, extracellular matrix composition and mechanical properties are location-specific within the NSC. Moreover, most articles included in the review investigate these properties in isolation and only very few articles demonstrate the interrelationship between multiple cartilage properties.

CONCLUSIONS

This scoping review presents a first comprehensive description of research on NSC properties with a focus on NSC growth, extracellular matrix and mechanical properties. It additionally identifies the needs (1) to understand how these various cartilage properties intersect and (2) for more granular, standardized assessment protocols to describe NSC.

Decreased Expression of Heat Shock Protein 47 Is Associated with T-2 Toxin and Low Selenium-Induced Matrix Degradation in Cartilages of Kashin-Beck Disease

Recent evidence suggests a role of type II collagen in Kashin-Beck disease (KBD) degeneration. We aimed to assess the abnormal expression of heat shock protein 47 (HSP47) which is associated with a decrease in type II collagen and an increase in cartilage degradation in KBD. Hand phalange cartilages were collected from KBD and healthy children. Rats were administered with T-2 toxin under the selenium (Se)-deficient diet. ATDC5 cells were seeded on bone matrix gelatin to construct engineered cartilaginous tissue. C28/I2 and ATDC5 cells and engineered tissue were exposed to different concentrations of T-2 toxin with or without Se. Cartilage degeneration was determined through histological evaluation. The distribution and expression of type II collagen and HSP47 were investigated through immunohistochemistry, western blotting, and real-time PCR. KBD cartilages showed increased chondronecrosis and extracellular matrix degradation in deep zone with decreased type II collagen and HSP47 expression. The low-Se + T-2 toxin animal group showed a significantly lower type II collagen expression along with decreased HSP47 expression. Decreased type II collagen and HSP47 in C28/I2 and ATDC5 cells induced by T-2 toxin showed a dose-dependent manner. Hyaline-like cartilage with zonal layers was developed in engineered cartilaginous tissues, with decreased type II collagen and HSP47 expression found in T-2 toxin-treated group. Se-supplementation partially antagonized the inhibitory effects of T-2 toxin in chondrocytes and cartilages. HSP47 plays a role in the degenerative changes of KBD and associated with T-2 toxin-induced decreased type II collagen expression, further promoting matrix degradation.

Structure and composition of arytenoid cartilage of the bullfrog (Lithobates catesbeianus) during maturation and aging

The aging process induces progressive and irreversible changes in the structural and functional organization of animals. The objective of this study was to evaluate the effects of aging on the structure and composition of the extracellular matrix of the arytenoid cartilage found in the larynx of male bullfrogs (Lithobates catesbeianus) kept in captivity for commercial purposes. Animals at 7, 180 and 1080 days post-metamorphosis (n=10/age) were euthanized and the cartilage was removed and processed for structural and biochemical analysis. For the structural analyses, cartilage sections were stained with picrosirius, toluidine blue, Weigert's resorcin-fuchsin and Von Kossa stain. The sections were also submitted to immunohistochemistry for detection of collagen types I and II. Other samples were processed for the ultrastructural and cytochemical analysis of proteoglycans. Histological sections were used to chondrocyte count. The number of positive stainings for proteoglycans was quantified by ultrastructural analysis. For quantification and analysis of glycosaminoglycans were used the dimethyl methylene blue and agarose gel electrophoresis methods. The chloramine T method was used for hydroxyproline quantification. At 7 days, basophilia was observed in the pericellular and territorial matrix, which decreased in the latter over the period studied. Collagen fibers were arranged perpendicular to the major axis of the cartilaginous plate and were thicker in older animals. Few calcification areas were observed at the periphery of the cartilage specimens in 1080-day-old animals. Type II collagen was present throughout the stroma at the different ages. Elastic fibers were found in the stroma and perichondrium and increased with age in the two regions. Proteoglycan staining significantly increased from 7 to 180 days and reduced at 1080 days. The amount of total glycosaminoglycans was higher in 180-day-old animals compared to the other ages, with marked presence of chondroitin- and dermatan-sulfate especially in this age. The content of hydroxyproline, which infers the total collagen concentration, was higher in 1080-day-old animals compared to the other ages. The results demonstrated the elastic nature of the arytenoid cartilage of L. catesbeianus and the occurrence of age-related changes in the structural organization and composition of the extracellular matrix. These changes may contribute to alter the function of the larynx in the animal during aging.

Pannexin 3 is a novel target for Runx2, expressed by osteoblasts and mature growth plate chondrocytes

Pannexins are a class of chordate channel proteins identified by their homology to insect gap junction proteins. The pannexin family consists of three members, Panx1, Panx2, and Panx3, and the role each of these proteins plays in cellular processes is still under investigation. Previous reports of Panx3 expression indicate enrichment in skeletal tissues, so we have further investigated this distribution by surveying the developing mouse embryo with immunofluorescence. High levels of Panx3 were detected in intramembranous craniofacial flat bones, as well as long bones of the appendicular and axial skeleton. This distribution is the result of expression in both osteoblasts and hypertrophic chondrocytes. Furthermore, the Panx3 promoter contains putative binding sites for transcription factors involved in bone formation, and we show that the sequence between bases -275 and -283 is responsive to Runx2 activation. Taken together, our data suggests that Panx3 may serve an important role in bone development, and is a novel target for Runx2-dependent signaling.

Immunochemical and mechanical characterization of cartilage subtypes in rabbit

Cartilage is categorized into three general subgroups, hyaline, elastic, and fibrocartilage, based primarily on morphologic criteria and secondarily on collagen (Types I and II) and elastin content. To more precisely define the different cartilage subtypes, rabbit cartilage isolated from joint, nose, auricle, epiglottis, and meniscus was characterized by immunohistochemical (IHC) localization of elastin and of collagen Types I, II, V, VI, and X, by biochemical analysis of total glycosaminoglycan (GAG) content, and by biomechanical indentation assay. Toluidine blue staining and safranin-O staining were used for morphological assessment of the cartilage subtypes. IHC staining of the cartilage samples showed a characteristic pattern of staining for the collagen antibodies that varied in both location and intensity. Auricular cartilage is discriminated from other subtypes by interterritorial elastin staining and no staining for Type VI collagen. Epiglottal cartilage is characterized by positive elastin staining and intense staining for Type VI collagen. The unique pattern for nasal cartilage is intense staining for Type V collagen and collagen X, whereas articular cartilage is negative for elastin (interterritorially) and only weakly positive for collagen Types V and VI. Meniscal cartilage shows the greatest intensity of staining for Type I collagen, weak staining for collagens V and VI, and no staining with antibody to collagen Type X. Matching cartilage samples were categorized by total GAG content, which showed increasing total GAG content from elastic cartilage (auricle, epiglottis) to fibrocartilage (meniscus) to hyaline cartilage (nose, knee joint). Analysis of aggregate modulus showed nasal and auricular cartilage to have the greatest stiffness, epiglottal and meniscal tissue the lowest, and articular cartilage intermediate. This study illustrates the differences and identifies unique characteristics of the different cartilage subtypes in rabbits. The results provide a baseline of data for generating and evaluating engineered repair cartilage tissue synthesized in vitro or for post-implantation analysis.

Ultrastructural localization of type VI collagen in normal adult and osteoarthritic human articular cartilage

OBJECTIVE

Type VI collagen is a major component of the pericellular matrix compartment in articular cartilage and shows severe alterations in osteoarthritic cartilage degeneration. In this study, we analysed the exact localization of type VI collagen in its relationship to the chondrocyte and the (inter)territorial cartilage matrix. Additionally, we were interested in its ultrastructural appearance in normal and osteoarthritic cartilage.

DESIGN

Distribution and molecular appearance was investigated by conventional immunostaining, by multilabeling confocal scanning microscopy, conventional transmission, and immunoelectron microscopy.

RESULTS

Our analysis confirmed the pericellular concentration of type VI collagen in normal and degenerated cartilage. Type VI collagen formed an interface in between the cell surface and the type II collagen network. The type VI collagen and the type II collagen networks appeared to have a slight physical overlap in both normal and diseased cartilage. Additionally, some epitope staining was observed in the cell-associated interterritorial cartilage matrix, which did not appear to have an immediate relation to the type II collagen fibrillar network as evaluated by immunoelectron microscopy. In osteoarthritic cartilage, significant differences were found compared with normal articular cartilage: the overall dimension of the lacunar volume increased, and a significantly increased type VI collagen epitope staining was observed in the interterritorial cartilage matrix. Also, the banded isoform of type VI collagen was found around many chondrocytes.

CONCLUSIONS

Our study confirms the close association of type VI collagen with both, the chondrocyte cell surface and the territorial cartilage matrix. They show severe alterations in type VI collagen distribution and appearance in osteoarthritic cartilage. Our immunohistochemical and ultrastructural data are compatible with two ways of degradation of type VI collagen in osteoarthritic cartilage: (1) the pathologically increased physiological molecular degradation leading to the complete loss of type VI collagen filaments from the pericellular chondrocyte matrix and (2) the transformation of the fine filaments to the band-like form of type VI collagen. Both might implicate a significant loss of function of the pericellular microenvironment in osteoarthritic cartilage.

Fibronectin fragments and their role in inflammatory arthritis

OBJECTIVES

To identify potential immunopathogenic links between fibronectin (Fn) fragmentation and the inflammatory response in chronic joint disease.

METHODS

Scientific papers involving studies of Fn fragments and inflammatory processes important in the pathogenesis of arthritis, including chondrolysis, synoviocyte growth and adhesion, polymorphonuclear leukocyte (PMN) and monocyte function, proteolysis, and immune complex activation were reviewed. In addition, reports identifying Fn fragments in synovial fluid (SF) were assessed.

RESULTS

A series of Fn fragments have been identified in arthritic SF by several investigators. Fn and fragments ranging from 30 to 200 kd are present in elevated concentrations in inflammatory SF. SF Fn fragments display reduced affinity for fibrin and collagen. The 29- and 50-kd amino terminal fragments mediate release of proteoglycan from articular cartilage by RGD-independent mechanisms. Fn fragments can induce fibroblast gene expression of metalloproteinases or can act as proteinases themselves. A 90-kd plasmin generated fragment possesses homology with streptokinase. Fragments mediate PMN chemotaxis and enhance proliferation of CD4+ lymphocytes as well as binding to the C1q component of complement and influencing the behavior of immune complexes.

CONCLUSIONS

Fn fragments can be functionally and biochemically characterized in diseased SF. Modification of fragment formation and inhibition of fragment function may have potential therapeutic value in the interruption of chronic synovial inflammation.

Severe disturbance of the distribution and expression of type VI collagen chains in osteoarthritic articular cartilage

OBJECTIVE

The aim of this study was to evaluate the messenger RNA (mRNA) expression and distribution of the major pericellular type VI collagen in normal and osteoarthritic (OA) cartilage.

METHODS

Conventional and confocal laser scanning immunohistochemistry, as well as in situ hybridization experiments, were performed for all 3 collagen type VI chains in sections of normal and OA articular cartilage.

RESULTS

Normal adult articular chondrocytes were surrounded by a type VI collagen-positive pericellular matrix and showed significant levels of mRNA expression for all 3 type VI collagen chains. In OA cartilage, the expression and overall distribution of type VI collagen was largely increased in the lower middle and upper deep zones. In contrast, the upper zones showed a significant loss of pericellular type VI collagen staining.

CONCLUSION

Our results suggest that there is a significant basic turnover of type VI collagen in normal articular cartilage. In OA cartilage, the chondrocytes of the lower middle and upper deep zones account for a net increase in type VI collagen synthesis. The loss of type VI collagen staining in the upper zones is most likely the result of increased protein degradation rather than reduced synthetic activity.

Structural colocalisation of type VI collagen and fibronectin in agarose cultured chondrocytes and isolated chondrons extracted from adult canine tibial cartilage

Cell-matrix and matrix-matrix interactions are of critical importance in regulating the development, maintenance and repair of articular cartilage. In this study, we examined the structural colocalisation of type VI collagen and fibronectin in isolated chondrons and long-term agarose cultured chondrocytes extracted from normal adult canine articular cartilage. Using double labelling immunohistochemistry in conjunction with dual channel confocal microscopy and digital image processing we demonstrate that type VI collagen and fibronectin are distributed in a similar staining pattern and are colocalised at the surface of cultured chondrocytes and isolated chondrons. The results suggest that type VI collagen and fibronectin may play a role in both cell-matrix adhesion and matrix-matrix cohesion in the pericellular microenvironment surrounding articular cartilage chondrocytes.

Development and ageing of the articular cartilage of the rabbit knee joint: distribution of the fibrillar collagens

The development of the articular cartilage of the rabbit knee joint from the 17-day fetus to the 2-year adult rabbit has been examined. At 17 days, the developing femur and tibia are separated by the interzone. Cavitation occurs around 25 days; the cells of the intermediate layer flatten and move onto those of the chondogenous layers to create the articular surfaces. After birth, growth of the cartilage is mainly the result of matrix production. Ossification of the epiphyses is complete by 6 weeks postpartum. Horizontal zones can be distinguished in the articular cartilage; the superficial cells are aligned parallel to the surface, but in the deep layers the cells are in columns. The tidemark is first seen at 12-14 weeks. The matrix of the interzone in the 17-day fetus contains types I, III and V collagens, but no type II. After cavitation at 25 days, the surface layer of the articular cartilage still contains type I, but no type II collagen. From 6 weeks postnatal onwards, type II collagen is present throughout the cartilage and type I disappears. Type III collagen is initially in the inter-territorial matrix, but later it is mainly pericellular. Type V collagen is pericellular both in the chondrogenous layers and later in the articular cartilage, but is not present in the epiphyseal cartilage below. From 6 weeks onwards, types III and V collagens create a capsule around all the chondrocytes above the tidemark. The relationship of types V and XI collagens is discussed. It is concluded that the articular chondrocytes form a unique subset of cells from the earliest stages of joint formation in the fetal rabbit.

An immunohistochemical study of collagen types III, VI and IX in rabbit craniomandibular joint tissues following surgical induction of anterior disk displacement

The purpose of this study was to determine the effect of surgical induction of anterior disk displacement (ADD) on type-III, VI and IX collagens of the rabbit craniomandibular joint (CMJ) tissues using an immunohistochemical technique. The right joint was exposed surgically, all discal attachments were severed except for the posterior discal attachment (bilaminar zone). The disk was then repositioned anteriorly and sutured to the zygomatic arch. The left joint served as a sham-operated control. Ten additional joints were used as non-operated controls. Deeply anesthetized rabbits were perfused with 2% buffered formalin 2 weeks (10 rabbits) or 6 weeks (10 rabbits) following surgery. The articular disk, bilaminar zone, mandibular condyle and articular eminence were excised. The last two were decalcified in EDTA. All tissues were then sectioned at 10 microns in a cryostat. Sections were incubated with monoclonal antibodies directed against type-III, VI or IX collagens. Following incubation in the appropriate FITC-labelled secondary antibodies, all sections were studied under the fluorescence microscope. The results showed a reduction in immunostaining for type-VI and IX collagens in the condylar cartilage, disk and articular eminence at 2 weeks, followed by an increase in their immunostaining at 6 weeks and the appearance of a de novo type-III collagen in the condylar cartilage and the articular eminence. It is concluded that surgical induction of ADD in the rabbit CMJ leads to alterations in its type-III, VI and IX collagens.

Another look at collagen V and XI molecules

The fibrillar collagens are the most abundant proteins of extracellular matrices. Among them, collagens V and XI are quantitatively minor components which participate in the formation of the fibrillar collagen network. Since these collagens were discovered, studies have demonstrated that they may play a fundamental role in the control of fibrillogenesis, probably by forming a core within the fibrils. Another characteristic of these collagens is the partial retention of their N-propeptide extensions in tissue forms, an unusual observation in comparison to the other known fibrillar collagens. The tissue locations of collagens V and XI are different, but their structural and biological properties seem to be closely related. It has been shown that their primary structures are highly conserved at both the gene and protein levels, and that these conserved features are the bases of their similar biological properties. In particular, they are both resistant to mammalian collagenases, and surprisingly sensitive to trypsin treatment. Collagens V and XI are usually buried within the major collagen fibrils, although they have both cell adhesion and heparin binding sites which could be of crucial importance in physiological processes such as development and wound healing. It has became evident that several molecules are in fact heterotypic associations of chains from both collagens V and XI, demonstrating that these two collagens are not distinct types but a single type which can be called collagen V/XI.

Ultrastructural organization of type XI collagen in fetal bovine epiphyseal cartilage

Type XI collagen was localized with polyclonal antibodies specific for alpha 1 (XI) and alpha 2 (XI) chains in the resting zone of epiphyseal cartilage from calf fetuses. The immunofluorescence technique was used on sections of cartilage, and the immunogold labelling technique for electron microscopy on fibrils isolated from cartilage and, for the first time, in situ on blocks of cartilage fractured in liquid nitrogen. Immunofluorescence showed that without pepsin treatment the staining of type XI collagen was restricted to the pericellular zones; after pepsin treatment, the staining was co-distributed with that of type II collagen. Immunoelectron microscopy performed on isolated fibrils and on cartilage blocks showed that after disruption of fibrils with pepsin, type XI collagen was labelled on small filaments on the fibrils. When the fibrils were not disrupted, labelling was observed in situ only at the ends of the fibrils or on cross-sections of some fibrils. These results indicate that type XI collagen is located inside type II collagen fibrils in fetal bovine epiphyseal cartilage, as already postulated for embryonic chicken sterna.

Fibronectin, cartilage, and osteoarthritis

Fibronectin is a multifunctional glycoprotein present at low levels in the extracellular matrix of normal cartilage. In this tissue, as in others, it may be a component of a cell matrix adhesion complex together with cell-surface proteoglycans but also may play a role in the organization of the extracellular matrix. In osteoarthritis (OA), fibronectin content is markedly increased in the altered matrix because of an increased synthesis by the chondrocytes and accumulation in the extracellular matrix. At least part of the fibronectins synthesized in one degenerated cartilage is composed of isoforms more sensitive to proteolytic cleavage that are absent in normal cartilage. This increased content of fibronectin during osteoarthritic processes might entail several consequences related to the multiple functions of fibronectin and its generated fragments, namely a change in chondrocyte phenotype, a switch in synthesis of collagen type, an increased activity of locally secreted metalloproteases, and induction of self-proteolytic activities against gelatin and fibronectin. However, it is not yet clearly understood whether the early increased synthesis of fibronectin in OA acts as an "agent" in an attempt to repair cartilage by the chondrocytes or whether it acts as a deleterious "agent."

Monoclonal antibodies against two epitopes in the human alpha 1 (IX) collagen chain

Type IX collagen is a component of cartilage and vitreous humor. Its structure and matrix localization suggest it may serve to mediate interactions between fibrillar collagen, proteoglycan and other matrix components. Consequently, abnormalities in type IX collagen may result in chondrodysplasia. In this paper we describe the preparation and use of two monoclonal antibodies which recognize peptide sequences within the human cartilage alpha 1 (IX) collagen chain. Antibody 23-5D1 is highly sensitive and highly specific. It permits the immunoblot detection of type IX collagen extracted from milligram amounts of normal and chondrodysplastic cartilage; it also identifies the "short" form of the alpha 1 (IX) chain in human vitreous humor. Antibody 37-10H7 is highly specific, but of low sensitivity. It was used to make the new observation that an N-linked oligosaccharide is present in the amino-terminal globular domain of the alpha 1 (IX) chain. We anticipate that these antibodies may be valuable tools in the study of human and other mammalian chondrodysplasias.

Immunolocalization of types V and XI collagen in cartilage using monoclonal antibodies

Monoclonal antibodies produced against pepsin-solubilized newborn rat skin type V collagen [alpha 1(V)]2 alpha 2(V), and chondrosarcoma type XI collagen [alpha 1(XI) alpha 2(XI) alpha 3(XI)] are used to localize the collagens in sections of the chondrosarcoma as well as the normal rat knee joint by indirect immunofluorescence. Immunostaining for type V collagen shows strong cellular staining of chondrocytes; while the interstitial matrix as well as the lacunae are not stained. In contrast, antitype XI stains not only chondrocytes, but the extracellular compartments as well. In ELISA, rat anti-type XI collagen reacts with its native antigen, but does not cross-react with native types I, II, III, or V collagen from rat. The distinct locations of type V and XI collagens in cartilaginous tissue suggest varied functional roles for these constituents in the tissue.

Cleavage of type XI collagen fibers by gelatinase and by extracts of osteoarthritic canine cartilage

Gelatinase (matrix metalloproteinase 2) purified from culture medium of MDCK cells by affinity chromatography on gelatin-sepharose was tested against type XI collagen. The purified enzyme-digested native type XI collagen in solution, and as reconstituted fibers, at 30, 34, and 37 degrees C. Both substrates yielded the same digestion products, as characterized by SDS-polyacrylamide gel electrophoresis, but the soluble collagen was cleaved at a higher rate. The first major product seen was an 87-kDa peptide, which was usually associated with one or two peptides migrating between it and alpha 3(XI). With time, a second group of 3 peptides appeared at 78, 75, and 73 kDa. After continued digestion, a third group of peptides was detected with prominent 69- and 67-kDa peptides and minor peptides at 71, 65, and 62 kDa. In overnight (20 hour) digestions, the 60-kDa digestion product accumulated and most of the larger digestion products could no longer be detected. Minor products at 71, 55, and 50 kDa were also noted in these limited digestions. Under the same conditions, denatured type XI was digested to fragments smaller than 13.5 kDa. The enzyme was inhibited by 1,10-phenanthroline or EDTA. Two purified components of cartilage matrix, type II collagen and proteoglycan subunit, as well as crude cartilage homogenates, were not effective inhibitors of the purified enzyme. Similar activity was extracted from canine articular cartilage, and the activity was much stronger in cartilage from osteoarthritic joints than from control joints.

Type XI collagen-degrading activity in human osteoarthritic cartilage

Homogenates of 6 samples of human osteoarthritic cartilage were shown to degrade exogenous type XI collagen. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the cleavage products generated by each homogenate were similar, and they were identical to those obtained by cleavage of the substrate with purified gelatinase. Enzyme activity, which was inhibited by EDTA, was greater in extracts of fibrillated osteoarthritic cartilage than in extracts of grossly normal cartilage from the same joint or in extracts of cartilage from joints with osteonecrosis. Activation with APMA enhanced digestion, but breakdown was apparent in extracts of fibrillated osteoarthritic cartilage even without APMA. Enzymatic degradation of type XI collagen could play a significant role in the turnover of articular cartilage in health and disease states.

Quantitative assessment by competitive ELISA of fibronectin (Fn) in tendons and ligaments

A method utilizing the enzyme-linked immunosorbent assay (ELISA) is presented for quantitating fibronectin in periarticular soft tissues from rabbits. The concentrations of fibronectin were determined in the medial collateral ligament, anterior cruciate ligament, posterior cruciate ligament, and patellar tendon. The anterior cruciate and posterior cruciate ligaments, surrounded by a synovial sheath, had similar amounts of fibronectin that were each over twice as high as that found in either medial collateral ligament or patellar tendon which have no sheath covering.

Bovine cartilage types VI and IX collagens. Characterization of their forms in vivo

1. Collagens were extracted from bovine cartilage by 4 M-guanidinium chloride in the presence of proteinase inhibitors and identified by immunoblotting with specific anti-collagen sera. 2. The collagens retained their native conformations (shown by the resistance of their triple-helical domains to pepsin digestion), and the molecular masses of their component alpha-chains indicated that the chains were intact. 3. Type VI collagen was extracted as a large-molecular-mass disulphide-bonded aggregate composed of components of molecular mass 140 kDa and 200-240 kDa, and was therefore similar to type VI collagen identified in noncartilaginous tissues. Immunoblotting established the 200-240 kDa components as intact forms of the alpha 3(VI) chain. 4. Type IX collagen consisted of three clearly separable components of molecular mass 84 kDa, 72 kDa and 66 kDa, which were assigned to the alpha 1(IX)-, alpha 3(IX)- and alpha 2(IX)-chains respectively, and a large proportion of this collagen had no covalently bound glycosaminoglycan attached to the alpha 2(IX)-chain. 5. Differences between the type IX collagen extracted from bovine cartilage and that identified in biosynthetic studies on chick cartilage are discussed.

Type XI collagen is associated with the chondrocyte surface in suspension culture

Chondrocytes from bovine articular cartilage were stripped of matrix, then allowed to reconstitute their pericellular matrix in suspension culture. After incubation, the cells were centrifuged through a Percoll (TM) cushion and separated into a cell fraction, a medium fraction, and an interface fraction. The collagen in each fraction was analyzed by SDS-polyacrylamide gel electrophoresis and immunolocation with antisera against type XI and type II. Under these conditions, type XI collagen was recovered in the cell fraction, but was not detectable by immunolocation in the medium fraction or the interface fraction. In contrast, type II collagen was found in all three of these fractions. Insoluble type XI fibers subjected to the same fractionation scheme in the absence of cells were recovered in the medium and interface fractions, but not in the cell fraction. Incubation of intact cells with collagenase digested the cell-associated collagen, indicating that it was outside of the cells. The type XI collagen was removed from the cells by extraction with 4 M guanidinium chloride. These results indicate that type XI collagen is preferentially retained at the chondrocyte surface, and are consistent with our proposal that it is involved in organization of the pericellular matrix.

Molecular and immunologic differences in canine fibronectins from articular cartilage and plasma

Two new monoclonal antibodies (Mabs) which reacted with canine fibronectin were produced and characterized. Data supported the conclusion that the epitope recognized by Mab 1H9A4 is within the first three Type III homology repeats of the Hep 2 domain and that the epitope for Mab 13G3B7 is within the last Type III homology repeat of fibronectin. These antibodies, along with three others, Mabs IST-2, IST-7, and IST-9, produced and characterized in the laboratories of L. Zardi of Genoa, Italy, were used to characterize canine cartilage and plasma fibronectin. In addition, cartilage explants were labeled with [35S]methionine in order to characterize newly synthesized cartilage fibronectin. The following observations were made. (i) Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (NaDodSO4-PAGE) of reduced canine plasma fibronectin revealed a characteristic doublet; reduced cartilage fibronectin revealed two major bands and one minor band. The lower molecular weight band was 10 kDa less than the beta subunit of plasma fibronectin. In Western blots, this band stained with Mab 1H9A4 but failed to react with Mab 13G3B7. (ii) Western blots of thermolysin and trypsin digests of cartilage fibronectin revealed cleavage patterns which differed from those obtained from digestions of plasma fibronectin. (iii) The ED-A sequence, detected by Mab IST-9, was present in less than 2% of the cartilage fibronectins. (iv) NaDodSO4-PAGE of purified and reduced 35S-labeled fibronectin revealed two major radioactive bands and one minor radioactive band which comigrated with the fibronectin from the cartilage but not with plasma fibronectin. We concluded that like "cellular" fibronectin, the ratio of alpha-type subunits to beta subunits was greater than 4 to 1 in cartilage fibronectin compared to 1.25 to 1 for plasma fibronectin; however, cartilage fibronectin was not a cellular fibronectin by the criterion of the presence of the ED-A sequence. Another difference between plasma and cartilage fibronectin was the presence in cartilage fibronectin of a subpopulation of subunits on which the last Type III homology repeat could not be detected. Biosynthetic data were consistent with the concept that cartilage fibronectin originates from local synthesis by the chondrocyte.

Cartilage contains mixed fibrils of collagen types II, IX, and XI

The distribution of collagen XI in fibril fragments from 17-d chick embryo sternal cartilage was determined by immunoelectron microscopy using specific polyclonal antibodies. The protein was distributed throughout the fibril fragments but was antigenically masked due to the tight packing of collagen molecules and could be identified only at sites where the fibril structure was partially disrupted. Collagens II and IX were also distributed uniformly along fibrils but, in contrast to collagen XI, were accessible to the antibodies in intact fibrils. Therefore, cartilage fibrils are heterotypically assembled from collagens II, IX, and XI. This implies that collagen XI is an integral component of the cartilage fibrillar network and homogeneously distributed throughout the tissue. This was confirmed by immunofluorescence.

Problems in the immunolocalization of type IX collagen in fetal calf cartilage using a monoclonal antibody

Monoclonal antibodies were prepared against the pepsin-resistant fragments (X1-X3) of bovine type IX collagen. One of the five hybridomas that gave a positive reaction in an enzyme-linked immunosorbent assay was selected (H1a) for structural analysis and immunolocalization of type IX collagen. The location of the epitope for H1a was deducted from immunoblots and electron microscopic observations after rotary shadowing. The H1a antibody binds to one end of the longest X2, X3, X4 molecules, and preferentially 40-55nm from one end of X1 molecules thus, on or near the noncollagenous domain, NC2. Different immunolocalizations of type IX collagen in the superficial, middle and deep zones of fetal calf epiphyseal cartilage were observed depending on the thickness of the section and on hyaluronidase digestion conditions. In the middle and deep zones, staining with H1a throughout the matrix was obtained only with thin sections (5 microns) and digestion for 1 h at 37 degrees C. With thick sections (15 microns) or with digestion for 1 h at 24 degrees C, staining was restricted to the pericellular regions. Staining throughout the matrix was obtained in the superficial zone under all experimental conditions. Without hyaluronidase treatment, no immunofluorescent staining was seen with either H1a or polyclonal antibody to type II collagen, indicating that type IX collagen is present throughout the matrix in the different zones of fetal calf cartilage. This result is in good accordance with the recent demonstration of common cross-links between type II and type IX collagen in chicken and bovine cartilage. However, the preferential unmasking of type IX collagen antigenic sites in the pericellular regions of middle and deep zones of fetal calf cartilage does not preclude the presence in that region of a special pericellular organization of the collagenous network.

Antibodies to type II and XI collagens: evidence for the formation of antigen specific as well as cross reacting antibodies in patients with rheumatoid arthritis

Antigen specific and cross reacting antibodies to native and denatured types II and XI collagen were detected in the sera of rats immunised with either of these antigens. The antibodies from rats immunised with type XI collagen initially showed the strongest binding to the alpha 2(XI) chain of type XI collagen but later binding to the alpha 3(XI) chain was seen. Sera from patients with rheumatoid arthritis had antibodies that bound to both type II and XI collagens. Immunoblotting studies showed that most patients had antibodies which bound to the alpha 1(II) chain of type II collagen and to the alpha 3(XI) chain of type XI collagen. Some patients also had antibodies which bound to the alpha 1(XI) and to the alpha 2(XI) chains of type XI collagen. Thus antibodies to unique as well as to common epitopes on each of the two types of collagen molecule occur in some patients with rheumatoid arthritis.

Localization of type IX collagen in chondrons isolated from porcine articular cartilage and rat chondrosarcoma

Chondrocytes, each with their pericellular matrix bounded by a fibrous capsule, can be extracted singly or in groups from both mature pig articular cartilage and chondrosarcoma tissue. These structures, termed chondrons, are thought to anchor the chondrocytes in the matrix and protect them from the compressive forces experienced when articular cartilage is under load. The capsule of these chondrons contains both type II and type IX collagens and is composed of fine fibrillar material, unlike the large banded fibres of type II collagen found in the rest of the matrix. This suggests a role for type IX collagen in regulating the diameter of type II fibres to produce the fine fibrillar structure of the chondron capsules.

An immunohistochemical study of human endometrial extracellular matrix during the menstrual cycle and first trimester of pregnancy

Changes in the organisation and composition of extracellular matrix in human endometrium during the menstrual cycle and early pregnancy have been assessed by immunofluorescence. Amongst interstitial components, type-III and type V-collagens and fibronectin are present in endometrial stroma throughout the menstrual cycle as well as in first trimester decidua. Type V-collagen epitopes are masked early in the cycle, but become accessible in first trimester decidua. Type VI-collagen is abundant in endometrium in the proliferative phase, but is progressively lost in the secretory phase and decidua, in which it is retained only in blood vessel walls. Vitronectin is present in some blood vessels in decidua. Decidualising stromal cells also produce basement membrane components (type IV-collagen, laminin, heparan sulphate proteoglycan and a glycoprotein family recognised by monoclonal antibody G71) and these become organised into a pericellular aura.

Cartilage proteoglycan aggregate and fibronectin can modulate the expression of type X collagen by embryonic chick chondrocytes cultured in collagen gels

Chick embryo sternal chondrocytes from the caudal and cephalic regions were cultured within type I collagen gels and type I collagen/proteoglycan aggregate composite gels in normal serum. Caudal region chondrocytes were also cultured within type I collagen gels in the presence of fibronectin-depleted serum. There was a marked stimulation of type X collagen synthesis by the caudal region chondrocytes after 9 days in the presence of fibronectin-depleted serum and after 14 days in the presence of proteoglycan aggregate. These results provide evidence for the ability of chondrocytes from a zone of permanent cartilage to synthesise type X collagen and for the involvement of extracellular matrix components in the control of type X collagen gene expression.

Chondrons extracted from canine tibial cartilage: preliminary report on their isolation and structure

We report on the morphology and structure of single and multiple chondrons isolated from homogenized samples of fresh and fixed canine tibial cartilage. Phase contrast, Nomarski, and scanning electron microscopy observations show each chondron to be composed of a chondrocyte and its pericellular matrix enclosed within a "felt-like" pericellular capsule. The extraction of intact chondrons from cartilage homogenates confirms the structural validity of the chondron concept and emphasizes the intrinsic mechanical strength of the capsule. Frayed collagen fibers radiate from multiple chondron columns suggesting a shear-resistant, structural interrelationship between capsular components and type II collagen fibers. Future development of chondron extraction procedures could provide a unique model with which to study the structure, biochemistry, and function of articular cartilage chondrocytes and their pericellular microenvironment.

Extracellular matrix alterations during endochondral ossification in humans

Immunohistochemical methods were employed to examine alterations in the cartilage extracellular matrix constituents associated with endochondral ossification in humans. The distributions of chondroitin 4- and 6-sulfate and keratan sulfate proteoglycan (PG) determinants, cartilage PG link protein, collagen types I and II, and fibronectin were determined in iliac crest growth-plate specimens using the avidin-biotin-horseradish peroxidase system. Collagen type II was distributed throughout the growth plate, providing a framework within which chondrocytes divided and formed clusters of differentiating (hypertrophic) cells. The septa between these clusters and their subchondral extensions into underlying bone trabeculae were rich in PG, PG link protein, and collagen type II and resembled the extracellular matrix of reserve cartilage. The territorial matrix associated with the differentiating cells within the clusters contained reduced amounts of collagen type II, PG link protein, and possibly cartilage PG. Collagen type I and fibronectin were detected within the cytoplasm of the maturing and degenerating cells, and fibronectin localized intensely to the pericellular matrix envelopes of these cells. These alterations presumably facilitate the degradation of the matrix associated with the cell clusters by invading vascular tissue, while the septa, which retain the characteristics of more typical cartilage matrix, are not degraded and firmly anchor the cartilage to the subchondral bone.

Tenascin is associated with chondrogenic and osteogenic differentiation in vivo and promotes chondrogenesis in vitro

The tissue distribution of the extracellular matrix glycoprotein, tenascin, during cartilage and bone development in rodents has been investigated by immunohistochemistry. Tenascin was present in condensing mesenchyme of cartilage anlagen, but not in the surrounding mesenchyme. In fully differentiated cartilages, tenascin was only present in the perichondrium. In bones that form by endochondral ossification, tenascin reappeared around the osteogenic cells invading the cartilage model. Tenascin was also present in the condensing mesenchyme of developing bones that form by intramembranous ossification and later was present around the spicules of forming bone. Tenascin was absent from mature bone matrix but persisted on periosteal and endosteal surfaces. Immunofluorescent staining of wing bud cultures from chick embryos showed large amounts of tenascin in the forming cartilage nodules. Cultures grown on a substrate of tenascin produced more cartilage nodules than cultures grown on tissue culture plastic. Tenascin in the culture medium inhibited the attachment of wing bud cells to fibronectin-coated substrates. We propose that tenascin plays an important role in chondrogenesis by modulating fibronectin-cell interactions and causing cell rounding and condensation.

Ultrastructural localisation of fibronectin in human osteoarthritic articular cartilage

A protein A-gold immunolocalisation technique has been used on sections of femoral head articular cartilage to localise fibronectin. Chondroitinase treatment enhanced gold staining, particularly when tissue samples were digested before fixation. The greatest accumulations of fibronectin were seen in the surface zone of osteoarthritic cartilage. Disease free cartilage contained very little fibronectin in this region. Cells which appeared to produce fibronectin were rare in normal specimens but common in the superficial region of osteoarthritic cartilage. These chrondrocytes appeared to release fibronectin as part of an amorphous material which accumulated in the pericellular region. This is the first ultrastructural demonstration of fibronectin synthesis by articular cartilage chondrocytes.

An immunohistochemical study of fibronectin in human osteoarthritic and disease free articular cartilage

Fibronectin is a minor component of cartilage connective tissue matrix, which is reported to accumulate in increased amounts in osteoarthritis. The presence of raised levels of fibronectin in human osteoarthritic cartilage by immunoperoxidase localisation is confirmed. Residual femoral head articular cartilage from 17 patients with osteoarthritis contained variable but substantial amounts of fibronectin. This was localised mainly in a band within the matrix of the surface zone. No significant deposits of fibronectin were found in this or any other area of the normal specimens. Intracellular fibronectin was identified in some cells of the surface zone, indicating that it was, in part, synthesised locally. The presence and distribution of locally produced fibronectin in osteoarthritic cartilage suggest that its synthesis is a response by chondrocytes to changes in the cartilage matrix.

Effect of polyanions on fibrillogenesis by type XI collagen

Type XI collagen (1 alpha,2 alpha,3 alpha) from bovine articular cartilage form fibrils at 4 degrees C in 0.15 M NaCl at pH 7.4, but fibrillogenesis is inhibited by the addition of 1 M glucose or by raising the NaCl concentration to 1 M. Removal of the glucose or NaCl by dialysis allows fibril formation. When proteoglycans, heparin, or chondroitin sulfate were added to type XI collagen in 1 M NaCl both fibrillogenesis and polyanion-collagen interaction were inhibited by the high NaCl concentration. When the mixture was dialysed against 0.15 M NaCl, a new aggregate type was seen, scattered among shortened and branched fibers. The new aggregates were either X-, Y-, or wheel-shaped structures with electron dense cores. They were apparently formed by collagen molecules intersecting approximately 200 nm from one end. In contrast, when the polyanion was mixed with the collagen in 1 M glucose, which inhibits fibrillogenesis but not polyanion-collagen interaction, a different type of aggregate appeared following dialysis. These aggregates were discrete 280 X 40 nm structures with an asymmetric banding pattern. They are similar to SLS aggregates, and probably are composed of collagen molecules lined up in register. The results are different from those seen with the interstitial collagens and emphasize the unique character of the interaction of polyanions, including proteoglycan, with type XI collagen.

Chondrons in cartilage: ultrastructural analysis of the pericellular microenvironment in adult human articular cartilages

A combination of scanning and transmission electron microscopy was used to investigate the morphology and ultrastructure of normal human articular cartilage sampled from adult amputation specimens. This study confirms our previous observations on canine articular cartilage, which showed middle and deep layer chondrocytes surrounded by a pericellular matrix and enclosed within a pericellular capsule composed of filamentous and fine fibrillar materials. Pores in the "felt-like" organization of the capsular weave progressively decreased in size from the inner to the outer border of the capsule. Matrix vesicles were found embedded within the capsular weave and distributed throughout the territorial matrix. It is suggested that the chondrocyte, its pericellular matrix, and capsule together constitute the "chondron," a primary functional and metabolic unit of cartilage that acts hydrodynamically to protect the integrity of the chondrocyte and its pericellular microenvironment during compressive loading.

The immunoperoxidase localization of type X collagen in chick cartilage and lung

We have localized type X collagen in chick cartilage and lung by use of affinity-purified antibodies raised against the purified protein. Examination of the centers of primary endochondral development in the embryonic sternum, developing cartilage in the embryonic tubular bones and growth plate cartilages demonstrated the specific association of type X collagen with regions of hypertrophic chondrocytes in these tissues. Furthermore these studies revealed a tendency for type X collagen to accumulate adjacent to regions of active vascular invasion and an apparent lag between the initial intracellular accumulation and the matrix deposition of type X collagen. A lag in the matrix accumulation of type X collagen was also shown biochemically and immunohistochemically in chondrocytes in culture. In chick lung type X collagen was localized to the smooth muscle of the blood vessels and to smooth muscle surrounding alveolar ducts.

On the role of type IX collagen in the extracellular matrix of cartilage: type IX collagen is localized to intersections of collagen fibrils

The tissue distribution of type II and type IX collagen in 17-d-old chicken embryo was studied by immunofluorescence using polyclonal antibodies against type II collagen and a peptic fragment of type IX collagen (HMW), respectively. Both proteins were found only in cartilage where they were co-distributed. They occurred uniformly throughout the extracellular matrix, i.e., without distinction between pericellular, territorial, and interterritorial matrices. Tissues that undergo endochondral bone formation contained type IX collagen, whereas periosteal and membranous bones were negative. The thin collagenous fibrils in cartilage consisted of type II collagen as determined by immunoelectron microscopy. Type IX collagen was associated with the fibrils but essentially was restricted to intersections of the fibrils. These observations suggested that type IX collagen contributes to the stabilization of the network of thin fibers of the extracellular matrix of cartilage by interactions of its triple helical domains with several fibrils at or close to their intersections.

Immunoperoxidase localization of type X collagen in chick tibiae

Type X collagen was prepared from medium of long-term cultures of embryonic chick tibiotarsal chondrocytes. Antibodies to type X collagen were raised and used in immunoperoxidase localization studies with embryonic and growing chick tibiotarsus. Strong anti-type X collagen reactivity was detected mainly in the region of hypertrophic chondrocytes, and to a lesser extent in the zone of calcified cartilage. No reactivity was detected in the proliferative zone nor the superficial layer of the cartilage growth plate. These results suggest that type X collagen may play a key role in matrix calcification during growth and development of the skeletal system.

Isolation from bovine elastic tissues of collagen type VI and characterization of its form in vivo

Foetal-bovine nuchal ligament and aorta, together with adult-bovine aorta and pregnant uterus, were extracted under dissociative conditions in the absence and in the presence of a reducing agent. A collagenous glycoprotein of Mr 140000 [designated component 140K(VI)], identified in these extracts as the major periodate/Schiff-positive component, was shown to be related to collagen type VI. Digestion of non-reduced extracts with pepsin yielded periodate/Schiff-positive peptides that, on the basis of their electrophoretic mobilities, amino acid analyses and peptide 'maps', were identical with type VI collagen fragments prepared by standard procedures. It is concluded that collagen type VI occurs in vivo as molecule comprising three chains of Mr 140000 in which the helical domains account for about one-third of each polypeptide. Biosynthetic experiments with nuchal-ligament fibroblasts in culture demonstrated that a bacterial-collagenase-sensitive [3H]fucose-labelled glycoprotein, Mr 140000, was immunoprecipitated from culture medium by a specific antibody to the pepsin-derived form of collagen type VI. This result suggests that the collagenous polypeptides [140K(VI) components] represent the biosynthetic precursors of type VI collagen that do not undergo processing to smaller species before deposition in the extracellular matrix. Analyses of 5M-guanidinium chloride extracts of tissues with markedly different elastin contents and at different stages of development suggested that there was no relationship between collagen type VI and elastic-fibre microfibrils, a conclusion supported by the observation that the immunoprecipitated glycoprotein, Mr 140000, was distinct from the glycoprotein MFPI, Mr 150000, believed to be a constituent of these microfibrils [Sear, Grant & Jackson (1981) Biochem. J. 194, 587-598].

Monoclonal antibody against chicken type IX collagen: preparation, characterization, and recognition of the intact form of type IX collagen secreted by chondrocytes

A series of monoclonal antibodies was prepared against the pepsin-resistant fragment of type IX collagen designated HMW. One of these antibodies (called 2C2) was selected for further analysis. Antibody 2C2 showed no cross-reactivity with other collagen types by inhibition enzyme-linked immunosorbent assays. It recognized an epitope present in native HMW, but failed to recognize any of the three chains of HMW fractionated after denaturation followed by reduction and alkylation of interchain disulfide bridges. Electron microscopic observations after rotary shadowing showed that the location of the epitope for antibody 2C2 was close to the carboxy-terminus of HMW. Immunofluorescent staining of sections of embryonic and adult cartilage with antibody 2C2 after removal of proteoglycans by testicular hyaluronidase digestion showed that type IX collagen is distributed throughout the cartilage matrix, and is not present in other connective tissues or skeletal muscle. The intact type IX collagen molecule, which was secreted by a suspension culture of freshly isolated embryonic chick chondrocytes, was recognized by rotary shadowing in the presence of antibody 2C2 after first precipitating the procollagens from the culture medium with ammonium sulfate (30%). Two different collagenous molecules were present in the precipitate: a longer molecule of type II procollagen (average length, 335 nm) with both amino- and carboxy-propeptides still remaining uncleaved, and a shorter molecule (average length, 190 nm) which was identified as type IX collagen. Antibody 2C2 consistently bound to the shorter molecules at a site located 136 nm from a distinctive knob at one end of the molecule, and did not bind to any specific site on the type II procollagen molecules. The structure of the intact type IX collagen molecule with the location of both collagenous and noncollagenous domains was as predicted after converting the nucleotide sequence of a cDNA clone encoding for one of the chains of type IX collagen to an amino acid sequence (Ninomiya, Y., and B. R. Olsen, 1984, Proc. Natl. Acad. Sci. USA, 81:3014-3018).

Isolation and partial characterization of precursors to minor cartilage collagens

Suspension cultures of cartilage cells were prepared from 17-day chick embryo sterna and radiolabeled with [14C]-proline under conditions which sought to minimize proteolytic conversion of procollagen to collagen. Collagenous proteins were isolated from the culture medium and cell fraction, were purified in their native state by (NH4)2SO4 precipitation and DEAE-cellulose chromatography, and were characterized by protease susceptibility, SDS-gel-filtration and SDS-polyacrylamide gel electrophoresis. Qualitatively, the precursor components present in the medium were similar to those in the cell extract; quantitatively, it appeared that the minor cartilage collagen precursor components derived from 1 alpha, 2 alpha, 3 alpha and type IX collagens were more prevalent in the cell extract. SDS-PAGE of unreduced samples showed that precursors to both of these collagens migrated as distinct high-molecular-weight aggregates. After chymotrypsin digestion, unreduced type IX collagen migrated as two disulfide-bonded aggregates--a large one (Mr approximately 210K) and a small one (Mr approximately 43K); whereas 1 alpha, 2 alpha, 3 alpha chains migrated identically whether reduced or unreduced. Reduction of undigested type IX aggregate yielded two components of Mr approximately 97K and 78K; whereas reduction of the chymotrypsin resistant 210K and 43 K aggregates gave a single component of Mr approximately 61K and a component which migrated at the dye front, respectively. The molecular origin of these components was confirmed by differential NaCl precipitation. It was concluded that this culture system synthesized precursors to 1 alpha, 2 alpha, 3 alpha and type IX collagens in addition to type II; type X collagen was not detected even though the 17-day sternum contained a population of cells morphologically similar to hypertrophic chondrocytes. The precursor chains to 1 alpha, 2 alpha, 3 alpha collagen had an apparent Mr greater than pro-alpha (II) and could be isolated as a disulfide-bonded aggregate(s); the precursor chains to type IX collagen had an apparent Mr less than pro alpha (II) and could also be isolated as a disulfide-bonded aggregate. All of the cartilage collagen precursors had protease-susceptible regions, but those in type IX appeared to be more sensitive to pepsin than to chymotrypsin.

Morphology of the pericellular capsule in articular cartilage revealed by hyaluronidase digestion

To allow a more valid comparison between our previous ultrastructural data and the immunolocalization of type IX and other minor collagen species in cryosectioned cartilage, we examined both normal and testicular hyaluronidase-digested canine tibial cartilage by electron microscopy. Removal of matrix proteoglycans caused the pericellular capsule to collapse against the cell surface, suggesting that its normal anatomical position is mediated by pericellular matrix hydration. Detailed examination of the pericellular capsule and pericellular channel revealed fine, faintly banded fibrils and an amorphous component somewhat similar in structure to basement membrane collagens. Matrix vesicles and the electron-dense material of the interterritorial matrix were only partially digested by hyaluronidase. We propose that the pericellular capsule is composed of a "felt-like" network of minor collagen species which act synergistically to maintain both the composition of the pericellular matrix and the integrity of the chondrocyte/pericellular matrix complex during compressive loading.

Immunohistochemical localization of short chain cartilage collagen (type X) in avian tissues

Monoclonal antibodies were produced against the recently described short chain cartilage collagen (type X collagen), and one (AC9) was extensively characterized and used for immunohistochemical localization studies on chick tissues. By competition enzyme-linked immunosorbent assay, antibody AC9 was observed to bind to an epitope within the helical domain of type X collagen and did not react with the other collagen types tested, including the minor cartilage collagens 1 alpha, 2 alpha, 3 alpha, and HMW-LMW. Indirect immunofluorescence analyses with this antibody were performed on unfixed cryostat sections from various skeletal and nonskeletal tissues. Only those of skeletal origin showed detectable reactivity. Within the cartilage portion of the 13-d-old embryonic tibiotarsus (a developing long bone) fluorescence was observed only in that region of the diaphysis containing hypertrophic chondrocytes. None was detectable in adjacent regions or in the epiphysis. Slight fluorescence was also present within the surrounding sleeve of periosteal bone. Consistent with these results, the antibody did not react with the cartilages of the trachea and sclera, which do not undergo hypertrophy during the stages examined. It did, however, lightly react with the parietal bones of the head, which form by intramembranous ossification. These results are consistent with our earlier biochemical analyses, which showed type X collagen to be a product of that subpopulation of chondrocytes that have undergone hypertrophy. In addition, either it or an immunologically cross-reactive molecule is also present in bone, and exhibits a diminished fluorescent intensity as compared with hypertrophic cartilage.

Further biochemical and physicochemical characterization of minor disulfide-bonded (type IX) collagen, extracted from foetal calf cartilage

Minor disulfide-bonded collagen (previously termed X1-X7 and now called type IX collagen) was isolated from foetal calf cartilage after pepsin treatment. At least three native fractions, containing, respectively, the X1X2X3, X4, and X5X6X7 chains, were separated; and from further biochemical and physicochemical experiments (differential scanning calorimetry, electrical birefringence, rotary shadowing), we propose a tentative model for their organization within a parent molecule. X1 and X2 are molecules composed of three chains of apparent Mr 62,000 and 50,000 linked by interchain disulfide bonds and containing pepsin-sensitive regions. The cleavage of at least three of these sites, present within X2, gives rise to the X3 and X5X6X7 fractions composed of molecules 80-100 nm and 40-55 nm in length, respectively. The X5X6X7 fraction is not digested by pepsin at 30 degrees C owing to its high thermal stability (certainly explained by its high hydroxyproline + proline content). This organization is in good accordance with that proposed for chicken cartilage type IX collagen; differences could only exist in the number and (or) the location of the pepsin-sensitive sites.

Appearance and persistence of fibronectin in cartilage. Specific interaction of fibronectin with collagen type II

Binding of fibronectins (FN) to collagen types I-IV were studied using polyclonal antibodies against human and chicken FNs, proteoglycan monomers, collagen type II and monoclonal antibodies reacting with both soluble and insoluble forms of human FN. Plasma fibronectin and type II collagen were shown to interact specifically in a homologous system. Type II collagen, however, proved to be less effective in inhibition assays compared to other types of collagen. In high density cultures of chicken limb bud cells, fibronectin was first localized within the fibroblast-like cells of 4 hr cultures and an extensive extracellular filamentous network developed by the end of day 1. Fibronectin was present in the newly formed cartilage nodules although it seemed to disappear by day 6, when the proteoglycan accumulation became more intensive. Enzyme treatments (testicular hyaluronidase, chondroitinase ABC) helped to localize FN at this stage of development of chicken cartilage, in microdroplet high density cultures of human fetal chondrocytes and in articular cartilage. Fibronectin was localized only in the pericellular ring of intact human articular cartilage using monoclonal antibodies with the biotin-avidin system.

The in vitro cell culture age and cell density of articular chondrocytes alter sulfated-proteoglycan biosynthesis

The effect of cell culture age and concomitant changes in cell density on the biosynthesis of sulfated-proteoglycan by rabbit articular chondrocytes in secondary monolayer culture was studied. Low density (LD, 2 d), middle density (MD, 5-7 d), and high density (HD, 12-15 d) cultures demonstrated changes in cellular morphology and rates of DNA synthesis. DNA synthesis was highest at LD to MD densities, but HD cultures continued to incorporate [3H]-thymidine. LD cultures incorporated 35SO4 into sulfated-proteoglycans at a higher rate than MD or LD cultures. The qualitative nature of the sulfated-proteoglycans synthesized at the different culture ages were analyzed by assessing the distribution of incorporated 35SO4 in associative and dissociative CsCl density gradients and by elution profiles on Sepharose CL-2B. Chondrocytes deposited into the extracellular matrix (cell-associated fraction) 35SO4-labeled proteoglycan aggregate. More aggregated proteoglycan was found in the MD and HD cultures than at LD. A 35SO4-labeled aggregated proteoglycan of smaller hydrodynamic size than that found in the cell-associated fraction was secreted into the culture medium at each culture age. The proteoglycan monomer (A1D1) of young and older cultures had similar hydrodynamic sizes at all cell culture ages and cell densities. The glycosaminoglycan chains of A1D1 were hydrodynamically larger in the younger LD cultures than in the older HD cultures and consisted of only chondroitin 6 and 4 sulfate chains. A small amount of chondroitin 4,6 sulfate was detected, but no keratan sulfate was measured. The A1D2 fractions of young LD cultures contained measurable amounts of dermatan sulfate; no dermatan sulfate was found in older MD or HD cultures. These studies indicated that chondrocytes at LD synthesized a proteoglycan monomer with many of the characteristics of young immature articular cartilage of rabbits. These results also indicated that rapidly dividing chondrocytes were capable of synthesizing proteoglycans which form aggregates with hyaluronic acid. Culture age and cell density appears primarily to modulate the synthesis of glycosaminoglycan types and chain length. Whether or not these glycosaminoglycans are found on the same or different core proteins remains to be determined.

The collagens of the developing bovine cornea

The morphology of the developing bovine eye has been examined and the collagens in fetal bovine eyes from three months' gestation to maturity have been solubilized by pepsin treatment and analyzed to determine the ratios of the predominant types of collagen. The type I collagen decreased, while the type V collagen increased with age. Type III collagen comprised less than 1% of all the corneas, except for the three-month fetal calf. The anterior to posterior thickness of the paraffin-embedded fetal calf cornea increased from the third to the seventh month, decreased from the seventh month to birth, and then increased after birth. Descemet's membrane increased in thickness with age. Analysis of dissected regions of the calf cornea showed a uniform distribution of the collagen populations from the center to the limbus (89% type I, 10% type V and less than 1% type III collagen) and uniformity through the depth of the stroma, except that type III was concentrated around Bowman's layer, and type IV in Descement's membrane. The localization of the different collagens was consistent with the immunofluorescent staining studies with anticollagen antibodies, but the ratios of the intensities of the fluorescence did not correspond to the quantitative analyses. These results are concordant with other studies that have shown that antibody binding may be masked or diminished in certain tissues and therefore immunofluorescence cannot be used reliably for quantitative measurements.

A new look at vitreous-humour collagen

The collagens of bovine vitreous-humour and nasal-septum cartilage have been extracted, fractionated and compared. Both tissues show the same heterogeneity of collagen types, consisting of type II, 1 alpha, 2 alpha, 3 alpha and C-PS collagens. The type II collagen of the vitreous humour was significantly more hydroxylated both in the lysine and proline residues than was that of cartilage. C-PS1 collagen, together with higher-Mr forms were present in the vitreous humour, but the higher-Mr forms were not seen in cartilage. Both C-PS1 and C-PS2 were present in vitreous humour and cartilage, but vitreous humour contained three times more of these collagens than did cartilage. Despite the difference in amount, the molar ratio C-PS1/C-PS2 was approx. 1 in both tissues, suggesting that they are components of a larger molecule. The 1 alpha, 2 alpha, 3 alpha collagens were present in the same concentration in both tissues. These three chains co-precipitated on dialysis against phosphate-buffered saline, pH 7.2, in a manner analogous to type V collagen.