Localization of collagen type VI in articular cartilage of young and adult mice

Primary culture of chondrocytes after collagenase IA or II treatment of articular cartilage from elderly patients undergoing arthroplasty

Background

Joint replacement surgery provides articular cartilage samples for chondrocyte isolation. To our knowledge, the effect of the collagenase type on releasing of chondrocytes from the extracellular matrix of cartilage is not reported.

Objectives

To determine whether cartilage digested with collagenase IA yielded more chondrocytes than that digested with collagenase II and determine whether chondrocytes isolated with collagenase IA could be cultured in vitro.

Methods

Cartilage slices collected from 18 elderly patients who received joint replacement surgery (16 hips, 2 knees) were digested sequentially with 0.4% pronase E and 0.02% collagenase IA, or with 0.15% collagenase II alone, or sequentially with 0.4% pronase E and 0.02% collagenase II. We compared cell yield from each method. Cell viability by the most effective method was calculated and plotted. The morphology of cultured monolayer chondrocytes was recorded with a light microscope.

Results

Sequential digestion with pronase E and collagenase IA yielded 2566 ± 873 chondrocytes per mg wet cartilage, which was more effective than the other isolation methods (P = 0.018). The average chondrocyte viability could reach 84% ± 8% (n = 11). Light microscopic images showed typical chondrocyte morphology in monolayer cultures.

Conclusion

Sequential digestion of human articular cartilage with pronase E and collagenase IA was more effective than collagenase II alone or collagenase II combined with pronase E for releasing chondrocytes from extracellular matrix of cartilage. Chondrocytes isolated with this method could be maintained in monolayer cultures for at least 2 passages with unaltered morphology.

Characterization of the chondrocyte secretome in photoclickable poly(ethylene glycol) hydrogels

Poly(ethylene glycol) (PEG) hydrogels are highly tunable platforms that are promising cell delivery vehicles for chondrocytes and cartilage tissue engineering. In addition to characterizing the type of extracellular matrix (ECM) that forms, understanding the types of proteins that are secreted by encapsulated cells may be important. Thus, the objectives for this study were to characterize the secretome of chondrocytes encapsulated in PEG hydrogels and determine whether the secretome varies as a function of hydrogel stiffness and culture condition. Bovine chondrocytes were encapsulated in photoclickable PEG hydrogels with a compressive modulus of 8 and 46 kPa and cultured under free swelling or dynamic compressive loading conditions. Cartilage ECM deposition was assessed by biochemical assays and immunohistochemistry. The conditioned medium was analyzed by liquid chromatography-tandem mass spectrometry. Chondrocytes maintained their phenotype within the hydrogels and deposited cartilage-specific ECM that increased over time and included aggrecan and collagens II and VI. Analysis of the secretome revealed a total of 64 proteins, which were largely similar among all experimental conditions. The identified proteins have diverse functions such as biological regulation, response to stress, and collagen fibril organization. Notably, many of the proteins important to the assembly of a collagen-rich cartilage ECM were identified and included collagen types II(α1), VI (α1, α2, and α3), IX (α1), XI (α1 and α2), and biglycan. In addition, many of the other identified proteins have been reported to be present within cell-secreted exosomes. In summary, chondrocytes encapsulated within photoclickable PEG hydrogels secrete many types of proteins that diffuse out of the hydrogel and which have diverse functions, but which are largely preserved across different hydrogel culture environments. Biotechnol. Bioeng. 2017;114: 2096-2108. © 2017 Wiley Periodicals, Inc.

The structure and function of the pericellular matrix of articular cartilage

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. While the PCM was first observed nearly a century ago, its role is still under investigation. In support of early hypotheses regarding its function, increasing evidence indicates that the PCM serves as a transducer of biochemical and biomechanical signals to the chondrocyte. Work over the past two decades has established that the PCM in adult tissue is defined biochemically by several molecular components, including type VI collagen and perlecan. On the other hand, the biomechanical properties of this structure have only recently been measured. Techniques such as micropipette aspiration, in situ imaging, computational modeling, and atomic force microscopy have determined that the PCM exhibits distinct mechanical properties as compared to the ECM, and that these properties are influenced by specific PCM components as well as disease state. Importantly, the unique relationships among the mechanical properties of the chondrocyte, PCM, and ECM in different zones of cartilage suggest that this region significantly influences the stress-strain environment of the chondrocyte. In this review, we discuss recent advances in the measurement of PCM mechanical properties and structure that further increase our understanding of PCM function. Taken together, these studies suggest that the PCM plays a critical role in controlling the mechanical environment and mechanobiology of cells in cartilage and other cartilaginous tissues, such as the meniscus or intervertebral disc.

Immunofluorescence-guided atomic force microscopy to measure the micromechanical properties of the pericellular matrix of porcine articular cartilage

The pericellular matrix (PCM) is a narrow region that is rich in type VI collagen that surrounds each chondrocyte within the extracellular matrix (ECM) of articular cartilage. Previous studies have demonstrated that the chondrocyte micromechanical environment depends on the relative properties of the chondrocyte, its PCM and the ECM. The objective of this study was to measure the influence of type VI collagen on site-specific micromechanical properties of cartilage in situ by combining atomic force microscopy stiffness mapping with immunofluorescence imaging of PCM and ECM regions in cryo-sectioned tissue samples. This method was used to test the hypotheses that PCM biomechanical properties correlate with the presence of type VI collagen and are uniform with depth from the articular surface. Control experiments verified that immunolabelling did not affect the properties of the ECM or PCM. PCM biomechanical properties correlated with the presence of type VI collagen, and matrix regions lacking type VI collagen immediately adjacent to the PCM exhibited higher elastic moduli than regions positive for type VI collagen. PCM elastic moduli were similar in all three zones. Our findings provide further support for type VI collagen in defining the chondrocyte PCM and contributing to its biological and biomechanical properties.

Type II and VI collagen in nasal and articular cartilage and the effect of IL-1alpha on the distribution of these collagens

The distribution of type II and VI collagen was immunocytochemically investigated in bovine articular and nasal cartilage. Cartilage explants were used either fresh or cultured for up to 4 weeks with or without interleukin 1α (IL-1α). Sections of the explants were incubated with antibodies for both types of collagen. Microscopic analyses revealed that type II collagen was preferentially localized in the interchondron matrix whereas type VI collagen was primarily found in the direct vicinity of the chondrocytes. Treatment of the sections with hyaluronidase greatly enhanced the signal for both types of collagen. Also in sections of explants cultured with IL-1α a higher level of labeling of the collagens was found. This was apparent without any pre-treatment with hyaluronidase. Under the influence of IL-1α the area positive for type VI collagen that surrounded the chondrocytes broadened. Although the two collagens in both types of cartilage were distributed similarly, a remarkable difference was the higher degree of staining of type VI collagen in articular cartilage. Concomitantly we noted that digestion of this type of cartilage hardly occurred in the presence of IL-1α whereas nasal cartilage was almost completely degraded within 18 days of culture. Since type VI collagen is known to be relatively resistant to proteolysis we speculate that the higher level of type VI collagen in articular cartilage is important in protecting cartilage from digestion.

Pathogenesis of osteoarthritis-like changes in the joints of mice deficient in type IX collagen

OBJECTIVE

To examine the pathogenetic mechanisms of osteoarthritis (OA)-like changes in Col9a1-/- mice, which are deficient in type IX collagen.

METHODS

Knee joints and temporomandibular joints (TMJs) from Col9a1-/- mice and their wild-type (Col9a1+/+) littermates were examined by light microscopy. Immunohistochemical staining was performed to examine the expression of matrix metalloproteinase 3 (MMP-3) and MMP-13, degraded type II collagen, and the discoidin domain receptor 2 (DDR-2) in knee joints. Cartilage mechanics were also evaluated for compressive properties by microindentation testing of the tibial plateau and for tensile properties by osmotic loading of the femoral condyle.

RESULTS

Histologic analysis showed age-dependent OA-like changes in the knee and TMJs of Col9a1-/- mice starting at the age of 3 months. At the age of 6 months, enhanced proteoglycan degradation was observed in the articular cartilage of the knee and TMJs of the mutant mice. The expression of MMP-13 and DDR-2 protein and the amount of degraded type II collagen were higher in the knee joints of Col9a1-/- mice than in their wild-type littermates at the age of 6 months. Changes in cartilage mechanics were observed in the femoral and tibial plateaus of Col9a1-/- mice at 6 months, including a decrease in the compressive modulus and uniaxial modulus. At 3 and 6 months of age, tibial cartilage in Col9a1-/- mice was found to be more permeable to fluid flow, with an associated compromise in the fluid pressurization mechanism of load support. All of these changes occurred only at medial sites.

CONCLUSION

Lack of type IX collagen in Col9a1-/- mice results in age-dependent OA-like changes in the knee joints and TMJs.

The role of type X collagen in facilitating and regulating endochondral ossification of articular cartilage

AUTHOR: Shen G Objective -This review was compiled to explore the role of type X collagen in growth, development and remodeling of articular cartilage by elucidating the linkage between the synthesis of this protein and the phenotypic changes in chondrogenesis and the onset of endochondral ossification.

DESIGN

The current studies closely dedicated to elucidating the role of type X collagen incorporating into chondrogenesis and endochondral ossification of articular cartilage were assessed and analyzed to allow for obtaining the mainstream consensus on the bio-molecular mechanism with which type X collagen functions in articular cartilage.

RESULTS

There are spatial and temporal correlations between synthesis of type X collagen and occurrence of endochondral ossification. The expression of type X collagen is confined within hypertrophic condrocytes and precedes the embark of endochondral bone formation. Type X collagen facilitates endochondral ossification by regulating matrix mineralization and compartmentalizing matrix components.

CONCLUSION

Type X collagen is a reliable marker for new bone formation in articular cartilage. The future clinical application of this collagen in inducing or mediating endochondral ossification is perceived, e.g. the fracture healing of synovial joints and adaptive remodeling of madibular condyle.

Distribution of Type VI Collagen in the Cartilaginous Tissue of the Proximal Tibia in the Domestic Cat

To investigate the distribution of the early stage chondrocytes during the formation and closure of epiphyseal growth plate (EGP) of the domestic cat, we examined the EGP of proximal tibiae by immunohistochemistry for type VI collagen. In the epiphyseal cartilage without the secondary ossification center (SOC) and EGP in newborn cats aged 1 and 10 days, type VI collagen-positive chondrocytes were located around the cartilage canals and articular surface. In the epiphyseal cartilage with the SOC and EGP in young cats aged 1 to 3 months, type VI collagen-positive chondrocytes were located in the upper resting zone of the EGP, and then increased throughout the resting zone along with maturation. In the adult cats with the partially closed EGP, type VI collagen-positive chondrocytes were distributed throughout the remaining EGP. These findings indicate that the early stage chondrocytes characterized with type VI collagen are continuously located in the EGP during maturation. In addition, the increase of the early stage chondrocytes and the decrease of the reserve chondrocytes in the EGP along with maturation may cause the cessation of the longitudinal growth of the EGP, and finally bring about the EGP closure.

Experimental induction of anterior disk displacement of the rabbit craniomandibular joint: an immuno-electron microscopic study of collagen and proteoglycan occurrence in the condylar cartilage

BACKGROUND

Results from our previous studies suggest that surgical induction of anterior disk displacement (ADD) in the rabbit craniomandibular joint (CMJ) leads to histopathological alterations consistent with osteoarthritis. In addition, molecular changes in collagens and glycosaminoglycans (GAGs) were observed using immunohistochemistry. The purpose of the present study was to further characterize those molecular changes in collagens and GAGs using immuno-electron microscopy.

METHODS

The right joint of 15 rabbits was exposed surgically and all discal attachments were cut except for the posterior attachment (the bilaminar zone). The disc was then repositioned anteriorly and sutured to the zygomatic arch. The left joint was used as a sham-operated control. Ten additional joints were used as non-operated controls. Mandibular condyles were removed 2 weeks following surgery and processed for light and immuno-electron microscopy using colloidal gold-labeled antibodies against collagen type I, II, VI and IX and against keratan sulfate, chondroitin-4 and -6-sulfate, and link protein.

RESULTS

Light microscopic results showed osteoarthritic changes. Immuno-electron microscopy of osteoarthritic cartilage demonstrated a decline in type II collagen, the abnormal presence of type I collagen and loss of type VI and IX collagens. Quantitative colloidal gold immuno-electron microscopy confirmed the depletion of keratan sulfate, chondroitin-4 and -6-sulfate, and link protein in osteoarthritic cartilage.

CONCLUSION

Anterior disk displacement leads to molecular alterations in both the collagen and the proteoglycans of rabbit condylar cartilage characteristic of osteoarthritis in other synovial joints. These alterations are consistent with loss of the shock absorber function of the cartilage and injury of the underlying bone.

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.

The C5 domain of Col6A3 is cleaved off from the Col6 fibrils immediately after secretion

In articular cartilage, type VI collagen is concentrated in the pericellular matrix compartment. During protein synthesis and processing at least the alpha3(VI) chain undergoes significant posttranslational modification and cleavage. In this study, we investigated the processing of type VI collagen in articular cartilage. Immunostaining with a specific polyclonal antiserum against the C5 domain of alpha3(VI) showed strong cellular staining seen in nearly all chondrocytes of articular cartilage. Confocal laser-scanning microscopy and immunoelectron microscopy allowed localization of this staining mainly to the cytoplasm and the immediate pericellular matrix. Double-labeling experiments showed a narrow overlap of the C5 domain and the pericellular mature type VI collagen. Our results suggest that at least in human adult articular cartilage the C5 domain of alpha3(VI) collagen is synthesized and initially incorporated into the newly formed type VI collagen fibrils, but immediately after secretion is cut off and is not present in the mature pericellular type VI matrix of articular cartilage.

De-differentiated chondrosarcoma is not a 'de-differentiated' chondrosarcoma

AIMS

De-differentiated chondrosarcoma is characterized by the presence of two distinct chondroid and nonchondroid tumour portions. The aim of our study was to investigate the distribution of extracellular matrix components in this tumour entity and thus to shed light on its histogenetic origin.

METHODS AND RESULTS

Histochemical and immunohistochemical analyses were performed for collagen subtypes I, II, III and VI and cartilage proteoglycans in three samples of de-differentiated as well as conventional chondrosarcomas (various grades). In the chondroid tumour areas of de-differentiated chondrosarcoma, typical cartilage matrix components could be detected similar to chondroid areas of grade 1 and 2 conventional chondrosarcomas. In contrast, the tumour matrix of the nonchondroid portions of de-differentiated chondrosarcomas contained matrix molecules which are typical for fibroblastic tissue. This matrix composition was not identical with less differentiated (nonchondroid) areas of grades 2 and 3 conventional chondrosarcomas.

CONCLUSIONS

Our results confirm the chondroid nature of the differentiated portion of de-differentiated chondrosarcoma and indicate a nonchondrocytic nature of the nonchondroid portion. De-differentiated chondrosarcoma should not be considered as a 'de'-differentiated chondrosarcoma (grade 4 neoplasm), but as a tumour entity showing two types of mesenchymal differentiation.

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.

Type VI collagen anchors endothelial basement membranes by interacting with type IV collagen

Type VI collagen filaments are found associated with interstitial collagen fibers, around cells, and in contact with endothelial basement membranes. To identify type VI collagen binding proteins, the amino-terminal domains of the alpha1(VI) and alpha2(VI) chains and a part of the carboxyl-terminal domain of the alpha3(VI) chain were used as bait in a yeast two-hybrid system to screen a human placenta library. Eight persistently positive clones were identified, two coding the known matrix proteins fibronectin and basement membrane type IV collagen and the rest coding new proteins. The amino-terminal domain of alpha1(VI) was shown to interact with the carboxyl-terminal globular domain of type IV collagen. The specificity of this interaction was further studied using the yeast two-hybrid system in a one-on-one format and confirmed by using isolated protein domains in immunoprecipitation, affinity blots, and enzyme-linked immunosorbent assay-based binding studies. Co-distribution of type VI and type IV collagens in human muscle was demonstrated using double labeling immunofluorescent microscopy and immunoelectron microscopy. The strong interaction of type VI collagen filaments with basement membrane collagen provided a possible molecular pathogenesis for the heritable disorder Bethlem myopathy.

Cartilage fibronectin isoforms: in search of functions for a special population of matrix glycoproteins

Fibronectins are a part of the repertoire of matrix molecules produced by the chondrocyte in order to assemble a functional cartilage matrix. They are encoded by a single gene, but significant protein heterogeneity results from alternative RNA splicing. The population of fibronectin isofroms in adult cartilage is significantly different from fibronectins in other tissues and includes relatively high levels (20-30%) of ED-B(+) fibronectins and high levels (50-80%) of the cartilage specific (V + C)- isoform which lacks the V, III-15 and I-10 segments. Less than 4% of the fibronectins in cartilage are ED-A(+). The synthesis and accumulation of cartilage fibronectins are modulated in response to matrix pathology and to biochemical and mechanical mediators. In addition, alternative splicing patterns are altered when chondrocytes are allowed to dedifferentiate in monolayer culture such that the (V + C)- isoform is lost but the ED-A(+) isoform is reexpressed at high levels. Cartilage fibronectins have the potential to participate in cell signalling via integrin mediated pathways and to interact with other cartilage matrix macromolecules. The tissue-specific splicing pattern gives rise to a unique population of fibronectins within the cartilage. Together, this points to a critical role for cartilage fibronectins in chondrocyte cell biology and the organization of a biomechanically sound matrix. However, the precise function (or functions) of the cartilage fibronectins has (or have) not been defined. This minireview examines current information about the structure, synthesis and interactions of cartilage fibronectins. When possible, potential consequences of the inclusion of the ED-B segment or the exclusion of the V, III-15 and I-10 segments are discussed. The goal is to stimulate critical thought and discussion in the field about cartilage fibronectin isoforms, their function(s) in normal cartilage, and their role(s) in the pathogenesis of cartilage diseases.

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.

Attachment of articular cartilage chondrocytes to the tissue form of type VI collagen

Type VI collagen is composed of a short triple helix rich in RGD sequences with globular domains at each extremity of the helix. Disulfide-bonded tetramers of the monomeric molecule associate non-covalently to form networks of microfibrils in connective tissues, including cartilage. The disulfide-bonded tetramer can be extracted with 6 M guanidine HCl and purified without pepsin digestion and is referred to here as the tissue form of type VI collagen. Type VI collagen in mature articular cartilage appears to be concentrated pericellularly. We undertook a systematic investigation using solid phase assays to establish the nature of the attachment of bovine articular cartilage chondrocytes to the intact, tissue form of bovine type VI collagen. The tissue form of type VI collagen was extracted from bovine meniscus cartilage with 6 M guanidine HCl and purified by polyethylene glycol precipitation. When equal molar quantities were coated on microwells, the tissue form of type VI collagen attached more cells than the pepsin-digested form of the molecule that lacked the globular domains. The attachment to the intact, tissue form was dose-dependent and saturable and was not inhibited by heparin or type II collagen. A linear GRGDSP peptide failed to inhibit attachment of the chondrocytes to the intact, tissue or pepsin-digested forms of type VI collagen, but totally inhibited the interaction when the intact molecule was reduced and alkylated. In contrast, a cyclic C*GRGDSPC* peptide inhibited attachment to the tissue form of type VI collagen, but not to fibronectin. The attachment had a metal ion dependence that could be satisfied by MnCl2, slightly less by MgCl2, but not at all by CaCl2. A direct interaction between the tissue form of type VI collagen and a chondrocyte cell surface receptor or receptors is a structural feature of the pericellular matrix in cartilage.

An ultrastructural study of extracellular fibrillar components of developing mouse mandibular condyle with special reference to type VI collagen

The localization of type VI collagen was examined from birth to 8 weeks of age. Immunohistochemical staining with anti-type VI collagen antibody was strongly positive in the hypertrophic zone and moderately positive in the fibrous zone and the outer periphery of the proliferative zone, but negative in the inner area of the proliferative zone and mature zone. After ATP treatment, type VI collagen periodic fibrils with about 80-nm intervals were frequently observed but only in the fibrous zone. They occurred mainly in the superficial area of this zone, where striated collagen fibrils were sparse, while a few were noted in the inner area, where bundles of collagen fibrils were abundant. From these distributional differences of both components, a subzonation of the fibrous zone into superficial and inner area is suggested. Moreover, with ATP treatment there were fewer type VI collagen periodic fibrils formed with increasing age (8 weeks). Testicular hyaluronidase digestion before ATP treatment facilitated the formation of periodic fibrils, in all the ages examined, in the intercellular space and around the fibroblastic cells. The interaction of type VI collagen with other components such as collagen fibrils, glycosaminoglycans or proteoglycans may play a part in maintaining the structural integrity of extracellular matrix in the mouse mandibular condyle.

Morphology of the basement membrane

The aim of this contribution is to summarize our knowledge of the morphology of the basement membrane (BM). The first step in this direction is the attempt to define this term. The BM is composed of the Lamina lucida, densa, and fibroreticularis. Subsequently, the historical development of this term is discussed. Our main interest is, of course, focused on the description of the BM-structure up to the macromolecular level and the special forms of this structure. This is supplemented by discussing its chemical composition and establishing a relationship between morphology and biochemistry. The obtained findings yielded some indications as to the molecular composition of the BM which may serve for the construction of "models." The composition of the Lamina lucida (L.l.) and the Lamina or Pars fibroreticularis (L.f.) must be discussed separately, since, if present, they show a different and strongly varying structure (L.f.). An important aspect is the function of this extracellular layer which comprises mechanical tasks up to inductive effects. Finally, the concepts of the formation of the BM, especially of the Lamina densa (L.d.), are summarized. It obviously consists of a sequence of individual steps which starts with expression and secretion of the L.d.-components and is followed by an induction of integrin expression.