Beaded filaments and long-spacing fibrils: relation to type VI collagen

Rotary shadowing of elastic system microfibrils in the ocular zonule, vitreous, and ligamentum nuchae

Rotary shadowing of zonular fibrils in human and bovine eyes revealed a "string of beads" configuration with multiple interconnecting filaments, identical to that recently reported in fibrils of unknown type within the vitreous. These 29 nm beaded fibrils were the only macrostructures present in zonular samples, showing ultrastructural features correlating with both the macro and microperiodicity of zonular fibrils in tissues. Interbead periodicity varied from 30-57 nm and interbead filaments appeared capable of stretching even further, possibly explaining the inherent elasticity of zonular fibrils. The junctions between outer filaments and beads were fibrillin-positive. Similar beaded fibrils were found in the human and bovine anterior vitreous along with type II and IX collagen fibrils, proteoglycan filaments and other unidentified fibrils. After collagenase and elastase digestion, bovine ligamentum nuchae showed type VI collagen fibrils and clumps of beaded fibrils like those in zonule and vitreous. This distribution indicates that the beaded fibril is the microfibril which constitutes the basic unit of the elastic system.

Molecular composition of type VI collagen. Evidence for chain heterogeneity in mammalian tissues and cultured cells

The chain composition and relative abundance of type VI collagen synthesized by cells cultured from foetal bovine nuchal ligament and skin were compared with those of the type VI collagen present in these foetal tissues. Immunoprecipitation of intact collagen VI from medium and cell layers of nuchal ligament fibroblasts and skin fibroblasts at confluence revealed collagen type VI molecules with a chain composition consistent with an [alpha 1(VI)alpha 2(VI)alpha 3(VI)] monomeric assembly. Maintenance of cells in a post-confluent quiescent state promoted a marked phenotypic change in these ratios, with increased concentrations of assemblies composed of equimolar ratios of alpha 1(VI) and alpha 2(VI) chains detected in the medium of these cultures. Analysis of steady-state concentrations of mRNA for alpha 1(VI) and alpha 2(VI) chains revealed these species to be present in increased abundance at post-confluence in all the cultures, but no corresponding increase was observed in the alpha 3(VI) mRNA. In order to assess the physiological significance of these observations, the chain composition of the collagen VI content of the corresponding foetal tissues was assessed by Western blotting after extraction in guanidinium isothiocyanate under reducing conditions. Extracts of nuchal ligament revealed a collagen VI chain composition consistent with a heterotrimeric chain assembly. In contrast, the skin extracts revealed an abundance of alpha 1(VI) and alpha 2(VI) chains with only traces of the alpha 3(VI) chain detected. Increased equimolar concentrations of the alpha 1(VI)-chain and alpha 2(VI)-chain mRNAs in skin again reflected the increased concentrations of these polypeptide chains. Type VI collagen was present in greater abundance both in the nuchal ligament and in the corresponding nuchal-ligament fibroblast cultures. The results indicate that the chain composition of type VI collagen is subject to modulation at the level of transcription as a result of variations in the proliferative state of the cells, and demonstrate that different isoforms of collagen VI occur in foetal development.

Multiple forms of chicken alpha 3(VI) collagen chain generated by alternative splicing in type A repeated domains

Type VI collagen is a structurally unique component widely distributed in connective tissues. Its molecular structure consists of monomers that have the potential to assemble intracellularly into dimers and tetramers which, once secreted, can form microfilaments by end-to-end association. Individual monomers are composed of chains of Mr = approximately 140,000 (alpha 1 and alpha 2) and greater than 300,000 (alpha 3). Type VI collagen molecules contain a short triple helix with large globular domains at both ends. These domains are made for their greatest part of repetitive units similar to type A repeats of von Willebrand Factor. The alpha 3(VI) chain, contributing most of the mass of the NH2-terminal globule, appeared heterogenous both at the mRNA and protein level. Several alpha 3(VI)-specific clones that lack the sequences corresponding to repeats A8 and A6 were isolated from a chicken aorta cDNA library. Northern blot hybridization of poly (A+)-enriched RNA from chicken gizzard with cDNA fragments corresponding to several individual type A repeats showed that A8- and A6-specific probes did not hybridize to the lower Mr transcripts. Clones spanning approximately 20 kb of the 5'-end of the alpha 3(VI) gene were isolated from a chicken genomic library and subjected to analysis by restriction mapping, Southern blotting, and selective sequencing of the intron-exon boundaries. At the most 5'-end of the gene an additional type A repeat (A9), previously undetected in cDNA clones, was identified. Furthermore, it was determined that the presumed signal peptide and repeats A9 through A6 are encoded within individual exons. Reverse transcription and polymerase chain reaction of aorta RNA suggested that a mechanism of alternative mRNA splicing by a phenomenon of exon skipping generates alpha 3(VI) isoform variants that contain different numbers of type A repeats. Immunohistochemistry of frozen sections of chicken embryo tissues with repeat-specific mAbs showed that an antibody directed against a conditional exon has a more restricted tissue distribution compared to an antibody against a constitutive exon.

Structural and functional features of the alpha 3 chain indicate a bridging role for chicken collagen VI in connective tissues

Type VI collagen is a component of 100 nm long periodic filaments with a widespread distribution around collagen fibers and on the surface of cells. It is an unusual collagen constituted by three distinct chains, one of which (alpha 3) is much larger than the others and is encoded by a 9-kb mRNA. The amino acid sequence of the alpha 3(VI) deduced from the present cDNA clones specifies for a multidomain protein of at least 2648 residues made of a short collagenous sequence (336 residues), flanked at the N-terminus by nine 200 residue long repeating motifs and at the C-terminus by two similar motifs that share extensive identities with the collagen-binding type A repeats of von Willebrand factor. Type VI collagen and alpha 3(VI) fusion proteins bound to insolubilized type I collagen in a specific, time-dependent, and saturable manner. The alpha 3(VI) chain has three Arg-Gly-Asp sequences in the collagenous domain, and cell attachment was stimulated by the triple helix of type VI collagen and by alpha 3(VI) fusion proteins containing Arg-Gly-Asp sequences. This function was specifically inhibited by the Arg-Gly-Asp-Ser synthetic peptide. The type I collagen-binding and the cell-attachment properties of the alpha 3(VI) chain provide direct information for the role of type VI collagen in connective tissues.

Type-VI collagen in the human iris and ciliary body

The distribution of type-VI collagen in the human iris and ciliary body was investigated by means of immunohistochemical techniques and compared with that of type-IV collagen, fibronectin and laminin. As has been described for other tissues, type-VI collagen surrounds type-I and -III collagen fibers. The aggregated form of type-VI collagen (the "long-spacing" or "curly" collagen), which has already been described in the trabecular meshwork and sclera, was also observed at the ciliary muscle tips surrounding the anterior elastic tendons of this muscle. In addition, staining for type-VI collagen was seen directly adjacent to the basement membranes of the ciliary muscle cells, the iris muscles, the uveal vascular endothelia and nerves, but not adjacent to the epithelial basement membranes. The staining did not form a discrete line like the immunoreaction for type-IV collagen, but bundles of marked fibrils extended into the surrounding connective tissue. We assume that type-VI collagen similar to type-VII collagen forms part of an anchoring system for these tissues. As type-VII collagen has been described only in connection with epithelial basement membranes, both type-VI and type-VII collagens may represent anchoring fibrils, however for different tissue components.

Microfibrillar meshwork of the synovial lining and associated broad banded collagen: a clue to identity

The surface layer of synovial interstitium lining the rabbit knee was studied by transmission electron microscopy. Over a distance of 2-3 microns normal to the surface the interstitium contained a network of fine microfibrils (diameter 9.3 (0.7) nm, mean (SEM] which was quite dense in places (fractional area of projection 0.189 (0.023], and stained with ruthenium red. Periodic collagen fibrils were relatively scanty and fine (diameter 32 (2) nm) in this surface layer. Broad cross-striated bundles occurred in association with the microfibrils and B cells. These fibrous long spacing bundles (FLS) had a single period of 92.8 (2.8) nm with a broad dark band (37.6) (1.8) nm--so called 'zebra collagen'. Both the periodicity of the FLS and the morphological characteristics of the microfibrils are typical of type VI collagen, a widespread constituent of soft connective tissues. The functional importance of the inner microfibril network is likely to be mechanical, biochemical (glycosaminoglycan and glycoprotein entrapment), and to a very minor degree hydraulic resistance.

Cytochemical evidence for a proteoglycan-associated filamentous network in ligament extracellular matrix

The purpose of this investigation was to examine the extracellular matrix of rabbit ligament before and after digestion with glycosaminoglycan degrading enzymes. In order to preserve and enhance the visibility of negatively charged tissue components, particularly the glycosaminoglycan-containing proteoglycans, the cationic stains ruthenium red (RR) and ruthenium hexamine trichloride (RHT) were used. Cross-sections of the midsubstance of 10-month-old (mature) rabbit medial collateral ligaments fixed using conventional procedures revealed a sparse population of stellate-shaped cells that did not appear to be interconnected. Similar tissue fixed in either RR or RHT showed an extensive network of thin, electron-dense "seams" that interconnected cells and appeared to irregularly subdivide the extracellular matrix (ECM). These seams mainly consisted of a meshwork of microfilaments throughout which small granules were dispersed. Numerous 14-nm microfibrils, as well as mature elastic fibers were also present within the seams. The size and shape of the microfilaments, together with their threadlike, beaded appearance suggested that they could be Type VI collagen. The seam granules were easily removed with chondroitinase ABC, chondroitinase AC II, and mild (0.18 M) salt treatment. Only chondroitinase ABC succeeded in removing additional granules, tentatively identified as proteodermatan sulphate molecules, that were periodically located at d band sites along the Type I collagen fibrils. These results suggest that the seam granules are not dermatan sulphate containing proteoglycans, and further, that these proteoglycans may be sequestered into specific zones within the ECM through loose association with the seam microfilaments. While the functional significance of the seams remains unknown and their specific composition clearly requires further study, it is likely that they represent important functional (e.g., viscoelastic) or biological (e.g., nutritional) subdivisions of ligament substance.

Characterization of the collagen in the hexagonal lattice of Descemet's membrane: its relation to type VIII collagen

To investigate the nature of the hexagonal lattice structure in Descemet's membrane, monoclonal antibodies were raised against a homogenate of bovine Descemet's membranes. They were screened by immunofluorescence microscopy to obtain antibodies that label Descement's membrane. Some monoclonal antibodies labeled both Descemet's membrane and fine filaments within the stroma. In electron microscopy, with immunogold labeling on a critical point dried specimen, the antibodies labeled the hexagonal lattices and long-spacing structures produced by the bovine corneal endothelial cells in culture; 6A2 antibodies labeled the nodes of the lattice and 9H3 antibodies labeled the sides of the lattice. These antibodies also labeled the hexagonal lattice of Descemet's membrane in situ in ultrathin frozen sectioning. In immunofluorescence, these antibodies stained the sclera, choroid, and optic nerve sheath and its septum. They also labeled the dura mater of the spinal cord, and the perichondrium of the tracheal cartilage. In immunoblotting, the antibodies recognized 64-kD collagenous peptides both in tissue culture and in Descemet's membrane in vivo. They also recognized 50-kD pepsin-resistant fragments from Descemet's membranes that are related to type VIII collagen. However, they did not react either in immunoblotting or in immunoprecipitation with medium of subconfluent cultures from which type VIII collagen had been obtained. The results are discussed with reference to the nature of type VIII collagen, which is currently under dispute. This lattice collagen may be a member of a novel class of long-spacing fibrils.

Scheie's syndrome. An ultrastructural analysis of the cornea

The histopathology of a corneal graft specimen obtained from a patient with Scheie's syndrome (systemic mucopolysaccharidosis, type IS) is described with particular emphasis on the ultrastructural findings. Numerous vacuoles containing fibrillogranular material were found in the corneal epithelial cells, the keratocytes, and the endothelial cells. The basement membrane of the epithelium contained frequent breaks and peg-like undulations, and Bowman's layer was markedly attenuated. Fibrous long-spacing (FLS) collagen featured prominently in the stroma. Descemet's membrane was normal. The findings of a markedly attenuated Bowman's layer and FLS collagen may be abnormalities specific to Scheie's syndrome resulting from the altered glycosaminoglycan composition of the extracellular matrix.

Skin is a window on heritable disorders of connective tissue

A skin biopsy contains the macromolecules present in most connective tissues: collagens, elastin, glycoproteins, and proteoglycans. The specific combination and assembly of these matrix components and their interactions with other structures (e.g., epidermal appendages, nerve and vascular networks) and cells are responsible for the distinction among specific regions of the dermis. The matrix components are interactive and interdependent and modification of one of them, by extrinsic (environmental) and/or intrinsic (systemic, genetic, age-related) factors, may have consequences on the tissue as a whole. The skin, therefore, provides a window through which it is possible to examine how mutations in one connective tissue macromolecule can change the interactions among matrix components and affect tissue structure and organization. Light and electron microscopic studies of skin from patients with inherited connective tissue disorders (e.g., Ehlers-Danlos syndrome, osteogenesis imperfecta, Marfan syndrome, cutis laxa) have led us to the following generalizations about what components change, how individual collagen or elastic fibers are altered and how individual alterations affect overall dermal organization: 1) There is a limited change in the repertoire of collagen fibrils in the skin; 2) there appears to be a greater range of abnormal structure in dermal elastic fibers than in the collagen fibrils; 3) the morphology of the fibroblastic cells may provide clues to the defect in matrix components; 4) similar structural abnormalities result from different molecular defect; 5) a molecular defect in one connective tissue molecule has consequences for the structural properties of other connective tissue components; and 6) although structural alterations in connective tissue fibers are rarely specific for a given disease, there are characteristic patterns of structural change in the matrix that may be used to confirm a diagnosis. These generalizations show that mutations rarely affect only a single aspect of macromolecular function and because of the interactions of matrix components in this complex organ (skin) often disturb the organization of the entire dermis. Genotype-phenotype relationships are important to understand if effective therapies are to be designed. The structure of skin should provide the next level of integration in our efforts to determine how mutations produce disease.

Immunomicroscopical study of type VI collagen in the trabecular meshwork of normal and glaucomatous eyes

Cross-strained fiber bundles called long-spacing collagen or curly collagen occur in normal eyes in the trabecular meshwork. It can be seen in the basement membrane of the trabecular lamellae, in the sheath of the elastic-like fibers and underneath the inner wall of Schlemm's canal, where it forms part of the so called plaque material. The amount of this long-spacing collagen increases with age and is significantly more pronounced in glaucomatous eyes. Using immunohistochemical and immuno-electronmicroscopic methods, we have been able to show that type VI collagen is present in the aggregates called long-spacing collagen.

Ultrastructure of type VI collagen in human skin and cartilage suggests an anchoring function for this filamentous network

An mAb was used in conjunction with immunoelectron microscopy to study the ultrastructure and distribution of the type VI collagen network. Type VI collagen in femoral head and costal cartilage was found distributed throughout the matrix but concentrated in areas surrounding chondrocytes. Three-dimensional information gained from high voltage stereo pair electron microscopy showed that the type VI collagen network in skin was organized into a highly branched, open, filamentous network that encircled interstitial collagen fibers, but did not appear to interact directly with them. Type VI collagen was also found concentrated near basement membranes of nerves, blood vessels, and fat cells although in a less organized state. Labeling was conspicuously reduced close to the epithelial basement membrane in the region of the anchoring fibrils. No labeling of basement membranes was seen. Based on these observations it is suggested that the type VI collagen forms a flexible network that anchors large interstitial structures such as nerves, blood vessels, and collagen fibers into surrounding connective tissues.

Vitreous humor of chicken contains two fibrillar systems: an analysis of their structure

An analysis of the structure of chicken vitreous humor after brief homogenization of the tissue was performed. Electron micrographs prepared after rotary shadowing with platinum showed the presence of two distinct fibrils. The collagen fibril was coated by glycosaminoglycan which could be removed by chondroitinase ABC digestion. In addition, individual molecules of tenascin were observed wrapped around some of the collagen fibrils. A second beaded fibril was present and several fine filaments were observed to extend from each bead. The beaded fibril is formed by the overlap of these filaments, and beaded fibrils were observed in either a "closed" or an "open" form dependent on whether all of the filaments are brought together to form the overlap. A schematic diagram is presented for the structure of the beaded fibril. The potential relationship of the beaded fibril to the zonular fibrils and the elastin microfibrils is briefly discussed.

Comparative ultrastructural localization of collagen types III, IV, VI and laminin in rat uterus and kidney

Antibodies against collagen types III and VI have been localized by electron immunohistochemistry with two different techniques in normal rat uterus and kidney. Antibodies directed against two components of the extracellular matrix with known localization, laminin and type IV collagen, were used as controls for the specificity of the localization. The results demonstrate that types III and VI are found in the interstitium as fine (10- to 15-nm), beaded fibrils and filaments (6- to 10-nm), respectively. Both are often found associated with thick, crossbanded type I collagen fibers (30- to 35-nm) and occasionally associated with some basement membranes adjacent to the interstitium. Further, the findings suggest that collagens III and VI may connect the various components of the extracellular matrix, such as type I fibers with basement membranes and other structures, thus forming an integrated functional unit.

Retention of carboxypropeptides in type-II collagen fibrils in chick embryo chondrocyte cultures

An antibody reacting with the C-propeptide of chick type-II procollagen was used in an attempt to localize this terminal extension of the procollagen molecule (by immunogold labelling) during early collagen fibrillogenesis in chondrocyte cultures. After 2 days in culture the chondrocytes were surrounded by pericellular type-II collagen, as demonstrated by an indirect immunofluorescence labelling technique. An electron microscopy study of these cultures showed that the collagen fibrils were thin (approximately 15 nm diameter), with a poorly visible cross striation, sometimes enhanced by slight thickenings. The antibody against the C-propeptide of type-II procollagen labelled most of the collagen fibrils, according to a very regular pattern constituting a 60 nm periodicity. After 3 days the label was still present on the pericellular collagen fibrils but disappeared from the collagen fibrils of the extracellular matrix. Our results indicate that the C-propeptide of type-II procollagen is retained in the newly formed fibrils.

Type VI collagen of the intervertebral disc. Biochemical and electron-microscopic characterization of the native protein

The collagen framework of the intervertebral disc contains two major fibril-forming collagens, types I and II. Smaller amounts of other types of collagen are also present. On examination of the nature and distribution of these minor collagens within bovine disc tissue, type VI collagen was found to be unusually abundant. It accounted for about 20% of the total collagen in calf nucleus pulposus, and about 5% in the annulus fibrosus. It was discovered by serially digesting disc tissue with chondroitin ABC lyase and Streptomyces hyaluronidase that native covalent polymers of type VI collagen could be extracted. Electron micrographs of this material prepared by rotary shadowing revealed the characteristic dimensions of tetramers and double tetramers of type VI molecules, with their central rods and terminal globular domains. Molecular-sieve column chromatography on agarose under non-reducing non-denaturing conditions gave a series of protein peaks with molecular sizes equivalent to the tetramer, double tetramer and higher multimers. On SDS/polyacrylamide-gel electrophoresis after disulphide cleavage, these fractions of type VI collagen all showed a main band at Mr 140,000 and four lesser bands between Mr 180,000 and 240,000. On electrophoresis without disulphide cleavage in agarose/2.4% polyacrylamide only dimeric (six chains) and tetrameric (12 chains) forms of type VI molecules were present. The ability to extract all the type VI collagen of the tissue in 4 M-guanidinium chloride, and absence of aldehyde-mediated cross-linking residues on direct analysis, showed that, in contrast with most matrix collagens, type VI collagen does not function as a covalently cross-linked structural polymer.

High production of type VI collagen in multiple fibromatosis with multiple articular dysplasia

A patient with multiple fibromatosis occurring at the sites of multiple cartilagenous dysplasia was described. Collagen types solubilized with pepsin from the fibromatous tissue were fractionated by a different salt concentration and analyzed by SDS-polyacrylamide gel electrophoresis, which indicated that the tissue produces predominantly "short-chain" collagen. Western blotting of the subunits indicated a cross reaction with antisera of the type VI collagen. The results of rotatory shadowing electron microscopy confirmed the characteristic short-chain structure.

Immunoelectronmicroscopic localization of extracellular matrix components produced by bovine corneal endothelial cells in vitro

Bovine corneal endothelial cells deposit an extracellular matrix in short-term cultures, which contains various morphologically distinct structures when analysed by electron microscopy after negative staining. Amongst these were long-spacing fibers with a 150 nm periodicity, which appeared also to be assembled into more complex hexagonal lattices. Another structure was fine filaments, 10-40 nm in diameter, which occasionally exhibited 67 nm periodic cross-striation. Non-striated 10-20 nm filaments sometimes formed radially oriented bundles arranged in networks and fuzzy granular material was associated with the filaments in the bundles. Often, these bundles extended into solitary filaments, 10-20 nm in diameter, with a smooth surface. In addition, amorphous patches were seen, which contained dense aggregates of fibrillar and granular material. In longer-term cultures, some of the structures coalesced to form large fibrillar bundles. By using specific antibodies to various extracellular matrix components and immunolabeling with gold some of these structures could be identified as to their protein composition. Whereas fibronectin antibodies labeled a variety of structures--fine filaments with granular materials, radially oriented bundles, patchy amorphous aggregates and small granular material scattered throughout the background--type III collagen antibody predominantly labeled filaments with periodic banding (10-40 nm in diameter). A small amount of type III specific labeling was also observed over the networks of radially oriented fibrils and fine filaments associated with granular material. Type IV collagen and laminin antibodies localized in areas of the patchy amorphous aggregates. Type VI collagen antibodies, on the other hand, labeled fine filaments and the gold particles showed a pattern of 100 nm periodicity. Many of the fine 10-20 nm filaments exhibited a tubular appearance on cross-section, but they were not reactive with any of the antibodies used. Also negative were the long-spacing fibers and assemblies--including hexagonal lattices--containing this structural element.

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.

Type VI collagen: immunohistochemical identification as a filamentous component of the extracellular matrix of the developing avian corneal stroma

Selected stages of the developing chicken cornea have been examined for type VI collagen, employing monoclonal antibodies specific for this molecule. By immunofluorescence, the molecule is not detectable in 5 1/2 day corneas, a time at which the epithelial-derived, acellular primary stroma is the only corneal matrix present. One day later, the presumptive stromal fibroblasts have invaded this stroma and have initiated synthesis of the secondary (mature) stroma. By that time, a strong fluorescent signal for the type VI collagen molecule is detectable throughout the stroma. It is present in all subsequent ages examined. The molecule is not restricted to the cornea, and is present in most stromal matrices examined, including those of the sclera, eyelid, and nictitating membrane. Immunoelectron microscopy was also performed, utilizing a colloidal gold-labeled secondary antibody. These data show that the type VI collagen is not a component of the striated collagen fibrils, but instead is assembled in the form of thin filaments. The monoclonal antibody bound to the filaments at periodic intervals of about 100 nm.

Fibrillin, a new 350-kD glycoprotein, is a component of extracellular microfibrils

A new connective tissue protein, which we call fibrillin, has been isolated from the medium of human fibroblast cell cultures. Electrophoresis of the disulfide bond-reduced protein gave a single band with an estimated molecular mass of 350,000 D. This 350-kD protein appeared to possess intrachain disulfide bonds. It could be stained with periodic acid-Schiff reagent, and after metabolic labeling, it contained [3H]glucosamine. It could not be labeled with [35S]sulfate. It was resistant to digestion by bacterial collagenase. Using mAbs specific for fibrillin, we demonstrated its widespread distribution in the connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule. Electron microscopic immunolocalization with colloidal gold conjugates specified its location to a class of extracellular structural elements described as microfibrils. These microfibrils possessed a characteristic appearance and averaged 10 nm in diameter. Microfibrils around the amorphous cores of the elastic fiber system as well as bundles of microfibrils without elastin cores were labeled equally well with antibody. Immunolocalization suggested that fibrillin is arrayed periodically along the individual microfibril and that individual microfibrils may be aligned within bundles. The periodicity of the epitope appeared to match the interstitial collagen band periodicity. In contrast, type VI collagen, which has been proposed as a possible microfibrillar component, was immunolocalized with a specific mAb to small diameter microfilaments that interweave among the large, banded collagen fibers; it was not associated with the system of microfibrils identified by the presence of fibrillin.

Avian type VI collagen. Monoclonal antibody production and immunohistochemical identification as a major connective tissue component of cornea and skeletal muscle

Two monoclonal antibodies have been characterized as being against avian type VI collagen. By competition ELISA, the antibodies bound to the native type VI collagen molecule but not to its separated chains or to any of the other native collagen types tested. By rotary shadowing analysis of complexes of antibody-type VI collagen monomers, one of the antibodies (VI-EC6) has been shown to bind to a site in the triple helical domain of the molecule. The site at which this antibody binds to the dimeric form of type VI collagen is consistent with the previously proposed model for a supramolecular organization of the molecule (Furthmayr et al., Biochem j 211 (1983) 303) in which the monomers are arranged in an antiparallel, slightly staggered overlap. Immunofluorescence analyses of sections of chicken eyes and skeletal muscle demonstrate that type VI collagen is a major component of most stromal matrices.

Type VI collagen in extracellular, 100-nm periodic filaments and fibrils: identification by immunoelectron microscopy

Filaments and fibrils that exhibit a 100-nm axial periodicity and occur in the medium and in the deposited extracellular matrix of chicken embryo and human fibroblast cultures have been tentatively identified with type VI collagen on the basis of their similar structural characteristics (Bruns, R. R., 1984, J. Ultrastruct. Res., 89:136-145). Using indirect immunoelectron microscopy and specific monoclonal and polyclonal antibodies, we now report their positive identification with collagen VI and their distribution in fibroblast cultures and in tendon. Primary human foreskin fibroblast cultures, labeled with anti-type VI antibody and studied by fluorescence microscopy, showed a progressive increase in labeling and changes in distribution with time up to 8 d in culture. With immunoelectron microscopy and monoclonal antibodies to human type VI collagen followed by goat anti-mouse IgG coupled to colloidal gold, they showed in thin sections specific 100-nm periodic labeling on extracellular filaments and fibrils: one monoclonal antibody (3C4) attached to the band region and another (4B10) to the interband region of the filaments and fibrils. Rabbit antiserum to type VI collagen also localized on the band region, but the staining was less well defined. Control experiments with antibodies to fibronectin and to procollagen types I and III labeled other filaments and fibrils, but not those with a 100-nm period. Heavy metal-stained fibrils with the same periodic and structural characteristics also have been found in both adult rat tail tendon and embryonic chicken tendon subjected to prolonged incubation in culture medium or treatment with adenosine 5'-triphosphate at pH 4.6. We conclude that the 100-nm periodic filaments and fibrils represent the native aggregate form of type VI collagen. It is likely that banded fibrils of the same periodicity and appearance, reported by many observers over the years in a wide range of normal and pathological tissues, are at least in part, type VI collagen.

Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation

The formation of collagen fibrils, fibril bundles, and tissue-specific collagen macroaggregates by chick embryo tendon fibroblasts was studied using conventional and high voltage electron microscopy. During chick tendon morphogenesis, there are at least three extracellular compartments responsible for three levels of matrix organization: collagen fibrils, bundles, and collagen macroaggregates. Our observations indicate that the initial extracellular events in collagen fibrillogenesis occur within narrow cytoplasmic recesses, presumably under close cellular regulation. Collagen fibrils are formed within these deep, narrow recesses, which are continuous with the extracellular space. Where these narrow recesses fuse with the cell surface, it becomes highly convoluted with folds and processes that envelope forming fibril bundles. The bundles laterally associate and coalesce, forming aggregates within a third cell-defined extracellular compartment. Our interpretation is that this third compartment forms as cell processes retract and cytoplasm is withdrawn between bundles. These studies define a hierarchical organization within the tendon, extending from fibril assembly to fascicle formation. Correlation of different levels of extracellular compartmentalization with tissue architecture provides insight into the cellular controls involved in collagen fibril and higher order assembly and a better understanding of how collagen fibrils are collected into structural groups, positioned, and woven into functional tissue-specific collagen macroaggregates.

Structure and macromolecular organization of type VI collagen

Collagen VI is a large, disulfide-bonded protein complex which is widely distributed in connective tissue. The constituent polypeptide chains (Mr = 110,000-140,000) consist of collagenous and noncollagenous segments, are degraded to chains of about half the size when collagen VI is solubilized by pepsin, and assemble to a unique pattern of oligomers. As revealed by electron microscopy, the triple-stranded protomer consists of a triple helix 105 nm in length flanked on each side by globular domains of similar size (diameter about 7 nm). Protomers are assembled to dimers by an antiparallel staggered alignment of triple-helical segments. This leads to inner regions, 75 nm in length, of two slightly supercoiled triple helices flanked by globular domains. At both sides 30-nm-long outer triple-helical segments emerge that are terminated by globules. Tetramers are formed from laterally aligned dimers that cross with their outer triple-helical segments in a scissors-like fashion. The same structures, except with much smaller globular domains, are found in pepsin-treated collagen VI. Disulfide-linked collagen VI produced by cultured fibroblasts has a size similar to that of genuine collagen VI found in tissue extracts. Larger forms of collagen VI are assembled from tetramers by end-to-end aggregation which because of an overlap of the outer segments brings all globular domains close together. This arrangement predicts microfibrillar structures in tissues with a periodicity of 100-110 nm and a diameter of 5-10 nm. Structures consistent with this proposal were indeed found by immunoelectron microscopy of placenta and aorta using the ferritin technique. Large, lateral aggregates of collagen VI microfibrils may in addition exist in cell cultures and tissues ("zebra collagen," "Luse bodies") and are presumably maintained by contacts between globular domains.