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

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.

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.

Acquisition of type IX collagen by the developing avian primary corneal stroma and vitreous

Previous investigations from our laboratory and others have demonstrated that type II collagen, once thought to be a cartilage-specific molecule, is also a component of both the primary corneal stroma and the vitreous of embryonic chickens. In the present immunohistochemical study we have examined the expression in these embryonic matrices of another "cartilage-specific" collagen, type IX, along with type II. In the cornea, type IX collagen is in the primary stroma, but is not detectable in the mature, secondary stroma. Even within the primary stroma this collagen has a brief, transitory existence. It first appears in the peripheral stroma at the time the endothelial cells begin to migrate along its posterior surface, and spreads throughout the stroma during the following 24-36 hr. The epitopes on type IX collagen then suddenly become undetectable just before this matrix swells and becomes populated by the periocular mesenchymal cells (future keratocytes). In comparison, collagen type II (along with type I) is present in the stroma before and long after these events. Deposition of immunodetectable type IX collagen in the developing corneal stroma thus seems to be independent of type II. In the vitreous, we observed type IX collagen along with type II as soon as authentic vitreous could be identified and at all subsequent stages of development. In this tissue, therefore, the expression of collagen types IX and II appears to be coordinate.

Type IX collagen: a possible function in articular cartilage

The effect of type IX on in vitro fibrillogenesis of type II collagen indicated that, while not preventing fibrillogenesis, the presence of type IX collagen reduced the size of the type II fibre aggregates. This observation is consistent with the in vivo localisation studies of type IX collagen. Using the immunogold labelling technique, type IX collagen was shown to be located evenly on small fibrils which occur at higher concentration closer to the cell. Therefore type IX collagen may function as a regulator of fibre diameter in articular cartilage.

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.

Preparation and applications of monoclonal antibodies to different collagen types

Monoclonal antibodies have recently been developed against all of the major collagen types isolated from both human and other species. These antibodies have several potential advantages over polyclonal antibodies, and a brief survey will be made of the different antibodies that have so far been developed. In addition, various successful applications of these antibodies to biological investigations will be briefly discussed.

D-periodic distribution of collagen type IX along cartilage fibrils

It has recently become apparent that collagen fibrils may be composed of more than one kind of macromolecule. To explore this possibility, we developed a procedure to purify fibril fragments from 17-d embryonic chicken sternal cartilage. The fibril population obtained shows, after negative staining, a uniformity in the banding pattern and diameter similar to the fibrils in situ. Pepsin digestion of this fibril preparation releases collagen types II, IX, and XI in the proportion of 8:1:1. Rotary shadowing of the fibrils reveals a d-periodic distribution of 35-40-nm long projections, each capped with a globular domain, which resemble in form and dimensions the aminoterminal globular and collagenous domains, NC4 and COL3, of type IX collagen. The monoclonal antibody (4D6) specific for an epitope close to the amino terminal of the COL3 domain of type IX collagen bound to these projections, thus confirming their identity. Type IX collagen is therefore distributed in a regular d-periodic arrangement along cartilage fibrils, with the chondroitin sulfate chain of type IX collagen in intimate contact with the fibril.

An enzymatically induced structural transformation in articular cartilage. Its significance with respect to matrix breakdown

It was demonstrated in this study that the 3-dimensional, "pseudo-random" architecture of collagen in the general matrix of normal cartilage can be transformed enzymatically into a radial array of fibril aggregates or fibers. By first degrading the proteoglycans and then attacking the collagen, it is possible to produce a collagenous structure almost identical to that observed in matrices exhibiting both nonprogressive softening and osteoarthritic changes, and in matrices subjected to dynamic overloading. This structural transformation is explained as a breakdown in the fibril interlinking system.

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.

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.