No abstract

Fibronectin, the major cell surface glycoprotein of fibroblasts, is absent from differentiated cartilage matrix and chondrocytes in situ. However, dissociation of embryonic chick sternal cartilage with collagenase and trypsin, followed by inoculation in vitro reinitiates fibronectin synthesis by chondrocytes. Immunofluorescence microscopy with antibodies prepared against plasma fibronectin (cold insoluble globulin [CIG]) reveals fibronectin associated with the chondrocyte surface. Synthesis and secretion of fibronectin into the medium are shown by anabolic labeling with [3~S]methionine or [3H]glycine, and identification of the secreted proteins by immunoprecipitation and sodium dodecyl sulfate (SDS)-disc gel electrophoresis.When chondrocytes are plated onto tissue culture dishes, the pattern of surfaceassociated fibronectin changes from a patchy into a strandlike appearance. Where epithelioid clones of polygonal chondrocytes develop, only short strands of fibronectin appear preferentially at cellular interfaces. This pattern is observed as long as cells continue to produce type II collagen that fails to precipitate as extracellular collagen fibers for some time in culture. Using the immunofluorescence double-labeling technique, we demonstrate that fibroblasts as well as chondrocytes which synthesize type I collagen and deposit this collagen as extracellular fibers show a different pattern of extracellular fibronectin that codistributes in large parts with collagen fibers. Where chondrocytes begin to accumulate extracellular cartilage matrix, fibronectin strands disappear.From these observations, we conclude (a) that chondrocytes synthesize fibronectin only in the absence of extracellular cartilage matrix, and (b) that fibronectin forms only short intercellular "stitches" in the absence of extracellular collagen fibers in vitro. KEY

No abstract

The distribution and expression of type X collagen, a calcium-binding collagen, which is a marker of hypertrophic chondrocytes and thought to be involved in cartilage calcification, was examined in situ in nondegenerate (grade I or II) human discs taken at autopsy over a wide age range (fetal->80 years) and also in scoliotic discs removed at surgery. In the fetal vertebral column, type X collagen was strongly expressed in the hypertrophic chondrocytes of the endplate, but was not seen in other areas. In the cartilaginous endplate of adults, it was found over the whole age range examined, with intensity increasing with age. In the disc matrix itself, type X collagen was demonstrated around individual cells from all individuals older than 50 years, but not in any fetal or autopsy disc from individuals younger than 40 years. In scoliotic discs, however, focal type X collagen expression was seen in 3/8 patients younger than 40 years including one 12-year-old. No type X collagen was found in the outer annulus in any autopsy or scoliotic disc, supporting the idea that cells of the outer annulus are phenotypically distinct from cells of the inner annulus and the nucleus. Our results demonstrate for the first time that type X collagen is a possible gene product of the intervertebral disc cells and a potential biochemical component of the disc matrix. They indicate that with age or in scoliosis, some cells from the inner annulus or nucleus of the disc differentiate to the hypertrophic chondrocyte phenotype. This might be the initiating event for the abnormal calcification described in aged and scoliotic discs in other studies.