Cell-associated type I collagen in nondegenerate and degenerate human articular cartilage

Chondrocytes with abnormal morphology are present in nondegenerate human cartilage suggesting dedifferentiation to a fibroblastic phenotype and production of a mechanically-weakened matrix of unknown composition. We determined the relationship between in situ chondrocyte morphology, chondrocyte clusters, and levels of cell-associated collagen type I. Chondrocyte morphology in fresh femoral head cartilage from 19 patients with femoral neck fracture and collagen type I labelling was identified with Cell Tracker^TM fluorescence and immunofluorescence, respectively, in axial/coronal orientations using confocal microscopy with images analysed by Imaris^TM . In axial images of grade 0 cartilage, 87 ± 8% were normal chondrocytes with a small (10 ± 6%) abnormal population possessing ≥1 cytoplasmic process. More normal chondrocytes (78 ± 11%) were collagen type I negative than those labelling positively (p < 0.001). For abnormal chondrocytes, 81 ± 14% labelled negatively for collagen type I compared to those labelling positively (19 ± 3%; p = 0.007; N(n)=11(3)). Overall, approximately 9% of the cells in normal cartilage labelled for collagen type I. With degeneration, the percentage of normal chondrocytes decreased (p < 0.001) but increased for abnormal cells (p = 0.036) and clusters (p = 0.003). A larger percentage of normal, abnormal and clustered chondrocytes now demonstrated collagen type I labelling (p = 0.004; p = 0.009; p = 0.001 respectively). Coronal images exhibited increased (p = 0.001) collagen type I labelling in the superficial zone of mildly degenerate cartilage with none in the mid or deep zones. These results show that collagen type I was identified around normal and abnormal chondrocytes in nondegenerate cartilage, which increased with degeneration. This suggested the presence of mechanically weak fibro-cartilaginous repair tissue in otherwise macroscopically nondegenerate human cartilage which progressed with degeneration as occurs in osteoarthritis.