Cells and matrix expressing cartilage components in fibroblastic tissue in rheumatoid pannus.

Our studies of the cartilage-pannus junction in rheumatoid joints have demonstrated the frequent presence of a transitional fibroblastic zone (TFZ) overlying articular cartilage. Using immunohistochemical techniques this zone has been shown to contain cells and matrix expressing specific cartilage components but not antigens present on cells in invasive vascular pannus. These findings support the concept that fibroblastic pannus is derived from the underlying articular cartilage rather than adjacent tissues. This type of reaction involving a metaplastic change in the chondrocyte is particularly common in large weight-bearing joints and may represent a different mechanism of cartilage degeneration to the classically described direct invasion of cartilage by hypertrophic synovial tissue.