Early changes within articular cartilage during human idiopathic osteoarthritis are poorly understood. However alterations to chondrocyte morphology occur with the development of fine cytoplasmic processes and cell clusters, potentially playing a role in cartilage degeneration. The aggrecanase ADAMTS-4 (A disintegrin and metalloproteinase with thrombospondin motifs-4) has been implicated as an important factor in cartilage degradation, so we investigated the relationship between chondrocyte morphology and levels of ADAMTS-4 in both non-degenerate and mildly osteoarthritic human cartilage.

Human femoral heads were obtained following consent from patients undergoing hip arthroplasty following femoral neck fracture. Cartilage explants of normal (grade 0; G0) and mildly osteoarthritic (grade 1; G1) cartilage were labelled with the cytoplasmic dye CMFDA (5-chloromethylfluorescein-diacetate). Explants were cryosectioned (30μm sections), and labelled for ADAMTS-4 by fluorescence immunohistochemistry. Sections were imaged with confocal microscopy, allowing the semi-quantitative analysis of ADAMTS-4 and 3D visualisation of in situ cell morphology.

With cartilage degeneration from G0 to G1, there was a decrease in the proportion of chondrocytes with normal rounded morphology (P<0.001) but an increase in the proportion of cells with processes (P<0.01) and those in clusters (P<0.001;[4(1653)]; femoral heads:cells). Although average levels of ADAMTS-4 for all cells was the same between G0 and G1 (P>0.05), a change was evident in the distribution curves for cell-specific ADAMTS-4 labelling. Cell-by-cell analysis showed that ADAMTS-4 levels were higher in chondrocytes with cytoplasmic processes compared to normal cells (P=0.044) however cells in clusters had lower levels than normal cells (P=0.003;[3(436)]). Preliminary data suggested that ADAMTS-4 levels increased with larger chondrocyte clusters.

These results suggest complex heterogeneous changes to levels of cell-associated ADAMTS-4 with early cartilage degeneration – increasing in cells with processes and initially decreasing in clusters. Increased levels of ADAMTS-4 are likely to produce focal areas of matrix weakness potentially leading to early cartilage degeneration.

The development of cytoplasmic processes from in situ chondrocytes is a characteristic feature of early osteoarthritis in human cartilage. The processes involve cytoskeletal elements and are distinct from the short primary cilia described in human chondrocytes. Vimentin is an intermediate filament playing an essential structural and signal-transduction role. We determined cellular levels and distribution of vimentin in chondrocytes of different morphologies in non-degenerate and mildly osteoarthritic cartilage.

Femoral heads were obtained after consent from patients undergoing hip arthroplasty following femoral neck fracture. Cartilage explants were graded as non-degenerate (grade 0;G0) or mildly osteoarthritic (grade 1;G1) and labelled with the cytoplasmic dye CMFDA (5-chloromethylfluorescein-diacetate) for cell shape. Explants were cryosectioned and labelled for vimentin by fluorescence immunohistochemistry. In situ chondrocyte morphology was identified by confocal microscopy as either normal (rounded/elliptical) or abnormal (with one or more cytoplasmic process of ≥2µm) and vimentin levels and distribution determined semi-quantitatively and related to chondrocyte morphology.

When all cells in G0 and G1 cartilage were compared, there was no difference between average levels of vimentin per cell (P=0.144)[6(261)];femoral heads:cells). When cells were separated on the basis of morphology, there was no difference between vimentin levels in cells with one or more cytoplasmic process compared to those of normal morphology (P>0.05;[6(261)]). However vimentin levels were much greater at the base of cytoplasmic processes compared to distant areas of the same cells (P=0.021)[5(29)]).

Although overall levels of chondrocyte vimentin do not change in these early stages of osteoarthritis, the formation and structure of these substantial chondrocyte cytoplasmic processes involves changes to its distribution. These morphological changes are similar to those occurring during chondrocyte de-differentiation to fibroblasts reported in osteoarthritis which results in the formation of mechanically-inferior fibro-cartilage. Alterations to chondrocyte vimentin distribution either directly or indirectly may play a role in cartilage degeneration.