The F-Actin of In Situ Chondrocytes in Non-Degenerate and Mildly-Degenerate Human Cartilage: Changes With Cell Volume, and Cytoplasmic Processes.

The F-actin cytoskeleton regulates chondrocyte morphology, cell-matrix interactions, and phenotype. Abnormal chondrocyte properties observed in non-degenerate human cartilage suggest early phenotypic de-differentiation that may contribute to weakening of the extracellular matrix. This study investigated the relationships between in situ chondrocyte volume, clustering, morphology, and F-actin organization, and examined whether the formation of cytoplasmic processes was an active cellular event. Fresh ex vivo human femoral head cartilage explants from 22 patients were graded individually as non-degenerate (G0) or mildly degenerate (G1). Explants were labeled with Cell Tracker and phalloidin to visualize chondrocytes and F-actin, respectively, and imaged using confocal laser scanning microscopy. G1 explants were additionally exposed to hypo-osmotic pressure (170 mOsm, 10 min) to assess changes to the morphology of the cytoplasmic processes. Large chondrocytes with spheroidal morphology demonstrated increased F-actin levels and redistribution in both G0 and G1 cartilage (p < 0.05). Clustered chondrocytes exhibited significantly increased F-actin content and altered distribution with degeneration (p ≤ 0.0004). Chondrocytes possessing cytoplasmic processes exhibited marked focal F-actin accumulation within the extensions (p < 0.0001). In G0 cartilage, F-actin localized predominantly at the base of processes, whereas in G1 cartilage, it extended throughout both the base and body of the process. Hypo-osmotic challenge significantly increased chondrocyte volume and process width (p < 0.01; p < 0.0001), but not process length, despite clear swelling of the cell body. These findings demonstrated that active cytoskeletal F-actin remodeling may accompany early chondrocyte morphological changes associated with human cartilage degeneration, as occurs in osteoarthritis.