gms | German Medical Science

Objectives: Articular cartilage chondrocytes are spatially arranged throughout the extracellular matrix and build complex patterns such as single cells, pairs, strings, and clusters, the 'spatial organization'. This spatial organization is typical for specific joint types much like biological finger prints but changes extensively with tissue degeneration. A major factor for inducing tissue degeneration is compressive injury during joint trauma, which can lead to extensive cell death and to post-traumatic osteoarthritis (PTOA). Associations between the spatial organization and the extent of cell death after trauma have not been examined. However, better understanding whether trauma-induced cell death is associated with certain patterns of spatial organization or more pronounced in certain patterns is important, as this could identify areas that are more or less susceptible to cell death. Such information is unavailable yet important for developing novel diagnostic or therapeutic strategies for PTOA.

Methods: Articular cartilage from patients undergoing joint replacement procedures (n=17) was cut into standardized disc-shaped explants with a diameter of 3mm and a thickness of 300µm (n=150) containing the articular (neo-)surface and the deeper zones. Chondrocyte nuclei within the explants were stained with the CellTracker Blue CMAC fluorescent dye for live cell imaging, visualized with a fluorescence microscope (Zeiss), and sorted into explants with the spatial patterns "strings" representing remnants of a healthy organization, "double strings" representing an early osteoarthritic (OA) organization, and "clusters" representing advanced OA. 'Injury' was applied to human cartilage explants using an incubator-housed, PC-controlled loading machine for applying a single injurious compression down to 50% of the explant height within ½ second and subsequent release of the compression. A second explant groups served as non-injured control group. Immediately after injury, all explants were stained for viability (Invitrogen) and dead cells were determined at the articular surface. Statistical analyses were performed with SigmaPlot (Systat, Chicago) with p<0.05 as significance level.

Results and Conclusion: After injury, chondrocyte death was 21% (of all cells) in strings, 18% in double strings, and 70% in clusters. The percentage of injury-surviving chondrocytes was 79% in strings, 82% in double strings, and 30% in clusters. The difference between strings and double strings vs clusters was significant for both surviving and dead chondrocytes (p<0.001). In non-injured control explants 84.2% of chondrocytes were vital in strings, 84.2% in double strings, and 74.8% in clusters. The difference between dead chondrocytes in injured vs. non-injured explants was significant for all experimental groups (p<0.001). This study demonstrated for the first time that specific patterns of the spatial organization of joint surface chondrocytes are associated with and, thus, predict the extent of injury-induced cell death.