Determination of percentage of apoptotic cells in sections of articular cartilage using the TUNEL method. Cartilage explants were cultured for 2 days, impact loaded and then cultured for a further 6 days before being frozen, sectioned (8 μm) and TUNEL stained. (a) Unloaded control showed very few TUNEL positive cells. (b) The percentage of apoptotic cells increased significantly following an impact load. (c) Addition of celecoxib (0.1 μM) to the culture medium decreased the number of TUNEL positive cells in impact loaded explants. (d) The total cell count for each section was determined by counterstaining the nuclei red with propidium iodide. The bar represents 100 μm in all panels.