Effect of Cl2 exposure on airway cell apoptosis; TUNEL technique. A: Control mouse airway, showing baseline airway epithelial cell apoptosis (arrowheads). B: 12 h post-exposure. Cytoplasmic TUNEL signal in damaged epithelium. The high magnification inset details the cytoplasmic localisation of the TUNEL stain on cells with methyl green counterstained nuclei. These cells lack a TUNEL signal attributable to apoptosis-related DNA fragmentation. The arrowheads indicate examples of cells that appear truly apoptotic. C: 24 h post-exposure. Some clusters of basal cells undergoing apoptosis are visible. Inset shows high magnification detail. D: 5 d post-exposure. The frequency of TUNEL positive cells at 5 d is back to baseline level. Scale bars: 100 μm in I; 50 μm in A, B, C inset and D.