Figure 4. The results of the TUNEL assay of porcine corneal stroma. The TUNEL assay before treatment (A) and after incubation in N₂ gas for three (B), five (C), and seven (D) days is shown. Cellular nuclei were stained with propidium iodide. Many positively stained nuclei (yellow dots in C) were observed at day 3 whereas no staining was recognized in the porcine corneas before treatment or after N₂ incubation for five or seven days. All figures are at the same magnification, and the bar is 100 µm.