Figure 4. Light exposure induces apoptotic photoreceptor death. Photomicrographs of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive nuclei (red signal) in cross-sections from the mid-dorsal (left column) and midventral (right column) retina of control animals (A, B) and of light-exposed animals processed 0 h (C, D), 7 days (E, F), and 1 month (G, H) after light exposure (ALE). All retinal nuclei have been counterstained with 4’,6-diamidino-2-phenylindole (DAPI; blue). Immediately after ALE, TUNEL-positive nuclei were very abundant in the mid-dorsal retina (C), while in the ventral retina, only a few scattered TUNEL-positive photoreceptor nuclei were observed (D). Seven days ALE, the number of TUNEL-positive photoreceptor nuclei decreased, but there were still more apoptotic photoreceptors in the dorsal (E) than in the ventral (F) retina. One month ALE (G, H), no TUNEL-positive photoreceptor nuclei were observed. The scale bar represents 100 µm.