Photoreceptor apoptosis following metronidazole treatment of Tg(zop:nfsB-EGFP)nt19 fish. Cell death in the dorsal retina was assessed in metronidazole-treated Tg(zop:nfsB-EGFP)nt19 retinal cryosections using the TUNEL assay (magenta). A: TUNEL-positive nuclei were not observed in untreated control retinas. B: Immediately following metronidazole treatment (24h Met.), TUNEL-positive, EGFP-positive cells were detected in the ONL of the dorsal retina. C: The ONL TUNEL signal increased after 24 hours of recovery. D: At 48 hours of recovery, the number of TUNEL-positive, EGFP-positive dorsal cells decreased. E,F: By 72 hours of recovery, TUNEL-positive nuclei were largely absent (E) and newly regenerated EGFP-expressing immature rods were observed after 96 hours of recovery (F). ONL, outer nuclear layer; INL, inner nuclear layer. Scale bar = 20 μm in A (applies to A–F).