Defining PDT duration for mice. (A-C) Hematoxylin-eosin staining of a retinal section 7 days after PDT. The photoreceptor cell layer was thin in the area irradiated for 42 seconds using low-level laser light during PDT (A). No obvious changes were observed in retinal sections exposed to 20 or 10 seconds of laser irradiation (B,C). (D-G) TUNEL (red) and Hoechst (blue) stainings of retinal sections 3 days after PDT. TUNEL-positive cells were observed in the photoreceptor cell layer in only the irradiated area of retinas exposed to 42 seconds of laser light (D). Few TUNEL-positive cells were observed in areas exposed to 20 or 10 seconds of laser irradiation (E,F). TUNEL-positive cells in retinal sections were counted (G). Scale bar, 50 μm. **p < 0.01.