Retinal cells undergoing apoptosis were identified by TUNEL assay in retinal cryosections using the DeadEnd Fluorimetric TUNEL system (Promega Corporation, WI) as previously described [24 (link)]. Cells were stained with DAPI to label the nuclei. The images were acquired with a fluorescence microscope (Axio HXP 120, Zeiss, Oberkochen, Germany).