Fig. 2. Protection of photoreceptors by 12 hr preconditioning. A, Normal retina (superior region) of a rat kept in cyclic light. The photoreceptor outer segments (OS) are opposed to the retinal pigment epithelium (RPE), distinct photoreceptor inner segments (IS) are present, and the outer nuclear layer (ONL) consists of 10–11 rows of photoreceptor cell nuclei. B, Retina of a rat exposed to 7 d of constant light without preconditioning (superior region). TheONL is reduced to one to two rows of nuclei; the inner segments are missing or are reduced to short stumps; the few remaining outer segments are in the form of large rounded or oblong profiles.C, Superior retina of a rat that received a 12 hr preconditioning and then was exposed to 7 d of constant light. TheONL shows six to seven rows of nuclei; the inner segments are shorter than normal, and the outer segments, although better preserved than in B, are disorganized, many showing rounded and oblong profiles. Toluidine blue stain was used.OPL, Outer plexiform layer; INL, inner nuclear layer. Scale bar, 20 μm.