Littermate Crx-/- and wildtype pups were perfused in 1% formaldehyde and 0.5% glutaraldehyde at various postnatal stages. The eyes were then enucleated, and the cornea and lens were removed. The eye cup was immersed in fixative (1% formaldehyde and 2.5% glutaraldehyde) for 3 to 4 hours at 4°C. The sclera was then partially removed and the retinas were sliced into small pieces and fixed (1% paraformaldehyde and 2.5% glutaraldehyde) overnight at 4°C. These procedures were found optimal for maintaining the structural integrity of the photoreceptor outer segments.