Influence of cage shelf level on retinal atrophy in mice.

The correlation between cage shelf level and retinal atrophy was examined by analyzing data obtained from a chronic study carried out in 24,192 female BALB/c mice. Each cage of animals was maintained on a cage rack at a given shelf level throughout the course of the experiment, thus allowing analysis of data by shelf level. Retinal atrophy was first observed in a mouse killed at 12 months but subsequently was observed only at and after 16 months. This lesion was confined largely to animals on the top shelf. At 24 months, 19.7% of the mice on the top shelf had retinal atrophy while 0.2% of animals on all other shelves had the lesion. By 33 months, retinal atrophy was present in 30.2% of the mice on the top shelf, 12.0% on the next shelf, and 0.7% on the lower shelves. Differences in retinal morphology were probably caused by differences in lighting intensity. It was concluded that cage shelf level should be considered as an important factor in any chronic studies with mice.