Age-associated pathological changes in male rats.

The selection of rats for aging research and the interpretation of experimental results require knowledge of spontaneous age-associated lesions in specific stocks and strains. We have compared selected age-associated pathological changes, as determined by postmortem examination of 113 outbred Crl:CD(SD)BR male retired breeder rats 12--39 months old, with those reported by others in a group of 122 virgin male 12- to 33-month-old F344 rats, an inbred strain from the same commercial source as the outbred rats. Numerous degenerative, inflammatory, and neoplastic lesions occurred in both groups. The wide prevalence and severity of many of the lesions suggest that they could be the source of significant variability in the results of specific aging studies, by their direct and indirect effect(s) on target organs. The prevalence of spontaneous lesions should be defined for each stock or strain intended for use in aging research by postmortem examination of representative groups of animals at various ages. Antemortem diagnostic screening of old rats may also be useful in culling animals bearing lesions that could add to the variability of experimental results, or in selecting for specific lesions that are of research interest themselves.