Spinal nerve root degeneration in aging laboratory rats: A light microscopic study

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Abstract
Degeneration of unknown etiology was noted in the spinal nerve roots of aging rats. These animals (Charles River CD® rats) were included in a study of long‐term effects of ionizing radiation. The initial lesion was a de‐myelination of an individual axon or a few, isolated, scattered axons in the ventral roots of rats between 18 and 20 months of age. As the lesion progressed, the dorsal roots showed involvement, and by 24 months the usual finding was marked degeneration in ventral roots with degenerative changes of a lesser degree in the dorsal roots. When degeneration was complete, connective tissue filled the regions normally occupied by neural tissue. The lesion was unrelated to the radiation procedure, occurring with equal frequency in both irradiated and non‐irradiated animals. Furthermore, lesions occurred with equal frequency in both males and females. The one factor to which the lesion appeared to be related was age since it was not observed in animals less than 18 months old but was seen in 66.7% of those between 18 and 20 months and in 96.0% of those older than 20 months. These data indicate that this lesion needs to be considered when using aging laboratory rats.