Stereological investigation of the age-related changes of the myelinated fibers in the hippocampus of male rats.

The decline of hippocampus-dependent learning and memory during normal aging is not associated with neuron death and synapse loss. Until now, age-related changes in the myelinated fibers of the hippocampus have not been investigated. Therefore, in this study, the myelinated fibers in the hippocampi of young (6 months), middle-aged (18 months), and old-aged (28 months) male Sprague-Dawley rats were studied with transmission electron microscope and stereological methods, following spatial learning tests in a Morris water maze. The results showed that hippocampus-dependent spatial learning was impaired in old-aged rats but that the total volume, length, and mean diameter of the myelinated fibers in the hippocampus, as well as the hippocampal volume, remained constant during the normal aging process. Our results suggest that the age-related decline in hippocampus-dependent spatial learning is not attributable to myelinated fiber changes in the hippocampus and that other, undetermined factors are responsible.