Abstract: Objective To explore the changes of the hippocampal formation and the myelinated nerve fibers in the hippocampal formation of aged female Long-Evans rat. Methods The hippocampal formation and the myelinated nerve fibers in the hippocampal formation of 5 young (6-month old), 5 middle-aged (18-month old) and 6 aged (28-month old) female Long-Evans rats were quantitatively investigated with the stereological techniques and transmission electronic microscope technique. Results There were no significant changes in the volume of hippocampal formation, the volume density, the total volume, the length density and the mean diameter of the myelinated nerve fibers in the hippocampal formation among young, middle-aged and aged rats. When compared to that of young rats, the total length of the myelinated nerve fibers in the hippocampal formation of middle-aged rats was significantly increased by 63.6%. When compared to that of middle-aged rats, the total length of the myelinated nerve fibers in the hippocampal formation of aged rats was significantly decreased by 47.5%. When compared to that of young rats, the total length of the myelinated nerve fibers in the hippocampal formation of aged rats was non-significantly decreased by 13.8%. Conclusion Although the reason why the total length of the myelinated nerve fibers in the hippocampal formation of middle-aged rats was longer than that of young rats needs further investigations, the present results together with our previous findings in white matter and cortex further suggest that there are age-related changes of the myelinated nerve fibers in the normal aged brains.

Key words: Hippocampal formation, Myelinated nerve fibers, Age-related changes, Stereology, Rat