Objective       To explore the age-related changes of myelin sheath microstructure in the frontal region of corpus callosum.       Methods       Myelin sheath microstructure in the frontal region of corpus callosum of young (6 months old)

middle-aged (18 months old) and old-aged female (27 months old) Long-Evans rats were studied by electron microscope and unbiased stereological techniques.       Results       There was no significant change in mean diameter of axons (inner diameter)

mean diameter of myelinated fibers (external diameter)

mean inner perimeter

mean outer perimeter and mean thickness of myelin sheathes of myelinated nerve fibers in the frontal region of corpus callosum among the young

middle-aged and old-aged rats. The thicknesses of myelin sheaths of myelinated nerve fibers were divided into six intervals according to diameter of myelinated fibers (0.2 μm

0.2-0.4 μm

0.4-0.6 μm

0.6-0.8 μm

0.8-1.0 μm and ≥1.0 μm)

and the mean thickness of myelinated fibers in each interval was calculated. Within diameter from 0.2 μm to 0.4 μm

the mean thickness of myelin sheaths of the middle-aged and old-aged rats significantly decreased compared with that of the young rats. Within the diameter from 0.4 μm to 0.6 μm

the mean thickness of myelin sheaths of the old-aged rats significantly decreased compared with that of the young rats. Within the diameter from 0.6 μm to 0.8 μm

the mean thickness of myelin sheaths of the old-aged rats significantly decreased compared with that of the middle-aged rats.       Conclusion       There is a significant age-related decrease in myelin sheath thickness of myelinated fibers in the frontal region of corpus callosum

especially myelinated fibers with small diameters.