Objective  To investigate the age-related changes in the volumes of the mid-region and splenium of corpus callosum and the myelinated fibers in these areas of corpus callosum.     Methods  The volume of the mid-region and splenium of corpus callosum

and the length density

total length and diameters of myelinated fibers in these area of adult

middle-aged and old-aged female Long-Evans rats were studied by the use of electron microscope technique and unbiased stereological techniques.    Results  When compared to adult rats and middle-aged rats

the volume of the mid-region in corpus callosum of old-aged rats was significantly decreased by 21.5% and 18.1% respectively. However

there was no significant change in the volume of the splenium in corpus callosum among adult

middle-aged and old-aged rats. There was no significant change in the length density

total length and mean diameter of the myelinated fibers in the mid-region and splenium in corpus callosum among adult

middle-aged and old-aged rats. In the mid-region of corpus callosum of old-aged rats

the total length of the myelinated fibers at a diameter of 0.7 to 0.8 μm was significantly decreased by 3.45 km

and the total length of the myelinated fibers at a diameter of 1.5 to 1.6 μm was significantly increased by 0.5 km

respectively

when compared to adult rats. In the splenium of corpus callosum of old-aged rat

the total lengths of the myelinated fibers at a diameter of 1.2 to 1.3 μm and 1.4 to 1.5 μm were significantly increased by 0.123 km and 0.108 km

respectively

when compared to young rats.    Conclusion  The splenium region of rat corpus callosum doesn’t significantly shrinks with age. The mid-region of rat corpus callosum significantly shrunks with age

which might mainly be due to the significant decline of total length of myelinated fibers with smaller diameters in this region.