Stereological Study Of The Cerebral Cortex And The Myelinated Nerve Fibers In The Cerebral Cortex Of Rat Brain

It has been generally believed for several decades that there is significant death of the neurons in the cortex of aged brain. However, using the new stereological methods, Denmark scientists found that there was no significant death of the neurons in the cortex of aged human brain, but there was significant decline of the white matter volume in the aged human brain. We found that the total length of the myelinated nerve fibers in the white matter of aged human brain was significantly decreased when compared to that of young brain. Until now, however, there is no study of the myelinated nerve fibers in cortex with the new stereological methods. In order to figure out whether the aged-related changes of the myelinated fibers happened widely in the central nervous system, we investigated the effects of aging on the myelinated fibers in the cortex of Long-Evans rats, using transmission electron microscope (TEM) and stereological methods.PARTâ… 1. Materials and methods 1.1 Ten young Long-Evans rats (five females and five males, 6 month old), nine middle-aged Long-Evans rats (five females and 4 males, 18 month old) and ten aged Long-Evans rats (five females and five males, 27 month old) were used.1.2 One hemisphere was sampled from each rat brain. From the sampled hemisphere, 6 tissue blocks were sampled from cortex in a systematic random fashion. One ultra-thin section was cut from each sampled block. From each cut section, 5 fields of view were randomly photographed under TEM with a magnification of 4850.1.3 In both female and male rats, the cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex were investigated with the new stereological methods.2. Results2.1 In the young group, the cortex volume, the total length and total volume of the myelinated nerve fibers in the cortex of male rats were larger than those of the female rats. The mean diameter of the myelinated nerve fibers in the cortex of male rats was smaller than that of the females. However, the differences mentioned above were not statistically significant.2.2 In the middle-aged group, the cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex of male rats were bigger than those of female rats, but the differences were not statistically significant.2.3 In the aged group, the cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex of male rats were smaller than those of female rats. However, the differences were not statistically significant.PARTâ…¡1. Materials and methods1.1 Ten young Long-Evans rats (five females and five males, 6 month old), nine middle-aged Long-Evans rats (five females and four males, 18 month old) and ten aged Long-Evans rats (five females and five males, 27 month old) were used.1.2 One hemisphere was sampled from each rat brain. From the sampled hemisphere, 6 tissue blocks were sampled from cortex in a systematic random fashion. One ultra-thin section was cut from each sampled block. From each cut section, 5 fields of view were randomly photographed under TEM with a magnification of 4850.1.3 In the young, middle-aged and aged rats, the cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex were investigated with the new stereological methods. .2. Results2.1 There were no significant differences of the cortex volume, the total length, total volume and mean diameter of the myelinated fibers between young rats and middle-aged rats (p>0.05).2.2 When compared to young rats, the cortex volume and total length of the myelinated fibers in the cortex of aged rats were significantly decreased by 18.5% and 35.4% (p<0.01), respectively. However, there was no significant difference of the total volume of the myelinated fibers in the cortex between young rats and aged rats. The mean diameter of the myelinated fibers in the cortex of aged rats was significantly increased by 23.3% (p<0.05) when compared to that of young rats.2.3 The cortex volume and total length of the myelinated fibers in the cortex of aged rats were significantly decreased by 15.5% and 33.9% (p<0.01), respectively, when compared to those of middle-aged rats. There was no significant difference of the total volume of the myelinated fibers in the cortex between aged rats and middle-aged rats. The mean diameter of the myelinated fibers in the cortex of aged rats was significantly increased by 20.0% (p<0.05) when compared to that of middle-aged rats.General Conclusions1. The present study for the first time in the world combined the new stereological techniques and transmission electron microscope technique to quantitatively study the sex differences and the age-related changes of the cerebral cortex and the myelinated nerve fibers in the cortex of rat brain.2. The present study provided the normal data of the cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex of Long-Evans rats. These data will provide the important baseline values for future studies.3. The present results indicated that there was no sexual dimorphism in the cerebral cortex volume, the total length, total volume and mean diameter of the myelinated nerve fibers in the cortex of young, middle-aged and aged Long-Evans rats.4. The present results indicated that there was significant age-related decrease in the cortex volume and in the total length of the myelinated fibers in the cortex of Long-Evans rats. The loss of the total length of the myelinated fibers in the cortex of middle-aged and aged rats was mainly due to the loss of the myelinated fibers with smaller diameter.5. The present study suggested that the age-related changes of the myelinated fibers in cortex happened later than those in white matter.