Age-related Changes Of The Myelinated Fibers In The White Matter Of Rat Brain And The Effects Of Enriched Environment On The Myelinated Fibers

PART ONE STEREOLOGICAL INVESTIGATIONS OF THE AGE-RELATED CHANGES IN THE WHITE MATTER AND THE MYELINATED FIBERS IN THE WHITE MATTER OF RAT BRAINObjective: To investigate the changes of the white matter and the myelinated fibers in the white matter of 28-month (old-aged) female rats using new stereological methods in order to further understand in order to further understand how the white matter and the myelinated fibers in the white matter of old-aged female rats changed during aging process.Methods: Five young (6-month), five middle-aged (18-month) and six old-aged (28-month) female Long-Evans rats were used. Four white matter blocks were sampled in a systematic random fashion for electronic microscope use. Fields of view were randomly photographed with the magnification of 6000. The white matter volume, the total length of the myelinated fibers in the white matter, the total volume of the myelinated fibers in the white matter, the total volume of the myelin sheaths in the white matter and the mean diameter of the myelinated fibers in the white matter were investigated with the new stereological methods.Results: The white matter volume of old-aged female rats was significantly reduced by 42.7% (p<0.00001) when compared to young female rats and was significantly reduced by 37.7% (p<0.0001) when compared to middle-aged female rats, respectively. The total length of the myelinated fibers in the white matter of old-aged female rats was significantly decreased by 52.5% (p<0.01) when compared to young female rats and was significantly decreased by 48.6% (p<0.05) when compared to middle-aged female rats, respectively. The total volume of the myelinated fibers in the white matter of old-aged female rats was significantly decreased by 26.7% (p<0.05) when compared to young female rats and was significantly decreased by 43.0% (p<0.05) when compared to middle-aged female rats, respectively. The total volume of the myelin sheaths in the white matter of old-aged female rats was significantly decreased by 30.7% (p<0.05) when compared to middle-aged female rats and was significantly decreased by 46.7% (p<0.05) when compared to middle-aged female rats, respectively. The mean diameter of the myelinated fibers in the white matter of old-aged female rats was significantly increased 37.5% (p<0.05) when compared to young female rats.Conclusiton: When compared to young and middle-aged female rats, the white matter volume, the myelinated fiber length, the myelinated fiber volume and the myelin sheath volume of old-aged female rats were all significantly decreased. The present results together with our previous findings indicated that the age-related white matter changes in female rats happened much later than in male rats. PART TWO EFFECTS OF THE ENRICHED ENVIRONMENT ON THE SPATIAL MEMORY AND THE MYELINATED FIBERS IN THE WHITE MATTER OF RATObjective: To investigate the effects of enriched environment on the spatial learning ability of middle-aged rats and old-aged rats, and then for the first time use the new stereological methods to investigate if the enriched environment can delay the age-related changes of the white matter and the age-related changes of the myelinated fibers in the white matter.Methods: Forty-eight (24 females and 24 males) middle-aged (14 month) SD rats and forty-eight (24 females and 24 males) old-aged (24 month) SD rats were randomly divided into enriched environment (EE) group and standard environment (SE) group. Rats were reared under either EE or SE for following 4 months. Spatial memory was examined with Morris water maze tasks for five consecutive days. Time latency was recorded. After the behavioral tests, five rats of each group were randomly sampled for stereological study. Four white matter blocks were sampled in a systematic random fashion for electronic microscope use. Fields of view were randomly photographed with the magnification of 6000. The white matter volume, the total length of the myelinated fibers, the total volume of the myelinated fibers and the mean diameter of the myelinated fibers in the white matter were investigated with the new stereological methods.Results: The time latency in Morris water maze tasks of middle-aged female rats in EE group was significantly shorter than that of controls in SE group (p<0.05). When compared to SE rats, the white matter volume was significantly increased by 46.9% (p<0.001) in middle-aged female EE rats, 68.1% (p<0.05) in old-aged male EE rats and 18.4% (p<0.001) in old-aged female EE rats, respectively. When compared to SE rats, the myelinated fiber length in the white matter was significantly increased by 34.1% (p<0.01) in middle-aged EE males, 43.2% (p<0.05) in middle-aged EE females, 68.8% (p<0.01) in old-aged EE males and 62.9% (p<0.001) in old-aged EE females, respectively. When compared to SE rats, the myelinated fiber volume in the white matter was significantly increased by 54.0% (p<0.05) in middle-aged female EE rats and 64.3% (p<0.001) in old-aged male EE rats, respectively. There were no significant effects of enriched environment on the mean diameters of the myelinated fibers in the white matter between SE rats and EE rats.Conclusion: The spatial memory of middle-aged female rats was significantly improved by the exposure to short-term enriched environment, which provided an important scientific basis for the future study to search strategy to delay the progress of brain function decline with aging. The short-term enriched environment induced the increases in the white matter volume, the total volume and total length of the myelinated fibers in the white matter middle-aged rats and old-aged rats. The changes of the myelinated fibers in the white matter induced by enriched environment might provide one of the important structural bases for the enriched environment-induced improvement of the spatial learning ability of aged rats. PART THREE THE EFFECTS OF THE ENRICHED ENVIRONMENT ON THE MYELIN SHEATHS OF THE MYELINATED FIBERS IN THE WHITE MATTER OF RAT BRAINObjective: To further investigate the structural bases of the short-term enriched environment induced significant increases of the white matter volume, myelinated fiber length and myelinated fiber volume in the white matter of middle-aged rats and old-aged rats.Methods: The materials, enriched environment protocol and sampling were the same as in part two. Fields of view were randomly photographed with the magnification of 20000. The white matter volume, the total length of the myelinated fibers, the total volume of the myelin sheaths, the total volume of the axons, as well as the mean inner diameter and mean outer diameter of the myelinated fibers, the mean inner perimeter and mean outer perimeter of the myelin sheaths, and the mean thickness of the myelin sheaths in the white matter were investigated with the new stereological methods.Results: When compared to SE rats, the total volume of the myelin sheaths in the white matter was significantly increased by 27.1% (p<0.05) in middle-aged male EE rats, 39.7% (p<0.05) in middle-aged female EE rats and 29.7% (p<0.05) in old-aged female EE rats, respectively. When compared to SE rats, the total volume of the axons in the white matter was significantly increased by 73.2% (p<0.05) in middle-aged female EE rats and 93.2% (p<0.05) in old-aged male EE rats. There were no significant differences in the mean inner diameter and mean outer diameter of the myelinated fibers, in the mean inner perimeter and mean outer perimeter of the myelin sheaths and in the mean thickness of the myelin sheaths in the white matter between SE rats and EE rats, respectively. The enrichment-induced significant increase of the myelinated fiber length in the white matter was mainly due to the marked increases of the myelinated fibers with the size of less than 0.6μm and with the myelin sheath thickness of less than 0.16μm. For those myelinated fibers that their lengths were significantly changed, the ratio between inner diameter and outer diameter (axonal diameter and myelinated fiber diameter) was computed. The average value was 0.62 in SE groups and 0.60 in EE groups. The myelin sheath thickness was linearly proportional to the axonal diameter. The myelin sheath thicknesses in the white matter of EE rats were more or less thicker than those of SE rats.Conclusion: Short-term enriched environment induced marked increases of the myelin sheaths, which resulted in the significant increases of the myelinated fibers with small size and with thin myelin sheaths. This type of myelinated fibers was mostly located in frontal white matter, which was closely related to the cognition. The myelin sheath thickness of the remyelinated fibers induced by short-term enriched environment was positively correlated to the axonal diameter of the remyelinated fibers. The ratio between axonal diameter and fiber diameter was around 0.60, which was considered to be optimal ratio to maximize the conduction velocity. The results might provide the structural basis for the enrichment-induced functional improvement of aging brain. The myelin sheaths of the myelinated fibers in the white matter were one of the important targets for the future study to search the new ways to delay the process of brain aging.