The myelinated fiber changes in the white matter of aged female Long-Evans rats.

Age-related changes of the white matter and the myelinated fibers in white matter of female Long-Evans rats were investigated by the use of design-based stereological methods. The white matter volume in middle-aged rats was not significantly different from that in young rats, but the white matter volume in aged rats was significantly decreased by 42.7% and 37.7%, respectively, when compared with young and middle-aged rats. When compared with young rats, the total volumes of the myelinated fibers and myelin sheaths in white matter of aged rats were significantly decreased by 26.7% and 30.7%, respectively. When compared with young and middle-aged rats, the total length of the myelinated fibers in white matter of aged rats was significantly decreased by 52.5% and 48.6%, respectively. The mean diameter of the myelinated fibers in white matter of aged rats was significantly increased by 37.5% when compared with young rats. The absolute diameter distributions of the myelinated fiber length in the white matter of young, middle-aged, and aged rats indicated that the age-related loss of the myelinated fiber length in the white matter of aged rats was mainly as a result of the marked loss of the myelinated fibers with smaller diameter. The age-related changes of the white matter and the myelinated fibers in white matter may have important implications for age-related cognitive impairments.