Abstract
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. © 2009 Wiley‐Liss, Inc.