Sex differences in the white matter and myelinated nerve fibers of Long-Evans rats.

We investigated sex differences in the white matter and the myelinated nerve fibers within the white matter and detected an effect of sex on the age-related changes of the white matter. Ten young Long-Evans rats (five females and five males, aged 6-8 months) and 9 middle-aged Long-Evans rats (five females and four males, aged 18 months) were studied using electron microscopy and stereology. The white matter volume, the volume of the myelinated nerve fibers and the volume of the myelin sheaths in the white matter of young male rats were all significantly larger than those of young female rats. However, the white matter volume, the volume of the myelinated fibers, and the volume of the myelin sheaths in the white matter of middle-aged female rats were significantly larger than those of middle-aged male rats. There was sexual dimorphism in the white matter volume, the volume of the myelinated fibers, and the volume of the myelin sheaths in the white matter both in young and middle-aged Long-Evans rats. Age-related changes of the white matter volume and the total length of the myelinated fibers within the white matter in male Long-Evans rats were more prominent than those in female Long-Evans rats. The results obtained in the present study will provide baseline data for future studies related to white matter changes in various experimental conditions and in various neurodegenerative diseases.