Effects of an enriched environment on myelin sheaths in the white matter of rats during normal aging: A stereological study

We found that an enriched environment (EE) could delay the loss of myelinated fibers in the white matter of rats during normal aging. However, the reasons for the protective effects of EE on the myelinated fibers were unclear. In this present study, via the use of stereological methods, we quantitatively investigated the myelin sheaths and the axons of myelinated fibers in the white matter of rats reared in an EE or a standard environment (SE) during the aging process. The results showed that an EE induced significant increases in the lengths of myelinated fibers, the axon volumes and the myelin sheath volumes of aging rats when compared with SE rats and that the enrichment effects, with the exception of the axon volumes, were sex- and age-independent. The mean diameter of the myelinated fibers, the mean perimeter of the myelin sheaths and the mean thicknesses of the myelin sheaths were not significantly changed. The EE-induced increase in myelinated fibers was mostly observed in those of smaller diameter (<1μm) with thinner myelin sheaths (<0.16μm), which had an optimal axon-fiber ratio (g=0.61). Our results suggest that EE-induced an increase in myelinated fibers in the white matter of aging rats primarily due to marked remyelination and some ongoing myelination.