Morphometry of myelin fibers in corpus callosum and optic nerve of aging rats.

Results of morphometric studies are presented, conducted on myelin fibers of corpus callosum and optic nerve of old rats. The studies were performed on 2 and 2.5 years old rats of Wistar strain. The control group consisted of 4 months old animals. The measurements were conducted on electron micrographs taken at magnification of 35,000 X. Aging was associated with increased thickness of myelin sheaths resulting both from increased number of myelin lamellae and from increased interlamellae distances (i.e. decreased compactness of myelin). The increased transversal size of myelin fibers, analogous to that observed in human material, was particularly evident in 2 years old rats. In the oldest, 2.5 years old rats size of myelin fibers decreased compared to 2 years old rats, approaching values observed in the control group. Moreover, disturbances in axon-myelin sheath ratio took place in the course od aging of rat brain white matter; axon cross-section area ratio to number of myelin lamellae in corpus callosum fibers was decreased while it was increased in fibers of optic nerve in 2 years old animals. In the process of aging most pronounced alterations were noted in thin fibers but thick fibers became more numerous and showed greater number of myelin lamellae.