Different patterns in age-related morphometric alteration of myelinated fibers and capillaries of the tibial nerve: a longitudinal study in normal rats.

Age-related regression of myelinated fibers in peripheral nerves of the lower limbs is strongly influenced by capillaries and results in balance dysfunction and falls. However, the temporal relationships between alteration patterns of myelinated fibers and capillaries have not yet been clarified. This study aimed to investigate age-related morphological and histological changes of both myelinated fibers and capillaries in peripheral nerves to clarify whether myelinated fibers or capillaries change earlier. Seven male Wistar rats each were randomly selected at 20 weeks (young group), 70 weeks (middle group), and 97 weeks (old group) for histological evaluations. The left and right tibial nerves were removed morphologically and histologically to examine myelinated fibers and capillaries. Axon diameter and myelin thickness were almost unaltered in the middle group compared with the young group but were significantly reduced in the old group when compared with the other two groups. However, the capillary diameter and number of microvascular branch points were substantially reduced in the middle group. The current study demonstrates that myelinated fibers of peripheral nerves show signs of regression in elderly rats, whereas capillaries start to reduce in middle-aged animals. In normal aging of the tibial nerve, capillaries may regress before myelinated fibers.