Regression of the myelinated fibers and capillaries in the tibial nerve associated with aging in rats
Authors: Sakita Masahiro , Murakami Shinichiro , Huo Ming , Saito Takafumi , Fujino Hidemi 
Published: November 2023

PurposeThe sarcopenia of elderly people is associated with muscle weakness, balance and gait disorders. In the findings from a recent animal study, it has been shown that the atrophy and dysfunction of myelinated fibers in the peripheral nerves precede sarcopenia. This atrophy predominantly appears in myelin sheath and the proximal site of peripheral nerve. In addition, degeneration of peripheral nerve with aging is linked to failure of interactions with capillary metabolism. However, it is not histologically obvious whether distal peripheral nerves and capillaries regress with aging. The purpose of the present study is to investigate histological changes in the myelinated fibers and capillary architecture of the distal tibial nerve in rats with aging.Materials and MethodsThe male of wistar rats were used in the present study. A series of nerves from the sciatic to the distal end of the tibia in the bilateral hind limbs were perfused for three minutes to wash out the intravascular blood with a 0.9% physiological saline solution containing 10,000 IU/L of heparin, and 10% glucose. Then, contrast medium, consisting of 1% fluorescent material (PUSR80; Mitsubishi Pencil, Tokyo, Japan), 8% gelatin (Nakalai Tesque, Kyoto, Japan) and distilled water was administered into the circulation to bilateral hind limbs. After perfusion with contrast medium via the abdominal aorta, transverse sections of 5μm thick were sliced using a cryostat microtome and were stained with 1 % toluidine blue. Moreover, the frozen tibial nerves were fixed with 4% paraformaldehyde. Then, confocal laser scanning microscopy was used in fluorescence mode with an argon laser to observe the 3‐D architecture in the capillary of tibial nerve. The myelinated fibers were divided into three fiber types (large, middle and small) according to histogram of the fiber diameter. A two‐way analysis of variance (ANOVA) was used under three fiber types to compare the myelin sheath thickness, axon diameter and G‐ratio (axon diameter/fiber diameter) between the young and elderly groups. An unpaired t‐test was used to compare the capillary diameter and the number of microvascular ramifications between the young and elderly groups.ResultsThere were the major effects in the fiber types (p< 0.0001) and groups (p< 0.0001) in two‐way ANOVA. The myelin sheath thickness and axon diameter of all fiber types in the elderly group were significantly smaller (p<0.01) than those of the young group. The capillary diameter and number of microvascular branch points in the elderly group significantly decreased in comparison with those of the young group (p<0.01).ConclusionsThe finding revealed that the myelinated fibers and capillaries in the distal tibial nerve degenerate with aging. The failure of cross‐talk of key biomarkers (such as BDNF and VEGF) between Schwann and vascular endothelial cells will synchronously cause regressions both the distal myelinated fiber and capillary.Support or Funding InformationThe present study was supported by Grants‐in‐Aid for Scientific Research (C) from Japan Society for the Promotion of Science (Grant number; 26350853).