Morphometric changes in vagal nerves of fourth generation mice passage-bred in a 2-G environment.
BACKGROUND
Previous studies have shown that microgravity induces both functional and structural adaptations in the autonomic nerves. Functional adaptation to hypergravity has also been reported, but structural change has not yet been isolated. The purpose of this study was to evaluate structural adaptation to hypergravity in the parasympathetic nerve.


METHODS
We selected fourth generation mice which were passage-bred in a 2-G environment by cycles of coupling, delivery, and growth. Complete left cervical vagal nerves of these mice were studied in transverse sections by electron microscopy. The number of small (diameter < 5 microm, thin and light-stained myelin sheath) and large (diameter > 5 microm, thick and dark-stained myelin sheath) myelinated fibers was counted.


RESULTS
The total number of all myelinated fibers (2 G: 795 +/- 103, 1 G: 644 +/- 60) and the number of small myelinated fibers (2 G: 657 +/- 95, 1 G: 522 +/- 66) were significantly greater in the 2-G mice than those in the 1-G mice (p < 0.05). The number of large myelinated fibers in the 2-G mice was greater than that in the 1-G mice, although it was not statistically significant (2 G: 138 +/- 15, 1-G: 122 +/- 16; p = 0.091).


CONCLUSIONS
The results show that the autonomic nerves can adapt structurally to hypergravity. We contend that the present results are due to the fact that the mice were passage-bred. As far as we know, this is the first report to show an increase in myelinated fibers in autonomic nerves under prolonged exposure to an increased G environment.
Shibata
Shigeki
S
Department of Hygiene/Space Medicine, Nihon University School of Medicine, Itabashi-ku, Tokyo, Japan.
Kita
Sayaka
S
Kim
Hyeteok
H
Ito
Masao
M
Iwasaki
Ken-ichi
K
D000222:Adaptation, Physiological; D000818:Animals; D018471:Hypergravity; D051379:Mice; D008854:Microscopy, Electron; D009413:Nerve Fibers, Myelinated / Q000648:ultrastructure; D036421:Nerve Fibers, Unmyelinated / Q000648:ultrastructure; D014630:Vagus Nerve / Q000254:growth & development* / Q000648:ultrastructure*