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Journal of Electron Microscopy Advance Access originally published online on January 14, 2008
Journal of Electron Microscopy 2008 57(2):77-82; doi:10.1093/jmicro/dfm039
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Novel muscle spindles containing muscle fibers devoid of sensory innervation in the extensor digitorum longus muscle of aged rats

Junzo Desaki1,* and Naoya Nishida2

1 Department of Integrated Basic Medical Research, Ehime University School of Medicine, Toon, Ehime 791-0295, Japan
2 Department of Otolaryngology, Ehime University School of Medicine, Toon, Ehime 791-0295, Japan

* To whom correspondence should be addressed. E-mail: desaki{at}m.ehime-u.ac.jp

We examined the structural features of muscle spindles at the equatorial and juxtaequatorial regions in the extensor digitorum longus muscle of adult (12 months) and aged (25 months) rats. In aged muscle spindles, the lamellated layers of the spindle capsule were a little increased in number compared to those in the adult ones. Two novel muscle spindles were observed in the aged muscle. In one muscle spindle, the spindle capsule contained four thin intrafusal muscle fibers invested by the inner capsule and two muscle fibers between the layers of the spindle capsule. Serial semithin sections revealed that the latter lacked the investment of the spindle capsule at the polar region. The other muscle spindle contained four intrafusal muscle fibers: two thin sensory-innervated muscle fibers invested by the inner capsule and two thick muscle fibers similar in structural features to neighboring extrafusal muscle fibers and lacking sensory innervation within the wide periaxial space. These findings suggest that two muscle fibers between the layers of the spindle capsule may be invested by the newly formed capsular cells during aging, while two thick fibers within the periaxial space may fail to receive the sensory innervation during the early development and follow the course of extrafusal fiber differentiation.

Keywords     muscle spindle, spindle capsule (outer capsule), intrafusal muscle fiber, extrafusal muscle fiber, extensor digitorum longus muscle, aged rat

Received     13 October 2007, accepted 12 December 2007


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