The mammalian neuromuscular junction (NMJ) appears to undergo a dynamic remodeling process with advancing age. Previous studies suggest that the NMJ becomes progressively larger with age, to a greater degree in fast-twitch muscles than slow-twitch muscles. To better understand this process, quantitative morphometry was conducted on soleus and extensor digitorum longus (EDL) nerve terminals from C57BL/6NNia mice aged 4, 8, 10, 12, 18, 22, 24, and 32 months. Computer-aided morphometry of camera lucida drawings of aging nerve terminals indicated significant differences in the size and complexity of NMJs in both muscles between different age groups; however, there were no significant net trends with age. The magnitude and direction of the morphometric changes depended on the particular ages compared, and occurred to a greater degree in the EDL. This study is the first comprehensive morphometric study of C57BL/6NNia nerve terminals at so many ages. The results are simlar to those obtained for the Fischer 344 rat, and support the hypothesis that nerve terminal remodeling is a dynamic process, not a progressive elaboration.
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