Abstract
The present study aimed to analyze the morphology of the peripheral nerve, postsynaptic compartment, skeletal muscles and weight-bearing capacity of Wistar rats at specific ages. Twenty rats were divided into groups: 10 months-old (ADULT) and 24 months-old (OLD). After euthanasia, we prepared and analyzed the tibial nerve using transmission electron microscopy and the soleus and plantaris muscles for cytofluorescence and histochemistry. For the comparison of the results between groups we used dependent and independent Student's t-test with level of significance set at p ≤ 0.05. For the tibial nerve, the OLD group presented the following alterations compared to the ADULT group: larger area and diameter of both myelinated fibers and axons, smaller area occupied by myelinated and unmyelinated axons, lower numerical density of myelinated fibers, and fewer myelinated fibers with normal morphology. Both aged soleus and plantaris end-plate showed greater total perimeter, stained perimeter, total area and stained area compared to ADULT group (p < 0.05). Yet, aged soleus end-plate presented greater dispersion than ADULT samples (p < 0.05). For the morphology of soleus and plantaris muscles, density of the interstitial volume was greater in the OLD group (p < 0.05). No statistical difference was found between groups in the weight-bearing tests. The results of the present study demonstrated that the aging process induces changes in the peripheral nerve and postsynaptic compartment without any change in skeletal muscles and ability to carry load in Wistar rats.
Highlights
Evidence suggests that alterations such as motorneuron degeneration, functional denervation, accompanied by structural and functionality changes in the neuromuscular junction (NMJ), and loss of motor units contribute to the progression of muscular aging process in rodents and humans (Deschenes, 2011; Jang and Van Remmen, 2011; Gonzalez-Freire et al, 2014; Tudorascu et al, 2014; Hepple and Rice, 2016; Piasecki et al, 2016; Rygiel et al, 2016; Gilmore et al, 2017)
Area of Skeletal Muscles For both soleus and plantaris, the interstitial volume density increased significantly in the OLD group compared to the ADULT (p < 0.05)
The nuclei ratio of OLD group was statistically lower than ADULT (p < 0.05)
Summary
Aging is a natural phenomenon associated with structural, functional, biochemical, molecular, and genetic alterations in many cells and tissues (Verdú et al, 1996; Ceballos et al, 1999; Shokouhi et al, 2008; Shen et al, 2011; Ibebunjo et al, 2012; Marzetti et al, 2013; Moldovan et al, 2016; Pannérec et al, 2016; Klaips et al, 2017). Numerous mechanisms are proposed to explain age-related muscle fiber loss, perhaps the most reasonable is that peripheral denervation is caused by the loss of motoneurons and NMJ remodeling (Vandervoort, 2002; Deschenes et al, 2010; Pannérec et al, 2016). Such a process is proposed to occur in humans and several lineages of laboratory rodents. According to Pannérec et al (2016), as expected for muscle phenotype in hindlimbs, Wistar rats might be divided under the following categories: adult (8–10 months-old), early-sarcopenic (18–20 months-old), and sarcopenic (22–24 months-old)
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