Changes in cross sectional area per myonucleus on mice soleus muscle during one week of hindlimb suspension reinforce the concept of myonuclear domain.

Objective: The effects of hindlimb unweighting by tail suspension on mice’s soleus muscle cross-sectional area (CSA) and myonuclei content were studied. Methods: Five groups of male Charles River CD 1 mice (n=10 per group) were assigned to varying hindlimb suspension (HS) periods: control (cont), 1 (1HS), 2 (2HS), 3 (3HS) and 8 (8HS) days. Immediately after the suspension period, the soleus muscles were removed in order to assess morphometry data. The CSA and number of myonuclei were determined in a total of 5011 fibers (1098, 793, 1363, 1281, 1276 fibers for the cont, 1HS, 2HS, 3HS, 8HS groups, respectively) from different areas in the mid-portion region of the soleus muscle, and the CSA per myonucleus ratio was calculated. Results: The results showed that the removing of weight bearing had a negative impact on muscle fiber CSA (1686 ± 516 μm2, 1506 ± 470 μm2, 1235 ± 340 μm2, 1277 ± 412 μm2 and 1075 ± 356 μm2 in the cont, 1HS, 2HS, 3HS and 8HS groups, respectively) and myonuclei number per fiber (1.91 ± 1.17, 1.94 ± 1.30, 1.55 ± 1.01, 1.48 ± 1.00 and 0.83 ± 0.76 in the cont, 1HS, 2HS, 3HS and 8HS groups, respectively). The CSA per myonucleus ratio was 993±522μm2,854±443μm2,815±374μm2,881±442μm2 and957±367μm2 inthecont, 1HS, 2HS, 3HS and 8HS groups, respectively. Thus, there were no significant differences between the control and 8HS groups. Conclusion: The morphological data is consistent with the hypothesis that, despite the muscle fiber atrophy induced by HS, the CSA per myonucleus ratio is apparently maintained, reinforcing the concept of myonuclear domain.