Essential role of satellite cells in the growth of rat soleus muscle fibers

Abstract

Effects of gravitational loading or unloading on the growth-associated increase in the cross-sectional area and length of fibers, as well as the total fiber number, in soleus muscle were studied in rats. Furthermore, the roles of satellite cells and myonuclei in growth of these properties were also investigated. The hindlimb unloading by tail suspension was performed in newborn rats from postnatal day 4 to month 3 with or without 3-mo reloading. The morphological properties were measured in whole muscle and/or single fibers sampled from tendon to tendon. Growth-associated increases of soleus weight and fiber cross-sectional area in the unloaded group were ∼68% and 69% less than the age-matched controls. However, the increases of number and length of fibers were not influenced by unloading. Growth-related increases of the number of quiescent satellite cells and myonuclei were inhibited by unloading. And the growth-related decrease of mitotically active satellite cells, seen even in controls (20%, P > 0.05), was also stimulated (80%). The increase of myonuclei during 3-mo unloading was only 40 times vs. 92 times in controls. Inhibited increase of myonuclear number was not related to apoptosis. The size of myonuclear domain in the unloaded group was less and that of single nuclei, which was decreased by growth, was larger than controls. However, all of these parameters, inhibited by unloading, were increased toward the control levels generally by reloading. It is suggested that the satellite cell-related stimulation in response to gravitational loading plays an essential role in the cross-sectional growth of soleus muscle fibers.

Role(s) of Mechanical Load and Satellite Cells in The Regulation of The Size of Soleus Muscle Fiber in Rats

“…But fiber CSA increased to the levels, even greater than the age-matched controls, in response to 4-week ambulation. Similar phenomena were also observed in rat soleus following 3-month hindlimb unloading and reloading (Kawano et al, 2008). Body weight and the absolute soleus weight in control rats were increased ~34 and 65 times, respectively, during the 3-month growth (vs. 4-day old pups, p<0.05).…”

“…2A and B, Kawano et al, 2008). Although they were still less than the age-matched controls after 3 month, both parameters increased toward the control levels during ambulation recovery.…”

“…Roles of gravitational loading or unloading on the differentiation and development of soleus muscle fibers of rats were studied (Kawano et al, 2008;Ohira et al, 2001). Hindlimb unloading by tail suspension was carried out during 3 weeks of lactation period (Ohira et al, 2001) or 3 months (Kawano et al, 2008) from postnatal day 4 to inhibit the weight-supporting activities of hindlimb muscles.…”

“…*, †, and §: p<0.05 vs. the levels at 4-day, immediately after 3-month unloading or cage housing (3-mo), and the age-matched control, respectively. Cited from Kawano et al (2008). − 137 − control level, which was stable between month 3 and 6, in response to re-loading. But it was not fully normalized even after 3-month recovery.…”

Steep Increase in Myonuclear Domain Size During Infancy

“…The myonuclear domain size also increases during maturational growth. In rodents it increases its size 4-fold between 4 days and 3 months after birth (Kelly, 1978;Kawano et al, 2008;White et al, 2010). Though in humans the myonuclear domain has also been found to increase 2.5-fold from toddler to adolescent (Vassilopoulos et al, 1977), a detailed time course of this increase is yet lacking, especially during infancy.…”