Analyses of muscle spindles in the soleus of six inbred mouse strains

Abstract

Adult muscle size and fibre-type composition are heritable traits that vary substantially between individuals. We used inbred mouse strains in which soleus muscle mass varied by an order of magnitude to explore whether properties of muscle spindles can also be influenced by genetic factors. Skip-serial cross-sections of soleus muscles dissected from 15 male mice of BEH, BEL, C57BL/6J, DUH, LG/J and SM/J strains were analysed for number of muscle spindles and characteristics of intrafusal and extrafusal fibres following ATPase staining. The BEL and DUH strains determined the range of: soleus mean size, a 10-fold difference from 2.1 to 22.3 mg, respectively; the mean number of extrafusal fibres, a 2.5-fold difference from 497 to 1249; and mean fibrecross-sectional area, three-fold difference, e.g. for type 1 fibres, from 678 to 1948 lm 2 . The range of mean proportion of type 1 fibres was determined by C57BL/6J (31%) and DUH (64%) strains. The mean number of spindles per muscle ranged between nine (LG/J) and 13 (BEL) (strain effect P < 0.02). Genetic correlations between spindle count and muscle weight or properties of extrafusal fibres were weak and not statistically significant. However, there was a strong correlation between the proportion of spindles with more than one bag 2 fibre and the proportion of extrafusal fibres that were of type 1, and strain-dependent variation in the numbers of such spindles was statistically significant. The numbers of intrafusal fibres per spindle ranged from 2 to 8, with the most common complement of four found in 75.6% of spindles. There were no significant differences between the strains in the mean numbers of intrafusal fibres; however, the variance of the number was significantly less for the C57BL/6J strain than for any of the others. We conclude that abundance of muscle spindles and their intrafusal-fibre composition are substantially determined by genetic factors, which are different from those affecting muscle size and properties of the extrafusal fibres.

Baseline Muscle Mass Is a Poor Predictor of Functional Overload-Induced Gain in the Mouse Model

“…However, such a model is not consistent with our findings (Figure 2). The D2 strain soleus increased 1.7-fold compared to ~1.3-fold for the DUHi or BEH (this strain's soleus contains ~1063 fibers; Kilikevicius et al, 2013; Lionikas et al, 2013a,b). Thus, it is not the number of fibers per se , that is positively associated with muscle mass gain.…”

“…For instance, there is a considerable difference in the number of soleus fibers among the studied strains, ranging from ~760 in the D2 through ~1249 in the DUHi (Kilikevicius et al, 2013; Lionikas et al, 2013a,b). It is reasonable to hypothesize that, other factors remaining constant, more numerous fibers should confer greater gains due to a cumulative effect of fiber hypertrophy.…”

“…Comparison of relevant traits between animals of different strains reared under the same environmental conditions permits assessment of the role of the genetic background. Up to a 10-fold difference in muscle mass has been reported between inbred strains (Kilikevicius et al, 2013; Lionikas et al, 2013a,b). The number of fibers can differ by over two-fold (Lionikas et al, 2013b).…”

“…Up to a 10-fold difference in muscle mass has been reported between inbred strains (Kilikevicius et al, 2013; Lionikas et al, 2013a,b). The number of fibers can differ by over two-fold (Lionikas et al, 2013b). Collectively that indicates a substantial impact of genetic variability on these traits.…”

Analysis of Proprioceptive Sensory Innervation of the Mouse Soleus: A Whole-Mount Muscle Approach

“…There is limited anatomical data beyond receptor counts for rodents, and more particularly for the mouse, despite the opportunities afforded by transgenic mouse models to investigate both the form and function of these receptors [16,17]. In this study we exploited mouse genetic tools to analyze the structure and afferent supply of MS and GTOs in a whole-mount preparation of the soleus muscle, a muscle thoroughly examined in the cat literature.…”