Abnormal distribution of fiber types in the slow‐twitch soleus muscle of the C57BL/6J DY^2J/DY^2J dystrophic mouse during postnatal development

Abstract

The postnatal development of extrafusal fibers in the slow-twitch soleus muscle of genetically dystrophic C57BL/6J dy2J/dy2J mice and their normal age-matched controls was investigated by histochemical and quantitative methods at selected ages of 4, 8, 12, and 32 weeks. The majority of fibers in the soleus consisted of two kinds, fast-twitch oxidative-glycolytic (FOG) and slow-twitch oxidative (SO), according to reactions for alkaline-stable and acid-stable myosin ATPase and the oxidative enzyme, NADH-tetrazolium reductase. A minor population of fibers, stable for both alkaline- and acid-preincubated ATPase, but variable in staining intensity for NADH-TR, were designated "atypical" fibers. With age, the normal soleus exhibited a gradual increase in the number and proportion of SO fibers and a reciprocal, steady decline in the percentage of FOG fibers. Atypical fibers were numerous at 4 weeks, but were substantially diminished at later ages. Since total extrafusal fiber number remained relatively constant between the periods examined, this change in relative proportions reflects an adaptive transformation of fiber types characteristic of normal postnatal growth. A striking alteration in the number and distribution of fiber types was associated with the dystrophic soleus. At 4 weeks an 18% reduction in total fiber number was already noted. Subsequently, by 32 weeks a further 22% diminution in overall fiber number had occurred. With age, the absolute number and proportion of dystrophic SO fibers were drastically reduced. In contrast, the percentage of dystrophic FOG fibers increased significantly while their absolute numbers between 4 and 32 weeks remained relatively constant. Atypical fibers in the dystrophic solei were found in elevated numbers at all age groups, particularly at 12 weeks. They may, in part, represent attempts at regeneration or an intermediate stage in fiber-type transformation. Microscopically, both of the major fiber types appeared affected, albeit differently, by the dystrophic process. We suggest that a failure or retardation in the normal postnatal conversion of fiber types within the soleus muscle occurs in this murine model for muscular dystrophy.

Activation of the MEF2 transcription factor in skeletal muscles from myotonic mice

“…MEF2 activation was obvious in all the myotonic mice examined. Interestingly, MEF2 activation was limited to muscles composed mostly of fast glycolytic fibers (in wild-type mice), such as plantaris, extensor digitorum longus (EDL), and WV, but not in soleus, which has very few fast glycolytic fibers (34). MEF2 activation was observed in two strains of ADR myotonic mice: SWR/J-Clcn1 adr-mto and BALB/cByJ-Clcn1 adr-mto .…”

Stereological analysis of the muscle‐tendon junction in the aging mouse

“…Since it has been shown that all the fibers of the murine soleus extend from tendon of origin to tendon of insertion, this difference could not be due to a series-connectedness of physiologically different fibers (Schwarzacher, 1957). A possible explanation arises from the evidence that the soleus is actually a mixed muscle containing both fasttwitch and slow-twitch fibers (te Kronnie et al, 1980;Ovalle et al, 1983) which are regionally concentrated. Our sampling protocol could have resulted in our analyzing different mixtures of fiber types at the insertions and origins.…”

Genetic regulation of slowly progressing mild muscle atrophy in fast-twitch muscles of BUF/Mna rats

“…Human Duchenne dystrophy and murine mdx dystrophy inherit by X chromosome-linked recessive trait and murine dy and dy 2J myopathy inherit by autosomal recessive trait. [21][22][23] It is unique that the susceptible genes for the development of the mild-moderate and progressing muscle atrophy in B rats are autosomal dominant, as in dystrophy chicken. 2 Chromosomal mappings of these loci are under way.…”