Electron microscopic and autoradiographic characterization of hindlimb muscle regeneration in the mdx mouse

Abstract

The pattern of postnatal growth and development of skeletal muscle in mdx mice was studied by light and transmission electron microscopy and by autoradiography and was compared with that in their normal age-matched controls at 4 and 32 weeks of age. The muscle weights of both the extensor digitorum longus (EDL) and soleus muscles of mdx mice were significantly greater than those in control mice at both ages. Body weights of male and female mdx mice were also increased over controls up to 12 weeks of age. At 4 weeks, both the EDL and soleus muscles exhibited focal areas of degeneration, necrosis, and regeneration of centrally nucleated extrafusal fibers resulting in a wide range of fiber sizes. By 32 weeks, the majority of fibers in both muscles were centrally nucleated, and focal areas of recent regeneration were observed. By electron microscopy, the course of macrophage infiltration into areas of degenerating fibers and the ongoing regeneration of myofibers within redundant cylinders of external lamina were noted. This pattern was frequent in 4-week-old mdx muscles and was present to a lesser degree at 32 weeks. A notable lack of both adipose tissue infiltration and fibrotic change in the endomysium were observed in muscles at both ages. Autoradiograms of muscles from 4-week-old mdx mice injected with tritiated thymidine showed an increased proportion of labeled sublaminal nuclei at 24 and 48 hours after injection compared to controls. At 32 weeks of age, labeling of nuclei in muscles of mdx mice was also greater than in controls, but was reduced compared to muscle labeling in 4-week-old mdx mice. The observed features of mdx muscle tissue suggest that this animal model is more applicable to the study of regeneration dynamics than to Duchenne-type human muscular dystrophy.

Age-Dependent Effect of Myostatin Blockade on Disease Severity in a Murine Model of Limb-Girdle Muscular Dystrophy

“…First, mdx mice might have greater regenerative capacity in their skeletal muscle compared with humans with muscular dystrophy. [17][18][19][20][21][22] Second, mdx mice can show up-regulation of utrophin that may partially compensate for the lack of dystrophin, thus reducing pathological manifestations of disease somewhat. In contrast, humans with DMD show no significant utrophin up-regulation and are characterized by progressive muscle degeneration with satellite cell senescence.…”

MyoD is required for myogenic stem cell function in adult skeletal muscle.

“…This regeneration results in the formation of additional muscle fibers, which leads to a significant level of muscle hypertrophy. Hypertrophy in rndx muscle is characterized by a 25% increase in the number of fibers, a four-fold increase in the distribution of the fiber calibers, centrally located nuclei within 70%-80% of all fibers, and a 1 .-/-fold increase in muscle mass (Anderson et al 1987;Carnwath and Shotton 1987;Coulton et al 1988; e.g., see Fig. 2, below; Table 1).…”

“…These data suggest that MyoD has a role in the postnatal growth of skeletal muscle. The rndx phenotype is characterized by an -1.7-fold increase in the mass of individual skeletal muscles compared with wild-type mice (Anderson et al 1987;Carnwath and Shotton 1987;Coulton et al 1988; e.g., see Fig. 2 and Table 1).…”

“…Detection of cell proliferation by tritiated thymidine incorporation in vivo was performed as described previously (Anderson et al 1987). Mice were intraperitonealy injected 3.5 days following injury with tritiated thymidine (Amersham) at a dose of 2.0 pCi/gram body weight.…”

Lmo7 is dispensable for skeletal muscle and cardiac function

“…Therefore, we generated a novel Lmo7-null (lmo7 Ϫ/Ϫ ) mouse with deletion of all three isoforms. Using these mice, we examined skeletal and cardiac muscle physiology, as well as the role of Lmo7 in a model of muscular dystrophy and regeneration using the dystrophindeficient mdx mouse model (1,6). Despite loss of all Lmo7 isoforms, our results demonstrated that lmo7 Ϫ/Ϫ mice had no abnormalities in skeletal muscle morphology, physiological function, or regeneration.…”