Morphometric and capillary analyses of panniculus carnosus skeletal muscle in the mdx mouse

Duchenne muscular dystrophy (DMD) is the most prevalent fatal genetic disease of children and one of the most common neuromuscular disorders. DMD is diagnosed as progressive muscle degeneration due to mutations in the dystrophin gene resulting in loss of the dystrophin protein. Dystrophin while present in skeletal and cardiac muscle, is also expressed in the smooth muscle and endothelial cells of blood vessels1. Dystrophin provides structural stability to the sarcolemmal membrane especially important under contractile stress.  It also localizes neuronal nitric oxide synthase to the sarcolemma, which regulates vasorelaxation via nitric oxide release. Therefore, dystrophin absence has been implicated in vascular defects and ischemia in DMD2.  These defects have not been fully explored and their contribution to the pathology is ill-defined. For this reason we chose to characterize the dorsal skin panniculus carnosus (PC) skeletal muscle in the mdx mouse model of DMD.  The PC muscle was chosen due to the high visibility of the microcirculation and relatively high regenerative activity3 . The PC in mdx is a poorly documented skeletal muscle. Therefore, to assess that PC exemplifies the dystrophic phenotype, we applied two morphological indexes to transverse sections from both mdx and wild-type (wt) mice; heterogeneity of myofiber cross-sectional-area (CSA) and the percentage of central nucleated fibres (CNF). Herein, we found that PC muscle from 5 week and 11 week old male mdx and wt mice showed high muscle regeneration. PC in mdx demonstrated a higher percentage of CNF by 4-fold at 5-weeks and by 5-fold at 11-weeks of age compared to wt. The mean distribution data of CSA revealed a significant shift towards increased fibre area in mdx at both time points. Two dimensional analyses of capillaries in PC muscles were performed at from 5 week old mice revealed that the capillary number area was similar between mdx and wt.  However, capillary-to-fibre ratio was higher in mdx compared to wt muscles at this 5 week time point. In conclusion, the PC muscle shows a high regenerative activity and exemplifies the DMD dystrophic phenotype. Therefore, future studies on PC muscle in mdx mice will allow us to gain a better insight into the vascular defects of DMD, and the testing of therapeutic agents in ameliorating the disease using established techniques such as the dorsal skinfold chamber model.