Histological and Molecular Characteristics
Necrosis and Regeneration:
An event in the progression of DMD is initiated by disruption of the muscle cell membrane due to the missing cytoskeletal protein Dys. This disruption of the plasma membrane of the muscle cell, results in necrosis of the affected muscle fibers. Following necrosis, attempts at regeneration by proliferation and fusion of satellite cells with the remaining myotube occurs, which results in the formation of branched fibers. Branched fibers are susceptible to damage from skeletal muscle induced-contraction, which creates a secondary cycle of muscle degeneration.
Branching Muscle Fibers
Histological analyses of regenerating muscle fibers of persons with DMD were performed by Henning. Muscle biopsies were obtained from two males with DMD, both eight years of age, and one healthy 13-year-old. Biopsies were obtained from the quadriceps of DMD Male 1, biceps brachii of DMD Male 2, and the quadriceps of Normal Male 3. CK levels were 17,600 U/l and 7,900 U/l for boys 1 and 2 respectively which indicate muscle damage in these boys (average CK levels for males is 55-170 U/L).
Muscle biopsies of the males with DMD showed numerous branching fibers amongst strands of dense connective tissue. Motor end plate position could not be determined in these branching fibers and none of the cross sections studied contained nerve fibers. These results suggest that branching fibers may not have been innervated after necrosis and regeneration. Non-innervation would suggest that these fibers were non-functional, and this may explain the muscle atrophy weakness that is seen in males with DMD by the ages of eight to 12 years.
The symptoms of DMD are manifested by an imbalance between muscle damage and regeneration. Sandri et al. suggested that programmed cell death or apoptosis might play a role in the progression of DMD in humans. Biopsies taken from males with DMD (ages 9 months to 8 years) were analyzed by in situ nick-end labeling for apoptotic muscle cell nuclei.
The results indicate that all the DMD samples showed apoptotic nuclei compared to the control sample which had none. Apoptotic nuclei increased with age up to seven years in the males with DMD. This increase in apoptotic nuclei parallels the onset of severe muscle weakness usually seen in DMD males between the ages of eight to 12. Increased loading of the skeletal muscles and muscle damage may be responsible for this phenomenon. This muscle damage or membrane disruption may cause an influx of Ca+2 into the cytoplasm triggering protease activation and apoptosis. The question of whether exercise may induce progression of DMD due to the increased muscle damage and loading can be raised. Exercise-induced muscle damage may activate apoptosis of muscle nuclei, thus triggering myofiber instability and necrosis.