Ultrastructural findings at the satellite cell-myofiber border in normal and diseased human muscle biopsy specimens.

Satellite cells (SC) are mononuclear myoblasts located between the plasma membrane and the basement membrane of a myofiber. In normal adult muscle, SC are quiescent in the G0 phase of the cell cycle. The contact of the SC with the myofiber plasma membrane imposes a mitotic inhibition on the SC. Sarcolemmal molecules which might explain this membrane-imparted mitotic inhibition have not yet been identified. In this study we examined the border of the SC and the adjacent myofiber electron microscopically, assessing the number of SC showing encroachments of basement membrane (BM) material, secretion of cellular degradation products into the intercellular space, and caveolae. We studied normal and diseased muscle including Duchenne muscular dystrophy, Becker muscular dystrophy, idiopathic inflammatory myopathies, and neurogenic atrophies. Caveolae were present in SC from normal muscle, but they were more abundant in SC from diseased muscle, and they significantly prevailed at the outer surface of SC in all of the diseased muscle groups. Encroachments of BM material was only present in SC from diseased muscle, and mostly so in neurogenic atrophies. Secretion of cellular degradation products into the intercellular cleft occurred in normal and diseased muscle. We conclude that degradation products in the intercellular cleft do not disturb SC adhesion and that there is a neural influence on SC adhesion. The significance of the abundance of caveolae at the outer surface of the SC when compared with the inner surface in diseased muscle remains at present unknown.

Abundant minute myotubes in a patient who later developed centronuclear myopathy.

Centronuclear myopathy (CNM) is a congenital myopathy which manifests itself as a severe neonatal (also termed myotubular myopathy), early-onset, or adult form. The histological pattern of each is marked by a considerable number of nuclei of muscle fibers being internally placed. Owing to their remote resemblance to myotubes, and their expression of developmentally regulated proteins, most authors now favor the concept that myogenesis is arrested or delayed in this disease. We here present two muscle biopsy specimens of a patient with early-onset CNM, taken at the age of 5 months and 14 years, respectively. The first biopsy sample contained internally placed nuclei in 7% of the muscle fibers, abundant minute myotubes, and hypertrophic muscle fibers. The second biopsy sample showed internally placed nuclei in 40% of the muscle fibers, and hypotrophic fibers. We suggest that the histological findings in early-onset CNM are the result of a complex dynamic process, which includes a delay in maturation.