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.