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 m embrane and the basement membrane of a myofiber. In normal adult muscle, SC are quiescent in the Go phase of the cell cycle. The contact of the SC wit h the myofiber plasma membrane imposes a mitotic inhibition on the SC. Sarc olemmal molecules which might explain this membrane-imparted mitotic inhibi tion have not yet been identified. In this study we examined the border of the SC and the adjacent myofiber electron microscopically, assessing the nu mber of SC showing encroachments of basement membrane (BM) material, secret ion of cellular degradation products into the intercellular space, and cave olae. We studied normal and diseased muscle including Duchenne muscular dys trophy, Becker muscular dystrophy, idiopathic inflammatory myopathies, and neurogenic atrophies. Caveolae were present in SC from normal muscle, but t hey were more abundant in SC from diseased muscle, and they significantly p revailed 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, a nd mostly so in neurogenic atrophies. Secretion of cellular degradation pro ducts into the intercellular cleft occurred in normal and diseased muscle. We conclude that: degradation products in the intercellular cleft do not di sturb 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 wh en compared with the inner surface in diseased muscle remains at present un known.