EM of a satellite cell in adult skeletal muscle. The cell sits on the surface of a mature muscle fiber. They share a thin, common external lamina (arrows). The narrow space between their apposed membranes is difficult to distinguish at this magnification. The cytoplasm contains a sparse collection of mitochondria, rough endoplasmic reticulum, and free ribosomes. Well-defined myofibrils with intervening elements of sarcoplasmic reticulum and mitochondria characterize the underlying muscle fiber. 24,000×