Details of the I band structure as revealed by the localization of ferritin.

Introduction of ferritin inside muscle fibres deprived of the sarcolemma puts in evidence the presence of two lines, here called N lines, which cross the I band. In correspondence of the N(1) line, which is nearer to the Z line, the thin filaments are disposed in a square pattern. In correspondence of the N(2) line the thin filaments are irregularly disposed and glycogen granules occupy the spaces between them. Small projections of the thin filaments, which occur at the distance of approximately 400 A along the filaments are interpreted as due to the presence of tropomyosin.