Ultrastructure of the different fibre types in axial muscles of the sharks Etmopterus spinax and Galeus melastomus.

Red, intermediate and white axial muscle fibres of the sharks Etmopterus spinax and Galeus melastomus were studied by electron microscopy and morphometry. The mitochondrial content is more than thirty percent in red, less than one percent in white, and up to fifteen percent in intermediate fibres. About one third of the mitochondria in red fibres are accumulated close to the sarcolemma. Red fibres contain much glycogen, present as rosettes (alpha particles). Intermediate fibres contain less glycogen (as beta particles). White fibres have scarcely any visible energy reserves. Red fibres contain slightly less (4-5%) of the sarcotubular system than the other fibre types (6-8%). In all fibre types, the terminal cisternae of the SR are regularly divided by clefts. Triads or dyads are generally positioned at the Z discs, but in Galeus white fibres two dyads may be present, one on each side of the Z disc. The morphology is discussed in relation to current views on the functions of different muscle fibre types.