Morphometry of muscle fibre types in the carp (Cyprinus carpio L.). Relationships between structural and contractile characteristics.

Ultrastructural parameters of muscle fibre types of the carp (Cyprinus carpio L.) were measured and compared with their contractile properties. In red fibres, which are slower than pink fibres, the relative length of the junction between the T system and the sarcoplasmic reticulum (T-SR junction) is smaller and the Z lines are thicker than in pink fibres. Pink fibres have a smaller relative length of T-SR junction than white fibres from the axial muscles. The two types of red fibres present in carp muscle also differ in their relative lengths of T-SR junction. Significant differences in the relative areas of the SR were not found. The relative volume of myofibrils in red fibres is two-thirds that in pink fibres, a difference that is not reflected in the maximal isometric tetanic tensions of these types. Red fibres, which are less easily fatigued than pink fibres, have larger relative volumes of subsarcolemmal and intermyofibrillar mitochondria. Small pink fibres have a larger relative volume of subsarcolemmal mitochondria than large pink fibres, but have a similar relative volume of intermyofibrillar mitochondria. Small and large pink fibres differ in the relative volumes of their membrane systems, but have similar relative lengths of T-SR junction.