After denervation, the usual cytochemical criteria for identifying fibre types in mammalian skeletal muscle are lost, and thus ultrastructural analysis becomes essential. The width of the Z-line is a valuable criterion for the identification of fibre types in normal muscle, and the differences in width remain apparent even after denervation. In the red portion of the normal rat semitendinosus muscle, there is a mixture of red, intermediate and white fibres. The red fibre has the highest mitochondrial content and the widest Z-line, whereas the white fibre has the lowest mitochondrial content and the narrowest Z-line (about half as wide as that of the red fibre). The intermediate fibre is intermediate in both characteristics. At 14 days after denervation, there is a shift toward a more homogeneous population of fibres, which are rich in mitochondria, but which have an ultrastructural appearance distinct from that of normal fibres. Most of the fibres resemble normal red or intermediate fibres, but lack the characteristic subsarcolemmal aggregations of mitochondria. There are also some fibres which exhibit severe myofibrillar disruption. These have narrow Z-lines, and are therefore classified as white fibres. This suggests that there is a preferential alteration of white fibres, and that their degradation may be responsible for the apparent increase in the proportion of red or intermediate fibres at this stage of denervation. In these disrupted fibres the sarcoplasmic membrane systems lose their usual organization, and triads become aligned parallel to the longitudinal axis of the myofibrils. All fibres, therefore, are, to some extent, altered by denervation, though the changes differ according to the type of fibre.
In normal fibres, ribosomes are sparse, but following denervation they are abundant, particularly at the periphery. This is a general response to removal of the nerve supply, and does not appear to be related to fibre type; but it is more apparent in red and intermediate fibres, which possess an extensive superficial sarcoplasm. Conspicuous aggregations of large mitochondria normally occur in the subsarcolemmal region of these fibres, but after denervation, there are, instead, massive accumulations of free ribosomes and a few cisternae of rough-surfaced endoplasmic reticulum. Our observations suggest, moreover, that this concentration of protein-synthetic machinery corresponds to a spread in the sensitivity to acetylcholine along the surface of the fibres.