Ultrastructural fiber typing in normal and diseased human muscle.
The differential staining of the 3 fiber types for oxidative enzyme activity at the histochemical level provides the basis of their identification at the ultrastructural level. Type I fibers have the largest and most numerous mitochondria, the Type IIA smaller and less numerous mitochondria, and Type IIB have the smallest and least numerous mitochondria as studied in 4 patients without neuromuscular disease. Type I fibers could be distinguished from Type II fibers on the basis of mean Z-line width and IIA fibers could be distinguished from IIB fibers on the basis of mean M-line width. Type I fibers had wide Z-lines (95 nm) and wide M-lines (89 nm), Type IIA had narrow Z-lines (74 nm) and wide M-lines (79 nm) and Type IIB fibers had narrow Z-lines (69 nm) and narrow M-lines (60 nm). This system of fiber typing based on relative Z-line and M-line widths was applied to several abnormal muscle biopsies each of which showed a deficiency in at least 1 of the major histochemical fiber types. In each case, the same deficiency was revealed at the ultrastructural level by measuring the relative Z-line and M-line widths of the remaining fiber types.