Concentric laminated bodies. Ultrastructural demonstration of muscle fiber type specificity.

Concentric laminated bodies were identified in the skeletal muscle of 3 children affected by muscle weakness and hypotonia with probable cerebral involvement. The mean Z-line and M-line widths from each of 11 muscle fibers containing the concentric laminated bodies were calculated. The mean Z-line widths were 61-81 nm and the mean M-line widths from 7 out of 11 fibers were 54-65 nm. According to our present system of fiber typing, the majority of these fibers would be classified as subtype IIB. The possible pathogenesis and fiber type specificity of the concentric laminated body is discussed.