Five different types of centrally nucleated muscle fibres in man: elemental composition and morphological criteria. A study of myopathies, fetal tissue and muscle spindle.

Muscle biopsy material from patients with two types of benign hereditary myopathy was explored with electron probe X-ray microanalysis performed on thick cryosections observed in a scanning electron microscope equipped for the analyses. The elemental spectra thus obtained were related to the ratio between cross-sectional areas of centrally located nuclei and corresponding muscle fibres. Fetal myotubes and intrafusal muscle fibres were examined by the same methods. One of the myopathies was characterized by fibres resembling myotubes ('myotubular myopathy') and exhibited elementary spectra with i.a. higher sodium and chlorine and lower potassium signals than normally found. These spectra have a tinge of those found in fetal muscle and in extrafusal muscle fibres. The other type of myopathy had less specific morphological characteristics and exhibited different pathological changes in the muscle fibres and i.a. central nuclei in a considerable number of fibres. These nuclei were, however, smaller in relation to the cross-sectional area of the corresponding fibres than in fetal myotubes, fibres in the myotubular myopathy and in the intrafusal muscle fibres. Elemental spectra of muscle fibres in the latter myopathy conformed to the normal state. We conclude that central nuclei of muscle fibres is a pathological phenomenon which can mirror an arrest in fibre development as well as a muscle fibre degeneration. An arrest in the development is suspected in myopathic muscle fibres with large central nuclei and an elevated content of sodium chlorine in the sarcoplasm.