Human myasthenia gravis thymic myoid cells: de novo immunohistochemical and intracellular electrophysiological studies.

Thymic myoid cells from myasthenia gravis (MG) patients and controls were successfully grown in explant cultures: we have compared them with skeletal muscle cells cultured from biopsies in morphological, immunohistochemical and electrophysiological studies. Some mononucleate cells in thymus cultures were myoglobin- or desmin-positive, but they were much rarer than the otherwise similar fusing myoblasts in muscle cultures. Frequencies of cultured myoglobin-positive cells showed no difference between MG and control and male or female, but were lower in samples of malignant thymoma, in younger cases and in those with less severe MG. Electrophysiologically the resting membrane potentials of cultured thymic multinucleate cells were significantly less than those of cultured skeletal muscle cells, and action potentials by electrical stimulation were rarely observed. In thymus cultures from only one case with malignant thymoma, desmin-positive myotubes had spontaneous irregular contractions followed by electrical firings. It is concluded that there are myoid cells in MG and control thymuses which have the potential to become skeletal muscle fibers morphologically and electrophysiologically, although their frequency and proliferation in culture are quite low.