Establishment of a myoid cell clone from rat thymus.

In the course of rat thymus cultures, cloning of myoid cells (myoblasts) was successfully achieved. Myoblasts first appeared in the primary culture of the rat thymus in very limited number, but proliferated rapidly, started to fuse, and formed multinucleated myotubes contracting spontaneously. In the 15th month of culture, myoblasts were detached from the flasks to establish clones by the limiting dilution method. The cloned cells of IT-45R92 were identified as myoid cells; the small spindle-shaped cells proliferated rapidly and fused with each other forming large cross-striated myotubes. Ultrastructurally, thick and thin filaments with A, I, and Z bands as well as H and M bands occupied the cytoplasm of a large myotube. No epithelial origin of IT-45R92 cells was considered, and pluripotent stem cells were regarded as the most likely cells of origin. A difference is suggested between the factor stimulating fusion and the factor stimulating formation of fully differentiated myotubes. The significance of myoid cells is discussed in relation to acetylcholine receptors of IT-45R92 and the possible causes of myasthenia gravis.