Morphological characters and disturbance of myofibril assembly in avian myoblasts fused with HVJ (Sendai virus).

We have been studying myogenic differentiation, especially the mechanism of myotube formation, using quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus, QM-RSV cells. Myogenic differentiation of QM-RSV cells can be controlled by the incubation temperature. When cultured at 35.5 degrees C, a permissive temperature for the virus, the cells proliferate but do not differentiate, whereas, when incubated at 41.0 degrees C, a non-permissive temperature, they proceed through myogenic differentiation and then form myotubes. Presumptive QM-RSV cells cultured at 35.5 degrees C were fused with hemagglutinating virus of Japan (HVJ, Sendai virus) and the morphological characteristics of the fused myoblasts were compared with the myotubes formed spontaneously at 41.0 degrees C. The artificially fused myoblasts were tubular, like the spontaneously formed myotubes, and their tubular form was based on their microtubules. However, assembly of myofibrils was seen in spontaneously formed myotubes cultured at 41.0 degrees C, but not in artificially fused myoblasts cultured at 35.5 degrees C. Instead of myofibrils, round or rod-shaped structures were seen in artificially fused myoblasts. These structures were stained with rhodamine-labeled phalloidin but not anti-myosin antibody and were formed at 35.5 degrees C. On shift up to 41.0 degrees C, these structures disappeared, and myofibrils were formed, suggesting that the actin-positive structures in artificially fused myoblasts at 35.5 degrees C were disassembled forms of myofibrils.