Response of satellite cells and muscle fibers to long-term compensatory hypertrophy.

The ultrastructural changes of the plantaris muscle of the rat have been investigated during long-term compensatory hypertrophy. The latter was induced by the removal of the synergistic gastrocnemius muscle. Transmission electron microscope (TEM) observations were performed on the 20th, 40th and 60th days after surgery on overloaded and control muscles. The plantaris muscle had hypertrophied 54.8% after 20 days, 74.4% after 40 days and 79.8% after 60 days. It was observed that activation of the satellite cell developed as shown by an increase in cytoplasmic volume, together with diffuse presence of polyribosomes and a rough endoplasmic reticulum. On the 40th day masses of swollen mitochondria grouped at the edge of muscle fibers were observed. On the 60th day structures completely covered with basal lamina, whose cytoplasm was filled with mitochondria and polyribosomes, were partially detached from the fibers. The above ultrastructural changes were absent in control contralateral muscles. These findings suggest that surgically induced overload produces a relevant ultrastructural re-arrangement in muscle tissue.