Scanning electron microscopic study of the muscle fiber ends at the myotendinous junction in the posterior cricoarytenoid and cricothyroid muscles in rats.

The fine structural organization of muscle fiber ends at the myotendinous junction in the posterior cricoarytenoid (PCA) and cricothyroid (CT) muscles in adult rats was studied by scanning electron microscopy, after removal of tendon connective tissues using the HCl-hydrolysis method. The muscle fiber ends in the PCA muscle had a relatively simple conical appearance and contained a great number of longitudinal slits on the surface. In the CT muscle, the muscle fiber ends were classified into two types. One type had a conical appearance similar to the PCA muscle with many slits on the surface, while the other type was characterized by a complex arrangement of cylindrical cytoplasmic processes and deep clefts, as seen in common skeletal muscles. These findings suggest that the PCA muscle is evolutionarily primitive, and that the CT muscles represent a transitional form between primitive and evolved muscles.