Morphological transformation of slow to fast muscle fibers after tenotomy

Abstract

Recent investigations have reported a speeding of contraction in mammalian slow-twitch muscles after tenotomy, but others have questioned the validity of the reports. This paper reports the results of long-term tenotomy on avian slow skeletal muscle. The slow anterior latissimus dorsi muscle of pigeions were tenotomized and the proximal portion freed from the underlying connective tissue in its proximal half. The distal portion, in which the nerve enters, was not disturbed. The contralateral muscles were used as controls. The muscles were examined after a year. The proximal ends had reattached to the connective tissue of underlying muscles, so the length was reduced to 75% of the control. The muscles were also smaller than the control. The ultrastructure demonstrated that, in addition to various pathological changes, a large proportion of the fibers had developed triadic complexes and the Z-line had transformed from the typical slow type to one more characteristic of fast fibers. The fibers had also developed a fibrillar pattern, atypical of slow fibers. These morphological changes suggest that earlier investigations showing a speeding of contractions after tenotomy might be doing so by these changes. The various factors that might cause this transformation are discussed.