Ultrastructural changes of myotendinous junctions in tenotomized soleus muscles of the rat.

The effects of tenotomy on the ultrastructure of myotendinous junctions in the rat soleus muscle were studied by thin-section electron microscopy. When the soleus muscle was tenotomized at both distal and proximal ends, virtually all muscle fibers showed a characteristic alteration called central core lesion in their middle portions. The similar alteration was also found in the end portions of each muscle fiber. As early as 5 days after tenotomy, myofibrils at the fiber end were extensively disoriented and disintegrated with thickening of the Z-discs. The surface specializations, sarcoplasmic processes and invaginations of the fiber end were also altered in such a way that the sarcoplasmic processes retracted. The disintegration of myofibrils was most extensive at one week after tenotomy, often losing their association with the sarcolemma. The thickening and rod formation of the Z-discs at the fiber ends were prominent at 1 and 2 weeks. The thickened Z-discs and rod structures were never closely associated with the sarcolemmal undercoats, which were not significantly thickened. This suggests that the Z-disc and the sarcolemmal undercoat are discrete entities. Interestingly, the basal laminae were frequently seen to be separated from the sarcolemma forming free sheets, as the sarcoplasmic processes retracted. In the space surrounded by such free basal lamina were seen many vesicular or granular materials. At 3 and 4 weeks the fiber end showed a recovery process forming new myofibrils, as the sarcoplasmic processes grew long. Thus, the surface specializations of the fiber end were much more elaborate than those in the control muscle. At 5 and 6 weeks the myotendinous junctions returned to normal ultrastructure except that the surface specializations were still elaborated. These findings demonstrated that the myotendinous junction underwent a series of characteristic alterations and recovery of the surface specializations as well as myofibrils.