No abstract

Early morphological changes of the rat soleus muscle induced by tenotomy were examined by scanning and thin-section electron microscopy. The potassium hydroxide (KOH) and collagenase treatment was successfully used to remove extracellular materials and to examine the surface of muscle fibres with a scanning electron microscope. Morphological changes in the fibre surface appeared as early as 12 h after tenotomy, showing wrinkling of the sarcolemma in localized areas. At 24 h post-operative, transverse folds and grooves were conspicuous along the entire length of muscle fibres. Tenotomized muscles which were denervated simultaneously or within 24 h of tenotomy did not show any appreciable changes in the fibre surface. Thin-section electron microscopy revealed that the earliest change inside muscle fibres was the occurrence of focal or segmental areas of an irregular alignment of myofibrils representing myofibril disorganization with occasional disintegrated Z lines at 24 h after tenotomy. Segmental myofibril breakdown was occasionally found to extend obliquely from the fibre periphery to the central area. Such myofibril disorganization became more conspicuous along muscle fibres, especially in the central area, at 2 days after tenotomy, showing the loss of thick filaments within myofibrils and the disintegration of Z lines. Such myofibrillar disorganization can be correlated with the morphological changes in the fibre surface in view of the clinical implication.

Summary. The effects of tenotomy on the surface morphology of muscle fibers including myotendinous junctions in the rat soleus muscle were studied by scanning electron microscopy (SEM). Using potassium hydroxide (KOH) and collagenase, the extracellular materials were successfully removed to expose the surface of muscle fibers. When the soleus muscle was tenotomized at both proximal and distal ends, virtually all muscle fibers showed marked alterations of the fiber surface characterized by the formation of numerous transverse grooves and folds along their length. Narrow longitudinal grooves and folds of the sarcolemma were also observed. At myotendinous junctions, the fiber ends showed an overall rounded shape with several short sarcoplasmic processes, indicating that the processes were significantly retracted. These changes were clearly recognizable at 5 days after tenotomy, and most apparent at one week. Thin-section electron microscopy of the same SEM samples demonstrated that such folding of the sarcolemma was not directly related to the sarcomere pattern of the underlying myofibrils, suggesting that, once formed, the folds and grooves were retained for a certain period of time. At 2 and 3 weeks the surface morphology of the fibers underwent a recovery process of restoring the smooth surface on which the crossstriations of the underlying myofibrils were seen. At the fiber ends, sarcoplasmic processes regrew into slender, wavy and short forms. Such sarcoplasmic processes were greater in number and more elaborate than those in the control muscle. At 5 and 6 weeks the fiber surface resumed an almost normal morphology, except that the sarcoplasmic processes at the fiber end were still shorter and more numerous than those in the control. These observations support our previous results obtained by thin-section electron microscopy that the myotendinous junction undergoes a series of morphological changes of collapse and regrowth of the sarcoplasmic processes, reflecting changes in the underlying myofibrils. In conclusion, the changes in the surface morphology of tenotomized muscle fibers were well correlated chronologically to those of myofibrils such as the central core lesion.

The formation of unique vacuoles in tenotomized rat soleus muscle fibers was examined by light and electron microscopy. After tenotomy at both proximal and distal tendons, virtually all muscle fibers underwent characteristic degenerative changes with a disorganization of myofibrils called the central core lesion, but eventually recovered. At 3 days after tenotomy, some muscle fibers showed small vacuoles in the sarcoplasm of the end segments, which were larger in diameter and paler in staining than those of the control fibers in light microscopy. At 5 days, more fibers formed larger vacuoles together with the extensive disorganization of myofibrils. Such vacuole formation was more conspicuous in the distal end than in the proximal end. At 1 week the myofibrillar disorganization was most extensive in the central areas, and vacuoles were considerably enlarged in some fibers to occupy most of the sarcoplasm near the fiber ends. Vacuoles decreased in number and size with time and could rarely be seen at 4 weeks postoperative. In thin-section electron microscopy, the early forms of vacuoles were often connected with the T-system tubules. The limiting membrane of such vacuoles possessed many caveolae, some of which appeared to be continuous with the T-system networks. The vacuole membrane was closely associated with the sarcoplasmic reticulum to form dyadic connections. In later stages, the vacuole membrane was lined in part with the basal lamina. From these findings, it can be concluded that the vacuoles are sarcolemmal in nature and derived from the T-system. The significances of the vacuole formation are discussed with special reference to the mechanism and fate of the vacuoles and their clinical implications.